Encyclopedia of Earth Science

434 Triassic

signature. This can be difficult and requires precise dating Transgressive marine sequences record major marine
and correlation of events along different shorelines, plus a advances over the land at several times in the Phanerozoic.
detailed understanding of the local tectonic and sedimenta- Most transgressive sequences are preceded by an erosional
tion history. When the local effects are isolated they can be unconformity and show a progressive landward shift in sedi-
subtracted from the global sea-level curve, and the causes of mentary facies that, according to Walther’s Law, is also
global sea-level changes can be investigated. recorded in the vertical sequence. The base of transgressive
sequences is typically marked by a beach sandstone or con-
Plot showing the six major unconformity-bounded sequences for North glomerate unit, followed upward by an offshore muddy
America, formed when major transgressions occur. Sea levels have risen facies, then typically a deeper water limestone facies.
to more than 1,150 feet (350 m) above present levels and fallen to 655 feet
(200 m) below present levels. The six major transgressive sequences are In the 1950s, using index fossils and isotopic dates of key
known as the Sauk (S), Tippecanoe (T), Kaskaskia (K), Absaroka (A), Zuni horizons, an effort pioneered by Laurence L. Sloss and
(Z), and Tejas (T). coworkers in the petroleum industry, correlated many trans-
gressive sequences across North America and the world.
Many large, laterally extensive rock units that are bounded by
unconformities of regional or global significance were recog-
nized and precisely dated in many places. Some of these
unconformity-bounded sequences are so significant that they
are found in almost all shallow water deposits of that age in
the world. These sequences always occur where sea level has
dropped from high to low, and the overlying sequence is
transgressive. Index fossils were used to show that these
unconformities have the same age on all continents and are
clearly related to changes in sea level. Sea level has fluctuated
by as much as 1,150 feet (350 m) higher than the present level
and 655 feet (200 m) below the present level. Using these cor-
relations, six major transgressive sequences have been recog-
nized in the stratigraphic record of the continents. These
transgressive sequences include the Eocambrian-Cambrian
(600–500 Ma) Sauk Sequence, the Middle Ordovician–Lower
Devonian (470–410 Ma) Tippecanoe Sequence, the Middle
Devonian–Upper Mississippian (410–320 Ma) Kaskaskia
Sequence, the Lower Pennsylvania–Lower Jurassic (320–185
Ma) Absaroka Sequence, the Middle Jurassic–Upper Creta-
ceous (185–30 Ma) Zuni Sequence, and the Tertiary-Recent
(30–0 Ma) Tejas Sequence.

See also REGRESSION; SEQUENCE STRATIGRAPHY.

Triassic The oldest of three Mesozoic periods, and the cor-
responding system of rocks. The Triassic period is bounded
below by the Permian period of the Paleozoic era, and above
by the Jurassic period. The time span by the Triassic ranges
from 248 million years ago to 206 million years ago, divided
into Early (248–242 Ma), Middle (242–227 Ma), and Late
(227–206 Ma) epochs, and seven ages including, from oldest
to youngest, the Induan, Olenekian, Anisian, Ladinian, Car-
nian, Norian, and Rhaetian.

The base of the Triassic is also the base of the Mesozoic,
meaning middle life, so this boundary is marked by a pro-
found change in the fossil biota in the stratigraphic sequence.
The end of the Permian saw the extinction of 90 percent of
marine organisms, followed by a re-radiation of the pelecy-
pods, sea urchins, lobsters, and ammonoids. The ammonoids
have proven very useful in subdividing the Triassic in dozens

tropical climate 435

Late Triassic reconstruction of Tethys and the Panthalassic Ocean

of biozones, and the ages of the periods are under considera- activity with active volcanism stretching from Alaska through
tion for revision based on these higher resolution divisions. the North and South American cordillera and into Antarctica.
The first hexacorals appeared in the Early Triassic, whereas
oysters appeared in the Ladinian, and the first dinosaurs are Numerous flood basalts were erupted and swarms of
known from the Carnian. The first mammals, turtles, and diabase dikes intruded during the Triassic, including the
crocodiles all appeared in the Late Triassic. Many extinctions Karoo of South Africa, the Permo-Triassic Siberian traps, and
occurred throughout the Triassic, including the loss of dozens along the eastern coast of North America, western North
of families of marine gastropods, cephalopods, brachiopods, Africa, and northeastern South America.
bivalves, and sponges. Terrestrial extinctions include numer-
ous families of insects, freshwater fish, and reptiles. Some Triassic climates were generally warm, and sea levels
models have suggested that these numerous extinctions creat- started low, fluctuated in the Middle Triassic, and were gen-
ed opportunities for mammals to radiate and succeed in a pre- erally about 300 feet (100 m) higher than the present level in
viously hostile world. The cause of so many extinctions is not the Late Triassic. Deserts covered much of inland Pangea,
well known, but several seem to be grouped at the end of the with extensive evaporite basins, red beds, and coal swamps
Early Triassic, in the Carnian, and at the end of the Triassic. forming in different locations on land, and marine carbonates
The triggering mechanism for the first two events is unknown, deposited in much of the Tethys Ocean.
but some models suggest that a meteorite impact, identified as
the 50-mile (80-km) wide Manicuagan structure in Ontario, See also FLOOD BASALT; MESOZOIC; PANGEA; PANTHAL-
may have been responsible for the end-Triassic event. LASA; TETHYS.

The supercontinent of Pangea stretched nearly from pole tropical climate The climate in the tropics depends on
to pole in the Triassic, surrounded by the Panthalassa Ocean, which definition of the tropics is used. The equatorial belt
and partly surrounding the wedge-shaped Tethys Ocean that between 10° north and south latitude is characterized by
contained numerous reefs and carbonate platforms. The west- upwelling of warm moist air masses, and frequent thunder-
ern margin of Pangea was dominated by convergent margin storms. Most of the deserts of the world fall between 15° and
30° latitude, because this is where cool dry air descends in
Hadley Cell global circulation belts. The major deserts of

436 tropics

North Africa (Sahara) and South West Asia are included in The main components of the atmosphere are nitrogen,
this belt, as are the Kalahari of southern Africa, the Australian oxygen, argon, carbon dioxide, helium, krypton, neon, and
desert, and the Sonoron of the United States and Mexico. xenon. Increasing levels of photochemical smog in the tropo-
Thus, most of the true tropics are characterized by dry sunny sphere is an increasing problem in many places. Smog is
conditions, with the cool downwelling surface air dramatical- made mainly of the gas ozone (O3), produced by secondary
ly warmed on the surface. There is a large annual temperature chemical reactions from automobile and other pollution such
cycle, especially in the continental interiors, whereas coastal as hydrocarbons, although some is produced naturally. While
regions in tropical climates show a much smaller annual tem- ozone in the stratosphere forms a protective shield against
perature variation because of the moderating effects of the ultraviolet radiation, low-level ozone is harmful. Ozone irri-
ocean. Rainfall is extremely variable, with some years show- tates the respiratory system and also retards tree and crop
ing no rainfall in many places, and occasional intense rains in growth, and even causes rubber to break down. The forma-
others. Climate zones include dry tropical climate and tion of low-level ozone is particularly bad near major urban
Mediterranean-type including subdesert. These regions are areas such as Los Angeles, California.
characterized by a very large soil moisture deficiency and by
low amounts of soil water storage. Potential evapotranspira- See also ATMOSPHERE.
tion may be many times greater than the actual rainfall.
tsunami Long wavelength seismic sea waves generated by
Tropical rainy climates and seasonal wet/dry climates char- the sudden displacement of the seafloor. The name is of
acterize other parts of the tropics. Much of the Indian Ocean Japanese origin, meaning “harbor wave.” Tsunami are also
realm and southeast Asia are in the tropics and characterized by commonly called tidal waves, although this is improper
drenching monsoonal rains, and average temperatures in all because they have nothing to do with tides. Tsunami may rise
months greater than 64°F (18°C). Tropical rainy climates have unexpectedly out of the ocean and sweep over coastal com-
no rainy season, and large annual rainfall that exceeds the munities, killing hundreds of people and causing millions of
evaporation potential. Tropical rainforests, such as those in dollars in damage. Such events occurred in 1946, 1960,
southeast Asia, the southwest Pacific islands, and parts of South 1964, 1992, 1993, and 1998 in coastal Pacific areas. In 1998
and Central America receive more than 2.4 inches (6 cm) of a catastrophic 50-foot (15-m) high wave unexpectedly struck
rain in the driest season, whereas the monsoonal tropical cli- Papua New Guinea, killing more than 2,000 people and leav-
mate has a pronounced dry season with some months receiving ing more than 10,000 homeless. The December 2004 Indian
less than 2.4 inches of rain. Tropical savanna climates have at Ocean tsunami killed approximately 300,000 people, making
least one month with less than 2.4 inches of rain. it the most destructive tsunami known in history.

See also CLIMATE; DESERT; TROPICS. Tsunami are generated most often by thrust earth-
quakes along deep-ocean trenches and convergent plate
tropics The tropics fall between 10° and 25° north and boundaries. Tsunami therefore occur most frequently along
south latitude, bounded on the low-latitude side by equatori- the margins of the Pacific Ocean, a region characterized by
al regions, and on the high latitude side by subtropical then numerous thrust-type earthquakes. About 80 percent of all
mid-latitude belts. Most informal usages of the term tropics tsunami strike circum-Pacific shorelines, with the most
include the entire latitudinal belt between the tropics of Can- being generated in and striking southern Alaska. Volcanic
cer (23.5°N) and Capricorn (23.5°S). The tropics are charac- eruptions, giant submarine landslides, and the sudden
terized by a marked seasonal cycle, because the sun is directly release of gases from sediments on the seafloor may also
overhead the tropic of Cancer at the summer solstice, and generate tsunami. Tsunami are not rare on Pacific islands
above the tropic of Capricorn at the winter solstice. This including Hawaii and Japan, which now have extensive
causes a large variation in the solar insolation at different warning systems in place to alert residents when they are
times of the year in each tropical zone. likely to occur. Before these warning systems were in place,
residents would have no warning when the tsunami, in
troposphere The atmosphere is divided into several layers, some cases reaching 50 feet or more in height. would occa-
based mainly on the vertical temperature gradients that vary sionally strike coastal areas.
significantly with height. The lower 36,000 feet (11 km) of
the atmosphere is characterized by circulating air known as Some historical tsunami have been absolutely devastat-
the troposphere, where the temperature generally decreases ing to coastal communities, wiping out entire populations
gradually, at about 14°F (6.4°C) per kilometer, with increas- with little warning. One of the most devastating tsunami in
ing height above the surface. This is because the Sun heats the recent history was generated by the eruption of the Indone-
surface, which in turn warms the lower part of the tropo- sian volcano Krakatau in 1883. When Krakatau erupted, it
sphere. Most of the atmospheric and weather phenomena we blasted a large part of the center of the volcano out, and sea-
are familiar with occur in the troposphere. water rushed in to fill the hole. This seawater was immediate-
ly heated and it exploded outward in a steam eruption and a

tsunami 437

huge wave of hot water. The tsunami generated by this erup- tsunami that destroyed many Mediterranean coastal areas
tion reached more than 120 feet (37 m) in height and killed and probably led to the eventual downfall of the Minoan civ-
more than 36,500 people in nearby coastal regions. Another ilization on Crete. The tsunami deposited volcanic debris at
famous tsunami was also generated by a volcanic eruption of elevations of up to 800 feet (245 m) above the mean ocean
Santorin (now called Thira) on the Mediterranean island of level on the nearby island of Anaphi, and the wave was still
Crete. In 1600 B.C.E., this volcano was the site of the most more than 20 feet (6 m) high when it ran up the shorelines on
powerful eruption in recorded history, and it generated a the far side of the Mediterranean in Israel.

December 26, 2004: Indian Ocean Earthquake The tsunami traveled around the world, being measured as
and Tsunami minor (inches) changes in sea level more than 24 hours later in the
North Atlantic and Pacific. Overall, more than 300,000 people per-
One of the worst natural disasters of the 21st century unfolded on ished in the December 26th Indian Ocean tsunami, though many
December 26, 2004, following a magnitude 9.0–9.2 earthquake off the could have been saved, if a tsunami warning system had been in
coast of Sumatra in the Indian Ocean. During this catastrophic place in the Indian Ocean. Tsunami warning systems are capable of
earthquake, a segment of the seafloor the size of California suddenly saving lives by alerting residents of coastal areas that a tsunami is
moved upward and seaward by several tens of feet, releasing more approaching their location. These systems are most effective for
energy than all the earthquakes on the planet in the last 25 years areas located more than 500 miles (750 km), or one hour away from
combined. The sudden displacement of this volume of undersea the source region of the tsunami, but may also prove effective at
floor displaced huge volumes of water and generated the most saving lives in areas closer to a tsunami. The National Oceano-
destructive tsunami ever recorded. graphic and Atmospheric Administration operates the Pacific
Tsunami Warning Center in Honolulu, Hawaii, integrating data from
Within minutes of the earthquake, a mountain of water more several different sources, including seismic stations that record
than 100 feet high was ravaging northern Sumatra, sweeping into earthquakes and quickly sort out those earthquakes that are likely to
coastal villages and resort communities with a fury that crushed all be tsunamogenic. A series of tidal gauges placed around the Pacific
in its path, removing buildings and vegetation, and eroding shoreline monitors the passage of any tsunamis past their location, and if
areas down to bedrock. Similar scenes of destruction and devasta- these stations detect a tsunami, warnings are quickly issued for
tion rapidly moved up the coast of nearby Indonesia, where resi- local and regional areas likely to be affected. Analyzing all of this
dents and tourists were enjoying a holiday weekend. In some cases, information takes time, however, so this Pacific-wide system is most
the sea retreated to unprecedented low levels before the waves effective for areas located far from the earthquake source.
struck, drawing people to the shore to investigate the phenomena—
in other cases, the sea waves simply came crashing inland without Tsunami warning systems designed for shorter-term local
warning. Buildings, vehicles, trees, boats, and other debris were warnings are also in place in many communities, including Japan,
washed along with the ocean waters, forming projectiles that Alaska, Hawaii, and some Pacific islands. These warnings are
smashed at speeds of up to 30 miles per hour into other structures, based mainly on estimating the magnitude of nearby earthquakes
leveling all in its path, killing approximately 300,000 people. and on the ability of public authorities to rapidly issue warnings so
that the population has time to respond. For local earthquakes, the
The displaced water formed a deepwater tsunami that moved time between the shock event and the tsunami hitting the shoreline
at speeds of 500 miles per hour across the Indian Ocean, smashing may be only a few minutes. So if you are in a coastal area and feel a
within an hour into Sri Lanka and India, wiping away entire fishing strong earthquake, you should take that as a natural warning that a
communities and causing widespread destruction of the shore envi- tsunami may be imminent and leave low-lying coastal areas.
ronment. South of India are many small islands, including the Mal-
dives, Chagos, and Seychelles, many with maximum elevations only U.S. scientists detected the magnitude of the Sumatra earth-
tens of feet above sea level. As the tsunami approached these quake and tried to warn countries in soon-to-be-affected regions
islands, many wildlife species and tribal residents fled to the deep for- that a tsunami might be approaching. However, despite efforts by
est, perhaps sensing the danger as the sea retreated and the ground some scientists over the past few years, no systematic warning sys-
trembled with the approaching wall of water. As the tsunami heights tem was in place in the Indian Ocean. Initial cost estimates for a
were higher than many of the maximum elevations of some of these crude system were about $20 million, deemed too expensive by poor
islands, the forest was able to protect and save many lives in places nations that needed funds for more obviously pressing humanitarian
where the tsunami rose with less force than in places where the causes. When the earthquake struck on a Sunday, scientists who
shoreline geometry caused large breaking waves to crash ashore. tried contacting countries and communities surrounding the Indian
Ocean to warn them of the impending disaster typically found no
Several hours later the tsunami reached the shores of Africa one in the office and no systematic list of phone numbers of emer-
and Madagascar, and though distance diminished its height to less gency response personnel. Having such a simple phone-pyramid list
than 10 feet, several hundred people were killed by the waves and could potentially have saved tens of thousands of lives. Indian
high water. Kenya and Somalia were hit severely, with harbors Ocean communities are now planning to establish a tsunami warn-
experiencing rapid and unpredictable rises and falls in sea level, ing system before the next tsunami strikes.
and many boats and people washed to sea.

438 tsunami

Tsunami run-up is a measure of the height of the tsunami above sea level at the farthest point it reaches from the shore.

Movement of Tsunami When tsunami strike the coastal environment, the first
Tsunami are waves with exceptionally large distances between effect is sometimes a significant retreat or drawdown of the
individual crests, and they move like other waves across the water level, whereas in other cases the water just starts to rise
ocean. We define wavelength as the distance between crests, quickly. Since tsunami have long wavelengths, it typically takes
wave-height as the vertical distance from the crest to the bot- several minutes for the water to rise to its full height. Also,
tom of the trough, and the amplitude as one-half of the wave since there is no trough right behind the crest of the wave, on
height. Most ocean waves have wavelengths of 300 feet (100 account of the very long wavelength of tsunami, the water
m) or less; tsunami are exceptional in that they have wave- does not recede for a considerable time after the initial crest
lengths that can be 120 miles (200 km) or greater. When tsuna- rises onto land. The rate of rise of the water in a tsunami
mi are traveling across deep ocean water, their amplitudes are depends in part on the shape of the seafloor and coastline. If
typically less than three feet (1 m). You would probably not the seafloor rises slowly, the tsunami may crest slowly, giving
even notice even the largest of tsunami if you were on a boat in people time to outrun the rising water. In other cases, especial-
the deep ocean. Circular or elliptical paths that decrease in size ly where the seafloor rises steeply, or the shape of the bay caus-
with depth describe the motion of water in waves. All motion es the wave to be amplified, tsunami may come crashing in
from the waves stops at a depth equal to one-half the distance huge walls of water with breaking waves that pummel the
of the wavelength. Tsunami therefore are felt at much greater coast with a thundering roar and wreaking utmost destruction.
depths than ordinary waves, and this effect may be used with
deep ocean bottom tsunami detectors to help warn coastal Because tsunami are waves, they travel in successive
communities when tsunami are approaching. crests and troughs. Many deaths in tsunami events are related
to people going to the shoreline to investigate the effects of
Waves with long wavelengths travel faster than waves the first wave, or to rescue those injured or killed in the ini-
with short wavelengths. Since the longer the wavelength the tial crest, only to be drowned or swept away in a succeeding
faster the wave in deep open water, tsunami travel extremely crest. Tsunami have long wavelengths, so successive waves
fast across the ocean. Normal ocean waves travel at less than have a long lag time between individual crests. The period of
55 miles per hour (90 km/hr), whereas many tsunami travel a wave is the time between the passage of individual crests,
at 500 to 600 miles per hour (800 to 950 km/hr), faster than and for tsunami the period can be an hour or more. Thus, a
most commercial airliners! tsunami may devastate a shoreline area, retreat, and then
another crest may strike an hour later, then another, and
When waves encounter shallow water the friction of the another in sequence.
seafloor along the base of the wave causes them to slow down
dramatically, and the waves effectively pile up on themselves as Tsunami Run-Up
successive waves move into shore. This causes the wave height Run-up is the height of the tsunami above sea level at the far-
or amplitude to increase dramatically, sometimes 15 to 150 thest point it reaches on the shore. This height may be consid-
feet (4.5–45 m) above the normal still water line for tsunami.

tsunami 439

erably different from the height of the wave where it first hits slumps along convergent tectonic plate boundaries. For exam-
the shore. Many things influence the run-up of tsunami, ple, in 1896 a huge 75-foot (23-m) high tsunami was generat-
including the size of the wave, the shape of the shoreline, the ed by an earthquake-induced submarine slump in Sanriku,
profile of the water depth, and other irregularities particular Japan, killing 26,000 people in the wave. Another famous
to individual areas. Some bays and other places along some tsunami generated by a slump from an earthquake is the 1946
shorelines may amplify the effects of waves that come in from wave that hit Hilo, Hawaii. This tsunami was 50 feet (15 m)
a certain direction, making run-ups higher than average. high, killed 150 people, and caused about $25 million in dam-
These areas are called wave traps, and in many cases the age to Hilo and surrounding areas. The amazing thing about
incoming waves form a moving crest of breaking water, called this tsunami is that it was generated by an earthquake-induced
a bore. Tsunami magnitudes are commonly reported using the slump off Unimak Island in the Aleutian Chain of Alaska 4.5
maximum run-up height along a particular coastline. hours earlier! This tsunami traveled at 500 miles per hour (800
km/hr) across the Pacific, hitting Hawaii without warning.
Origin of Tsunami
Tsunami may be generated by any event that suddenly dis- Another potent kind of tsunami-generating earthquake
places the seafloor, which in turn causes the seawater to move occurs along subduction zones. Sometimes, when certain
suddenly to compensate for the displacement. Most tsunami kinds of earthquakes strike in this environment, the entire
are earthquake induced or caused by volcanic eruptions, forearc region above the subducting plate may snap upward
although giant submarine landslides have initiated others. It by up to a few tens of feet, displacing a huge amount of water.
is even possible that gases dissolved on the seafloor may sud- The tsunami generated during the 1964 magnitude 9.2
denly be released, forming a huge bubble that erupts upward Alaskan earthquake formed a tsunami of this sort, and it
to the surface, generating a tsunami. caused numerous deaths and extensive destruction in places as
far away as California. The 2004 Indian Ocean tsunami was
EARTHQUAKE-INDUCED TSUNAMI Earthquakes that strike likewise generated by a magnitude 9.0–9.2 subduction zone
offshore or near the coast have generated most of the world’s earthquake in the Sumatra trench-subduction zone system.
tsunami. In general, the larger the earthquake, the larger the
potential tsunami, but this is not always the case. Some earth- VOLCANIC ERUPTION-INDUCED TSUNAMI Some of the
quakes produce large tsunami, whereas others do not. Earth- largest recorded tsunamis have been generated by volcanic
quakes that have large amounts of vertical displacement of the eruptions. These may be associated with the collapse of vol-
seafloor result in larger tsunamis than earthquakes that have canic slopes, debris and ash flows that displace large amounts
predominantly horizontal movements of the seafloor. This dif- of water, or by submarine eruptions that explosively displace
ference is approximately a factor of 10, probably because water above the volcano. The most famous volcanic erup-
earthquakes with vertical displacements are much more effec- tion-induced tsunami include the series of huge waves gener-
tive at pushing large volumes of water upward or downward, ated by the eruption of Krakatau in 1883, which reached
generating tsunami. Another factor that influences how large run-up heights of 120 (36.5 m) feet and killed 36,500 people.
a tsunami may be generated by a specific earthquake is the The number of people that perished in the eruption of San-
speed at which the seafloor breaks during the earthquake— torin in 1600 B.C.E. is not known, but the toll must have been
slower ruptures tend to produce larger tsunami. huge. The waves reached 800 feet (245 m) in height on
islands close to the volcanic vent of Santorin. Flood deposits
Tsunami earthquakes are a special category of earth- have been found 300 feet (91 m) above sea level in parts of
quakes that generate tsunami that are unusually large for the the Mediterranean Sea and extend as far as 200 miles (320
earthquake’s magnitude. Tsunami earthquakes are generated km) up the Nile River to the south. Several geologists suggest
by large displacements that occur along faults near the that these were formed from a tsunami generated by the
seafloor. Most are generated on steeply dipping seafloor sur- eruption of Santorin. The floods from this eruption may also,
face penetrating faults that have vertical displacements along according to some scientists, account for some historical leg-
them during the earthquake, displacing the maximum amount ends such as the great biblical flood, the parting of the Red
of water. These types of earthquakes also frequently cause Sea during the exodus of the Israelites from Egypt, and the
large submarine (underwater) landslides or slumps, which also destruction of the Minoan civilization of the island of Crete.
generate tsunami. In contrast to tsunami generated by vertical
slip on vertical faults, which cause a small region to experience LANDSLIDE-INDUCED TSUNAMI Many tsunamis are gener-
a large uplift, other tsunami are generated by movement on ated by landslides that displace large amounts of water. These
very shallowly dipping faults. These are capable of causing may be from rock falls and other debris that falls off cliffs
large regions to experience minor uplift, displacing large vol- into the water, such as the huge avalanche that triggered a
umes of water and generating a tsunami. Some of the largest 200-foot (61-m) high tsunami in Lituya Bay, Alaska. Subma-
tsunami may have been generated by earthquake-induced rine landslides tend to be larger than avalanches that origi-
nate above the waterline, and they have generated some of

440 tsunami

Causes of tsunami, including vertical displacements of the seafloor and the observed size of the associated tsunami. Sediments near
slumping of large blocks into the water deep-sea trenches are often saturated in water, and close to
failure. When an earthquake strikes these areas, large parts of
the largest tsunami on record. Many submarine landslides are the submarine slopes may give out simultaneously, displacing
earthquake-induced, but tsunami are thought to be landslide- water and generating a tsunami. The 1964 magnitude 9.2
induced when the earthquake is not large enough to produce earthquake in Alaska generated more than 20 tsunami, and
these were responsible for most of the damage and deaths
from this earthquake.

Some steep submarine slopes that are not characterized
by earthquakes may also be capable of generating huge
tsunami. Recent studies along the east coast of North Ameri-
ca, off the coast of Atlantic City, New Jersey, have revealed
significant tsunami hazards. A thousands of feet thick pile of
unconsolidated sediments on the continental slope is so
porous and saturated with water that is on the verge of col-
lapsing under its own weight. A storm or minor earthquake
may be enough to trigger a giant submarine landslide in this
area, possibly generating a tsunami that could sweep across
the beaches of Long Island, New Jersey, Delaware, and much
of the east coast of the United States.

GAS HYDRATE ERUPTION-INDUCED TSUNAMI Decaying
organic matter on the seafloor releases large volumes of
gas. Under some circumstances, including cold water at
deep depths, these gases may coagulate forming gels called
gas hydrates. It has recently been recognized that these gas
hydrates occasionally spontaneously release their trapped
gases in giant bubbles that rapidly erupt to the surface.
Such catastrophic degassing of gas hydrates poses a signifi-
cant tsunami threat to regions not previously thought to
have a significant threat, such as along the east coast of the
United States.

OTHER TSUNAMI Giant tsunami may be generated by the
impact of asteroids with the Earth. These types of events do
not happen very often, but when they do they are cata-
clysmic. Geologists are beginning to recognize deposits of
impact-generated tsunami and now estimate that they may
reach several thousand feet in height. One such tsunami was
generated about 66 million years ago by an impact that
struck the shoreline of the Yucatán Peninsula, producing the
Chicxulub impact structure. This impact produced a huge
crater and sent a 3,000-foot (915-m) high tsunami around
the Atlantic, devastating the Caribbean and the U.S. Gulf
coast. Subsequent fires and atmospheric dust that blocked the
Sun for several years killed off much of the planet’s species,
including the dinosaurs.

See also BEACH; EARTHQUAKES; PLATE TECTONICS.

Further Reading
Bernard, E. N., ed., Tsunami Hazard: A Practical Guide for Tsunami

Hazard Reduction. Dordrecht, The Netherlands: Kluwer Aca-
demic Publishers, 1991.
Dawson, A. G., and S. Shi. “Tsunami Deposits.” Pure and Applied
Geophysics 157 (2000): 493–511.

Tunguska, Siberia 441

Driscoll, N. W., J. K. Weissel, and J. A. Goff. “Potential for Large- bedrock. Bedrock exposures are commonly shattered into
Scale Submarine Slope Failure and Tsunami Generation along the fields of angular boulders called blockfields or kurums.
United States Mid-Atlantic Coast.” Geology 28 (2000): 407–410.
See also GLACIER; MASS WASTING.
Dvorak, J., and T. Peek. “Swept Away.” Earth 2, no 4 (1993): 52–59.
Latter, J. H. “Tsunami of Volcanic Origin, Summary of Causes, with Tunguska, Siberia The location of what is thought to be
the site of a collision of a comet with the Earth. On June 30,
Particular Reference to Krakatau, 1883.” Journal of Volcanology 1908, a huge explosion rocked the Tunguska area of Siberia
44 (1981): 467–490. and devastated more than 1,158 square miles (3,000 km2) of
McCoy, F., and G. Heiken. “Tsunami Generated by the Late Bronze forest. The force of the blast is estimated to have been equal
Age Eruption of Thera (Santorini), Greece.” Pure and Applied to 15 megatons and is thought to have been produced by the
Geophysics 157 (2000): 1,227–1,256. explosion six miles (10 km) above the surface of Earth of an
Minoura, K., F. Inamura, T. Nakamura, A. Papadopoulos, T. Taka- asteroid with a diameter of 200 feet (61 m). Shock waves
hashi, and A. Yalciner. “Discovery of Minoan Tsunami Deposits.” were felt thousands of miles away, and people located closer
Geology 28 (2000): 59–62. than 60 miles (97 km) to the site of the explosion were
Revkin, A. C. “Tidal Waves Called Threat to East Coast.” The New knocked unconscious, and some were thrown into the air by
York Times (2000): A18. the force of the explosion. Fiery clouds and deafening explo-
Satake, K. “Tsunamis.” Encyclopedia of Earth System Science 4 sions were heard more than 300 miles (480 km) from Tun-
(1992): 389–397. guska. After years of study and debate it is now thought that
Steinbrugge, K. V. Earthquakes, Volcanoes, and Tsunamis, An this huge explosion was produced by a fragment of Comet
Anatomy of Hazards. New York: Skandia America Group, 1982 Encke that broke off the main body and exploded in the air
Tsuchiya, Y., and N. Shuto, eds., Tsunami: Progress in Prediction about five miles (8 km) above the Siberian Plains.
Disaster Prevention and Warning. Boston: Kluwer Academic Pub-
lishers, 1995. The sequence of events associated with the Tunguska
U.S. Geological Survey. “Surviving a Tsunami—Lesson from Chile, event has been reconstructed as follows. About 7:00 A.M. on
Hawaii, and Japan.” U.S. Geological Survey Circular 1187, 1987. June 30, a huge fireball was seen moving westward across
Siberia. Next, an explosion was heard that was centered on
tundra Treeless plains of the Arctic and subarctic regions, the remote Tunguska region, with reports of the explosion
characterized by a marshy surface with abundant growth of and pressure waves being felt more than 600 miles (965 km)
mosses and lichens, and a dark organic-rich soil overlying per- away. People who were hundreds of miles from the site of the
mafrost. When heated in the summer months, the permafrost explosion were knocked down, and a huge 12-mile (19-km)
layer remains impermeable and develops a layer of meltwater high column of fire was visible for more than 400 miles (640
on or below the surface forming wet swampy or mucky condi- km). Seismometers recorded the impact, and barometers
tions. Bacterial action is slow in these high-latitude swamps, around the world recorded the air pressure wave as it trav-
so organic material accumulates, adding to the soil layer. The eled two times around the globe. The impact caused a
size of most plants in the tundra is limited by the depth to strange, bright, unexplained glow that lit up the night skies in
which roots can penetrate and by the mechanical abrasion by Scotland and Sweden.
wind-driven snow and dry winds in the winter months.
Several years after the impact, a scientific expedition to
Some unique and unusual landforms characterize the tun- the remote region discovered that many trees were charred
dra environment. Frost action slowly brings rock fragments to and knocked down in a 2,000 square mile (5,180 km2) area,
the surface, as ice crystals form below the rocks and force with near total destruction in the center 500 square miles
them upward through the soil. Once on the surface many of (1,295 km2). Many theories were advanced to explain the
these rock particles may be pushed laterally by ice to form strange findings, and more than 50 years later, in 1958, a
stone polygons. Patterned ground may form through per- new expedition to Tunguska found small melted globules of
mafrost cracking into polygons by thermal contraction, then glass and metal, identified as pieces of an exploded meteorite
the cracks fill with water that freezes, forming ice wedges. or asteroid.
Eventually these subsurface ice wedges may form polygonal or
patterned ground where the surface is broken into regular One of the biggest puzzles at Tunguska is the absence
polygons separated by narrow ice wedges, and partly covered of an impact crater, despite all other evidence that points to
by thaw lakes. Pingos are large, sometimes several hundred an impact origin for this event. It is now thought that a
feet high, mounds of soil and rock cored by ice that grows as piece of a comet, Comet Encke, broke off the main body as
more and more ice accumulates under the surface. it was orbiting nearby Earth, and this fragment entered
Earth’s atmosphere and exploded about 5 miles (8 km)
Tundra regions are also susceptible to solifluction and above the Siberian plains at Tunguska. Comets are weaker
slumping of melt-ice layers particularly during the summer than metallic or stony meteorites, and they more easily
months when a melt layer is well developed. The slightest break up and explode in the atmosphere before they hit the
incline can cause soil layers to move downhill in solifluction Earth’s surface.
lobes. Downhill creep of permafrost layers is also common,
as is intense weathering and shattering of any exposed

442 turbidite

turbidite A deposit of a submarine turbidity current con- Graywacke and shale of a turbidite sequence, Chugach Mountains,
sisting of graded sandstone and shale, typically deposited in a southern Alaska (Photo by Timothy Kusky)
thick sequence of similar turbidites. Most turbidites are
thought to be deposited in various sub-environments of sub- interpreted to have been deposited closer to the slope or
marine fans, in shallow to deepwater settings. Typically, a channel, whereas turbidite sequences with more of the C-D-E
water-saturated sediment on a shelf or shallow water setting horizons are interpreted as more distal deposits.
is disturbed by a storm, earthquake, or some other mecha-
nism that triggers the sliding of the sediment downslope. The Many turbidite sequences are deposited in foreland
sediment-laden sediment/water mixture then moves rapidly basins and in deep-sea trench settings. These environments
downslope as a density current and may travel tens or even have steep slopes in the source areas, a virtually unlimited
hundreds of miles at tens of miles per hour until the slope source of sedimentary material, and many tectonic triggers to
decreases, and the velocity of the current decreases. As the initiate the turbidity current.
velocity of the current decreases, the ability of the current to
hold coarse material in suspension decreases, so the current See also CONVERGENT PLATE MARGIN PROCESSES; FORE-
drops first its coarsest load, then progressively finer material LAND BASIN; SEDIMENTARY ROCKS.
as the current decreases further. In this way, the coarsest
material is deposited closest to the channel or slope that the Further Reading
turbidity current flowed down, and the finest material is Bouma, Arnold, H. Sedimentology of Some Flysch Deposits. Amster-
deposited further away. The same sequence of coarse to fine
material is deposited upward in the turbidite bed as the cur- dam: Elsevier, 1962.
rent velocity decreases with time at any given location. This is Kuenen, Phillip H., and Carlo I. Migliorini. “Turbidite Currents as a
how graded beds are formed, with the coarsest material at
the base and finer material at the top. Cause of Graded Bedding.” Journal of Geology, v. 58 (1950):
91–127.
Classical complete turbidite beds consist of a sequence of Walker, Roger G. Facies Models. Toronto: Geoscience Canada
sedimentary structures divided into a regular A–E sequence Reprint Series 1, Geological Association of Canada, 1983.
known as the Bouma sequence, after the sedimentologist
Arnold Bouma who first described the sequence. The A hori- turbidity current See TURBIDITE.
zon consists of coarse to fine-grained graded sandstone beds,
representing material deposited rapidly from suspension. The typhoon See CYCLONE.
B horizon consists of parallel-laminated sandstones deposited
by material that moved in traction on the bed, whereas divi-
sion C contains cross-laminated sands deposited in the lower
flow regime. The D and E horizons represent the transition
from material deposited from the waning stages of the turbid-
ity current and background pelagic sedimentation.

Variations in the thickness and presence or absence of
individual horizons of the Bouma sequence have been related
to where on the submarine fan or slope the turbidite was
deposited. Turbidites with more of the A-B-C horizons are

U

unconformity A substantial break or gap in a stratigraph- ta. They are typically recognized by their irregular surfaces,
ic sequence that marks the absence of part of the rock record. missing strata, or large breaks between dated strata. Noncon-
These breaks may result from tectonic activity with uplift and formities are surfaces where strata overlies igneous or meta-
erosion of the land, sea-level changes, climate changes, or morphic rocks. Unconformities are significant in that they
simple hiatuses in deposition. Unconformities normally imply record an unusual event, such as tectonism, erosion, sea-level
that part of the stratigraphic sequence has been eroded but change, or climate change.
may also indicate that part of the sequence was not ever
deposited in that location. Unconformities are typically overlain by a progradation-
al marine sequence, starting with shallow-water sandstone,
There are several different types of unconformities. conglomerate or quartzite, and succeeded by progressively
Angular unconformities are angular discordances between deeper water deposits such as sandstone, shale, and lime-
older and younger rocks. Angular unconformities form in stone. Unconformities are often used by stratigraphers and
places where older layers were deformed and partly eroded other geologists to separate different packages of rocks
before the younger layers were deposited. Disconformities deposited during different tectonic, climatic, or time systems.
represent a significant erosion interval between parallel stra-
See also PLATE TECTONICS; STRATIGRAPHY.
Unconformity, Scotland. Steeply dipping beds in lower half of photograph
are overlain with a sharp angular discordance (at nose level of person) by uniformitarianism The doctrine that proposes that geo-
younger horizontal beds logic processes and natural laws that are operating at present
on the Earth have acted in a similar way and with similar
intensity throughout geologic time. The principle is common-
ly used by geologists to interpret ancient rocks, although con-
trasting or opposed views exist, including catastrophism and
other non-uniformitarian views. The term was coined by
William Whewell to explain James Hutton’s approach to
geology, best summarized in a famous quote from his 1795
book The Theory of the Earth: “The present is the key to the
past, and the past is the key to geological processes.” In 1830
another famous geologist named Charles Lyell announced
support for James Hutton’s approach, and the two became
the two main progenitors of uniformitarianism philosophy.

Uniformitarianism is one of two main ways to think
about old rocks and orogens. The uniformitarian view aims to
apply plate tectonic principles to old rocks to see how similar
they are to younger rocks and orogens. In this approach, the
scientist notes differences between then and now and searches

443

444 United States Geological Survey

for causes and secular trends. Some differences between the rare minerals and gems, yielding many excellent samples of
present and early Earth may be expected because of higher ear- emeralds, beryl, and topaz.
lier heat production and heat flow in the early part of Earth
history. In the opposing, non-uniformitarian view, the scientist The Urals form part of the Ural-Okhotsk mobile belt, a
assumes a model for the earlier history of the Earth, based on Late Proterozoic to Mesozoic orogen that bordered the Pale-
theoretical considerations of heat flow, biological evolution, oasian Ocean. The Ural Mountains section of this orogen
etc., and then applies this model to old rocks and orogens to saw a history that began with Early Paleozoic, probably
determine if the model is compatible with the observations. Cambrian rifting of Baikalian basement, and Late Ordovician
The two approaches can yield very different solutions to spreading to form a back arc or oceanic basin that was active
explain observations, and there is currently widespread debate until the Mid-Carboniferous. Oceanic arcs grew in this basin,
among scientists that work on the early history of the Earth as but by Middle Devonian they began colliding with the East
to which paradigm may be more appropriate. European continent, forming flysch basins. The Kazakhstan
microcontinent collided with the Laurussian continent in the
See also ARCHEAN; PLATE TECTONICS; PROTEROZOIC. Permian, forming a series of foredeep basins on the Russian
and Pechora platforms. These foredeeps are filled with
United States Geological Survey (USGS) The USGS molasse and economically important Middle to Late Permian
(http://www.usgs.gov/) was established in 1879 and has coal deposits, as well as potassium salts.
become a world leader in the natural sciences through scien-
tific excellence and responsiveness to society’s needs. It focus- The Urals show a tectonic zonation from the Permian
es into four major areas: natural hazards, resources, the flysch basins on the East European craton to the Permian
environment, and information and data management. The molasse basins on the western slopes of the Urals, then into
USGS stands as the sole science agency for the Department of belts of allochthonous carbonate platform rocks derived from
the Interior. The USGS serves the nation by providing reliable the East European craton and thrust to the west over the Per-
scientific information to describe and understand the Earth; mian foredeeps. These rocks are all involved in westward-ver-
minimize loss of life and property from natural disasters; gent fold-thrust belt structures, including duplex structures,
manage water, biological, energy, and mineral resources; and indicating westward tectonic transport in the Permian. The
enhance and protect the quality of life. axial zone of the Urals includes a chain of anticlinoria bring-
ing up Riphean rocks, whose eastern contact is known as the
Ural Mountains The boundary between Europe and Asia is Main Uralian fault. This major fault zone brings oceanic and
typically taken to be the Ural Mountains, a particularly island arc rocks in large nappe and klippen structures, plac-
straight mountain range that stretches 1,500 miles (2,400 km) ing them over the passive margin sequence.
from the Arctic tundra to the deserts north of the Caspian Sea.
Naroda (6,212 feet; 1,894 m) and Telpos-Iz (meaning “nest of The eastern slope of the Urals consists of a number of
winds,” 5,304 feet; 1,617 m) are the highest peaks, found in Ordovician to Carboniferous oceanic and island arc synformal
the barren rocky and tundra-covered northern parts of the nappes, imbricated with slices of the Precambrian crystalline
range. Southern parts of the mountain range rise to 5,377 feet basement. It is uncertain if these Precambrian gneisses are part
(1,639 m) at Yaman-Tau, in the Mugodzhar Hills. The south- of the East European craton, part of the accreted Kazakhstan
ern parts of the range are densely forested, whereas the north- microcontinent, or an exotic terrane. The eastern slopes of the
ern parts are barren and covered by tundra or bare rock. Urals are intruded by many Devonian-Permian granites.

The Ural River flows out of the southern Urals into the See also CASPIAN SEA.
Caspian Sea, and the western side of the range is drained by
the Kama and Belaya Rivers, which are tributaries that also Further Reading
feed into the Caspian Sea, providing more than 75 percent of Coleman, Robert G., ed. Reconstruction of the Paleo-Asian Ocean.
the water that flows into this shallow, closed basin. The east-
ern side of the range is drained by the Aob-Irtysh drainage The Netherlands: VSP International Science Publishers, 1992.
system that flows into the Ob Gulf on the Kara Sea.
uranium A radioactive mineral that spontaneously decays
The Urals are extremely rich in mineral resources, to lighter “daughter” elements by loosing high-energy parti-
including iron ore in the south, and large deposits of coal, cles at a predictable rate known as a half-life. The half-life
copper, manganese, gold, aluminum, and potash. Ophiolitic specifically measures how long it takes for half of the original
rocks in the south are also rich in chromite and platinum, or parent element to decay to the daughter element. 238U
plus deposits of bauxite, zinc, lead, silver, and tungsten are decays to 236Pb, and 235U decays to 207Pb, and radium,
mined. Basins on the western side of the Urals produce large through a long series of steps with a cumulative half-life of
amounts of oil, and regions to the south in the Caspian are 4.4 billion years. During these steps, intermediate daughter
yielding many new discoveries. The Urals are also very rich in products are produced, and high-energy particles including
alpha particles, consisting of two protons and two neutrons,
are released—this produces heat.

Uranus 445

The main ore of uranium is uraninite (UO2), a black, tionally larger rocky core than either one of these giant
brown, or steel gray mineral that has cubic or octahedral crys- gaseous planets.
tal forms. Uraninite typically occurs in veins with tin, lead,
and copper minerals, and as a detrital mineral in sandstones. Most of the planets in the solar system have their rota-
It is also a primary mineral in many granites and pegmatites. tional axis roughly perpendicular to the plane of the ecliptic,
the plane that the planets approximately orbit the Sun within.
See also MINERALOGY; RADIOACTIVE DECAY. However, the rotational axis of Uranus is one of the most
unusual in the solar system as it lies roughly within the plane
Uranus The seventh planet from the Sun, Uranus is a of the ecliptic, as if it is tipped over on its side. The cause of
giant gaseous sphere with a mass 15 times that of the Earth, this unusual orientation of the planet is not known, but some
and a diameter four times as large as Earth (51,100 km). astronomers have speculated that it may be a result of a large
The equatorial plane is circled by a system of rings, some impact early in the planet’s history. As a consequence of this
associated with the smaller of the 15 known moons circling unusual orientation, as Uranus orbits the Sun, it goes through
the planet. Uranus orbits the Sun at a distance of 19.2 astro- seasons where the north and south poles are alternately point-
nomical units (Earth–Sun distance) with a period of 84 ing directly at the Sun, and periods in between (spring and
Earth years and has a retrograde rotation of 0.69 Earth autumn) when the pole aligned in between these extremes.
days. Its density is only 1.2 grams per cubic centimeter, The poles experience very long summers and winters because
compared to the Earth’s average density of 5.5 grams per of the long orbital period of Uranus and are alternately
cubic centimeter. The density is higher, however, than plunged into icy cold darkness for 42 years, then exposed to
Jupiter’s or Saturn’s, suggesting that Uranus has a propor- the distant Sun for 42 years. With the rapid rotation rate and

These two pictures of Uranus—one in true color (left) and the other in false color—were compiled from images returned January 17, 1986, by the narrow-
angle camera of Voyager 2. The spacecraft was 5.7 million miles (9.1 million km) from the planet, several days from closest approach. The picture at left
has been processed to show Uranus as human eyes would see it from the vantage point of the spacecraft. The picture is a composite of images taken
through blue, green, and orange filters. The darker shadings at the upper right of the disk correspond to the day-night boundary on the planet. Beyond this
boundary lies the hidden northern hemisphere of Uranus, which currently remains in total darkness as the planet rotates. The blue-green color results
from the absorption of red light by methane gas in Uranus’s deep, cold, and remarkably clear atmosphere. The picture at right uses false color and
extreme contrast enhancement to bring out subtle details in the polar region of Uranus. Images obtained through ultraviolet, violet, and orange filters
were respectively converted to the same blue, green, and red colors used to produce the picture at left. The very slight contrasts visible in true color are
greatly exaggerated here. In this false-color picture, Uranus reveals a dark polar hood surrounded by a series of progressively lighter concentric bands.
One possible explanation is that a brownish haze or smog, concentrated over the pole, is arranged into bands by zonal motions of the upper atmosphere.
The bright orange and yellow strip at the lower edge of the planet’s limb is an artifact of the image enhancement. In fact, the limb is dark and uniform in
color around the planet. The Voyager project is managed for NASA by the Jet Propulsion Laboratory. (Photo by NASA)

446 urbanization and flash flooding

unusual orientation of the planet, an observer on the pole geologically inactive bodies, with the exception of Miranda,
experiencing the change from winter to spring would first which shows a series of ridges, valleys, and different morpho-
observe the distant Sun rising above the horizon and tracing logical terrains. One of the most unusual are a series of oval
out part of a circular path, then sinking below the horizon. wrinkled or faulted terrains of uncertain origin, but perhaps
Eventually the Sun would finally emerge totally and trace out related to subsurface magmatism, impacts, or volcanism.
complete circle paths every 17 hours. The position of the Sun
would progressively change over the next 42 years until it The ring system around Uranus was only discovered
sunk below the horizon for the following 42 years. recently, in 1977, when the planet passed in front of a bright
star and the rings were observed by astronomers studying the
The atmosphere of Uranus is roughly similar to Jupiter’s planet’s atmosphere. There are nine known rings at
and Saturn’s, consisting mostly of molecular hydrogen (84 27,340–31,690 miles (44,000–51,000 km) from the planet’s
percent), helium (14 percent), and methane (2 percent). center, and each of these rings appears to be made of many
Ammonia seems to be largely absent from the atmosphere of much smaller rings. The rings are generally dark and narrow
Uranus, part of a trend that has ammonia decreasing in abun- with widths of up to six miles (10 km), with wide spaces
dance outward in the outer solar system, with less at lower between the main rings ranging at 125–620 miles (200–1,000
temperatures. The reason is that ammonia freezes into km). The rings are only a few tens of meters thick. Most of
ammonia ice crystals at –335°F (70°K), and Jupiter’s and Sat- the particles that make up the rings are dust to boulder-sized,
urn’s upper atmospheres are warmer than this, whereas dark-colored, and trapped in place by the gravitational forces
Uranus’s is –355°F (58°K). Any ammonia therefore would between Uranus and its many moons.
have crystallized and fallen to the surface. The atmosphere of
Uranus appears a blue-green color because of the amount of urbanization and flash flooding Urbanization is the pro-
methane, but so far relatively few weather systems have been cess of building up and populating a natural habitat or envi-
detected on the planet. Enhancement of imagery, however, ronment, such that the habitat or environment no longer
has revealed that the atmosphere is characterized by winds responds to input the way it did before being altered by
that are blowing around the planet at 125 to 310 miles per humans. When heavy rains fall in an unaltered natural envi-
hour (200–500 km/hr) in the same sense as the planet’s rota- ronment, the land surface responds to accommodate the
tion, with detectable channeling of the winds into bands. The additional water. Desert regions may experience severe ero-
winds are responsible for transporting heat from the warm to sion in response to the force of falling raindrops that dislodge
the cold hemisphere during the long winter months. soil, and also by overland flow during heavy rains. This caus-
es upland channel areas to enlarge, becoming able to accom-
The magnetic field of Uranus is surprisingly strong, modate larger floods. Areas that frequently receive heavy
about 100 times as strong as the Earth’s. However, since the rains may develop lush vegetative cover, which helps to break
rocky core of the planet is so far below the cloud level, the the force of the raindrops and reduce soil erosion, and the
strength of the magnetic field at the cloud tops on Uranus is extensive root system holds the soil in place against erosion
actually similar to that on the surface of the Earth. Like the by overland flow. Stream channels may be large so that they
rotational axis, the orientation of the magnetic poles and can accommodate large volume floods.
field on Uranus are highly unusual. The magnetic axis is tilt-
ed at about 60° from the spin axis and is not centered on the When the natural system is altered in urban areas, the
core of the planet but is displaced about one-third of the result can be dangerous. Many municipalities have paved
planetary radius from the planet’s center. The origin of these over large parts of drainage basins, and covered much of the
unusual magnetic field properties is not well understood but recharge area with roads, buildings, parking lots, and other
may be related to a slurry of electrically conducting ammonia structures. The result is that much of the water that used to
clouds near the planet’s rocky surface. Whatever the cause, a seep into the ground and infiltrate into the groundwater sys-
similar unusual field exists on nearby Neptune. tem now flows overland into stream channels, which may
themselves be modified or even paved over. The net effect of
Uranus has 15 known large moons with diameters over these alterations is that flash floods may occur much more
25 miles (40 km), orbiting between 31,070 miles (50,000 km) frequently than in a natural system since more water flows
from the planet (for the smallest moon) to 362,260 miles into the stream system than before the alterations. The floods
(583,000 km) for the largest moon. From largest to smallest, may occur with significantly lower amounts of rainfall as
these moons include Oberon, Titania, Umbriel, Ariel, Miran- well, and since the water flows overland without slowly seep-
da, Puck, Belinda, Rosalind, Portia, Juliet, Desdemona, Cressi- ing into the ground, the flash floods may reach urban areas
da, Bianca, Ophelia, and Cordelia. The 10 smallest moons all more quickly than the floods did before the alterations to the
orbit inside the orbit of Miranda and are associated with the stream system. Overall the effect of urbanization is faster,
ring system around Uranus, and all of the moons rotate in the stronger, and bigger floods, which have greater erosive power
planet’s equatorial plane, not the ecliptic of the solar system. and do more damage. It is almost as if the natural environ-
Most of these moons are relatively dark, heavily cratered, and

urbanization and flash flooding 447

ment responds to urban growth by increasing its ability to event. However, in urbanized areas the floodwaters not only
return the environment to its natural state. rise quickly but also recede faster than in the natural environ-
ment. This is attributed to the lack of groundwater continuing
Many examples of the effects of urbanization on flood to recharge the stream after the flood peak in urbanized areas.
intensity have been documented from California and the desert
southwest. Urban areas such as Los Angeles, San Diego, Tuc- Many other modifications to stream channels have been
son, Phoenix, and other cities have documented the speed and made in urbanized areas, with limited success in changing
severity of floods from similar rainfall amounts along the same nature’s course to suit human needs. Many stream channels
drainage basin. What these studies have documented is that the have been straightened, which only causes the water to flow
floodwaters rise much more quickly after urbanization, and faster and have more erosive power. Straightening the stream
they rise up to four times the height they reached before urban- course also shortens the stream length and thereby steepens
ization, depending on the amount of paving over of the sur- the gradient. The stream may respond to this by aggrading
face. The increased speed at which the floodwaters rise and the and filling the channel with sediment, in an attempt to regain
increased height to which they rise are directly correlated with the natural gradient.
the amount of land surface that is now covered over by roads,
houses, and parking lots, blocking infiltration. See also DESERT; GEOMORPHOLOGY; GROUNDWATER.

In natural systems, floods gradually wane after the high- Further Reading
est peak passes, and the slow fall of the floodwaters is related Arnold, J. G., P. J. Boison, and P. C. Patton. “Sawmill Brook—An
to the stream system being recharged by groundwater that
seeped into the shallow surface area during the heavy rainfall Example of Rapid Geomorphic Change Related to Urbaniza-
tion.” Journal of Geology 90 (1982): 115–166.

V

valley breeze See KATABATIC WINDS. and 4,000 feet (1,200 m) deep on the top of Mount Katmai.
Magma from Katmai migrated underground to Novarupta,
Valley of Ten Thousand Smokes The eruption of mixed with the magma there, and erupted from Novarupta,
Novarupta volcano in the Aleutian Ranges on the Alaska draining the magma from beneath Katmai, leading to the for-
Peninsula in June 1912 produced a huge ash flow, hundreds mation of the giant caldera. Eruptions from the fissures filled
of feet thick covering more than 40 square miles (100 km2). the valley with more than 650 feet (200 m) of ash flows and
A foot of ash covered the distant port of Kodiak on Kodiak other pyroclastic material. An estimated 4.5 cubic miles (7
Island. The eruption was about 100 times greater than the km3) of material was erupted from the volcanic vents.
1982 eruption of Mount St. Helens and produced a Plinian
ash cloud that probably rose 20 miles (32 km) in the atmo- Studies of the deposits in the Valley of Ten Thousand
sphere. As the ash from the eruption cooled, vapors were Smokes have led to the recognition that many huge sheets of
released and made their way to the surface. The result was siliceous volcanic rocks are not lava flows but are pyroclastic or
that a huge area with thick ash developed thousands of ash flows. Some of the flows from Novarupta are dominantly
steaming fumaroles, with still hot gasses rising from the white rhyolitic ash and pumice, whereas others include brown
fumaroles when an expedition mounted by the National Geo- andesitic pumice. Some flows show strongly banded alterna-
graphic Society in 1916 explored the site of the eruptions. tions between the andesitic and rhyolitic types, and there are
During the expedition, botanist Robert Griggs gave the valley numerous blocks of solid andesite and exotic sedimentary
its name, and the Valley of Ten Thousand Smokes was rocks. The ash flows are dominantly unsorted and unstratified
declared a National Monument by President Woodrow Wil- (except at large distances from the vents) and are composed
son soon after the National Geographic expedition. The land- largely of sand and dust-sized pumice and lapilli particles. Some
scape in the Valley of Ten Thousand Smokes is still barren, of the ash flows are reworked by rivers and show cross-beds and
almost a century after the catastrophic eruptions. other fluvial features. Most of the ash flows are only weakly
indurated, but some are welded and exhibit columnar jointing.
The Valley of Ten Thousand Smokes is at the base of Kat-
mai and Novarupta volcanoes, in Alaska’s Aleutian chain of For such a large eruption, the eruption of Novarupta
subduction-related convergent margin volcanoes. On the was preceded by only a few days of earthquakes. Early erup-
Alaskan Peninsula, the main andesitic volcanoes are adjacent tions were minor pumice falls, followed rapidly by extremely
to a subsidiary chain of rhyolite domes, and the 1912 erup- violent eruptions that gradually over several days became
tions were unusual in that magma from the main andesitic sys- more gas poor and less explosive, leading to the extrusion of
tem beneath Mount Katmai migrated to and mixed with the Novarupta dome. A late-stage eruption was particularly
magma from the rhyolitic domes beneath Novarupta. Major violent and may have been related to the injection of a new
activity began on June 6, 1912, when a five-day-long swarm of batch of magma into the chamber beneath Novarupta.
earthquakes yielded to a series of spectacular fissure eruptions
around Novarupta, the opening of explosion craters, and the Mount Katmai, Novarupta, and the Valley of Ten Thou-
formation of a collapse caldera nearly two miles (3 km) across sand Smokes are located in the remote reaches of the Alaskan
Peninsula, and a few days warning was enough for the local
residents to evacuate, so no deaths resulted from this huge

448

Venus 449

eruption. However, if such a huge eruption were to occur in Lipps, Jere H., and Phillip W. Signor, eds., Origin and Early Evolu-
populated parts of the country, the results would be different. tion of the Metazoa. New York: Plenum, 1992.
If Mount Katmai were located in St. Louis, Missouri, then
Chicago would be buried under nearly a foot of ash, and the Venus The second planet from the Sun, Venus is the planet
eruption would be heard as far as New York City and Los in our solar system that most closely resembles Earth with a
Angeles. The gases from the eruption would reach New York planetary radius of 3,761 miles (6,053 km), or 95 percent of
and Boston, tarnishing brass doorknobs and causing breath- that of the Earth’s radius. Venus orbits the Sun in a nearly cir-
ing problems in these distant cities. cular path at 0.72 astronomical units (Earth-Sun distance) and
has a mass equal to 81 percent of Earth, and density of 5.2
See also CONVERGENT PLATE MARGIN PROCESSES; grams per cubic centimeter, very similar to Earth’s 5.5 grams
VOLCANO. per cubic centimeter. The orbital period (year) of Venus is
0.62 Earth years, but it has a retrograde rotation about its
Further Reading axis of 243 days, with its north pole turned essentially upside
Curtis, G. H. “The Stratigraphy of the Ejecta from the 1912 Erup- down so that its equatorial pole is inclined at 177.4° from the
orbital plane. One result of this tilt and the slow retrograde
tion of Mount Katmai and Novarupta, Alaska.” Geological Soci- rotation is that the two effects to combine such that each day
ety of America Memoir 116 (1968). on Venus takes the equivalent of 117 Earth days. Another
Fenner, Clarence N. “The Origin and Mode of Emplacement of the effect is that the slow rotation has not set up a geodynamo
Great Tuff Deposit of the Valley of Ten Thousand Smokes.” current in the planet’s core, so Venus has no detectable mag-
National Geographic Society, Contribution Technical Papers,
Katmai Series, no. 1 (1923). Mosaic of Magellan radar images showing the surface of Venus, with
Aphrodite Terra at the center of the image. This global view of the surface
Vendian In some classifications the latest Precambrian peri- of Venus is centered at 180° east longitude. Magellan synthetic aperture
od in the Neoproterozoic is called the Vendian, stretching from radar mosaics from the first cycle of Magellan mapping are imposed onto
610–570 million years ago. The period is divided into the a computer-simulated globe to create this image. Data gaps are filled
Varanger (610–590 million years ago) and Ediacara (590–570 with Pioneer Venus orbiter data, or a constant mid-range value. Simulated
million years ago) epochs, in turn divided into the Smalfjord, color is used to enhance small-scale structure. The simulated hues are
Mortensnes, Wonokan, and Poundian ages. The Vendian is based on color images recorded by the Soviet Venera 13 and 14
one of two periods divided from the Sinian sub-era—the Stur- spacecraft. The image was produced by the Solar System Visualization
tian (800–610 million years ago) and the Vendian. project and the Magellan science team at the Jet Propulsion Laboratory
(JPL) Multimission Image Processing Laboratory and is a single frame
The Vendian is an extremely important time in Earth his- from a video released at the October 29, 1991, JPL news conference.
tory as it preserves some of the earliest known animal fossils. (Photo by NASA)
The Varanger was a time of widespread and even global glacia-
tion reaching to low latitudes and leaving glacial deposits on
many continents. This glaciation may have been induced by
the breakup of Rodinia and the amalgamation of the super-
continent of Gondwana at the end of the Proterozoic. Carbon-
ate platforms on continental margins involved in collisions
were uplifted above sea level and eroded, causing eroded sili-
cates and carbonates to combine with and draw down atmo-
spheric carbon dioxide, lowering global temperatures.

In many places around the world the Vendian is recog-
nized as containing the oldest multicelled organisms. In south
China, well-preserved assemblages of early multicelled meta-
zoan fossils overlie rocks of the Varanger glaciation, making
them the oldest known animal fossils. Other Vendian rocks
and Ediacaran fossils are known from Russia, Ukraine,
Northwest Canada, Australia, Norway, England, Newfound-
land, Namibia, China, the southwest United States, and
North Carolina.

See also LIFE’S ORIGINS AND EARLY EVOLUTION; PRECAM-
BRIAN; PROTEROZOIC; SINIAN; SUPERCONTINENT CYCLES.

Further Reading
Glaessner, Martin F. The Dawn of Animal Life: A Biohistorical

Study. Cambridge: Cambridge University Press, 1984.

450 Venus

netic field. Thus, it lacks a magnetosphere to protect it from radiation as the Earth, which is enough to prevent the oceans
the solar wind so it is constantly bombarded by high-energy from condensing from vapor. Without the oceans, the carbon
particles from the Sun. These particles lead to constant ioniza- dioxide does not dissolve in the seawater or combine with
tion of the upper levels of the atmosphere. Venus is usually other ions to form carbonates, so the CO2 and water stayed in
one of the brightest objects in the sky (excepting the Sun and the atmosphere. As the water vapor was lighter than the CO2
Moon) and is usually visible just before sunrise or just after it rose to high atmospheric levels and was dissociated into H
sunset, since its orbit is close to the Sun. and O ions, and the hydrogen escaped to space, whereas the
O combined with other ions. Thus, the oceans never formed
The atmosphere of Venus is very dense and is composed on Venus, and the water that could have formed them dissoci-
mostly (96.5 percent) of carbon dioxide and is nearly opaque ated and is lost in space. Earth is only slightly further from the
to visible radiation, so most observations of the planet’s sur- Sun, but conditions here are exactly balanced to allow water
face are based on radar reflectivity. Spacecraft and earth- to condense, the atmosphere to remain near the equilibrium
based observations of Venus show that the atmospheric and (triple) point of solid, liquid, and vapor water. These condi-
cloud patterns on the planet are more visible in ultraviolet tions allowed life to develop on Earth, and life further modi-
wavelengths, since some of the outer clouds seem to be made fied the atmosphere-ocean system to maintain its ability to
of mostly sulfuric acid which absorbs UV radiation, whereas support further life. Venus never had a chance.
other clouds reflect this wavelength, producing a highly con-
trasted image. These observations show that the atmosphere With Venus’s thick atmosphere, the surface must be
contains many large fast-moving clouds that are moving mapped with cloud-penetrating radar from Earth and space-
around 250 miles per hour (400 km/hr) and rotate around craft. The surface shows many remarkable features including
the planet on average once every four days. The atmospheric a division into a bimodal crustal elevation distribution remi-
patterns on Venus resemble the jet stream systems on Earth. niscent of Earth’s continents and oceans. Most of the planet
Aside from carbon dioxide, the atmosphere contains nitrogen is topographically low, including about 27 percent flat vol-
plus minor or trace amounts of water vapor, carbon monox- canic lowlands and about 65 percent relatively flat plains,
ide, sulfur dioxide, and argon. probably basaltic in composition, surrounded by volcanic
flows. The plains are punctuated by thousands of volcanic
Although many basic physical Venutian properties are structures including volcanoes and elongate narrow flows,
similar to Earth’s, the atmosphere on Venus is about 90 times including one that stretches 4,225 miles (6,800 km) across
more massive and extends to much greater heights than the surface. Some of the volcanoes are huge, with more than
Earth’s atmosphere. The mass of the Venutian atmosphere 1,500 having diameters of more than 13 miles (20 km), and
causes pressures to be exceedingly high at the surface, a value one (Sapas Mons) more than 250 miles (400 km) across and
of 90 bars, compared to Earth’s one bar. The troposphere, or almost one mile (1.5 km) high. About 8 percent of the planet
region in which the weather occurs, extends to approximately consists of highlands made of elevated plateaus and mountain
62 miles (100 km) above the surface. The upper layers of the ranges. The largest continent-like elevated landmasses include
Venutian atmosphere between about 75–45-mile (120–70- the Australian-sized Ishtar Terra in the southern hemisphere,
km) height are composed of sulfuric acid cloud layers, under- and African-sized Aphrodite Terra in equatorial regions.
lain by a mixing zone that is underlain by a layer of sulfuric Ishtar Terra has interior plains rimmed by what appears to be
acid haze at 30–20 miles (50–30 km). Below about 30 kilo- folded mountain chains, and Venus’s tallest mountain,
meters the air is clear. Maxwell Mons, reaching 7 miles (11 km) in height above
surrounding plains. Aphrodite Terra also has large areas of
The carbon-dioxide–rich and water-poor nature of the linear folded mountain ranges, many lava flows, and some
Venutian atmosphere has several important consequences for fissures that probably formed from lava upwelling from
the planet. First of all, these gases are greenhouse gases that crustal magma chambers, then collapsing back into the cham-
trap solar infrared radiation, an effect that has raised the sur- ber instead of erupting.
face temperature to an astounding 750°K (900°F, or 475°C,
compared to 273°K for Earth). Second, surface process are The surface of Venus preserves numerous impact struc-
much different on Venus than on Earth because of the elevat- tures and unusual circular to oval structures and craters that
ed temperatures and pressures. There is no running water, are most likely volcanic in origin. Some of the most unusual
rock behaves differently mechanically under high tempera- appearing are a series of rounded pancake-like bulges that
tures and pressures, and heat flow from the interior is differ- overlap each other on a small northern hemisphere elevated
ent with such a drastically different surface temperature. terrane named Alpha Regio. These domes are about 15.5
miles (25 km) across, and probably represent lava domes that
Earth and Venus had essentially the same amounts of filled and then had the magma withdrawn from them, form-
gaseous carbon dioxide, nitrogen, and water in their atmo- ing a flat, cracked lava skin on the surface. There are many
spheres soon after the planets formed. However, since Venus basaltic shield volcanoes scattered about the surface and
is closer to the Sun (it is 72 percent of the distance from the
Sun that Earth is), it receives about two times as much solar

volcano 451

some huge volcanic structures known as coronae. These are See also DEFORMATION OF ROCKS; IGNEOUS ROCKS;
hundreds of kilometers across and are characterized by a STRUCTURAL GEOLOGY.
series of circular fractures reflecting a broad upwarped dome,
probably formed as a result of a plume from below. Many volcanic bomb Clots of magma that are more than 2.5
volcanoes dot the surface in and around coronae, and lava inches (64 mm) long and were partly or entirely plastic when
flows emanate and flow outward from some of them. Impact erupted from a volcanic vent. In contrast, volcanic blocks are
craters are known from many regions on Venus, but their of the same size but were entirely solid when erupted. Pyro-
abundance is much less than expected for a planet that has clastic fragments smaller than blocks and bombs are known
had no changes to its surface since formation. No impacts as lapilli and ash. The sizes and shapes of volcanic bombs are
less than 2 miles (3 km) across are known since small mete- extremely variable, with some bombs being more than 20 feet
orites burn up in the thick atmosphere before hitting the sur- (6 m) in diameter Shapes of these pyroclastic bombs are deter-
face. The paucity of other larger impacts reflects the fact that mined by the rheology of the magma when it is erupted as
the surface has been reworked and plated by basalt in the well as the length and speed of the flight path the bomb took
recent history of the planet, as confirmed by the abundant when it was ejected from the volcano, and any deformation
volcanoes, lava flows, and the atmospheric composition of that occurred during impact with the surface. Some bombs
the planet. It is likely that volcanism is still active on Venus. appear to show significant shape modification through the
expansion of bubbles or gas vesicles in the magma. Expansion
Many of the surface features on Venus indicate some of gases often occurs after the bombs have landed and cause
crustal movements. For instance, the folded mountain ranges the skin of the bomb to expand and crack.
show dramatic evidence of crustal shortening, and there are
many regions of parallel fractures. Despite these features Basaltic magmas typically show spheroidal bombs or
there have not been any features found that are indicative of lapilli, whereas less-fluid basaltic eruptions often produce
plate tectonic types of processes operating. Most of the struc- almond or spindle-shaped bombs. Ribbon-shaped bombs typi-
tures could be produced by crustal downsagging or conver- cally have vesicles that are elongate parallel to the length of
gence between rising convective plumes, in a manner similar the bomb and are often broken or shattered upon impact with
to that postulated for the early Earth before plate tectonics the surface. Some bombs have formed around older material
was recognized. included in the magma and are referred to as cored bombs.

viscosity The resistance of a material to flow, sometimes See also PYROCLASTIC.
measured as the ratio of tangential frictional force per unit
area (stress) to the velocity gradient perpendicular to the Further Reading
direction of flow. The study of the flow of matter and the rela- Fisher, Richard V., and Hans U. Schminke. Pyroclastic Rocks. Berlin:
tionships between stress, strain, strain rate, and material and
applied properties is known as rheology. Many geologic mate- Springer-Verlag, 1984.
rials behave viscously, where strain accumulates as a function Williams, Howell, and Alexander McBirney. Volcanology. San Fran-
of time. For instance, water that flows downhill exhibits vis-
cous behavior because it continuously accumulates strain with cisco: Freeman, Cooper and Company, 1979.
time as it flows. Likewise, many magmas and rocks continu-
ously flow under an applied stress such that strain continues volcaniclastic See PYROCLASTIC.
to build up with time under the applied stress. For viscous
materials stress is proportional to strain rate, such that volcano A mountain or other constructive landform built
increasing the stress causes the rate of flow to increase. Vis- by a singular eruption or a sequence of volcanic eruptions of
cous materials may exhibit Newtonian viscous behavior molten lava and pyroclastic material from a volcanic vent.
where a plot of stress versus strain rate is linear, or they may Volcanoes have many forms ranging from simple vents in the
exhibit nonlinear viscous behavior where a stress-strain rate Earth’s surface, through elongate fissures that erupt magma,
plot is nonlinear or curved. Some materials exhibit more com- to tall mountains with volcanic vents near their peaks. Vol-
plex nonviscous rheologies including some time-dependent canic landforms and landscapes are as varied as the volcanic
behavior where strain accumulation and strain recovery are rocks and eruptions that produce them. Shield volcanoes
delayed in visco-elastic behavior. Materials that are distorted include the largest and broadest mountains on Earth. Mauna
under applied stress but return to their undeformed state after Loa is the largest shield volcano on the planet, more than 100
the stress is removed exhibit elastic behavior, whereas plastic times as large as Mount Everest, a nonvolcanic mountain.
behavior describes a response of a material to stress where the Shield volcanoes have surface slopes of only a few degrees,
material develops a strain without the loss of continuity, or produced by basaltic lavas that flow long distances before
without the development of fractures. cooling and solidifying. Stratovolcanoes, in contrast, are the
familiar steep-sided cones like Mount Fuji of Japan, or
Mount Rainier of Washington. Stratovolcanoes are made of
stickier lavas such as andesites and rhyolites, and they may

452 volcano

have slopes of 30°. Other volcanic constructs include cinder includes both new magma from the volcano and older broken
or tephra cones, including the San Francisco Peaks in Ari- rock fragments that got caught in the eruption. The term
zona, which are loose piles of cinder and tephra. Calderas, includes pyroclasts, which are rocks ejected by the volcano,
like Crater Lake in Oregon, are huge circular depressions, and ash. Large pyroclasts are called volcanic bombs, smaller
often many kilometers in diameter, that are produced when fragments are lapilli, and the smallest grade into ash.
deep magma chambers under a volcano empty out during an
eruption, and the overlying land collapses inward producing While the most famous volcanic eruptions produce huge
a topographic depression. Yellowstone Valley occupies one of explosions, many eruptions are relatively quiet and nonex-
the largest calderas in the United States. This caldera contains plosive. Nonexplosive eruptions have magma types that have
many geysers, hot springs, and fumaroles related to ground- low amounts of dissolved gases, and they tend to be basaltic
water circulating to great depths, being heated by shallow in composition. Basalt flows easily and for long distances and
magma, and mixing with volcanic gases, which escape tends not to have difficulty flowing out of volcanic necks.
through minor cracks in the crust of the earth. Nonexplosive eruptions may still be spectacular, as any visi-
tor to Hawaii lucky enough to witness the fury of Pele, the
There is tremendous variety in the style of volcanic erup- Hawaiian goddess of the volcano, can testify. Mauna Loa,
tions, both between volcanoes and from a single volcano dur- Kilauea, and other nonexplosive volcanoes produce a variety
ing the course of an eruptive phase. This variety is related to of eruption styles including fast-moving flows and liquid
the different types of magma produced by partial melting rivers of lava, lava fountains that spew fingers of lava trailing
beneath the volcano, and by the amount of dissolved gases in streamers of light hundreds of feet into the air, and thick
the magma. Geologists have found it useful to classify vol- sticky lava flows that gradually creep downhill. The Hawai-
canic eruptions based on the explosive characteristics of the ians devised clever names for these flows, including aa lava
eruption, the materials erupted, and by the type of landform for blocky rubbly flows because walking across these flows in
produced by the volcanic eruption. bare feet makes one exclaim “a! a!” in pain. Pahoehoes are
ropy textured flows, after the Hawaiian term for rope.
Tephra is material that comes out of a volcano during an
eruption, and it may be thrown through the air or transport- Explosive volcanic eruptions are among the most dra-
ed over the land as part of a hot moving flow. Tephra matic of natural events on Earth. With little warning, long-

Volcanoes and Plate Tectonics spot type of volcano in the world, with the active volcanoes on the
island of Hawaii known as Kilauea and Mauna Loa. Mauna Loa is a
The types of volcanoes and associated volcanic eruptions are dif- huge shield volcano, characterized by a gentle slope of a few
ferent in different tectonic settings because each tectonic setting degrees from the base to the top. This gentle slope is produced by
produces a different type of magma. Mid-ocean ridges and basaltic lava flows that have a very low viscosity and can flow and
intraplate “hot spot” types of volcanoes typically produce nonexplo- thin out over large distances before they solidify. Magmas with
sive eruptions, whereas convergent tectonic margin volcanoes may high viscosity are much stickier and solidify in short distances, pro-
produce tremendously explosive and destructive eruptions. Much of ducing volcanoes with steep slopes. Measured from its base on the
the variability in the eruption style may be related to the different Pacific Ocean seafloor to its summit, Mauna Loa is the tallest
types of magma produced in these different settings and to the mountain in the world, a fact attributed to the large distances that
amount of dissolved gases in these magmas. Magmas with large its low-viscosity lavas flow and to the large volume of magma pro-
amounts of gases tend to be highly explosive, whereas magmas duced by this hot-spot volcano.
with low contents of dissolved gases tend to be nonexplosive.
Volcanoes associated with convergent plate boundaries pro-
Eruptions from mid-ocean ridges are mainly basaltic flows, duce by far the most violent and destructive eruptions. Recent con-
with low amounts of dissolved gases. These eruptions are relatively vergent margin eruptions include Mount Saint Helens in
quiet, with basaltic magma flowing in underwater tubes and break- Washington State, and Mount Pinatubo volcano in the Philippines.
ing off in bulbous shapes called pillow lavas. The eruption style in The magmas from these volcanoes tend to be much more viscous,
these underwater volcanoes resembles toothpaste being squeezed higher in silica content, and they have the highest concentration of
out of a tube. Eruptions from mid-ocean ridges may be observed in dissolved gases. Many of the dissolved gases such as water are
the few rare places where the ridges emerge above sea level, such released from the subducting oceanic plate as high mantle temper-
as Iceland. Eruptions there include lava fountains, where basaltic atures heat it up as it slides beneath the convergent margin volca-
cinders are thrown a few hundred feet in the air and accumulate as noes. These gases build up pressure beneath the volcano as it
cones of black, glassy fragments, and they also include long prepares to erupt, much like dissolved gas in a carbonated soda
streamlike flows of basalt. that is shaken before it is opened. When the gas and magma pres-
sure in these volcanoes exceeds the weight of overlying rocks the
Hot-spot volcanism tends to be much like that at mid-ocean volcano may suddenly explode, emitting enormous quantities of
ridges, particularly where the hot spots are located in the middle of magma, pulverized rocks, and gases.
oceanic plates. The Hawaiian Islands have the most famous hot-

volcano 453

dormant volcanoes can explode with the force of hundreds of and travel more than 62 miles (100 km) from the volcanic
atomic bombs, pulverizing whole mountains and sending the vent. Nuée ardentes have been the nemesis of many a volca-
existing material together with millions of tons of ash into nologist and curious observer, as well as thousands upon
the stratosphere. Explosive volcanic eruptions tend to be thousands of unsuspecting or trusting villagers who previ-
associated with volcanoes that produce andesitic or rhyolitic ously lived on the flanks of volcanoes. Nuée ardentes are but
magma and have high contents of dissolved gases. These are one type of pyroclastic flow, which include a variety of mix-
mostly associated with convergent plate boundaries. Volca- tures of volcanic blocks, ash, gas, and lapilli that produce
noes that erupt magma with high contents of dissolved gases volcanic rocks called ignimbrites.
often produce a distinctive type of volcanic rock known as
pumice, which is full of bubble wholes, in some cases making Most volcanic eruptions emanate from the central vents
the rock light enough to float on water. at the top of volcanic cones. However, many flank eruptions
have been recorded, where eruptions blast out of fissures on
When the most explosive volcanoes erupt they produce the side of the volcano. Occasionally volcanoes blow out
huge eruption columns known as Plinian Columns (named their sides, forming a lateral blast like the one that initiated
after Pliny the Elder, the Roman statesman who died in 79 the 1980 eruption of Mount Saint Helens in Washington
C.E. sampling volcanic gas during an eruption of Mount State. This blast was so forceful that it began at the speed of
Vesuvius). These eruption columns can reach 28 miles (45 sound, killing everything in the initial blast zone.
km) in height, and they spew hot turbulent mixtures of ash,
gas, and tephra into the atmosphere where winds may dis- See also NUÉE ARDENTE; PLATE TECTONICS.
perse them around the planet. Large ash falls and tephra
deposits may be spread across thousands of square kilome- Further Reading
ters. These explosive volcanoes also produce one of the Fisher, R. V. Out of the Crater: Chronicles of a Volcanologist. Prince-
scariest and most dangerous clouds on the planet. Nuée
ardentes are hot glowing clouds of dense gas and ash that ton: Princeton University Press, 2000.
may reach temperatures of nearly 1,832°F (1,000°C), rush Fisher, R. V., G. Heiken, and J. B. Hulen. Volcanoes: Crucibles of
down volcanic flanks at 435 miles per hour (700 km/hr),
Change. Princeton: Princeton University Press, 1998.
Simkin, T., and R. S. Fiske. Krakatau 1883: The Volcanic Eruption

and Its Effects. Washington D.C.: Smithsonian Institution Press,
1993.

W

wadi A term used for a dry streambed in desert regions of wells are dug and water is carried to villages by pumps or
North Africa, the Middle East, and southwest Asia. Most other gravity-fed irrigation systems.
wadis have gravelly bases, and many have steep to vertical
sides that cut through mountainous regions, older fluvial See also DESERT; GEOMORPHOLOGY; RIVER SYSTEM.
deposits, whereas others have more gentle sides in places
where the wadi cuts across plains and flatter regions. Wadis wash See WADI.
may be dry for many months or years and then be ravaged by
flash floods during brief but intense rainfalls somewhere in watershed See DRAINAGE BASIN.
the watershed, then quickly lose water to underlying porous
sediments. In desert regions it is often possible to find water water table The depth or level at which groundwater
at shallow depths beneath wadi channels because the gravels below the surface fills all of the pore spaces between indi-
and sands typically are very porous and may host a signifi- vidual grains of the regolith. The distribution of water in
cant amount of subsurface base flow of water, not visible the ground can be divided into the unsaturated and the sat-
from the surface. Thus, many small villages and settlements urated zones. The top of the water table is defined as the
in rural arid regions are located adjacent to wadis, where upper surface of the saturated zone. Below this surface, all
openings are filled with water. After a rainfall, much of the
Wadi Dayqa, northern Oman, showing small amount of water flowing over water stays near the surface, since clay in the near-surface
cobbles with more extensive subsurface base flow (Photo by Timothy horizons of the soil retains much water because of its molec-
Kusky) ular attraction. This forms a layer of soil moisture in many
regions and is able to sustain seasonal plant growth. Some
of this near surface water evaporates, and plants use some
of the near surface water; other water runs directly off into
streams. The remaining water seeps into the saturated zone,
or into the water table. Once in the saturated zone it moves
by percolation, gradually and slowly, from high areas to low
areas, under the influence of gravity. These lowest areas are
usually lakes or streams. Many streams form where the
water table intersects the surface of the land. Once in the
water table, the paths that individual particles follow vary,
the transit time from surface to stream may vary from days
to thousands of years along a single hillside! Water can flow
upward because of high pressure at depth and low pressure
in the stream. The level of the water table changes with dif-
ferent amounts of precipitation—in humid regions, it
reflects the topographic variation, whereas in dry times or

454

waves 455

places it tends to flatten out to the level of the streams of blows over, the longer the wavelength, the greater the wave
lakes. Water flows faster when the slope is greatest, so height, and the longer the period.
groundwater flows faster during wet times. The fastest rate
of groundwater flow yet observed in the United States is It is important to remember that waves are energy in
800 feet per year (250 m/yr). motion, and the water in the waves does not travel along
with the waves. The motion of individual water particles as a
See also ARTESIAN WELL; GROUNDWATER. wave passes is roughly a circular orbit that decreases in
radius with depth below the wave. You have probably experi-
waves Geometrically regular and repeating undulations on enced this motion sitting in waves in the ocean, feeling your-
the surface of water that move and transport energy from self moving roughly up and down, or in a circular path, as
one place to another. Waves are generated by winds that the waves pass you.
blow across the water surface, and the frictional drag of the
wind on the surface transfers energy from the air to the sea, The motion of water particles in waves changes as the
where it is expressed as waves. The waves may travel great water depth decreases, and the waves approach shore. At a
distances across entire ocean basins, and they may be depth equal to roughly one-half of the wavelength, the circu-
thought of as energy in motion. This energy is released or lar motion induced by the wave begins to feel the sea bottom,
transferred to the shoreline when the waves crash on the which exerts a frictional drag on the wave. This changes the
beach. It is this energy that is able to move entire beaches circular particle paths to elliptical paths and causes the upper
and erode cliffs grain-by-grain, slowly changing the appear- part of the wave to move ahead of the deeper parts. Eventual-
ance of the beach environment. ly, the wave becomes oversteepened and begins to break, as
the wave crashes into shore in the surf zone. In this surf zone,
When waves are generated by winds over deep water, the water is actually moving forward, causing the common
often from distant storms, they develop a characteristic spac- erosion, transportation, and deposition of sand along beaches.
ing and height, known as the wavelength and height. The
wave crest is the highest part of the wave, and the wave Most coastlines are irregular and have many headlands,
trough is the low point between waves. Wavelength is the dis- bays, bends, and changes in water depth from place to place.
tance between successive crests or troughs, the wave height is These variables cause waves that are similar in deep water to
the vertical distance between troughs and crests, and ampli- approach the shoreline at different angles in different places.
tude is one-half of the wave height. Wave fronts are (imagi- You may have noticed on beaches, how waves may come
nary) lines drawn parallel to the wave crests, and the wave ashore gently in one place, yet form nice breakers down the
moves perpendicular to the wave fronts. The time (in sec- beach. These changes can be attributed to changes in water
onds) that it takes successive wave crests to pass a point is depth, steepness of the underwater slope, and the shape of
known as the wave period. the beach. Wave refraction occurs when a straight wave front
approaches a shoreline obliquely. The part of the wave front
The height, wavelength and period of waves is deter- that first feels shallow water (with a depth of less than one-
mined by how strong the wind is that generates the waves, half of the wavelength, known as the wave base) begins to
how long it blows, and the distance over which the wind slow down while the rest of the wave continues at its previ-
blows (known as the fetch). As any sailor can tell you, the ous velocity. This causes the wave front to bend, or be
longer and stronger and the greater the distance the wind refracted. Refraction tends to cause waves to bend toward

Wave motion showing orbital paths followed by water particles. Note that wave motion dies out a distance equal to half of the wavelength of the waves.

456 weathering

with air and water migrating through cracks, fractures, and
pore space. The effects of weathering can often be seen in
outcrops on the side of the roads, where they cut through the
zone of alteration into underlying bedrock. These roadcuts
and weathered outcroppings of rock show some similar prop-
erties. The upper zone near the surface is made of soil or
regolith in which the texture of the fresh rock is not appar-
ent, a middle zone in which the rock is altered but retains
some of its organized appearance, and a lower zone, of fresh
unaltered bedrock.

Waves that crash obliquely on beaches wash water and sand particles Processes of Weathering
obliquely, parallel to the wave direction, up the beach face. When the There are three main types of weathering. Mechanical
backwash moves the water and sand particles back down the beach weathering is the disintegration of rocks, generally by abra-
face, gravity moves the particles parallel to the beach face slope. The net sion. Mechanical weathering is common in the talus slopes
effect is that the wash/backwash action on beaches moves sand particles at the bottom of the mountains, along beaches, and along
down the beach face, sometimes many hundreds of meters per day. river bottoms. Chemical weathering is the decomposition of
rocks through the alteration of individual mineral grains
headlands concentrating energy in those places, and to con- and is a common process in the soil profile. Biological
centrate less energy in bays. Material is eroded from the weathering involves the breaking down of rocks and miner-
headlands and transported to and deposited in the bays. als by biological agents. Some organisms attack rocks for
nutritional purposes; for instance, chitons bore holes
When waves approach a beach obliquely, a similar phe- through limestone along the seashore, extracting their nutri-
nomenon occurs. Even though the waves are refracted, they ents from the rock.
may still crash onto the beach obliquely, moving sand parti-
cles sideways up the beach with each wave. As the wave Generally, mechanical and chemical weathering are the
returns to the sea in the backwash, the wave energy has been most important, and they work hand-in-hand to break down
transferred to the shoreline (and has moved the sand grains), rocks into the regolith. The combination of mechanical,
and gravity is the driving force moving the water and sand, chemical, and biological weathering produces soils, or a
which then moves directly downhill. The net result is that weathering profile.
sand particles move obliquely up and straight down the
beach slope, moving slightly sideways along the beach with MECHANICAL WEATHERING There are several different
each passing wave. It is common for individual sand grains to types of mechanical weathering which may act separately or
move almost a mile per day along beaches, through this pro- together to break down rocks. The most common process of
cess known as longshore drift. If the supply or transportation mechanical weathering is abrasion, where movement of rock
route of the sand particles is altered by human activity, such particles in streams, along beaches, in deserts, or along the
as the construction of seawalls or groins, the beach will bases of slopes causes fragments to knock into each other.
respond by dramatically changing in some way. These collisions cause small pieces of each rock particle to
break off, gradually rounding the particles and making them
See also BEACH; CONTINENTAL MARGIN; TSUNAMI. smaller, and creating more surface area for processes of
chemical weathering to act upon.
weathering The process of mechanical and chemical alter-
ation marked by the interaction of the lithosphere, atmo- Some rocks develop joints or parallel sets of fractures,
sphere, hydrosphere, and biosphere. The resistance to from differential cooling, the pressures exerted by overlying
weathering varies with climate, composition, texture, and rocks, or tectonic forces. Joints are fractures along which no
how much a rock is exposed to the elements of weather. observable movement has occurred. Joints promote weather-
Weathering processes occur at the lithosphere/atmosphere ing in two ways: they are planes of weakness across which
interface. This is actually a zone that extends down into the the rock can break easily, and they act as passageways for
ground to the depth that air and water can penetrate—in fluids to percolate along, promoting chemical weathering.
some regions this is a few meters, in others it is a kilometer or
more. In this zone, the rocks make up a porous network, Crystal growth may aid mechanical weathering. When
water percolates through joints or fractures, it can precipitate
minerals such as salts, which grow larger and exert large
pressures on the rock along the joint planes. If the blocks of
rock are close enough to a free surface such as a cliff, large
pieces of rock may be forced off in a rockfall, initiated by the
gradual growth of small crystals along joints.

weathering 457

When water freezes to form ice, its volume increases by potassium, and bicarbonate are typically dissolved in water
9 percent. Water is constantly seeping into the open spaces and carried away during weathering.
provided by joints in rocks. When water filling the space in a
joint freezes, it exerts large pressures on the surrounding Much of the material produced during chemical weath-
rock. These forces are very effective agents of mechanical ering is carried away in solution and deposited elsewhere,
weathering, especially in areas with freeze-thaw cycles. They such as in the sea. The highest-temperature minerals are
are responsible for most rock debris on talus slopes of leached the easiest. Many minerals combine with oxygen in
mountains. the atmosphere to form another mineral, by oxidation. Iron
is very easily oxidized from the Fe+2 state to the Fe+3 state,
Heat may also aid mechanical weathering, especially in forming goethite or with the release of water, hematite.
desert regions where the daily temperature range may be
extreme. Rapid heating and cooling of rocks sometimes 2FeO + OH ¨ Fe2O3 + H2O
exerts enough pressures on the rocks to shatter them to
pieces, thus breaking large rocks into smaller fragments. Different types of rock weather in distinct ways. For
instance, granite contains K-feldspar and weathers to clays.
Plants and animals may also aid mechanical weathering. Building stones are selected to resist weathering in different
Plants grow in cracks and push rocks apart. This process may climates, but now, increasing acidic pollution is destroying
be accelerated if plants such as trees become uprooted, or many old landmarks. Chemical weathering results in the
blown over by wind, exposing more of the underlying rock to removal of unstable minerals and a consequent concentration
erosion. Burrowing animals, worms, and other organisms of stable minerals. Included in the remains are quartz, clay,
bring an enormous amount of chemically weathered soil to and other rare minerals such as gold and diamonds, which
the surface and continually turn the soils over and over, may be physically concentrated in placer deposits.
greatly assisting the weathering process.
On many boulders, weathering only penetrates a fraction
CHEMICAL WEATHERING Minerals that form in igneous of the diameter of the boulder, resulting in a rind of the
and metamorphic rocks at high temperatures and pressures altered products of the core. The thickness of the rind itself is
may be unstable at temperatures and pressures at the Earth’s useful for knowing the age of the boulder, if rates of weather-
surface, so they react with the water and atmosphere to pro- ing are known. These types of weathering rinds are useful for
duce new minerals. This process is known as chemical weath- determining the age of rock slides and falls, and the time
ering. The most effective chemical agents are weakly acidic interval between rockfalls in any specific area.
solutions in water. Therefore, chemical weathering is most
effective in hot and wet climates. Exfoliation is a weathering process where rocks spall off
in successive shells, like the skin of an onion. Exfoliation is
Rainwater mixes with CO2 from the atmosphere and caused by differential stresses within a rock formed during
from decaying organic matter, including smog, to produce chemical weathering processes. For instance, feldspar weath-
carbonic acid according to the reaction: ers to clay minerals, which take up a larger volume than the
original feldspar. When the feldspar minerals turn to clay,
H2O + CO2 ¨ H2CO3 they exert considerable outward stress on the surrounding
rock, which is able to form fractures parallel to the rock’s
Water + carbon dioxide ¨ carbonic acid surface. This need for increased space is accommodated by
the minerals through the formation of these fractures, and the
Carbonic acid ionizes to produce the hydrogen ion (H+), rocks on the hillslope or mountain are then detached from
which readily combines with rock-forming minerals to pro- their base and are more susceptible to sliding or falling in a
duce alteration products. These alteration products may then mass wasting event.
rest in place and become soils, or be eroded and accumulate
somewhere else. If weathering proceeds along two or more sets of joints
in the subsurface, it may result in shells of weathered rock,
Hydrolysis is a process that occurs when the hydrogen which surround unaltered rocks, looking like boulders. This
ion from carbonic acid combines with K-feldspar to produce is known as spheroidal weathering. The presence of the sever-
kaolinite, a clay mineral, according to the reaction: al sets of joint surfaces increases the effectiveness of chemical
weathering, because the joints increase the available surface
2 KAlSi3O8 + 2 H2CO3 + H2O ¨ Al2Si2O5(OH)4 + 4 SiO2 + area to be acted on by chemical processes. The more subdivi-
2 K+1 + 2 HCO3 sions within a given volume, the greater the surface area.

feldspar + carbonic acid + water ¨ kaolinite + silica + Factors That Influence Weathering
potassium + The effectiveness of weathering processes is dependent upon
bicarbonate ion several different factors, explaining why some rocks weather
one way at one location, and a different way in another loca-
This reaction is one of the most important reactions in chemi- tion. Rock type is an important factor in determining the
cal weathering. The product, kaolinite, is common in soils
and is virtually insoluble in water. The other products, silica,

458 Wegener, Alfred Lothar

weathering characteristics of a hillslope, because different interest in meteorology and geology led him on a Danish
minerals react differently to the same weathering conditions. expedition to northeastern Greenland in 1906–1908. This
For instance, quartz is resistant to weathering, and quartz- was the first of four Greenland expeditions he would make,
rich rocks typically form large mountain ridges. Conversely, and this area remained one of his dominant interests. Wegen-
shales readily weather to clay minerals, which are easily er is famous for being the first person to come up with the
washed away by water, so shale-rich rocks often occupy the idea for continental drift. He studied the apparent correspon-
bottoms of valleys. Examples of topography being closely dence between the shapes of the coastlines of western Africa
related to the underlying geology in this manner are abun- and eastern South America. Later on he learned that evidence
dant in the Appalachians, Rocky Mountains, and most other of paleontological similarities was being used to support the
mountain belts of the world. theory of the “land bridge” that had connected Brazil to
Africa. He continued to study the paleontological and geolog-
Rock texture and structure is important in determining ical evidence, concluded that these similarities demanded an
the weathering characteristics of a rock mass. Joints and explanation, and wrote an extended account of his continen-
other weaknesses promote weathering by increasing the sur- tal drift theory in his book Die Entstehung der Kontinente
face area for chemical reactions to take place on, as described und Ozeane (The Origin of the Continents and Oceans). As a
above. They also allow water, roots, and mineral precipitates meteorologist he began to look at ancient climates and used
to penetrate deeply into a rock mass, exerting outward pres- paleoclimatic evidence he found to strengthen his theory of
sures that can break off pieces of the rock mass in catastroph- continental drift. Wegener was by no means the first to think
ic rockfalls and slides. of the theory of continental drift. However, he was the first to
go to great lengths to develop and establish the theory. He is
The slope of a hillside is important for determining what also known for his work on dynamics and thermodynamics
types of weathering and mass wasting processes occur on that of the atmosphere, atmospheric refraction and mirages, opti-
slope. Steep slopes let the products of weathering get washed cal phenomena in clouds, acoustical waves, and the design of
away, whereas gentle slopes promote stagnation and the for- geophysical instruments.
mation of deep weathered horizons.
Werner, Abraham Gottlob (1749–1817) Polish Geolo-
Climate is one of the most important factors in deter- gist, Mineralogist Abraham Gottlob Werner was enormous-
mining how a site weathers. Moisture and heat promote ly influential in the field of geology. Werner developed
chemical reactions, so chemical weathering processes are techniques for identifying minerals using human senses, and
strong, fast, and dominant over mechanical processes in hot this appealed to a broad audience interested in learning more
wet climates. In cold climates, chemical weathering is much about geology. Werner also proposed a new classification for
less important. Mechanical weathering is very active during certain geologic formations. In the 18th century, rocks were
freezing and thawing, so mechanical processes such as ice explained and were classified into three categories with
wedging tend to dominate over chemical processes in cold cli- accordance to the “biblical flood,” including Primary for
mates. These differences are exemplified by two examples of ancient rocks without fossils (believed to precede the flood),
weathering. In much of New England, a hike over mountain Secondary for rocks containing fossils (often attributed to the
ridges will reveal fine, millimeter-thick striations that were flood itself) and Tertiary for sediments believed to have been
formed by glaciers moving over the region more than 10 deposited after the flood. Werner did not dispute the com-
thousand years ago. Chemical weathering has not removed monly held belief in the biblical flood, but he did discover a
even these one-millimeter thick marks in 10 thousand years. different group of rocks that did not fit this classification:
In contrast, new construction sites in the tropics, such as rocks with a few fossils that were younger than primary
roads cut through mountains, often expose fresh bedrock. In rocks but older than secondary rocks. He called these “transi-
a matter of 10 years these road cuts will be so deeply eroded tion” rocks. Geologists of succeeding generations classified
to a red soil-like material called gruse, that the original rock these rocks into the geologic periods still accepted today.
will not be recognizable.
white smoker chimneys See BLACK SMOKER CHIMNEYS.
As in most things, time is important. It takes tens of
thousands of years to wash away glacial grooves in cold cli- wildcat Oil and gas wells that are drilled on structures,
mates, but in tropics, weathered horizons that extend to hun- formations, depths, or regions not yet known to contain
dreds of meters may form over a few million years. hydrocarbons, or that have not yet yielded any oil or gas, are
known as wildcat wells. These risky wells are also called out-
See also SOILS. post wells, deeper-pool or shallower-pool wells, and
exploratory wells. Wildcat wells are routinely drilled by large
Wegener, Alfred Lothar (1880–1930) German Meteorol-
ogist, Geophysicist Alfred Wegener is well known for his
studies in meteorology and geophysics and is considered by
many to be the father of continental drift. He completed his
studies in Berlin and presented a thesis on astronomy. His

Witwatersrand basin 459

companies but may cost hundreds of thousands or even mil- it is correlated with many similar volcanic groups along the
lions of dollars to drill, so the risk is high. Some small northern margin of the Kaapvaal craton. The Dominion
petroleum exploration companies have been made or broken group and its correlatives, and a group of related plutons, has
by wildcat wells, and investors have gotten rich or bankrupt been interpreted as the products of Andean arc magmatism,
by investing fortunes in wildcatters. formed above a 2.8-billion-year-old subduction zone that
dipped beneath the Kaapvaal craton. The overlying West
See also HYDROCARBON; PETROLEUM. Rand and Central Rand groups were deposited in a basin at
least 50,000 square miles (80,000 km2) in area. Stratigraphic
Witwatersrand basin The Witwatersrand basin on South thicknesses of the West Rand group generally increase toward
Africa’s Kaapvaal craton is one of the best known of Archean the fault-bounded northwestern margin of the basin, whereas
sedimentary basins and contains some of the largest gold thicknesses of the Central Rand group increase toward the
reserves in the world, accounting for more than 55 percent of center of the basin. Strata of both groups thin considerably
all the gold ever mined in the world. Sediments in the basin toward the southeastern basin margin. The northeastern and
include a lower flysch-type sequence, and an upper molassic southwestern margins are poorly defined, but some correla-
facies, both containing abundant silicic volcanic detritus. The tions with other strata (such as the Godwan formation) indi-
strata are thicker and more proximal on the northwestern cate that the basin was originally larger than the present
side of the basin that is at least locally fault bounded. The basin. Strata that were originally deposited north of Johan-
Witwatersrand basin is a composite foreland basin that devel- nesburg are buried, removed by later uplift, omitted by
oped initially on the cratonward side of an Andean arc, simi- igneous intrusion, and cut out by faulting.
lar to retroarc basins forming presently behind the Andes. A
continental collision between the Kaapvaal and Zimbabwe The West Rand group consists of southeastward-taper-
cratons 2.7 billion years ago caused further subsidence and ing sedimentary wedges that overlie the Dominion group,
deposition in the Witwatersrand basin. Regional uplift during and onlap granitic basement in many places. The maximum
this later phase of development placed the basin on the cra- thickness of the West Rand group, 25,000 feet (7,500 m),
tonward edge of a collision-related plateau, now represented occurs along the northern margin of the basin, and the
by the Limpopo Province. There are many similarities group thins southeast to a preserved thickness of 2,700 feet
between this phase of development of the Witwatersrand (830 m) near the southern margin. Shale and sandstone in
basin and basins such as the Tarim and Tsaidam north of the approximately equal proportions characterize the West
Tibetan Plateau. Rand group, and a thin horizon of mafic volcanics is locally
present. This volcanic horizon thickens to 800 feet (250 m)
The Witwatersrand basin is an elongate trough filled near the northern margin of the basin but is absent in the
predominantly by 2.8–2.6-billion-year-old clastic sedimenta- south. The West Rand group contains mature quartzites,
ry rocks of the West Rand and Central Rand groups, togeth- minor chert, and sedimentation patterns indicating both
er constituting the Witwatersrand supergroup. These are tidal and aeolian reworking. Much of the West Rand group
locally, in the northwestern parts of the basin, underlain by is an ebb-dominated tidal deposit later influenced by beach-
the volcanosedimentary Dominion group. The structure swash deposition. More shales are preserved near the top of
trends in a northeasterly direction parallel with, but some the group. Overall, the West Rand group preserves a transi-
distance south of, the high-grade gneissic terrane of the tion from tidal flat to beach then deeper water deposition,
Limpopo Province. The high-grade metamorphism, calc- which indicates a deepening of the Witwatersrand basin
alkaline plutonism, uplift, and cooling in the Limpopo are of during deposition. Upper formations in the West Rand
the same age as and closely related to the evolution of the group contain magnetic shales and other fine-grained sedi-
Witwatersrand basin. Strata dip inward with dips greater on ments suggestive of a distal shelf or epicontinental sea envi-
the northwestern margin of the basin than on the southeast- ronment of deposition.
ern margin. The northwestern margin of the basin is a steep
fault that locally brings gneissic basement rocks into contact The lower West Rand group records subsidence of the
with Witwatersrand strata to the south. Dips are vertical to Witwatersrand basin since the sediments grade vertically
overturned at depth near the fault, but only 20° near the sur- from beach deposits to a distal shallow marine facies. This
face, demonstrating that this is a thrust fault. A number of inundation of shallow water sedimentary environments sug-
folds and thrust faults are oriented parallel to the northwest- gests that the subsidence was rapid, and the absence of
ern margin of the basin. coarse, immature, fanglomerate type sediments suggests that
the subsidence was accommodated by flexure and not fault-
The predominantly clastic fill of the Witwatersrand ing. A decreasing rate of subsidence and/or a higher rate of
basin has been divided into the West Rand and the overlying clastic sediment supply is indicated by the progressively shal-
Central Rand groups, which rest conformably on the largely lower water facies deposits in the upper West Rand group.
volcanic Dominion group. The Dominion group was deposit- Numerous silicic volcanic clasts in the West Rand group indi-
ed over approximately 9,000 square miles (15,000 km2), but

460 Witwatersrand basin

cate that a volcanic arc terrane to the north was contributing indicated by paleocurrent directions in a few locations. A
volcanic detritus to the Witwatersrand basin. Additionally, few tuffaceous horizons and a thin mafic lava unit are found
the presence of detrital ilmenite, fuchsite, and chromite indi- in the Central Rand group in the northeast part of the basin.
cate that an ultramafic source such as an elevated greenstone The great dispersion of unimodal paleocurrent directions
belt was also contributing detritus to the basin. derived from most of the Central Rand group indicates that
these sediments were deposited in shallow braided streams
The Central Rand group was deposited conformably on on coalescing alluvial fans. The paleorelief is estimated at 20
top of the West Rand group and attains a maximum pre- feet (6 m) in areas proximal to the source, and 1–2 feet (0.5
served thickness of 9,500 feet (2,880 m) northwest of the m) in more distal areas. Some of the placers in the Central
center of the basin, and north of the younger Vredefort Rand group have planar upper surfaces, commonly associat-
impact structure. Sediments of the Central Rand group con- ed with pebbles and heavy placer mineral concentrations,
sist of coarse-grained graywackes and conglomerates along which may be attributed to reworking by tidal currents.
with subordinate quartz arenite interbedded with local lacus- Clasts in the conglomerates include vein quartz, quartz aren-
trine or shallow marine shales and siltstones. The conglom- ite, chert, jasper, silicic volcanics, shales and schists, and
erates are typically poorly sorted and larger clasts are well other rare rocks.
rounded, while the smaller pebbles are angular to subangu-
lar. Paleocurrent indicators show that the sediments prograd- The Central Rand group contains a large amount of
ed into the basin from the northwestern margin in the form molassic type sediments disposed as sand and gravel bars in
of a fan-delta complex. This is economically important coalesced alluvial fans and fluvial systems. The West
because numerous goldfields in the Central Rand group are Rand/Central Rand division of the Witwatersrand basin into
closely associated with major entry points into the basin. a lower flysch-type sequence and an upper molasse facies is
Some transport of sediments along the axis of the basin is typical of foreland basins. Extensive mining of paleoplacers

Map of the Witwatersrand basin showing paleocurrent directions indicating several discrete sources for detrital sediments in the basin, located mostly
north of the basin

Witwatersrand basin 461

Two stages in the tectonic evolution of the Witwatersrand basin showing early formation as a foreland trough that developed behind an Andean arc
complex built on the Kaapvaal craton. A later stage shows the collision between the Kaapvaal and Zimbabwe cratons, forming an uplifted Tibetan-style
plateau, with further deposition in the Witwatersrand basin in a collisional foredeep.

for gold and uranium has enabled the dendritic pale- front with time, as in younger foreland basins. Stratigraphic
odrainage patterns to be mapped, and the points of entry into relationships within the underlying Dominion group, the
the basin to be determined. The source of the Central Rand presence of silicic volcanic clasts throughout the stratigraphy,
sediments was a mountain range located to the northwest of and minor lava flows within the basin suggest that the fore-
the basin, and this range contained a large amount of silicic land basin was developed behind a volcanic arc, partly pre-
volcanic material. served as the Dominion group. Sediments of the West Rand
group are interpreted as deposited in an actively subsiding
The growth of folds parallel to the basin margin during foreland basin developed adjacent to an Andean margin and
sedimentation and the preferential filling of synforms by fold thrust belt.
some of the mafic lava flows in the basin indicate that folding
was in progress during Central Rand group sedimentation. Deformation in the Limpopo Province and northern
Deformation of this kind is diagnostic of flexural foreland margin of the Kaapvaal craton are related to a collision
basins, and studies show that the depositional axis of the between the northern Andean margin of the Kaapvaal craton
basin migrated southeastward during sedimentation, with with a passive margin developed on the southern margin of
many local unconformities related to tilting during flexural the Zimbabwe craton that began before 2.64 billion years
migration of the depositional centers. ago, when Ventersdorp rifting, related to the collision, began.
It is possible that some of the rocks in the Witwatersrand
The Witwatersrand basin exhibits many features that are basin, particularly the molasse of the Central Rand group,
characteristic of foreland basins including an asymmetric pro- may represent erosion of a collisional plateau developed as a
file with thicker strata and steeper dips toward the mountain- consequence of this collision. The plateau would have been
ous flank, a basal flysch sequence overlain by molassic-type formed in the region between the Witwatersrand basin and
sediments, and thrust faults bounding one side of the basin. the Limpopo Province, a region characterized by a deeply
Compressional deformation was in part synsedimentary, and eroded gneiss terrane. A major change in the depositional
associated folds and faults trend parallel to the basin mar- style occurs in the Witwatersrand basin between the Central
gins, and the depositional axis migrated away from the thrust

462 wollastonite

Rand and West Rand groups, and this break may represent See also ARCHEAN; CONVERGENT PLATE MARGIN PRO-
the change from Andean arc retroarc foreland basin sedimen- CESSES; CRATONS; KAAPVAAL CRATON.
tation to collisional plateau erosion-related phases of fore-
land basin evolution. Further Reading
Antrobus, E. S. A., ed., Witwatersrand Gold—100 Years. Johannes-
Paleoplacers in the Witwatersrand basin have yielded
more than 850 million tons of gold, dwarfing all the world’s burg: Geological Society of South Africa, 1986.
other gold placer deposits put together. Many of the placer Burke, Kevin, William S. F. Kidd, and Timothy M. Kusky. “Is the
deposits (called reefs in local terminology) preserved detrital
gold grains on erosion surfaces, along foreset beds in cross- Ventersdorp Rift System of Southern Africa Related to a Conti-
laminated sandstone and conglomerate, in trough cross-beds, nental Collision between the Kaapvaal and Zimbabwe Cratons at
in gravel bars, and as detrital grains in sheet sands. Most of 2.64 Ga ago?” Tectonophysics 11 (1985).
the gold is located close to the northern margin of the basin ———. “Archean Foreland Basin Tectonics in the Witwatersrand,
in the fluvial channel systems. Some of the gold flakes in South Africa.” Tectonics 5 (1986).
more distal areas were trapped by stromatolite-like filamen- Tankard, Anthony J., M. P. A. Jackson, Ken A. Eriksson, David K.
tous algae, and some appears to have even have been precipi- Hobday, D. R. Hunter, and W. E. L. Minter. Crustal Evolution of
tated by some types of algae, although it is more likely that Southern Africa: 3.8 Billion Years of Earth History. New York:
these are fine recrystallized grains that were trapped by algal Springer-Verlag, 1982.
filaments. Besides gold there are more than 70 ore minerals
recognized in the Witwatersrand basin, and most of these are wollastonite A calc-silicate mineral of the pyroxenoid
detrital grains, and others are from metamorphic fluids. The group found in contact metamorphosed carbonate rocks, and
most abundant detrital grains include pyrite, uraninite, bran- in the invading igneous rocks near their contacts with the car-
nerite, gold, arsenopyrite, cobaltite, chromite, and zircon. bonates as a result of contamination. Named after Wollaston,
Gold mining operations in the Witwatersrand employ more Massachusetts, wollastonite may appear as tabular twinned
than 300,000 people and have led to the economic success of crystals or in cleavable masses and has colors that range from
South Africa. white to gray, brown, red and yellow. The chemical formula
for wollastonite is CaSiO3.

See also METAMORPHISM.

X

xenolith A foreign inclusion in an igneous rock, typically xenotime A tetragonal mineral that is isostructural with
a fragment of the local surrounding country rock. Xenoliths zircon and has the formula YPO4. Xenotime may be yellow,
are commonly found near the margins of igneous intrusions brown, or even red and typically has high concentrations of
and are typically flattened parallel to the margins of the plu- rare earth elements, including uranium, thorium, beryllium,
ton, reflecting high strains associated with emplacement and and zirconium, as well as aluminum and calcium. Xenotime
perhaps expansion of the pluton. Many studies of plutons is a moderately rare mineral but occurs as an accessory in
use their compositions to learn about the types of rocks that some granites and pegmatites.
the magma passed through from its source region to its
emplacement location. In most cases the xenoliths are from See also MINERALOGY; ZIRCON.
within a few kilometers below the pluton, but in other cases
the xenoliths come from much deeper and contain important X-ray fluorescence A geochemical technique that is com-
information about deep crustal or even mantle petrography monly used to determine the concentration of major and
and petrology. trace elements in minerals and rocks, at concentrations rang-
ing from less than one part per million to high concentra-
Suites of deep xenoliths found in continental and oceanic tions. Samples of the rocks and minerals are cleaned and
alkalic basalts are compositionally and mineralogically diverse powdered, then melted and fused into a glass disc or pressed
and include some mantle rocks including dunite, lherzolite, powder pellet. The sample is then irradiated with primary X
and spinel lherzolite. Kimberlites typically contain large rays and reemits secondary X rays with wavelengths and
amounts of xenoliths including many deep crustal and mantle energies that are characteristic of the minerals present in the
xenoliths, including diamonds. Diamonds are derived from sample. To determine the concentrations of the elements in
depths of approximately 93 miles (150 km), making kimber- the sample, the peaks from the analysis are compared to simi-
lites the host of the most deep-level xenoliths, and an impor- lar peaks from a standard sample with known concentrations
tant source of information about the deep mantle roots of the same elements.
beneath Archean cratons, and the upper mantle.
See also GEOCHEMISTRY.
See also IGNEOUS ROCKS; KIMBERLITE; PETROLOGY.

463

Y

yardangs Yardangs are elongate streamlined wind-eroded bison, numerous birds, and a diverse flora. The park sits on a
ridges, which resemble an overturned ship’s hull sticking out large upland plateau resting at about 8,000 feet (2,400 m) ele-
of the water. These unusual features are formed by abrasion, vation straddling the continental divide. The plateau is sur-
by the long-term sandblasting along specific corridors. The rounded by mountains that range 10,000–14,000 feet
sandblasting leaves erosionally resistant ridges but removes (3,000–4,250 m) above sea level. Most of the rocks in the
the softer material which itself will contribute to sandblasting park formed from a massive volcanic eruption that occurred
in the downwind direction and eventually contribute to the 600,000 years ago, forming a collapse caldera that is 28 miles
formation of sand, silt, and dust deposits. (45 km) wide and 46 miles (74 km) long. The deepest part of
the caldera is now occupied largely by Yellowstone Lake. The
See also DESERT; GEOMORPHOLOGY. region is still underlain by molten magma that contributes
heat to the groundwater system, which boasts more than
Yellowstone National Park The northwest corner of 10,000 hot springs, 200 geysers, and numerous fumaroles,
Wyoming, and adjacent parts of Idaho and Montana, was vents, and hot mud pools. The most famous geyser in the park
established as a national park in 1872 by President Ulysses S. is Old Faithful, which erupts an average of once every 64.5
Grant, and it remains the largest national park in the conter- minutes blowing 11,000 gallons (41,500 l) of water 150 feet
minous United States. The park serves as a large nature pre- (46 m) into the air. The most famous hot springs include
serve and has large populations of moose, bear, sheep, elk,

Yellowstone Falls on the Yellowstone River in Yellowstone National Park Fumaroles and geysers in a geyser basin, Yellowstone National Park
(Photo by Timothy Kusky) (Photo by Timothy Kusky)

464

Yosemite Valley 465

Mammoth hot springs, Yellowstone National Park, showing thick terraces has migrated 280 miles (450 km) southwestward in the past
and pools of travertine mineral deposits (Photo by Timothy Kusky) 16 million years with respect to this hot spot, the volcanic
effects migrated from the Snake River Plain to the Yellow-
Mammoth hot springs on the northern side of the park, where stone Plateau. There is currently a parabolic shaped area of
giant travertine and mineral terraces have formed from the seismicity, active faulting, and centers of igneous intrusion
spring, and where simple heat-loving (thermophyllic) organ- that is centered around the parabolic area, all of which are
isms live in the hot waters. Other remarkable features of the migrating northeastward. Heat and magma from this mantle
park include the petrified forests buried and preserved by the plume has emplaced as much as 7.5 miles (12 km) of mafic
volcanic ash, numerous volcanic formations including black magma into the continental crust overlying the plume along
obsidian cliff, and waterfalls and canyons including the spec- this trace, causing the surface eruptions of the massive Snake
tacular Lower Falls in the Grand Canyon of the Yellowstone. River Plain flood basalts and the Yellowstone volcanics. It is
likely that, on geological timescales, massive volcanism and
The massive eruption from Yellowstone caldera 600,000 other effects of this hot spot will continue and also will slow-
years ago covered huge amounts of the western United States ly move northeast.
with volcanic ash, and if such an eruption were to occur
today, the results would be devastating. There has been some See also GEYSER.
concern recently about some increase in some of the thermal
activity in Yellowstone, although it is probably related to Further Reading
normal changes within the complex system of heated ground- Leeman, William P. “Development of the Snake River Plain—Yellow-
water and seasonal or longer changes in the groundwater sys-
tem. First, Steamboat geyser, which had been quiet for two stone Plateau Province, Idaho and Wyoming: An Overview and
decades, began erupting in 2002. New lines of fumaroles Petrologic Model.” In Cenozoic Geology of Idaho, Bulletin 26,
formed around Nymph Lake, including one line 250 feet (75 edited by Bil Bonnichsen and R. M. Breckenridge. Moscow:
m) long that forced the closure of the visitor trail around the Idaho Bureau of Mines and Geology, 1982.
geyser basin. Other geysers have seen temperature increases Morgan, Lisa A., David J. Doherty, and William P. Leeman. “Ign-
from 152°F (67°C) to 190°F (88°C) over a several-month imbrites of the Eastern Snake River Plain: Evidence for Major
period. Other changes include a greater discharge of steam Caldera Forming Eruptions.” Journal of Geophysical Research
from some geysers, changes in the frequency of eruptions, 89 (1984).
and a greater turbidity of thermal pools. Perhaps most worry- Morgan, W. Jason. “Deep Mantle Convection Plume and Plate
ing is the discovery of a large bulge beneath Yellowstone Motions.” American Association of Petroleum Geologists Bul-
Lake, although its age and origin are uncertain. Fears are that letin 56 (1972).
the bulge may be related to the emplacement of magma to Rogers, David W., William R. Hackett, and H. Thomas Ore. “Exten-
shallow crustal levels, a process that sometimes precedes sion of the Yellowstone Plateau, Eastern Snake River Plain, and
eruptions. However, the bulge was recently discovered Owyhee Plateau.” Geology 18 (1990).
because new techniques are being used to map the lake bot-
tom. The feature has an unknown age and may have been Yosemite Valley Yosemite Valley and National Park are
there for decades or even hundreds of years. located in eastern California in the Sierra Nevada Mountains.
The main feature of the park is a beautiful glacially scoured
Yellowstone Park is underlain by a hot spot, the surface valley, surrounded by peaks, huge monoliths, and pinnacles
expression of a mantle plume. As the North American plate
Full moon over the Half Dome, Yosemite Park, California (Photo by
Timothy Kusky)

466 Yosemite Valley

including the famous Half Dome, reaching heights of 4,800 dropping 2,425 feet (740 m), making it the tallest waterfall in
feet (1,465 m). The park was established in 1890 largely as a North America. The park includes abundant stands of giant
result of the efforts of the conservationist John Muir and was and other sequoias, meadows, and acts as a preserve for many
designated a World Heritage Site in 1984. Yosemite Falls other fauna and flora, preserving great biological diversity.
flows out of a hanging valley into the main Yosemite Valley,
See also GLACIER; ROCKY MOUNTAINS.

Z

Zagros and Makran Mountains The Zagros are a sys- resting above subducting oceanic crust of the Gulf of Oman.
tem of folded mountains in western and southern Iran, A large ophiolitic sheet is thrust over the ophiolitic mélange
extending about 1,100 miles (177 km) from the Turkish-Rus- and flysch and is part of a large ophiolitic belt that stretches
sian-Iranian border, to Zendam fault north of the Straits of the length of the Makran-Zagros ranges, falling between the
Hormuz. The Makran Mountains extend east from the Cenozoic volcanics and accretionary wedge/folded platform
Zagros, through the Baluchistan region of Iran, Pakistan, and rocks of the Makran and Zagros. The main differences
Afghanistan. The mountains form the southern and western between the Zagros and the Makran can be attributed to the
borders of the Iranian plateau and Dasht-e-Kavir and Dasht- fact that continent/continent collision has begun in the
e-Lut Deserts. The northwestern Zagros are forested and Zagros but has not yet begun in the Makran.
snowcapped and include many volcanic cones, whereas the
central Zagros are characterized by many cylindrical folded Iran is seismically active, as shown by the devastating
ridges and interridge basins. The southwest Zagros and magnitude 6.7 earthquake that destroyed the ancient walled
Makran ranges are characterized by more subdued topogra- fortress city Bam on December 26, 2003, killing an estimated
phy with bare rock, sand dunes, and lowland salt marshes. 50,000 people. The Zagros belt is extremely active, where
Many major oil fields are located in the western foothills of thrust-style earthquakes occur beneath a relatively ductile
the Central Zagros, where many salt domes have punctured layer of folded sedimentary rocks on the surface. The
through overlying strata creating many oil traps. Makran accretionary wedge is also seismically active, where
subduction zone and upper plate accretionary wedge earth-
Southwestern Central Iran has been an active continen- quakes occur. The boundary between the Makran and
tal margin since the Mesozoic, with at least three main phas- Zagros is a structurally complex region where many strike-
es of magmatic activity related to subduction of Tethyan slip faults, including the Zendan fault and related structures,
oceanic crust beneath the mountain ranges. Late Cretaceous rupture to the surface. The Bam earthquake was a strike-slip
magmatism in the Makran formed above subducting oceanic earthquake, related to this system of structures. The Central
crust related to the Oman ophiolite preserved on the Arabi- Iranian plateau is also seismically active and experiences large
an continental margin. In the late Eocene, the axis of active magnitude earthquakes that rupture to the surface.
magmatism shifted inland away from the Mesozoic magmat-
ic belt, but then shifted back during the Oligocene-Miocene. See also CONVERGENT PLATE MARGIN PROCESSES; KUWAIT.
The Oligocene-Miocene magmas are also related to subduc-
tion of oceanic crust, suggesting that the Arabian-Iranian Further Reading
collision did not begin until the Miocene. Most of the south- Berberian, F., and Manuel Berberian. “Tectono-plutonic Episodes in
ern Zagros consist of folded continental margin sediments of
the Arabian platform deformed since the Miocene, and Iran.” In Zagros-Hindu Kush-Himalaya Geodynamic Evolution,
mostly since the Pliocene. In contrast, the Makran is an edited by Harsh K. Gupta and Frances M. Delany. American
oceanic accretionary wedge consisting of folded Cretaceous Geophysical Union Geodynamics Series 3, 1981.
to Eocene flysch and ribbon chert-bearing mélange that is Berberian, Manuel. “Active Faulting and Tectonics of Iran.” In
Zagros-Hindu Kush-Himalaya Geodynamic Evolution, edited by
Harsh K. Gupta and Frances M. Delany. American Geophysical
Union Geodynamics Series 3, 1981.

467

468 Zagros and Makran Mountains
Tectonic map of the Zagros, Makran, and Northern Oman Mountains

Zimbabwe craton 469

Glennie, Ken W., M. W. Hughes-Clarke, M. G. A. Boeuf, W. F. H. flows and pyroclastic rocks. This western section includes
Pilaar, and B. M. Reinhardt. “Interrelationship of Makran- bimodal volcanic rocks consisting of tholeiite and magnesium-
Oman Mountains belts of convergence.” In The Geology and rich pillow basalt and massive flows, with some peridotitic
Tectonics of the Oman Region, edited by A. H. F. Robertson, rocks alternating with dacite flows, tuffs, and agglomerates.
Mike P. Searle, and Alison C. Ries. Geological Society Special The eastern section of the Zimbabwe craton is characterized
Publication 49, 1990. by pillowed and massive tholeiitic basalt flows and less-abun-
dant magnesium-rich basalts and their metamorphic equiva-
zeolite Any of a group of white to colorless hydrous alumi- lents. The eastern section contains a number of phyllites,
nosilicates of alkali and alkaline earth metals, characterized banded iron formations, local conglomerate, and grit and rare
by easy and reversible loss of water of hydration, and fusion limestone. Wilson identified an area of well-preserved circa
and swelling when strongly heated. Zeolites have a composi- 3.5-billion-year-old gneissic rocks and greenstones in the
tion similar to feldspars, with sodium, potassium, and calci- southern part of the province and named this the Tokwe seg-
um or barium or strontium as their chief metals, and a ratio ment. He suggested that this may be a “mini-craton,” and
of (Al + Si) to non-hydrous oxygen of 1:2. Many different that the rest of the Zimbabwe craton stabilized around this
varieties of zeolites are found as secondary minerals in cavi- ancient nucleus.
ties in basalt and other vuggy rocks, in secondary hydrother-
mal deposits, in beds of volcanic tuff, and in sedimentary Despite these early hints that the Zimbabwe craton may
layers in saline lakes and even deep-sea sediments. Some of be composed of a number of distinct terranes, much of the
the common zeolite minerals include natrolite, heulandite, work on rocks of the Zimbabwe craton has been geared
analcime, thomsonite, stilbite, laumontite, harmonite, and toward making lithostratigraphic correlations between these
many others. Many of these zeolites are thought to form by different belts and attempting to link them all to a single
reaction of pore waters with solid aluminosilicate minerals supergroup style nomenclature. Many workers attempted to
such as feldspar and clay minerals, or with volcanic glass. pin the presumably correlatable 2.7-billion-year-old stratigra-
Many zeolites have been found over a narrow range of low- phy of the entire Zimbabwe craton to an unconformable rela-
grade metamorphism and diagenesis, resulting in the defini- tionship between older gneissic rocks and overlying
tion of a zeolite metamorphic facies. There are numerous sedimentary rocks exposed in the Belingwe greenstone belt.
industrial applications for zeolites that utilize their properties More recently, Timothy Kusky, Axel Hoffman, and others
of absorption and loss of water, including water softening, have emphasized that the sedimentary sequence uncon-
drying agents, or gas absorbers. formably overlying the gneissic basement may be separated
from the mafic/ultramafic magmatic sequences by a regional
See also DIAGENESIS; METAMORPHISM; MINERALOGY. structural break, and that the presence of a structural break
in the type stratigraphic section for the Zimbabwe craton
Zimbabwe craton The Zimbabwe craton is a classic gran- casts doubt on the significance of any lithostratigraphic cor-
ite greenstone terrane. In 1971 Clive W. Stowe proposed a relations across Stowe’s divisions of the craton.
division of the Zimbabwean (then Rhodesian) craton into
four main tectonic units. His first unit includes remnants of Central Gneissic Unit (Tokwe Terrane)
older gneissic basement in the central part of the craton, Three and a half billion-year-old gneissic and greenstone rocks
including the Rhodesdale, Shangani, and Chilimanzi gneissic are well-exposed in the area between Masvingo (Fort Victoria),
complexes. Stowe’s second (northern) unit includes mafic and Zvishavane (Shabani), and Shurugwi (Selukwe), in the Tokwe
ultramafic volcanics overlain by a mafic/felsic volcanic segment. The circa 3.5–3.6-billion-year-old Mashaba tonalite
sequence, iron formation, phyllites, and conglomerates of the forms a relatively central part of this early gneissic terrane, and
Bulawayan group all overlain by sandstones of the Shamvaian other rocks include mainly tonalitic to granodioritic, locally
group. The third or southern unit consists of mafic and ultra- migmatitic gneissic units such as the circa 3.475-billion-year-
mafic lavas of the Bulawayan group, overlain by sediments of old Tokwe River gneiss, Mushandike granodiorite (2.95 billion
the Shamvaian Group. The southern unit is folded about east- years old), the 3.0-billion-year-old Shabani gneiss, and the 3.5-
northeast axes. Stowe defined a fourth unit in the east, includ- billion-year-old Mount d’Or tonalite. Similar rocks extend in
ing remnants of schist and gneissic rocks, enclosed in a sea of both the northeast and southwest directions, but they are less
younger granitic rocks. In 1979 James F. Wilson proposed a well exposed and intruded by younger rocks in these direc-
regional correlation between the greenstone belts in the cra- tions. The Tokwe terrane probably extends to the northeast to
ton. His general comparison of the compositions of the upper include the area of circa 3.5 Ga greenstones and older gneissic
volcanics in the greenstone belts resulted in a distinction rocks southeast of Harare. The Tokwe segment represents the
between the greenstone belts located in the western part of the oldest known portion of the Tokwe terrane, which was acting
craton from those in the eastern section. The greenstone belts as a coherent terrane made up of 3.5–2.95-billion-year-old tec-
to the west of his division are composed of dominantly calc- tonic elements by circa 2.9 Ga.
alkaline rock suites including basalt, andesite, and dacite

470 Zimbabwe craton

The 3.500–2.950-billion-year-old Tokwe terrane also predates the regional cleavage forming event are also recog-
contains numerous narrow greenstone belt remnants, which nized. The Matsitama belt (Mosetse complex) is separated
are typically strongly deformed and multiply-folded along from the Tati belt to the east by an accretionary gneiss ter-
with interlayered gneiss. The area in northeastern-most part rane (Motloutse complex) formed during convergence of the
of the central gneissic terrane southeast of Harare best two crustal fragments. The Tati and Vumba greenstone belts
exhibits this style of deformation, although it continues (Francistown granite-greenstone complex) were overturned
southwest through Shurugwi. In the Mashava area west of prior to penetrative deformation, possibly indicating that
Masvingo, ultramafic rocks, iron formations, quartzites, and they represent lower limbs of large regional nappe structures.
mica schist are interpreted as 3.5-billion-year-old greenstone The mafic, oceanic-affinity basalts of the Tati belt are over-
remnants tightly infolded with the ancient gneissic rocks. The lain by andesites and other silicic igneous rocks and intruded
3.5-billion-year-old Shurugwi (Selukwe) greenstone belt was by syntectonic granitoids, typical of magmatic arc deposits.
the focus of Clive W. Stowe’s classic studies in the late 1960s, Similar arc-type rocks occur in the lower Gwanda greenstone
in which he identified Alpine-type inverted nappe structures belts to the east. The Lower Gwanda and Antelope green-
and proposed that the greenstone belt was thrust over older stone belts are allochthonously overlain by basement gneisses
gneissic basement rocks, forming an imbricated and inverted that were thrust over the greenstones prior to granite
mafic/ultramafic allochthon. This was subsequently folded emplacement.
and intruded by granitoids during younger tectonic events.
A second sequence of sedimentary rocks lies uncon-
The Tokwe terrane is in many places unconformably formably over the Lower Greenstone assemblage and over-
overlain by a heterogeneous assemblage of volcanic and sedi- laps onto basement gneisses in several greenstone belts, most
mentary rocks known as the Lower Greenstones. In the Bel- notably in the Belingwe belt where the younger sequence is
ingwe greenstone belt this Lower Greenstone assemblage is known as the Manjeri formation. The Manjeri formation
called the Mtshingwe group, composed of mafic, ultramafic, contains conglomerates and shallow water sandstones and
intermediate, and felsic volcanic rocks, pyroclastic deposits, locally carbonates at the base, and ranges stratigraphically up
and a wide variety of sedimentary rocks. Isotopic ages on into cherts, argillaceous beds, graywacke, and iron forma-
these rocks range from 2.9–2.83 billion years old, and the tion. The top of the Manjeri formation is marked by a
rocks are intruded by the 2.83-billion-year-old Chingezi regional fault.
tonalite. The Lower Greenstones are also well-developed in
the Midlands (Silobela), Filabusi, Antelope–Lower Gwanda, The Manjeri formation is between 2,000 and 800 feet
Shangani, Bubi, and Gweru-Mvuma greenstone belts. The (600 and 250 m) thick along most of the western side of the
upper part of the Lower Greenstones have yielded U-Pb ages Belingwe belt except where it is cut out by faulting, and it
of 2.8 billion years in the Gweru greenstone belt, and 2.79 thins northward to zero meters north of Zvishavane. It is
billion years in the Filabusi belt. considerably thinner on the western edge of the belt. On the
scale of the Belingwe belt, the Manjeri formation thickens
The Buhwa and Mweza greenstone belts contain the toward the southeast, with some variation in structural thick-
thickest section of three-billion-year-old shallow water sedi- ness attributed to either sedimentary or tectonic ramping.
mentary rocks in the Zimbabwe craton. The Buhwa belt con- The age of the Manjeri formation is poorly constrained and
tains a western shelf succession and an eastern deeper-water may be diachronous across strike. However, the Manjeri for-
basinal facies association. The shelf sequence is up to 2.5 mation must be younger than the unconformably underlying
miles (4 km) thick and includes units of quartzite and quartz circa 2.8-billion-year-old Ga Lower Greenstones, and it must
arenite, shale, and iron formation, whereas the eastern deep- be older than or in part contemporaneous with the thrusting
water association consists of strongly deformed shales, mafic- event that emplaced circa 2.7-billion-year-old magmatic rocks
ultramafic lavas, chert, iron formation, and possible of the Upper Greenstones over the Manjeri formation. The
carbonate rocks. The Buhwa greenstone belt is intruded by Manjeri formation overlaps onto gneissic basement of the
the Chipinda batholith, which has an estimated age of 2.9 Tokwe terrane on the eastern side of the Belingwe belt, and at
billion years. Shelf-facies rocks may have originally extended Masvingo, and rests on older (3.5-billion-year-old Sebakwian
along the southeastern margin of the Tokwe terrane into group) greenstones at Shurugwi. Regional stratigraphic rela-
Botswana, where a similar assemblage is preserved in the tionships suggest that the Manjeri formation forms a south-
Matsitama greenstone belt. Rocks of the Matsitama belt east thickening sedimentary wedge that prograded onto the
include interlayered quartzites, iron formations, marbles and Tokwe terrane.
metacarbonates, and quartzofeldspathic gneisses in a 6–12-
mile (10–20 km) thick structurally imbricated succession. The Northern Belt (Zwankendaba Arc)
strong penetrative fabric in this belt may be related to defor- The northern volcanic terrane includes the Harare (Salis-
mation associated with the formation of the Limpopo belt to bury), Mount Darwin, Chipuriro (Sipolilo), Midlands (Silo-
the south, but early nappes and structural imbrication that bela and Que Que), Chegutu (Gatoma), Bubi, Bulawayo, and

Zimbabwe craton 471

parts of the Filabusi and Gwanda greenstone belts. These served oceanic plateau and that there was no evidence for them
contain a lower volcanic series overlain by a calc-alkaline to have been derived from a convergent margin.
suite of basalts, andesites, dacites, and rhyolites. Pyroclastic,
tuffaceous, and volcaniclastic horizons are common. Also Craton-Wide Overlap Assemblage (Shamvaian Group)
common are iron formations and other sedimentary rocks The Shamvaian group consists of a sequence of coarse clastic
including slates, phyllites, and conglomerate. In the Bul- rocks that overlie the Upper Greenstones in several locations.
awayo-Silobela area (Mulangwane Range), the top of the These conglomerates, arkoses, and graywackes are well-
upper volcanics include a series of porphyritic and amyg- known from the Harare, Midlands, Masvingo, and Belingwe
daloidal andesitic and dacitic agglomerate and other pyro- greenstone belts. The Cheshire formation is the top unit of
clastic rocks. the Belingwe greenstone belt. It is a heterogeneous succession
of sedimentary rocks that contain a number of various litho-
U-Pb ages from felsic volcanics of the northern volcanic facies including conglomerate, sandstone, siltstone, argillite,
belt include 2.696, 2.698, 2.683, 2.702, and 2.697 billion limestone, cherty limestone, stromatolitic limestone, and
years. Isotopic data from the Harare-Shamva greenstone belt minor banded iron formation. The Shamvaian group is
and surrounding granitoids suggests that the greenstones intruded by the circa 2.6-billion-year-old Chilimanzi suite
evolved on older continental crust between 2.715 billion and granites, providing an upper age limit on deposition. In the
2.672 billion years ago. The age of deformation is con- Bindura-Shamva greenstone belt the Shamvaian group is 1.2
strained by a 2.667-billion-year-old syntectonic gneiss, a miles (2 km) thick, beginning with basal conglomerates and
2.664-billion-year-old late syntectonic intrusion, and 2.659- grading up into a thick sandstone sequence. Tonalitic clasts in
billion-year-old shear zone-related gold mineralization. Other the basal conglomerate have yielded igneous ages of 3.2, 2.9,
post-tectonic granitoids yielded U-Pb zircon ages of 2.649, 2.8, and 2.68 billion years. Felsic volcanics associated with
2.618, and 26.01 billion years. Isotopic data for the felsic the Shamvaian group in several greenstone belts have ages of
volcanics suggest that the felsic magmas were derived from a 2.66 billion to 2.64 billion years.
melt extracted from the mantle 200 million years before vol-
canism and saw considerable interaction between these melts Chilimanzi Suite
and older crustal material. The Chilimanzi suite of K-rich granitoids is one of the last
magmatic events in the Zimbabwe craton, with reported ages
Southern Belt of 2.57 billion to 2.6 billion years. These granites appear to
The southern belt of tholeiitic mafic-ultramafic dominated be associated with a system of large intracontinental shear
greenstones structurally overlies shallow water sedimentary zones that probably controlled their position and style of
sequences and gneissic rocks in parts of the Belingwe, Mutare intrusion. These relatively late structures are related to north-
(Umtali), Masvingo (Fort Victoria), Buhwa, Mweza, Antelope, northwest to south-southeast shortening and associated
and Lower Gwanda belts. The most extensively studied of southwestward extrusion of crust during the continental
these is the Belingwe belt, which many workers have used as a accretion and collision as recorded in the Limpopo belt.
stratigraphic archetype for the entire Zimbabwe craton. The
allochthonous Upper greenstones are here discussed separately Accretion of the Archean Zimbabwe Craton
from the structurally underlying rocks of the Manjeri forma- The oldest part of the Zimbabwe craton, the Tokwe terrane,
tion that rest unconformably on Tokwe terrane gneissic rocks. preserves evidence for a complex series of tectonomagmatic
events ranging in age of 3.6–2.95 billion years ago. These
The Negezi group of the Belingwe greenstone belt rests events resulted in complex deformation of the Sebakwian
allochthonously over the Manjeri formation, and it contains greenstones and intervening gneissic rocks. This may have
ultramafic and mafic volcanic and plutonic rocks of the involved convergent margin accretionary processes that led to
Reliance and Zeederbergs formations. The Reliance forma- the development of the Tokwe terrane as a stable continental
tion is composed of variably altered high-magnesium basalts, nucleii by 2.95 billion years ago.
komatiites, and their intrusive equivalents. High strain zones
are common within the Reliance formation especially in the A widespread unit of mixed volcanic and sedimentary
lower 650 feet (200 m). The Zeederbergs formation is com- rocks was deposited on the Tokwe terrane at circa 2.9 billion
posed almost entirely of 2.7-billion-year-old extrusive vol- years ago. These lower greenstones include mafic and felsic
canic rocks, which are typically pillowed tholeiitic basalts. volcanic rocks, coarse conglomerates, sandstones, and
shales. The large variation in volcanic and sedimentary rock
Geochemical studies have suggested that the komatiites of types, along with the rapid and significant lateral variations
the Reliance formation in Belingwe could not have been erupt- in stratigraphic thicknesses that typify the Lower Green-
ed through continental crust, but rather that they are similar to stones, are characteristic of rocks deposited in continental
intra-plate basalts and distinct from mid-ocean ridge and con- rift or rifted arc settings. The Tokwe terrane was subjected
vergent margin basalts. The geochemistry of the Ngezi group
in the Belingwe greenstone belt suggests that it could be a pre-

472 Zimbabwe craton

to rifting at 2.9 Ga leading to the formation of widespread southern half of the Tokwe terrane, as recorded in shallow
graben in which the Lower Greenstones were deposited. It water sandstones, carbonates, and iron formations of the
appears that the southeastern margin of the Tokwe terrane Manjeri-type units preserved in several greenstone belts. The
may have been rifted from another, perhaps larger fragment Manjeri-type units overlap basement of the Tokwe terrane in
at this time, along a line extending from the Buhwa-Mweza several places (e.g., Belingwe, Masvingo) and lie uncon-
greenstone belts to the Mutare belt, allowing a thick formably over the circa 3.5-billion-year-old and 2.9-billion-
sequence of passive margin-type sediments (preserved in the year-old greenstones. Regional stratigraphic relationships
Buhwa greenstone belt) to develop on this rifted margin. Age suggest that the Manjeri formation forms a southeast thick-
constraints on the timing of the passive margin development ening sedimentary wedge that prograded onto the Tokwe
are not good, but they appear to fall within in the range of terrane, in a manner analogous to the Ocoee-Chilhowee and
3.09 billion to 2.86 billion years ago. By 2.7 billion years correlative Sauk Sequence shallow-water progradational
ago, a major marine transgression covered much of the sequence of the Appalachians, and similar sequences in other

Tectonic map of the Zimbabwe craton showing the locations of greenstone belts of different ages, plus the Great Dike

Zimbabwe craton 473

Tectonic cross sections showing tectonic evolution of the Zimbabwe craton 2.7–2.6 billion years ago

mountain belts. The progradation could have been driven by The 2.7-billion-year-old greenstones are divided into a
sedimentary or tectonic flexural loading of the margin of the northwestern arc-like succession, and a southeastern
Tokwe terrane, but most evidence points to the latter cause. allochthonous succession. The northwestern arc succession
The top of the Manjeri-type units represents a regional contains lavas with strong signatures of eruption though
detachment surface, upon which allochthonous units of the older continental crust, and the arc appears to be a continen-
southern greenstones were emplaced. Loading of the passive tal margin type of magmatic province. In contrast, the south-
margin by these thrust sheets would have induced flexural ern greenstones are allochthonous and were thrust in place
subsidence and produced a foreland basin that migrated along a shear zone that is well exposed in several places,
onto the Tokwe terrane. including the Belingwe belt. These southern greenstones have

474 zinc

a stratigraphy reminiscent of thick oceanic crust, suggesting ed Rocks, edited by Timothy M. Kusky. Elsevier Amsterdam:
that they may represent an oceanic plateau that was obducted Elsevier, 2004.
onto the Tokwe terrane 2.7 billion years ago. All of the Jelsma, Hielke A., Michael L. Vinyu, Peter J. Valbracht, G. Davies,
southern greenstones are distributed in a zone confined to Jan R. Wijbrans, and Ed A. T. Verdurmen. “Constraints on
about 100 miles (150 km) from the line of passive margin- Archean Crustal Evolution of the Zimbabwe Craton: a U-Pb Zir-
type sediments extending from Buhwa-Mweza to Mutare. con, Sm-Nd and Pb-Pb Whole Rock Isotope Study.” Contribu-
This “Umtali line” may represent the place where a ocean or tions to Mineralogy and Petrology 124 (1996): 55–70.
back arc basin opened between 2.9 billion and 2.8 billion Kusky, Timothy M., and William S. F. Kidd. “Remnants of an
years ago, then closed at 2.7 billion years, and forms the root Archean Oceanic Plateau, Belingwe Greenstone Belt, Zimbabwe.”
zone from which the southern greenstones were obducted. Geology 20, no. 1 (1992): 43–46.
This zone contains numerous northeast-striking mylonitic Kusky, Timothy M., and Pamela A. Winsky. “Structural Relationships
shear zones in the quartzofeldspathic gneisses. Closure of the along a Greenstone/ Shallow Water Shelf Contact, Belingwe Green-
Sea of Umtali at circa 2.7 billion years ago deposited a flysch stone Belt, Zimbabwe.” Tectonics 14, no. 2 (1995): 448–471.
sequence of graywacke-argillite turbidites that forms the Kusky, Timothy M. “Tectonic Setting and Terrane Accretion of the
upper part of the Manjeri formation and formed a series of Archean Zimbabwe Craton.” Geology 26 (1998): 163–166.
northeast-striking folds. Stowe, Clive W. “Alpine Type Structures in the Rhodesian Basement
Complex at Selukwe.” Journal of the Geological Society of Lon-
The latest Archean tectonic events to affect the Zimbab- don (1974): 411–425.
we craton are associated with deposition of the Shamvaian ———. “The Structure of a Portion of the Rhodesian Basement,
group, and intrusion of the Chilimanzi suite granitoids at South and West of Selukwe.” Ph.D. thesis, University of London,
circa 2.6–2.57 billion years ago. These events appear to be 1968(a).
related to collision of the now-amalgamated Zimbabwe cra- ———. “The Geology of the Country South and West of Selukwe.”
ton with the northern Limpopo province, as the Zimbabwe Bulletin of the Geological Survey of Rhodesia 59 (1968b).
and Kaapvaal cratons collided. Interpretations of the Taylor, P. N., Jan D. Kramers, Stephen Moorbath, J. F. Wilson, J. L.
Limpopo orogeny suggest that the Central Zone of the Orpen, and A. Martin. “Pb/Pb, Sm-Nd, and Rb-Sr Geochronolo-
Limpopo province collided with the Kaapvaal craton at circa gy in the Archean Craton of Zimbabwe.” Chemical Geology (Iso-
2.68 billion years ago, and that this orogenic collage collided tope Geosciences) 87 (1991): 175–196.
with the southern part of the Zimbabwe craton at 2.58 bil- Wilson, James F., Robert W. Nesbitt, and C. Mark Fanning. “Zircon
lion years ago. Deposition of the Shamvaian group clastic Geochronology of Archean Felsic Sequences in the Zimbabwe
sediments occurred in a foreland basin related to this colli- Craton: a Revision of Greenstone Stratigraphy and a Model for
sion, and the intrusion of the Chilimanzi suite occurred when Crustal Growth.” In Early Precambrian Processes, edited by
this foreland became thickened by collisional processes and Mike P. Coward and Alison C. Ries, 109–126. Geological Society
was cut by sinistral intracontinental strike-slip faults. Late Special Publication 95, 1995.
folds in the Zimbabwe craton are oriented roughly parallel to Wilson, James F. “A Preliminary Reappraisal of the Rhodesian Base-
the collision zone and appear contemporaneous with this col- ment Complex.” Special Publication of the Geological Society of
lision. The map pattern of the southern Zimbabwe craton South Africa 5 (1979): 1–23.
shows some interference between folds of the early genera-
tion (related to the closure of the Sea of Umtali) and these zinc A native metallic element that forms a blue white min-
late folds related to the Limpopo orogeny. eral, commonly called zinc blende or sphalerite. Lead and
zinc typically form in ore deposits together and may occur
See also ARCHEAN; BELINGWE GREENSTONE BELT; CRA- with minerals including copper and other base sulfides. There
TONS; KAAPVAAL CRATON. is a large industrial demand for lead and zinc for use in bat-
teries, ammunition, electrical components, as a galvanizing
Further Reading agent, as a precipitating agent for gold extraction, and in
Bickle, Mike J., and Euan G. Nisbet, eds. The Geology of the Beling- medicines. Zinc forms six common minerals found in many
lead-zinc deposits, including sphalerite (ZnS), smithsonite
we Greenstone Belt, Geological Society of Zimbabwe Special (ZnCO3), Hemimorphite (Zn4Si2O7[OH]2•H2O), Zincite
Publication 2. Rotterdam: A.A. Balkema, Rotterdam, 1993(a). (ZnO), Willimenite (Zn2SiO4), and Franklinite (Fe, Zn, Mn,
Coward, Mike P., and P. R. James. “The Deformation Patterns of [Fe, Mn]2O4).
two Archean Greenstone Belts in Rhodesia and Botswana.” Pre-
cambrian Research 1 (1974): 235–258. Lead-zinc ore deposits are of several different types,
Fedo, Christopher M., and Kenneth A. Eriksson. “Stratigraphic including stratabound deposits of syngenetic origin including
Framework of the ~ 3.0 Ga Buhwa Greenstone Belt: a Unique deposits in Dzhezkazgan, stratabound deposits of secondary or
Stable Shelf Succession in the Zimbabwe Archean Craton.” Pre- epigenetic origin such as the deposits of southeast Missouri.
cambrian Research 77 (1996): 161–178. Many lead-zinc deposits are known from volcanosedimentary
Hoffman, A., and Timothy M. Kusky. “The Belingwe Greenstone terranes such as Archean greenstone belts and island arc asso-
Belt: Ensialic or Oceanic?” In Precambrian Ophiolites and Relat- ciations, including those of Kuroko, Japan, and Kidd Creek,

zircon 475

Canada. Replacement deposits include those of Cerro de Pasco that it is found in, if it grew during metamorphism, or if it is
in Peru, whereas veins and contact metasomatic or skarn a detrital mineral or xenolith incorporated in the rock from
deposits are known from many places throughout the world. an older source. Since zircon may contain significant quanti-
ties of radiogenic uranium, thorium, and lead, it is commonly
zircon A common accessory mineral in many rock types used to obtain U-Pb geochronologic ages of igneous rocks
that typically forms small tetragonal prisms, with the formula that it grew in, or to obtain ages of metamorphism of the
ZrSiO4. Zircon is the chief ore of zirconium, used as a refrac- host rock. Studies of zircon populations in sedimentary rocks
tory for smelting, and for gemstones that resemble diamonds. can also reveal an enormous amount about the ages of rocks
Studies of zircon morphology and growth can reveal whether in the source terrane for that sedimentary deposit.
it crystallized from the magma that formed igneous rocks
See also GEOCHRONOLOGY; MINERALOGY.



APPENDIX I

Periodic Table of Elements

12

H 1 atomic number He

1.008 H symbol 4.003

34 1.008 atomic weight 5 6 7 8 9 10

Li Be B C N O F Ne

6.941 9.012 10.81 12.01 14.01 16.00 19.00 20.18

11 12 Numbers in parentheses are the 13 14 15 16 17 18
atomic mass numbers of radioactive isotopes.
Na Mg Al Si P S Cl Ar

22.99 24.31 26.98 28.09 30.97 32.07 35.45 39.95

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

39.10 40.08 44.96 47.88 50.94 52.00 54.94 55.85 58.93 58.69 63.55 65.39 69.72 72.59 74.92 78.96 79.90 83.80

37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

85.47 87.62 88.91 91.22 92.91 95.94 (98) 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3

55 56 57-71* 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86

Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

132.9 137.3 178.5 180.9 183.9 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 (210) (210) (222)

87 88 89-103‡ 104 105 106 107 108 109 110 111 112 113 114 115

Fr Ra Rf Db Sg Bh Hs Mt Ds Uuu Uub Uut Uuq Uup

(223) (226) (261) (262) (263) (262) (265) (266) (271) (272) (285) (284) (289) (288)

*lanthanide 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
series
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

138.9 140.1 140.9 144.2 (145) 150.4 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.0 175.0

‡actinide 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103
series
Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

(227) 232.0 231.0 238.0 (237) (244) (243) (247) (247) (251) (252) (257) (258) (259) (260)

477

478 Encyclopedia of Earth Science

The Chemical Elements element symbol a.n. element symbol a.n. element symbol a.n.
element symbol a.n.

actinium Ac 89 erbium Er 68 molybdenum Mo 42 selenium Se 34
aluminum Al 13 europium Eu 63 neodymium Nd 60 silicon Si 14
americium Am 95 fermium Fm 100 neon Ne 10 silver Ag 47
antimony Sb 51 fluorine F 9 neptunium Np 93 sodium Na 11
argon Ar 18 francium Fr 87 nickel Ni 28 strontium Sr 38
arsenic As 33 gadolinium Gd 64 niobium Nb 41 sulfur S 16
astatine At 85 gallium Ga 31 nitrogen N 7 tantalum Ta 73
barium Ba 56 germanium Ge 32 nobelium No 102 technetium Tc 43
berkelium Bk 97 gold Au 79 osmium Os 76 tellurium Te 52
beryllium Be 4 hafnium Hf 72 oxygen O 8 terbium Tb 65
bismuth Bi 83 hassium Hs 108 palladium Pd 46 thallium Tl 81
bohrium Bh 107 helium He 2 phosphorus P 15 thorium Th 90
boron B 5 holmium Ho 67 platinum Pt 78 thulium Tm 69
bromine Br 35 hydrogen H 1 plutonium Pu 94 tin Sn 50
cadmium Cd 48 indium In 49 polonium Po 84 titanium Ti 22
calcium Ca 20 iodine I 53 potassium K 19 tungsten W 74
californium Cf 98 iridium Ir 77 praseodymium Pr 59 ununbium Uub 112
carbon C 6 iron Fe 26 promethium Pm 61 ununpentium Uup 115
cerium Ce 58 krypton Kr 36 protactinium Pa 91 ununquadium Uuq 114
cesium Cs 55 lanthanum La 57 radium Ra 88 ununtrium Uut 113
chlorine Cl 17 lawrencium Lr 103 radon Rn 86 unununium Uuu 111
chromium Cr 24 lead Pb 82 rhenium Re 75 uranium U 92
cobalt Co 27 lithium Li 3 rhodium Rh 45 vanadium V 23
copper Cu 29 lutetium Lu 71 rubidium Rb 37 xenon Xe 54
curium Cm 96 magnesium Mg 12 ruthenium Ru 44 ytterbium Yb 70
darmstadtium Ds 110 manganese Mn 25 rutherfordium Rf 104 yttrium Y 39
dubnium Db 105 meitnerium Mt 109 samarium Sm 62 zinc Zn 30
dysprosium Dy 66 mendelevium Md 101 scandium Sc 21 zirconium Zr 40
einsteinium Es 99 mercury Hg 80 seaborgium Sg 106
a.n. = atomic number

APPENDIX II

The Geologic Timescale

Era Period Epoch Age (millions First Geology
Cenozoic Quaternary of years) Life-forms
Holocene Ice age
Tertiary 0.01 Humans Cascades
Pleistocene Mastodons Alps
Pliocene 3
Neogene 11 Saber-toothed tigers Rockies
Miocene Sierra Nevada
Oligocene 26 Whales Atlantic
Paleogene 37 Horses, Alligators
Eocene Appalachians
Paleocene 54 Birds Ice age
65 Mammals
Mesozoic Cretaceous 135 Dinosaurs Pangaea

Paleozoic Jurassic 210 Reptiles Laursia
Gondwana
Proterozoic Triassic 250 Trees
Archean Permian 280 Oldest rocks
310 Amphibians Meteorites
Pennsylvanian Insects
345 Sharks
Carboniferous Land plants
Mississippian 400 Fish
435 Sea plants
Devonian 500 Shelled animals
Silurian 544 Invertebrates
Ordovician Metazoans
Cambrian 700 Earliest life
2500
3500
4000
4600

479

480 Encyclopedia of Earth Science

Classification of Species

Group Characteristics Geologic Age
Vertebrates
Spinal column and internal skeleton. About 70,000 living species. Fish, amphibians, reptiles, birds, Ordovician to recent
Echinoderms mammals. sand dollars, crinoids.

Arthropods Bottom dwellers with radial symmetry. About 5,000 living species. Starfish, sea cucumbers, Cambrian to recent
Cambrian to recent Cambrian to recent
Annelids Largest phylum of living species with more than 1 million known. Insects, spiders, shrimp, lobsters, Cambrian to recent
Mollusks Cambrian to recent
Brachiopods crabs, trilobites. Ordovician to recent
Bryozoans Segmented body with well-developed internal organs. About 7,000 living species. Worms and leeches. Cambrian to recent
Coelenterates Straight, curled, or two symmetrical shells. About 70,000 living species. Snails, clams, squids, ammonites. Proterozoic to recent
Porifera Two asymmetrical shells. About 120 living species. Precambrian to recent
Protozoans Moss animals. About 3,000 living species.
Tissues composed of three layers of cells. About 10,000 living species. Jellyfish, hydra, coral.
The sponges. About 3,000 living species.
Single-celled animals. Foraminifera and radiolarians.

Summary of Solar System Data

Body Orbit in Radius Mass Density Axis Year Temp.
millions miles tilt Rotation (°C) Atmospheric composition
Mercury of miles 0.1 5.1
Venus 1,500 0.8 5.3 10 58.6 days 88 days 425 Carbon dioxide
Earth 36 3,760 1.0 5.5 Carbon dioxide minor water
Mars 67 3,960 0.1 3.9 6 242.9 days 225 days 425 78% nitrogen 21% oxygen
Jupiter 93 2,110 318 1.3 Carbon dioxide minor water
141 44,350 23.5 24 hours 365 days 60% hydrogen, 36% helium, 3% neon,
Saturn 483 95 0.7
Uranus 14 1.6 25.2 24.5 hours 687 days –42 1% methane and ammonia
Neptune 18 2.3 Same as Jupiter
Pluto 0.1 1.5 3.1 9.9 hours 11.9 years 2000 Similar to Jupiter, no ammonia
Same as Uranus
886 37,500 26.7 10.2 hours 29.5 years 2000
1,783 14,500 98 10.8 hours 84 years
2,793 14,450 29 15.7 hours 165 years
3,666 6.4 days 248 years
1,800

Evolution of Life and the Atmosphere Origin Atmosphere
(millions of years)
Evolution Nitrogen, oxygen
Humans 2 Nitrogen, oxygen
Mammals 200 Nitrogen, oxygen
Land animals 350 Nitrogen, oxygen
Land plants 400 Nitrogen, oxygen
Metazoans 700 Nitrogen, oxygen, carbon dioxide
Sexual reproduction 1,100 Nitrogen, carbon dioxide, oxygen
Eukaryotic cells 1,400 Nitrogen, carbon dioxide, oxygen
Photosynthesis 2,300 Nitrogen, methane, carbon dioxide
Origin of life 3,800 Hydrogen, helium
Origin of Earth 4,600

INDEX

Note: Page numbers in boldface indicate main entries; italic page numbers indicate photographs and illustrations.

A Afif terrane, Arabian shield 19, 20 Ahmadi ridge, Kuwait 243
aa lava 1 Africa See also specific countries A-horizon sequences 442
abrasion 456, 464 A-horizon soil profiles 396, 397
Absaroka Sequence 434 Atlas Mountains 31 air pressure 7–8, 31
absolute dating system 165 desertification 115, 116, 117
Abu Hamth, Egypt 388, 389 East African orogen 59, 343 Venus 450
abyssal hills 1 East African rift system 6, 7, 62, Airy isostatic model 230
abyssal plains 1, 299 Alabama
Acadian orogeny, Appalachians 13, 355
flood basalt 153 sinkhole 240
14–15 Kaapvaal craton 25, 45, 46, Al-Amar-Idsas suture zone, Arabian
Acasta gneisses, Canada 22, 100,
100, 101, 236–238, 237, shield 19, 20
392, 394 459–462, 460, 461, 472, 474 Alaska
accretionary prisms 94, 95 Lake Victoria 246
accretionary processes, continental Limpopo Province 461, 461, Aleutian Islands and trench 8
472, 474 braided stream, Mount
crust formation 100–103 mantle plumes 261
accretionary wedges 1–3, 2, 94–95 Nile River 246, 293–295, 294 McKinley 54
achondrite 276 Sahara Desert 116, 360–362, Brooks Range 54
acid rain 32 367 Chugach Mountains 74–76, 75,
acritarchs 60–61, 73 Sahel 116, 117, 362–363
active arcs 94, 95 sand seas 367 77, 207, 286, 442
Adirondack Mountains 3–6, 4, 194, African craton 20 cirques 77
agates 6–7 conjugate joints 160
195–197 agglomerates 346 glaciers 176
advection 208 AGI (American Geological Institute) McCarty fiord 151
advection fog 155 11 McCarty tidewater glacier 176
aerial photographs 352 AGU (American Geophysical Union) mica schist 373
Afar Depression, Ethiopia 6, 7, 11 Mount McKinley 286, 286–287
Agulhas-Falkland fracture 148 North Slope and Arctic National
124–125 Ahaggar massif, Sahara 360
Afghanistan, Makran Mountains Wildlife Refuge (ANWR)
297–298
467–469, 468 oxbow lakes, Kuskokwim River
312

481

482 Index

Alaska (continued) Andean-style arc systems 94, 95 apparent polar wandering (APW)
permafrost 325–326 Anderson, Don 260 335–336
permafrost zone 326 Anderson, E. M. 160
seiche waves 381 Andes 10, 12 Aqiq-Tuluhah orogeny, Arabian
tsunami 436, 437, 439 andesite 12 shield 18
Valley of Ten Thousand Smokes andesitic magma 227
448–449 andradite 164 aquatic organisms
Andreanof Islands, Alaska 8 benthos 46–47, 50, 136
Aleutian Islands and trench 8 aneroid barometers 10 nektons 289
algae 405 angle of repose 266 plankton 329–330
Algeria, Atlas Mountains 31 angular shear 401
Algoma-type banded iron formation angular strain 402 aquicludes 17
angular unconformities 443 aquifers 16–17, 200
38, 39 angular velocity 332
Algonquin terrane, Grenville anorthosite 345 artesian wells 27
Antananarivo block, Madagascar cones of depression 86
province 192 fracture zone aquifers 161–162
Al-Jazirah, Sudan 293 255, 256, 257 Saharan 361
Alleghenian orogeny 13, 16 Antarctica Arabian folds 243
alluvial fans 8–9, 9 Arabian-Nubian Shield 17, 59, 343
alluvium 113, 147, 154 Dry Valleys 127 Arabian plate 6, 304
almandine 164 icebergs 222, 376 Arabian shield 17–22, 22
alpha decay 348 ice cap 223, 375, 376–377 intrusive rocks 18–19
alpha particles 348 ice sheet 175, 223, 376 Najd fault system 19, 20
Alpha Regio, Venus 450 ozone hole 313 ophiolite belts 19
Alpine fault, New Zealand 292–293 polynyas 340 rock unit classification 18
Alps 9–10, 111 Ross Ice Shelf 222–223, 359, tectonic evolution 19–20
alstonite 62 tectonic models, history of
altimeters 10 376
Altiplano, Andes 10 sea ice 375 17–18
altocumulus clouds 84 thermohaline circulation 424 aragonite 56, 62
altostratus clouds 84 Weddell Sea 424 Aral Sea 22
Amazon River 10 Antarctic Circumpolar Current 300 arc/continent collisions 96, 98
amber 10–11 Antarctic ridge 278 Archean (Archaean) era 22–27, 26
AMCG (anorthosites, mangerites, antecedent streams 126
anticlines 12, 12–13, 111 cratonic basins 25–26
charnokites, granitic gneisses) anticlinoria 12 cratons 22–23, 38, 99–103, 419
suite, Adirondack Mountains 3–5 antiforms 156 gneiss terranes 24–25, 26
American Geological Institute (AGI) antigorite 385 granite-greenstone terranes
11 Antler orogeny 357
American Geophysical Union (AGU) Anton complex, Slave craton 392 23–24, 25
11 Antongil block, Madagascar 255, 256 life 26–27
American Meteorological Anton terrane, Slave craton 392, ophiolites 307
Association (AMS) 11 393, 394, 395 Archimedes’ Principle 90
ammonia 446 ANWR (Arctic National Wildlife arcs 94–96, 97
ammonite 234 Refuge) 297–298 Arabian shield 17–18, 19–20
amniotes 66 Apex chert, Australia 26–27 and crustal growth 100, 101,
amphibians 65–66 aphanite 225
amphiboles 11–12, 273, 281 Aphrodite Terra, Venus 449, 450 333
amphibolite 12 Apollo objects 276 subduction zones 410
AMS (American Meteorological Appalachian-Caledonide orogen 58 Arctic National Wildlife Refuge
Association) 11 Appalachians 13–16, 14, 16 (ANWR) 297–298
Amu Darya River 22 Caledonides and 58 Arctic Ocean
analcime 469 Penobscottian orogeny icebergs 222
Ancient Gneiss complex, South ice cap 375, 376–377
Africa 236 321–325, 323, 324 sea ice 375
Svalbard and Spitzbergen Island

415–416
arenites 369
aretes 77

Index 483

Argand, Emily 91 atmosphere 31–35, 130 See also willy-willys 127
Argentina, Patagonia 320–321 winds Wittenoom asbestos disaster
Arizona air pressure 7–8
Archean 23 28
Grand Canyon 181 biosphere 49–50 australopithecines 338
Meteor Crater 228 carbon cycle and 63, 64 Austro-Asian feedback system 138
San Francisco Peaks 452 climate and 34–35 avalanches 265, 269–270
urbanization and flash flooding clouds 83–84 Avalon Composite terrane,
Coriolis effect 99
447 formation and evolution of Appalachians 15
arkoses 369 32–34, 203–204 Avalonia 58
Aroostook-Matapedia trough, gases 32–34 avulsion 248
general circulation 31, 33, 80, Awramik, Stanley 405–406
Appalachians 15, 16 138, 165, 205–206, 206, 431 axes of strain 402
Ar-Rayn terrane, Arabian shield 19, geostrophic currents 174 axis of rotation
greenhouse effect 185–187, 186
20 Hadley cells 31, 33, 80, Earth 332, 378
ARS (Attitude Reference System) 205–206, 206, 431 Uranus 445–446
inversions 229–230 Venus 449
371 ionosphere 230 Ayres Rock, Australia 229
artesian systems 17, 27, 200 Jupiter 234
artesian wells 27 layers of 31, 34 B
Arthropods 106 life’s origins and evolution back arc basins 95, 96, 102
Aruma Group, Oman Mountains 248–250, 480 backarc regions 94
Mars 262–263 backshore 43
304 mesosphere 271–272 backwash 456, 456
asbestos 27–28, 171 meteorology 277, 349, 458 bacteria
Asbestos Hazard Emergency ozone hole 313
precipitation See precipitation biosphere and 49–50
Response Act (1986) 27 Saturn 371–372 chemosynthetic 51, 71, 249
asbestosis 27–28 troposphere 31, 34, 436 stromatolites 405, 405
Asia Uranus 446 badlands 37, 37–38
Venus 450 Bahra anticline, Kuwait 243
Aral Sea 22 Baja California, Mexico 38
Caspian Sea 66–67 atolls 35, 351 bajada 9
Gobi Desert 179 atomic weight 231 Balcones escarpment, Texas 151
Himalaya Mountains 98–99, attitude 407 Baltica 58
Attitude Reference System (ARS) Baltic Sea 10–11
210, 210–211, 285–286, 428 Baltic shield 38, 241–242
monsoons 285 371 Bam earthquake, Iran 467
Tibetan Plateau 426–428 augen gneiss 179 Bancroft terrane, Grenville province
Ural Mountains 444 aulacogens 159 193
Asia-India convergence 98–99, 210, Aurora Australis 35–36 banded iron formations (BIFs)
211, 229, 334, 428 Aurora Borealis 35–36 38–40, 40, 70, 249, 250
Asir terrane, Arabian shield 19, 20 auroras (northern lights) 35–36, 36 Bangladesh, cyclones 217
assimilation 226, 339 Australia banner clouds 84
asteroids 28–30, 29, 171, 276, 440 Barberton greenstone belt, Belingwe,
asthenosphere 30 Apex chert 26–27 Zimbabwe 188, 189, 236
Atacama Desert 30–31 Ayres Rock 229 barchan dunes 367, 367
Athollian orogeny 374 Ediacarian fauna 276, 345 Barents shelf 415
Atlantic Ocean Great Barrier Reef 184 barometric altimeters 10
abyssal plains 1 Great Sandy Desert 367 barrier beaches 40–41
currents 300–301 greenstone belts 187, 188, 189, barrier chains 40
evaporites 144 barrier islands 40, 43, 164, 214
Mid-Atlantic ridge 51, 190, 191 barrier reefs 351
Lake Eyre 246
223–224, 278
North Atlantic cold bottom

water 80–82
ridges 125
thermohaline circulation 424
Atlas Mountains 31