Earth Science : An Illustrated Guide to Science

50 Paleocene epoch

EARTH’S HISTORY Alignment of the land of the USA in Paleocene times

Key words U.S. rocks formed

epoch
equator
period
volcanic

Land and sea North
South

possible igneous plutons
equator sedimentary rocks
volcanic rock
under sea mountains
land volcanoes

Land, sea, and the Paleocene life-forms
United States
These fossil organisms lived in Paleocene times (not shown to scale).
● By the start of the Paleogene period—
© Diagram Visual Information Ltd. the Paleocene epoch (65–55 million Odd-toed ungulate Dermopteran Rodent
years ago)—rotation had aligned Pantodont Condylarth Primate
North America almost as it is today
and it had drifted well to the north of
the equator.

● Mountain building, much of it
volcanic, continued in the west.

● The shallow sea bisecting North
America had drained away.

Paleocene life

● Primitive kinds of mammal waned as
the more advanced placentals took
their place.

● Early hoofed mammals such as
condylarths and pantodonts shared
lands with early odd-toed ungulates
related to the modern horse and
hippopotamus; flesh-eating creodonts;
rodents; and squirrel-like primates.

Eocene epoch 51

Alignment of the land of the USA in Eocene times EARTH’S HISTORY

U.S. rocks formed Key words

basin
epoch
volcano

Land and sea North
South

under sea possible igneous plutons
land equator sedimentary rocks
mountains volcanic rock
volcanoes

Eocene life-forms Land, sea, and the
United States
These fossil organisms lived in Eocene times (not shown to scale).
● In the Eocene epoch (55–34 million
Early carnivore Flightless bird years ago), volcanoes and other
mountains were rising in the west.
Bat
● Eroding uplands shed debris, filling
Odd-toed ungulate Proboscidean intermontane basins. © Diagram Visual Information Ltd.
Dinocerate Whale
● Accumulating sediments pushed out
the Mississippi shoreline.

● The widening North Atlantic cut off
North America from Europe.

Eocene life

● Insectivores gave rise to bats.
● The Carnivora diversified.
● Primates included ancestors of tarsiers

and lemurs.
● Hoofed mammals included ungainly

dinocerates but also early horses,
tapirs, and rhinoceroses.
● Whales and sea cows appeared.

52 Oligocene epoch

EARTH’S HISTORY Alignment of the land of the USA in Oligocene times
Key words
U.S. rocks formed
epoch
sediment North
temperate South
tropical

Land and sea

possible mountains sedimentary rocks
equator volcanoes volcanic rock

under sea
land

Land, sea, and the Oligocene life-forms
United States
These fossil organisms lived in Oligocene times (not shown to scale).
● In the Oligocene epoch (34–23.8
million years ago) the present-day Swift
western United States became
uplifted. Even-hoofed
ungulate
● Eroding western mountains dumped
sediments east to South Dakota. Creodont

© Diagram Visual Information Ltd. ● Mississippi sediments helped push the Brontothere Giant rhinoceros Pyrothere
Gulf Coast further south. Embrithopod Primate

Oligocene life

● Where climates cooled, grasses and
temperate trees began replacing
tropical vegetation.

● Grazing and browsing even-toed
ungulates multiplied, and
brontotheres (odd-toed ungulates)
roamed North America.

● Pyrotheres were among hoofed
mammals unique to what was then the
island continent of South America.

Miocene epoch 53

Alignment of the land of the USA in Miocene times EARTH’S HISTORY

Key words

epoch
grassland
uplift
volcanic

Land and sea U.S. rocks formed
North
South

under sea possible
land equator
mountains
volcanoes igneous plutons

Miocene life-forms sedimentary rocks

These fossil organisms lived in Miocene times (not shown to scale). volcanic rock

Seal Hippopotamus Flightless bird Land, sea, and the
Primate Hyena Chalicothere United States
Deinothere
Platypus ● In the Miocene epoch © Diagram Visual Information Ltd.
(23.8–5.3 million years ago) uplift and
volcanic action produced fresh
mountain building in the west.

● The Colorado Plateau, Rocky
Mountains, and Cascade Range rose,
and the coast of California extended
further west.

Miocene life

● Grasslands spread extensively.
● North American mammals included

horses, oreodonts, rhinoceroses,
pronghorns, camels, protoceratids,
chalicotheres, bear dogs, and saber-
toothed cats.
● Worldwide, mammals reached their
richest ever variety.

54 Pliocene epoch

EARTH’S HISTORY Alignment of the land of the USA in Pliocene times
Key words
U.S. rocks formed
epoch
equator North
lava South
uplift
volcanic

Land and sea

possible
equator

under sea mountains sedimentary rocks
land volcanoes volcanic rock

Land, sea, and the Pliocene life-forms
United States
These fossil organisms lived in Pliocene times (not shown to scale).
● In the Pliocene epoch (5.3–1.8 million
years ago) the west experienced Litoptern
renewed uplift, and volcanic activity
with lava flows. Hominid Vulture
Edentate Even-toed ungulate
● California’s Great Valley emerged
© Diagram Visual Information Ltd. above the sea, and a Californian Desmostylan Marsupicarnivore
coastal strip was moving north.
Hare
● By now the United States lay far to the
north of the equator and land linked
North and South America.

Pliocene life

● Mammals moved from North to South
America and vice versa.

● Grazing mammals replaced browsers
as North America grew cooler and
drier.

● Hominids evolved in Africa.

Pleistocene epoch 55

Land of the USA in Pleistocene times EARTH’S HISTORY

Key words

epoch
erosion
ice sheet
ocean

Land, sea, and the
United States

● In the Pleistocene epoch (1.8–0.1
million years ago) advancing ice sheets
eroded northern uplands.

● So much water was locked up in ice
sheets that ocean levels fell. Land now
extended out into the sea, especially
along the Atlantic and Gulf coasts.

ice mountains extended coastline
land volcanoes

Major temperature phases in the Pleistocene epoch Illinoian glacialSianntgeargmlaocniaialn Wgilsaccoinasl inian

Ngelbarcaiaskl an Aifntotenrigalnacial Kgalnascaianl Yainrtmerogultahciiaanl

Warm

Cold 1,700,000 1,400,000 Years ago 900,000 550,000 400,000 200,000

2,000,000

Pleistocene life-forms

These fossil organisms lived in Pleistocene times (not shown to scale).

Human Kangaroo Ice advance and retreat
Giraffid
Giant deer ● There were major temperature
fluctuations in the Pleistocene epoch:
Brown bear Giant sloth © Diagram Visual Information Ltd.
Woolly mammoth ● Cold phases—glacials—saw ice sheets
advancing south through North
Woolly rhinoceros America.

Sabertooth cat ● During warm phases—interglacials—
ice sheets retreated north.

Pleistocene life

● Horses, camels, deer, tapirs,
mastodonts, mammoths, bears, dogs,
and saber-toothed cats, and also giant
sloths from South America lived in
North America, but most of these
eventually died out there.

● By early in the Pleistocene, evolving
hominids had given rise to our own
species, Homo sapiens.

56

EARTH’S HISTORY Holocene (recent) epoch

Key words Some changes affecting the United States

coast sea level
epoch
ice sheet growth and decline of Great Lakes formed
earthquake Great Basin lakes
eruption

Some changes affecting Cascade glacial drift
the United States Range deposits left by
eruptions
● In recent times (the last 10,000 years), melting ice
earthquakes and volcanic eruptions
have featured in the west. Melted ice low coasts
sheets filled the Great Lakes, dumped drowned
rocky debris, and raised sea level,
drowning the Atlantic and Gulf of
Mexico coasts.

Worldwide sea level San Andreas Fault Mississippi Delta
changes earthquakes growth

● This graph shows how global sea level Worldwide sea level changes m feet
has risen from a low point more than 00
15,000 years ago, when vast quantities 20 15 10 5
of water lay locked up in ice sheets. Thousand years ago 30 100
Melting ice raised sea level 300 feet Below sea
(90 m) or more, and drowned low level now
coasts that had formerly been
exposed. This caused the Atlantic and 60 200
Gulf shorelines to retreat inland.
90 300
Evolution of the Great 0
Lakes

● The Great Lakes grew as a nearby ice
sheet shrank. At first they drained
south to the Gulf, later east to the
Atlantic.

Evolution of the Great Lakes Later ice retreat Post-glacial lakes

Early ice retreat

© Diagram Visual Information Ltd. ice sheet river outlets 0 300 miles
lakes Atlantic Ocean 0 500 km

Origins 2 57

1 EARTH’S ROCKS

Key words

atmosphere
element
meteorite
planetesimal

The origins of planet Earth

● Earth may have formed in four stages:
1 Clouds of particles revolved around

the Sun.
2 Where there were areas of extra-

dense particles, they gravitated
toward each other, forming a closer
group of spherical massed particles.
3 These in turn ultimately formed into
a dense iron and nickel core. Less
dense matter similar to that of the
3 meteorites called carbonaceous
chondrites formed around the core.
4 Ultimately this spinning ball gained a
solid crust supporting oceans and a
primeval atmosphere.

iron-nickel core Planetesimals

● Earth and other rocky planets arguably
formed from coalescing particles
orbiting the Sun.

● Aggregating particles in time formed
large asteroids or miniature planets
known as planetesimals.

● Collisions smashed planetesimals but
their coalescing debris then formed
larger planetary bodies.

● The early Earth grew bigger as its
gravitational attraction pulled in lesser
bodies including asteroids and comets.

4 Heating up

● Heat helped the early Earth evolve.

● Its spinning dust cloud generated heat

by the energy of motion.

iron-nickel core ● Asteroids and comets produced heat
as they bombarded Earth.

matter as in a carbonaceous ● Heavy elements gave off heat by
chondrite meteorite radioactive decay.

● Heat melting Earth’s early crust helped

mantle forming from to redistribute its elements. The © Diagram Visual Information Ltd.
melted chondrite heaviest became concentrated in

primeval crust Earth’s core. The lightest formed the
primeval ocean type of crust it has today.
● Volcanic gases including water vapor

primeval atmosphere and possibly ice from comets between
them fed the oceans and primeval

atmosphere.

58 Elements

EARTH’S ROCKS
Key words

degassing
element

© Diagram Visual Information Ltd. Abundances The universe Earth’s crust Whole Earth
compared
Percentage weights Percentage weights Percentage weights
● Three diagrams in order in order in order
contrast the relative of abundance of abundance of abundance
abundances by
weight of elements hydrogen 61.0 oxygen 46.6 iron 35.0
in the universe,
Earth’s crust, and helium 36.8 silicon 27.7 oxygen 30.0
Earth as a whole: oxygen 1.0
aluminum 8.1 silicon 15.0
Universe carbon 0.25 iron 5.0 magnesium 13.0
others 0.95
● The two lightest calcium 3.6 nickel 2.4
elements, hydrogen sodium 2.8 sulfur 1.9
and helium, are by potassium 2.6 aluminum 1.1
far the most magnesium 2.1 calcium 1.1
abundant elements titanium 0.4 others 0.5
in the universe.
others 1.1
Earth’s crust

● Oxygen and silicon
are the most
abundant elements
in Earth’s crust.

● Early on, free
hydrogen and most
helium, the lightest
elements, escaped
into space, a process
that is called
degassing.

Whole Earth

● Iron is the most
abundant element in
Earth as a whole.

● During Earth’s
formation, this
heavy element
became largely
concentrated in
Earth’s core.

59

Internal heat EARTH’S ROCKS

Temperature and depth Key words spreading ridge
trench
convection
°F (°C) mantle outer core inner core crust
isotope
5,000 mantle
(9,032) plate

4,000 Temperature and depth
(7,232)
● Below Earth’s crust of oceanic plates
Temperature 3,000 and continental plates is the mantle,
(5,432) which has a temperature of at least
1,600˚F (870˚C). At the center the
temperature is 9,000˚F (5,000˚C).

933 miles (1,400 km)

2,000
(3,632)

1,000 1,333 miles (2,000 km)
(1,832) 2,000 miles (3,000 km)

kilometers 1,000 2,000 3,000 4,000 5,000 6,000 6.6 miles (10 km)
(miles) (621) (1,242) (1,864) (2,485) (3,107) (3,728)

Depth

Areas of high and low heat flow Areas of high and low
heat flow
Heat flow units 5 © Diagram Visual Information Ltd.
4 ● Oceanic spreading ridges (where
3 coastal plates diverge), volcanic island
2 arcs, and other volcanoes, are areas of
1 high heat flow, where much heat from
Earth’s interior escapes up through
the crust. Oceanic trenches, where
crust descends into the mantle, are
low heat flow areas, and cool.

Heat sources

● The chief source of heat escaping from
Earth’s interior is the radioactive decay
of isotopes of the heavy elements
potassium, uranium, and thorium.

● Some 40 percent of continental heat
flow comes from these elements
located within the upper continental
crust.

● Below the oceans, heat flow stems
largely from the mantle.

● Heat escapes from Earth’s interior
mainly by convection.

spreading volcano ocean volcanic
ridge trench island arc

60 Periodic table

EARTH’S ROCKS

Key words Elements ● Elements 110 through 116 have been
produced artificially.
element ● Calcium (Ca), gold (Au), and
hydrogen (H) are chemical elements ● It was devised in 1869 by the
Hydrogen that are examples of basic chemical Russian chemist Dmitri Mendeleyev
substances: they cannot be broken (1834–1907). The table groups
1 down into simpler forms. elements into seven horizontal lines
or periods. As we read from left to
H ● The periodic table gives information right the elements become less
about all of the 116 known elements metallic. The elements in each
Light metals arranged in order of their atomic vertical group have similar chemical
number: the number of protons in properties.
34 each element’s atomic nucleus.

Li Be Nonmetals Inert
gases
56789
2
BCNO F
He

10

Ne

11 12 14 15 16 17 18
13
Na Mg Ar
Al Si P S Cl
19 20 36
Heavy metals 24 25 26 27 28 29 30 31 32 33 34 35
K Ca Kr
21 22 23 Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br

Sc Ti V

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

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

87 88 89– 104 105 106 107 108 109 110 111 112 113 114 115 116

103

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

© Diagram Visual Information Ltd. Rare earths

57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

89 90 91 92 93 94 95 96 97 98 99 100 101 102 103

Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Atoms Protons 61
(electrically positive)
Major subatomic particles EARTH’S ROCKS
Electrons
Examples of atoms (electrically negative) Key words

electron shell Proton and electron atom
nucleus electron
Neutrons element
(electrically neutral) neutron
proton
nucleus electron shells
Subatomic particles
Helium atom
● Atoms are an element’s indivisibly
smallest constituents, but each atom
comprises subatomic particles. The
chief subatomic particles are protons,
electrons, and neutrons.

Different atoms

● In an atom, electromagnetic force
attracts negatively charged electrons to
an equal number of positively charged
protons in the atomic nucleus. In
every element except the common
form of hydrogen, the nucleus also
contains electrically neutral neutrons,
held there by powerful nuclear force.

● Each kind of atom has a specific
number of protons: for instance, two
in helium, a light element; six in
carbon, a heavier element.

Electron shells

● Electrons are traditionally shown in
one or more energy levels called
shells, orbiting a nucleus. The heavier
an element is, the more electrons its
atoms contain and the more shells
these tend to occupy.

Carbon atom

Electron shells

Energy levels each hold a definite number of electrons orbiting the nucleus. For clarity only one orbit per shell is shown here.
Some atoms have as many as seven shells.

nucleus electron orbit

One shell Two shells Three shells Four shells © Diagram Visual Information Ltd.
e.g., helium e.g., carbon e.g., aluminum e.g., calcium

62 Compounds

EARTH’S ROCKS Outer-shell electrons in stable elements
Key words
The numbers of outer-shell electrons in common reactive elements are shown (e.g. argon has
compound eight). Elements combine as compounds to gain a full complement of electrons.

Compounds argon hydrogen, magnesium, calcium, aluminum,
potassium
● Compounds are chemically combined nickel, iron (one form) iron (one form)
elements. Covalent or ionic bonds
unite their atoms in fixed proportions,
forming molecules of substances with
their own special properties.

electron in outer shell of atom carbon, silicon nitrogen oxygen, sulfur chlorine
+6 protons in atomic nucleus
Covalent bonding
lost electron shell
electrical attraction hydrogen
atom (H)
Separate atoms
+6
+6

carbon atom (C)

Separate atoms The bonding of four hydrogen atoms and one carbon atom forms
one methane molecule (CH4).

Ionic bonding

© Diagram Visual Information Ltd. +11 +17 +11 +17

Sodium atom (Na) Chlorine atom (Cl) The bonding of one sodium atom and one chloride atom forms one
sodium chloride molecule (NaCl).

Isotopes and ions 63

Ions and isotopes ● Isotopes are atoms of an element with the EARTH’S ROCKS
same atomic number (number of protons Key words
● An ion is an electrically charged atom or in the nucleus) but different atomic mass
group of atoms. (number of neutrons in the nucleus). isotope
radioisotope
● Positive ions are atoms that have lost one ● Some elements occur in nature as a
or more electrons. Such ions are called mixture of different isotopes. Hydrogen isotopes
cations.
● All elements have artificially produced Hydrogen isotopes are hydrogen
● Negative ions are atoms that have gained radioisotopes. atoms with the same atomic number
one or more electrons. Such ions are (the number of protons in the nucleus)
called anions. but different atomic mass numbers
(the number of protons plus neutrons).
● Ions help living cells to function. Mass number: 1

Ions

Ions are atoms
that have lost
or gained
electrons.

Positively charged
sodium ion

Mass number: 2

Negatively charged Mass number: 3
chlorine ion
© Diagram Visual Information Ltd.
proton (+)
electron (-)
neutron

64 Crystals and minerals

EARTH’S ROCKS Interlocking mineral crystals forming igneous rock

Key words cooling magma
first mineral crystallizing
crystal
igneous rock
magma
mineral

Crystals in rock second mineral
third mineral
● As magma (molten rock) cools,
minerals are precipitated out of
solution as crystals.

● First crystals of one mineral are
formed, then another, then a third,
and so on, until interlocking crystals
form a solid igneous rock.

Sequence of minerals

● Crystallization follows a sequence in
which each mineral “freezes out” at a
particular temperature.

● A typical igneous rock contains a set of
minerals that solidified at roughly the
same temperature.

Sequence of minerals formed in igneous rocks as magma cools

Order of Iron-magnesium Feldspar Intrusive Extrusive
crystallization silicate minerals minerals rocks rocks

first 1 olivine anorthite 5 gabbro basalt
2 pyroxene labradorite

© Diagram Visual Information Ltd. 6
falling temperatures
amphibole 3 7 andesine

diorite andesite

4 8 albite
biotite mica

1, 2, 3, 4 9 orthoclase granite rhyolite
iron-magnesium 10 muscovite
silicates 11 quartz

5, 6, 7, 8 feldspars

9, 10, 11 residual
minerals

last

Crystal systems 65

System Example Ideal shape Lengths Angles of EARTH’S ROCKS
of axis axes'
intersection Key words

axis
crystal
mineral

Cubic all equal 90°
(isometric)
Crystals
halite
● Within Earth’s crust most elements
Tetragonal two 90° occur as minerals—natural substances
horizontal that differ chemically and have distinct
equal; atomic structures.
third
different ● Most minerals form from fluids that
have solidified—a process that
zircon arranges their atoms geometrically,
producing crystals.
Hexagonal three 60°
equal 90° Crystal systems
horizontal
axes; ● Scientists identify six crystal systems
fourth axis based on axes (singular: axis):
different imaginary lines passing through the
middle of a crystal.
quartz
● Each system yields crystals with
Orthorhombic three 90° distinctive symmetry.
sulfur axes;
unequal ● Within each system, each mineral
crystal grows in a special shape or
Monoclinic three axes; only two habit, though this can be modified by
orthoclase unequal 90° temperature, pressure, and impurities.

Six types of crystal

● Cubic, or isometric, crystals have three
axes of equal length that intersect at
right angles.

● Tetragonal crystals have three right-
angled axes, one longer or shorter
than the other two.

● Hexagonal crystals have four axes,
three in the same plane.

● Orthorhombic crystals have three
right-angled axes of different lengths.

● Monoclinic crystals have three axes of
different lengths, two intersecting at
an oblique angle.

● Triclinic crystals have three axes of
different lengths intersecting at three
different angles.

Triclinic three none © Diagram Visual Information Ltd.
axes; 90°
unequal

albite

66 Rock forming minerals

EARTH’S ROCKS Presence of minerals in Earth’s crust
Key words
plagioclase 39% others, mostly non-silicates:
feldspar carbonates, oxides, sulfides, halides 8.4%
silicate

Minerals in rocks olivine 3%
clay 4.6%
● Earth’s rocks hold many minerals. amphibole 5%
Most are silicates—silicon and
oxygen usually combined with a mica 5%
base or metal.
pyroxene 11%
● The chief silicates are feldspars
(silicates of aluminum quartz 12%
combined with certain other
elements), notably plagioclase
and orthoclase.

● Other silicates include pyroxene,
amphibole, quartz, mica, olivine.

● Silica-rich igneous rocks are
termed “acid.” Progressively less
silica-rich rocks are termed
intermediate, basic (mafic), and
ultrabasic (ultramafic).

orthoclase 12%

Silicate structures of common minerals

oxygen atoms
silicon atom

oxygen atoms

Oxygen-silicon tetrahedron Schematic representation Double tetrahedron Ring
of a tetrahedron e.g., epidote e.g., tourmaline
e.g., olivine

Chain
e.g., pyroxene

© Diagram Visual Information Ltd. Double chain Sheet Three-dimensional network
e.g., amphibole e.g., mica e.g., quartz

Hardness 67

Mohs’ scale Field scale of hardness EARTH’S ROCKS

Each reference mineral Used to determine the relative hardness Key words
scratches all with of sample outside the laboratory.
lower numbers. Mohs’ scale
sample scratches quartz sclerometer
diamond
10 Mohs’ scale

corundum sample ● Mineralogists often identify
9 scratches minerals by their hardness:
steel file their resistance to abrasion.

topaz sample is scratched ● In 1822 Austrian mineralogist
8 by steel file Friedrich Mohs published the
still widely used Mohs’ scale
quartz sample is for hardness, based on
7 scratched by scratch tests developed by
knife with miners.
orthoclase difficulty
6 ● The Mohs’ scale features ten
sample is easily numbered minerals, each of
apatite scratched by knife which scratches all the others
5 that have a lower number.
sample is On this scale the hardest
fluorite scratched mineral is diamond, the
4 by dime softest is talc.

calcite sample is scratched ● The Mohs’ scale is not © Diagram Visual Information Ltd.
3 by fingernail graduated in even steps: 4 is
little harder than 3 but 10 is
gypsum sample is much harder than 9.
2 flaked by
fingers ● Instruments called
talc sclerometers are used to
1 measure absolute hardness.

Field scale

● This scale determines relative
hardness with the use of
everyday objects including a
steel file, pocketknife, copper
penny, and fingernail.

● On the Mohs’ scale the steel
file would be numbered 7+;
the pocketknife, 6+; the
dime, 4; the fingernail 2+.

Other tests

● Minerals are also identified by
color, streak (color of the
powdered mineral), luster,
specific gravity, cleavage,
fracture, form, tenacity
(resistance to bending,
breaking, and other forces),
odor, taste, and feel.

● Sophisticated tests use
polarizing microscopes,
X-rays, and spectral analysis.

68 Igneous rocks

EARTH’S ROCKS Rocks of Earth’s crust
Key words
dunite and peridotite
igneous rock

Igneous and other rocks granite 27.4% 7.9%
64.7% basalt and gabbro
● Igneous rocks form from magma, metamorphic
either underground or on the surface. sedimentary
igneous
● They comprise nearly two thirds of all
rock in Earth’s crust.

● Sedimentary and metamorphic rocks
ultimately derive from igneous rocks.

Kinds of igneous rock

● Igneous rocks can feature coarse-
grained or fine-grained minerals.

● Many rocks hold the same minerals,
but in different proportions.

Igneous rocks classified

(pale) intermediate mafic (very dark)
felsic gabbro ultramafic
Coarse granite granodiorite diorite
grained syenite peridotite

100

Mineral content (%) orthoclase quartz
feldspar
plagioclase
75 feldspar

50 pyroxene

© Diagram Visual Information Ltd. 25 olivine
basalt
biotite

hornblende

Fine trachyte rhyolite rhyodacite andesite
grained

Intrusive igneous rocks 69

Fiery rocks formed underground EARTH’S ROCKS

How granite forms Key words
● Granite forms when molten
batholith
blobs called plutons rise, hypabyssal
coalesce, and cool as pluton
masses known as
batholiths—immense rock Intrusive rocks
masses in the cores of
mountain ranges such as ● Intrusive igneous rocks are produced
the Sierra Nevada of where magma cools and hardens
California. underground. Geologists place
● Erosion of overlying rock intrusive rocks in two categories:
exposes granite masses like plutonic and hypabyssal.
the domes above
California’s Yosemite Valley Plutonic rocks
and the tors of Dartmoor in
southwest England. ● Plutonic rocks include great masses
formed deep in mountain-building
Intrusive rocks produce these features Dike A wall of usually basic zones: some develop from the partial
igneous rock, such as fusion of lower continental crust;
Sill A sheet of usually diabase (dolerite), injected others come from magma rising
basic igneous rock up through a vertical crack from the mantle.
intruded horizontally in preexisting rock.
between rock layers. ● Slow cooling produces big mineral
crystals, forming coarse-textured rocks
Stock Like a including (acid) granite and
batholith but granodiorite; (intermediate) syenite
smaller, with an and diorite; (basic) gabbro; and
irregular surface (ultrabasic) peridotite. Granite—
area under mainly made of quartz, feldspar, and
40 square miles mica—is the chief igneous rock of
(about 100 km2). continental crust.

Hypabyssal rocks

● Hypabyssal rocks are relatively smaller
masses, that are often strips or sheets.
Such rocks cooled at a lesser depth
and faster than plutonic rock, so they
hold smaller crystals.

● Hypabyssal rocks include (acid)
microgranite and microgranodiorite;
(intermediate) microsyenite and
microdiorite; and (basic) diabase
(dolerite).

Laccolith A lens-shaped, Lopolith A saucer- © Diagram Visual Information Ltd.
usually acidic, igneous shaped intrusion
intrusion that domes
overlying strata. between rock strata;
can be hundreds of
Batholith A huge, deep-
seated, dome-shaped miles across.
intrusion, usually of acid
igneous rock. Boss A small
circular-surfaced
igneous intrusion
less than 16 miles

(26 km) across.

70 Magma production

EARTH’S ROCKS Magma loss and production

Key words constructive plate
boundaries
igneous rock (spreading ridges)
magma 3.2 cubic miles
spreading ridge (13.5 km3)

Magma output within destructive plate
oceanic plates boundaries
● Igneous, or “fiery,” rocks floor the 0.24 cubic miles (subduction zones)
world’s oceans and form rock masses (1 km3) 0.7 cubic miles
that rise from the roots of continents. (2.7 km3)
Such rocks arise directly from the
molten underground rock material—
magma—that occurs where heat melts
parts of Earth’s upper mantle and
lower crust.

● Most magma that has cooled and
solidified escaped up through the
crust from oceanic spreading ridges.
Smaller quantities came from
destructive plate boundaries and
colliding continents.
within
continental plates
0.17 cubic miles
(0.7 km3)

© Diagram Visual Information Ltd. Magma loss plate material
consumed at
● Old, solidified magma is lost from destructive plate
Earth’s crust by subduction below boundaries
oceanic trenches. Here, where 3.36 cubic miles
tectonic plates meet, oceanic plates (14 km3)
plunge beneath continental plates and
their rock melts in Earth’s mantle.

Global balance

● The estimated amounts of global
annual crustal output and loss shown
here imply that Earth’s crust is getting
thicker. However, some studies
suggest a balance between output and
loss. If true, the thickness of Earth’s
crust remains about the same.

Volcanoes: active 71

EARTH’S ROCKS

Key words

lithosphere
plate tectonics
volcano

Volcanic belts Volcanic belts

Laki, Iceland ● Most volcanoes occur in belts along
the edges of the lithospheric or
Katmai, Alaska tectonic plates that form Earth’s crust.
The most notable belt is the one
Vesuvius, Italy around the Pacific Ocean rim.

Mount Fuji, Japan ● A less obvious one lies along the
bottom of the Atlantic Ocean, visible
Mauna Loa, Hawaii Faial Island, Azores on the surface only as a few volcanic
islands.
Paricutín, Mexico
● Rift valleys—cracks within a tectonic
Rakata (Krakatau) plate—are also key locations.
However, some volcanoes occur away
Famous Villarrica, Chile from lines of tectonic activity, notably
volcanoes the Hawaiian Islands.

Great eruptions Some famous volcanoes

Volcanic eruptions release very large quantities of energy. This graph compares the ● Volcanoes generally achieve
energy released by three of the largest known volcanic events with the energy released prominence because of the great
by the most powerful nuclear explosion. Energy release is shown in Joules (J) and the violence of their past eruptions, or for
approximate TNT equivalent in megatons (Mt) is also given. their height, or for the beauty of their
shape.
Volcanic eruptions 8 x 1019 J (20,000 Mt)
Tambora, Indonesia (1815) ● Rakata (Krakatau), Indonesia, is
famous for its huge eruption of 1883,
Santorini, Greece (c. 1470 BCE) 6 x 1019 J (7,500 Mt) and Vesuvius for its destruction of
Pompeii and Herculaneum in 79 CE.
Krakatau, Indonesia (August 1883) 3 x 1018 J (1,500 Mt)
● Mauna Loa, Hawaii, is famous as a fine
Nuclear explosions 3 x 1017 J (60 Mt) example of a shield volcano. © Diagram Visual Information Ltd.
Novaya Zemlya, USSR (October 1961)
● Mount Fuji, Japan, is famous for the
beauty of its classic conical shape.

Great eruptions

● Volcanic eruptions release huge
quantities of energy.

● The energy released by the eruptions
of Tambora, Indonesia (1815),
Santorini, Greece (c. 1470 BC), and
Krakatau, Indonesia (1883) exceeded
that released in hydrogen bombs
exploded by the USA and USSR.

● Volcanic eruptions can also release
vast quantities of rock.

● About 600,000 years ago, the last
major eruption at Yellowstone,
Wyoming, released some 240 cubic
miles (1,000 km3) of rock, compared
to 36 cubic miles (150 km3) from
Tambora’s eruption, the greatest in
modern times.

72 Volcanic types = 1 mile (1.6 km)

EARTH’S ROCKS Flood or plateau basalt

Key words The lava is very liquid. Lava flows are very
widespread, and emitted from fractures.
caldera
lava
volcano

© Diagram Visual Information Ltd. Volcanic types Shield volcano

● The volcanic types below are Shield volcanoes (e.g., Mauna Loa, Hawaii) are
discussed in order of increasing lava built almost entirely from fluid lava flows.
viscosity and violence of eruption. Successive eruptions produce lava that flows
in all directions from a central vent. Over time
Gas eruption a broad, gently sloping cone or dome is formed.
Lava may also flow from fractures on the flanks
● It is unusual for gas alone to be of the cone. These volcanoes generally have
emitted during a volcanic eruption, diameters of 3 to 4 miles (5–6.5 km) and heights
but where this happens, the gas is of 1,500 to 2,000 feet (457–610 m).
usually carbon dioxide (CO2).
Cinder cone volcano
● For example, Lake Nyos (Cameroon,
West Africa) is a volcanic lake that in Composed entirely of basaltic fragments, cinder
1986 emitted a low-lying cloud of CO2 cones are Earth's most common volcanic
that killed 1,700 people. landform. Lava is thrown into the air during an
eruption where it cools into small, solid lumps
Hawaiian eruption filled with gas bubbles. These “cinders” rain
down and, over time, create a cone of material
● These commonly occur as lava rivers around the central vent. They commonly occur
emerging from elongated fissures or in clusters of a hundred or more in a region,
tubular vents. but are rarely more than 1,000 feet (305 m) in
altitude.
● They have a low gas content and are
the least violent type of eruption that Composite volcano or stratovolcano
produces solid emissions.
Composite volcanoes consist of successive
Strombolian eruption layers formed by lava flows, fragments, and
ash. Lava erupts from a central vent, or cluster
● These eruptions are characterized by of vents. Lava flows also escape via fissures in
frequent but comparatively small the flanks of the volcano. Lava that solidifies
emissions of glowing gas and ash that within fissures forms dikes that strengthen the
rise no more than 0.6 miles (1 km). cone. The rigidity of the cone often leads to
explosive eruptions as pressure builds up over
● They also produce viscous basaltic lava long periods between eruptions. Mount St
from the volcano’s throat. Helens, WA, is an example.

Plinian (Vesuvian) eruption Volcanic dome

● These are violent eruptions, Volcanic domes are formed as very viscous
characterized by the expulsion at high lava flows from a central vent to form a gradually
velocity of a vertical column of gas and expanding dome of material. They are often
rock, rising as high as 31 miles found on the outside slopes or within the craters
(50 km) into the atmosphere. of composite volcanoes. They can form a dense
“plug” over the central vent of a volcano that
● Ash and pumice rain back down, often can lead to pressure build-up and eventually
carried in the direction of the an explosive eruption.
prevailing wind and thus forming
asymmetric deposits. Caldera

A caldera is a very large crater formed from
the explosive eruption of a large composite
volcano. An eruption that forms a caldera can
eject hundreds of cubic miles (km3) of material.

Volcanoes: caldera 73

Lava plug blocks pipe EARTH’S ROCKS

pent-up gases lava plug volcanic cone Key words
in magma
chamber caldera
magma
A lava plug bottles up explosive gases below. volcano

Explosion blasts top of volcano Lava plug

escaping gases ● The first stage in caldera formation is
magma rising in the magma chamber
In time pent-up gas pressure blasts off the top of the volcano. deep beneath a volcano.

Caldera (basal wreck) ● If a lava plug blocks the volcano’s vent,
the gas-rich magma gradually exerts
caldera rim new volcano crater lake increasing pressure on the magma
chamber’s walls.
© Diagram Visual Information Ltd.
Explosion

● Over decades or centuries, the
underground pressure becomes so
great that gas-rich magma blasts a hole
through the top of the volcano.

● Gas and magma escape up through
the vent they have created, blasting off
some of the top of the volcano.

● As the magma chamber empties, the
rest of the volcano summit loses its
support, cracks up, and falls—a
process accompanied by more
explosions, and lasting days.

Caldera

● The resulting caldera is a great shallow
cavity left by the collapse of all or part
of a volcano summit.

● Calderas can occur where volcanoes
are dormant, or (as at Kilauea in
Hawaii), still active.

● Many calderas are circular in shape,
though some are elongated.

● They can be hundreds of feet (m) to
scores of miles (km) in diameter. The
Yellowstone Caldera, the world’s
largest, measures 28 x 47 miles
(45 x 76 km).

● Depth from crater rim to floor is often
several hundred feet (m).

● Lakes occupy some calderas, for
instance, Crater Lake in Oregon.

The explosion leaves a great shallow cavity known as a caldera, or basal wreck.

74

EARTH’S ROCKS Volcanoes: lava forms

Key words rhyolitic lava

andesitic lava
basalt
basaltic lava
lava
pyroclast

Basaltic lava Aa Columnar basalt

● Basaltic lavas are basic lavas low in These are used blocks of lava, The Giant’s Causeway
silica. Oozing from shield and fissure common at Villarrica, Chile. on the north coast
volcanoes, they flow far before they of Ireland is a
harden. Varying conditions produce Pahoehoe well-known
different forms: example.
“Ropy” lava, as seen in Iceland
● Aa has clinkery blocks shaped where and Hawaii.
gas spurted from sluggish molten rock
capped by crust. Pillow lava

● Pahoehoe has skin wrinkled by molten This is formed at underwater
lava flowing beneath it. spreading ridges.

● Pillow lava forms mounds at undersea
spreading ridges.

● Columnar basalt forms as lava cools
evenly, contracts, and cracks.

Andesitic lava

● Andesitic lava has a silica content
intermediate between those of basaltic
and rhyolitic lavas. It is rather viscous
and flows a short way down composite
volcanoes.

Rhyolitic lava

● Rhyolitic lava is an acid lava with a
high silica content. It is viscous and
hardens before flowing far. It can
produce explosive eruptions from
composite volcanoes.

● Solid products of explosive eruptions
are called pyroclasts. Pyroclasts
include spindle bombs and pumice.

Spindle bomb Pumice

A volcanic bomb shaped by a Rafts of this light bubble-filled
whizzing trajectory through the air. lava floated on the sea after
Krakatoa (now Krakatau)
exploded between Java
and Sumatra in 1883.

© Diagram Visual Information Ltd.

Volcanoes: central 75

Structure of a central volcano EARTH’S ROCKS
Key words

fissure
volcano

vent gas and ash lava Two forms of volcano
cone magma
side vent magma chamber side vent ● Volcanoes take two main forms: fissure
volcanoes and central volcanoes.
pipe subsidiary
cone ● Central volcanoes yield lava, ash,
and/or other products from a single
hole, building a rounded or cone-
shaped mound.

● In Mexico the cinder cone Paricutín
grew 1,500 feet (450 m) in a year. In
Argentina, the extinct composite
volcano Aconcagua towers 22,834 feet
(6,960 m) above sea level.

Features of a central
volcano

● A cross section through an active
central volcano would reveal the
following features as it develops:

● Deep below the surface lies the
magma chamber, a reservoir of gas-
rich molten rock under pressure. This
pressurized magma may “balloon”
outwards against the surrounding
solid rock until it relieves the pressure
by escaping through a weakness in
the crust.

● From the chamber the magma then
rises through a central conduit.

● As magma rises, the pressure on it is
reduced, and its dissolved gases are
freed as expanding bubbles.

● The force of gases blasts open a
circular vent on Earth’s surface.

● From this outlet ash, cinders, and/or
lava build the main volcano.

● Vent explosions shape its top as an
inverted cone or crater.

● From the central conduit, pipes to side
vents release ash or lava, which then
build subsidiary cones.

© Diagram Visual Information Ltd.

76 Volcanoes: fissure

EARTH’S ROCKS

Key words Fissure volcanoes

basalt

fissure Fissure leaks first
plateau Lava flows from fissures
cracks (fissures)
Fissure volcanoes to form a long later
low layer of fissures
● Most volcanic activity happens not at basalt covering
the sedimentary Deccan Plateau, India
layers below. Jaipur

central volcanoes, but at long fissures

in Earth’s crust.

● The chief fissure systems occur along

submarine spreading ridges. These Basalt plateaus
yield the free-flowing basic lava that Later cracks
creates new ocean floor. occur alongside
● Known also as Icelandic eruptions, older extrusions to
fissure eruptions built Iceland, an form a further
island on the Mid-Atlantic Ridge. covering of lava,
● Fissure systems also occur on which is called a
basalt plateau.

continents. Successive lava flows fed

by plumes of molten rock risen from

the mantle have covered vast tracts of

land in basalt rock. Basalt plateaus
● Such flood basalts cover great areas of

western North America, Columbia Plateau, USA
western India, and

Siberia.

WASHINGTON PAKISTAN

Ocean MONTANA

Pacific OREGON IDAHO WYOMING Nagpur
INDIA
Mumbai
Arabian Hyderabad
Sea
NEVADA UTAH
GOA
CALIFORNIA
200 400 miles
basalt

0 200 400 miles 0

© Diagram Visual Information Ltd. Two basalt plateaus

● In the western United States, the
Columbia River Flood Basalt province
covers 63,300 square miles
(164,000 km2) in basalt up to
1,150 feet (3,500 m) thick.

● In India the Deccan Plateau consists
largely of basaltic lava about 6,500 feet
(2,000 m) spread over some
200,000 square miles (518,000 km2).

Volcanoes: shield 77

Shield volcanoes EARTH’S ROCKS
Key words
volcanic islands
shield volcano
volcano

magma old lava flows sea Shield volcanoes

● Repeated flows of fluid basaltic lava
build the broad, gently sloping domes
of shield volcanoes. Those of the
Hawaiian Islands include the largest
volcano on Earth, Mauna Loa.

● Mauna Loa on Hawaii has a volume of
10,200 cubic miles (42,500 km3); it
rises 32,000 feet (9,750 m) above the
ocean floor.

Hawaii’s shield volcanoes

Mauna Loa, Hawaii

diameter above sea level
200 miles (350 km)

diameter below sea level
500 miles (800 km)

Hawaiian Islands Kauai Hawaiian Islands

Niihau ● In this volcanic island chain, the oldest
and lowest islands lie in the north
Oahu west, the youngest and highest in the
south east.
Molokai
Lanai Maui

Kahoolawe

Hawaii © Diagram Visual Information Ltd.

78

EARTH’S ROCKS Geysers and hot springs

Key words smoker Cavern geysers
solfatara
fissure steam heat
fumarole
geyser
mofette
mud volcano

Hot water underground steam heat

● Hot water, gas, and mud squirt or Cavern and sump geysers
dribble from vents in the ground
heated by volcanoes, often those steam steam heat
nearing extinction.
steam hot rock water heat
● Such features abound in parts of Italy,
Iceland, New Zealand, and the United Fumarole Hot spring
States.
steam moving water
Cavern geysers trapped water

© Diagram Visual Information Ltd. ● Water held in an underground cavity is
heated by the hot rocks deep under
the surface. Pressure builds up within
the cavity until steam and hot water
are ejected.

Cavern and sump geysers

● Fissures in rocks allow water to sink
deep underground. The water is
converted into steam by steam below
the sump and both are ejected from
the mouth of the fissure.

Fumarole

● Fumaroles are holes in volcanic areas
that emit steam and other gases under
pressure.

Hot springs

● Trapped by impermeable rock, heated
groundwater gushes from hillsides as
hot springs.

Related phenomena

● Mud volcanoes are low mud cones
deposited by mud-rich water escaping
from a vent.

● Solfataras are volcanic vents emitting
steam and sulfurous gas.

● Mofettes are small vents emitting gases
including carbon dioxide.

● Smokers are submarine hot springs at
oceanic spreading ridges. Emitted
sulfides build chimneys belching black
smoky clouds.

hot rock water moving impermeable rock hot rock
water

Sedimentary rocks: 79
formation
EARTH’S ROCKS
Compaction clay space
Key words
mineral
fragment authigenesis
compaction
The clay particles are compressed and diagenesis
Clay particles are mixed with mineral fragments. the spaces filled with minerals. lithification

Recrystallization Rocks from sediments

fine grains coarse grains ● Buried sediments may undergo
shell dissolved physical and chemical changes.
Solution pieces of shell
● Collectively called diagenesis, these
Authigenesis spaces precipitated mineral changes can result in lithification—
the conversion of loose sediments to
Cementation spaces natural cement solid sedimentary rock.

Compaction © Diagram Visual Information Ltd.

● Particles of sediment are forced closer
together by the weight of sediment
building up above them.

Recrystallization

● Certain minerals in sedimentary
deposits may be recrystallized, fine
particles giving rise to coarse crystals.
Thus lime mud forms calcite crystals,
and amorphous silica forms quartz
crystals.

Solution

● Certain minerals in a sedimentary
deposit may be dissolved away.
Solution occurs in evaporites,
limestones, and sandstones.

Authigenesis

● Authigenesis is the precipitation of
minerals within a sediment as this is
being formed. They come from watery
solutions percolating through pore
spaces in the sediment.

● New minerals may replace those that
have dissolved. For instance, clay
minerals replace feldspars.

Cementation

● Authigenic minerals filling pore spaces
can cement the particles of sediment
together, converting this to
sedimentary rock.

● Major natural cements are calcium
carbonate, iron oxides, and silica.

80 Sedimentary rocks:
clastic
EARTH’S ROCKS
Sedimentary rocks in Clastic rock textures
Key words Earth’s crust

breccia
clast
conglomerate

Clastic sedimentary rocks breccia

● Most sedimentary rocks form from conglomerate
particles eroded from rocks on land.
Their main ingredients are clasts (rock shale (nonclastic)
fragments) of quartz, feldspar, and clay 53% limestone
minerals. These range from tiny grains
to boulders. and dolomite
25%
Clastic rock textures
sandstone
● Lutites (fine-grained particles) produce 22%
the sedimentary rocks mudstone,
siltstone, and shale. shale

● The (medium-grained) arenites or sandstone
sandstones include arkose, greywacke,
and orthoquartzite.

● Rudites (coarse clasts) mixed with
finer particles can be consolidated into
conglomerates, with rounded
fragments, and breccias, with sharp-
edged fragments.

● Clasts of different sizes accumulate in
different environments.

Where rock-forming fragments accumulate

mountains river continental sea
shelf

large fragments

© Diagram Visual Information Ltd. sandy

gravel sands mud mud

81

Sedimentary rocks: EARTH’S ROCKS
organic and chemical
Key words sediment
Shelly limestone (organic rock)
coal
living corals of the dead coral coral
upper reef just base of reef evaporite
lignite
below the surface limestone

coral broken debris from the reef Organic rocks
from the reef and the remains of
● Organic sediments produce the rocks
Evaporites (chemical rocks) deeper-water animals we know as coals and shelly
and plants limestones.
In a deepwater sea basin evaporation
outstrips freshwater input. The basin water ● Coal forms as overlying sediments of
grows denser and sinks until evaporites are compact dead swamp vegetation,
precipitated in the sequence a, b, c. converting it first to peat, then
lignite, then coal.

Limestone

● Marine organisms’ shells rich in
calcium carbonate form organic
limestones.

● Reef limestones are largely made of
coral polyps’ skeletons.

● Coquina is a cemented mass of
shelly debris.

● Chalk is a white powdery limestone
formed from coccoliths—the shells
of microorganisms.

Rocks from chemicals

● Some rocks form from chemicals
formerly dissolved in water.

● Examples are oolitic limestone,
dolomite, gypsum, and halite (rock
salt). These last three form one after
another as evaporites.

c halite (rock salt) b gypsum barrier sill partly © Diagram Visual Information Ltd.
a dolomite shuts out the
open sea

82 Sedimentary rocks:

EARTH’S ROCKS bedding Bedding patterns formed by
water or wind
Key words
Fine Medium Coarse
bedding sand sand sand
sediment
Increase in depth and flow velocity sediment
Sedimentary rock layers unaffected

● Sedimentary rocks originate as Parallel bedding
sediments deposited in broadly
horizontal sheets. These layers are minor
known as beds or strata. cross
bedding
● Each stratum is separated from the
next by a so-called bedding Graded bedding parallel
plane. bedding

● Distinctive types of bedding major
can reveal the conditions cross
under which sediments were bedding
laid down.
Cross bedding
Parallel bedding
(current bedding)
● Parallel bedding layers contain
particles distributed evenly within the ripples
bedding and occur when there is no
wind or water turbulence. Increase in depth and flow velocity

Graded bedding

● Graded bedding layers form in seas
and rivers where the large, heavy
particles of sediment have time
to settle first. Subsequently,
increasingly smaller and smaller
pebbles and sand build a series
of layers with the contents
graded by size.

Cross bedding
(current bedding)

● Cross bedding is caused by
overlapping layers of sand deposited
by wind or moving water at an angle
to underlying layers, such as when
curved slopes develop on an old
storm-beveled bedding plane.

© Diagram Visual Information Ltd. parallel
bedding

Metamorphism Foliation 83

How metamorphic rocks are formed In shale, the tiny flakes of EARTH’S ROCKS
mica are not arranged in a
sedimentary rock regular position. After the Key words
process of foliation, (i.e.,
Contact metamorphism alignment by directed aureole
In contact metamorphism pressure), they align to form magma
igneous rocks intrude layers, as happens in slate. metamorphic
through sedimentary
rocks. Metamorphic rocks rock
are formed at the point of metamorphism
contact.
Rocks remade
metamorphic rock igneous
rock ● Great heat and pressure alter igneous
and sedimentary rocks into what is
pressure termed metamorphic (“changed
shape”) rocks.
Regional metamorphism
In regional metamorphism, ● Metamorphic rocks’ ingredients have
layered rocks come into undergone solid-state recrystallization
contact with high to produce new textures or minerals.
temperatures from below,
and high pressures from ● The greater the heat or pressure, the
above. greater the change and the higher the
grade of metamorphic rock produced.
metamorphic rock heat
● The best-known agents of change are
New rocks for old contact and regional metamorphism.

Heat and pressure can transform original rocks into fine-grained granular or foliated rocks Contact metamorphism

Granular or fine-particled Foliated and banded ● Contact or thermal metamorphism
metamorphic rocks metamorphic rocks occurs when molten magma invades
and bakes older rocks.
hornfels gneiss
● Beyond a narrow baked zone extends
shale granite a so-called contact aureole of altered
basalt diorite rock.

conglomerate ● Regional metamorphism covers a far
larger area, such as when colliding
marble schist continental plates build mountains.

limestone ● Intensely altered rocks produced this
dolomite way appear in the exposed roots of
eroded old mountain ranges; the
newer Alps and Himalayas; and in
some old subduction zones.

Six altered rocks

● Hornfels and slate are fine-grained
metamorphic rocks.

● Marble and quartzite are granular
metamorphic rocks.

● Schist and gneiss are examples of
foliated metamorphic rocks.

shale
basalt
andesite
gabbro
tuff

quartzite slate © Diagram Visual Information Ltd.

sandstone shale
tuff

84 Progressive

EARTH’S ROCKS metamorphism

Key words Metamorphosis of shale

igneous rock miles (km) shale
metamorphic
0
rock
sedimentary rock slate

New rocks from old 5 phyllite
(8)
● A single type of sedimentary or
igneous rock can produce a range of schist
metamorphic rocks.
10
● Those produced depend upon the (16)
parent rock, the amounts of heat and
pressure applied, and the fluids Depth 15
passing through the rock. (24)
gneiss
● Contact (thermal) metamorphism 20
tends to produce rocks with fine- (32)
grained textures.
25 molten
● Heat and pressure as in regional (40) rock
metamorphism produce coarse-
grained rocks with foliated minerals— 30
minerals flattened and aligned in (48)
bands at right angles to the stress
applied. 34
(56) 212 392 572 752 932 1,112 1,292 1,472 °F
Progressive regional
metamorphism (100) (200) (300) (400) (500) (600) (700) (800) (°C)
Temperature
● The sedimentary rocks sandstone,
limestone, and shale each turn into Progressive regional metamorphism sandstone
other rocks when subjected to intense
heat and pressure. miles km

● Sandstone becomes quartzite. 00
● Limestone becomes marble.
● Under increasing heat and pressure 1 shale
1
shale becomes first slate, then phyllite, limestone quartzite
then schist, then gneiss. 2 marble
● Similarly, heat and pressure can sandstone slate
convert certain different rocks to one phyllite
type of metamorphic rock. Thus shale, 23 schist
basalt, and granite can all be converted
into schist. 4

Metamorphosis of shale 35
quartzite
● Shale changes through four stages as
it is subjected to heat and pressure. 6
At five miles (8 km) and 3,632°F 4
(2,000°C), the stone metamorphoses
into slate. At more than 25 miles 7
(40 km) and 14,432°F (8,000°C) the
stone melts and forms molten rock.
© Diagram Visual Information Ltd. 58 marble quartzite
Depth below surface quartzite

9 gneiss
6

10
marble

7 11
12

8 13

Increased heat and pressure

85

The rock cycle EARTH’S ROCKS

The recycling of Earth’s rock Key words sediment

3 4 cementation
compaction
lithosphere
magma
mantle

sediments sedimentary Rocks recycled
rocks
erosion and ● Internal and external forces together
deposition compaction produce a rock cycle that constantly
and builds, destroys, and remakes
Earth’s crust.
cementation
● Internal forces are convection currents
erosion and heat and of magma (molten rock) in the
deposition pressure mantle. Their circulation injects new,
molten, rock into the crust’s
2 5 lithospheric plates and draws existing
rocks into the mantle.
igneous metamorphic
rocks rocks ● The main external forces acting on
Earth’s crust are those of the weather:
cooling and melting melting
crystallization 1 ● Water, ice, and wind erode rocks and
redeposit them as sediments.
© Diagram Visual Information Ltd.
1 Magma

● Magma within Earth’s interior is the
ultimate source of most rock forming
Earth’s lithospheric plates.

2 Igneous rocks

● Magma that has risen, cooled, and
crystallized forms igneous rocks.

3 Sediments

● Igneous rocks subjected to erosion,
transport, and deposition form layered
sediments.

4 Sedimentary rocks

● Sediments subjected to compaction
and cementation form layers of
sedimentary rock.

5 Metamorphic rocks

● Sedimentary and igneous rocks
subjected to great heat and pressure
form metamorphic rocks.

● Metamorphic rocks that melt inside
Earth’s interior revert to magma.

magma

86 Continental drift: fit

EARTH’S ROCKS Continental fit
Key words
overlaps and gaps submerged
continent continental rims
Pangaea

Continental fit South America Africa

● In 1915 German meteorologist Alfred submerged
Wegener (1880–1930) proposed that continental rims
the continents had drifted around the
world.

● One clue was continental fit: South
America and Africa almost fit together
along the submerged rims of their
continental shelves.

A global jigsaw puzzle

● Like pieces of a gigantic jigsaw puzzle,
most continents and part of Asia also
seem to fit together.

● This boosted the belief that continents
are fragments of a single prehistoric
supercontinent, Pangaea.

Europe Geographical evidence

Some continents’ coasts would almost
interlock if they were rearranged like pieces
of a jigsaw puzzle. For instance, South America
fits into Africa.

North America

Africa

India

© Diagram Visual Information Ltd. South America

Antarctica

Australia

Continental drift: geology 87

Geologic evidence EARTH’S ROCKS

early Paleozoic rocks Key words
early Mesozoic rocks
late Mesozoic–early Cenozoic craton

South America Africa Geologic evidence

India ● Three mountain-building phases
successively tacked rocks onto the
Antarctica shields or cratons (ancient cores) of
southern continents.

● These three old mountain zones
survive as three belts of rocks of
matching ages straddling the southern
continents, if these are shown joined
together in a particular way.

● Early Paleozoic mountains ran through
Antarctica and Australia.

● Early Mesozoic mountains ran through
South America, southern Africa, and
Antarctica.

● Late Mesozoic and early Cenozoic
mountains ran through South America
and Antarctica.

● These geologic linkages suggest that
southern continents were once joined
before being separated by continental
drift.

Caledonian Mountains

● Geologic clues to past links between
northern landmasses include rocks
folded in early Paleozoic times into the
Caledonian Mountains.

● The fold axes of this great chain
passed through West Africa, northeast
North America, Newfoundland,
Ireland, Wales, Scotland, Greenland,
and Norway.

● It is therefore logical to conclude that
these places formed a continuous
landmass before the Atlantic Ocean
separated them.

Australia

© Diagram Visual Information Ltd.

88

EARTH’S ROCKS Continental drift: biology

Key words Biological evidence

continent Glossopteris
fossil Lystrosaurus
glacier Mesosaurus
tillite

Permian land plants and Africa
animals
India
● Identical fossil land plants and animals Antarctica
appear in the southern continents,
which are now widely separated
by the sea.

● Examples are Glossopteris, a tree;
Lystrosaurus, a synapsid; and
Mesosaurus, a reptile.

South America

Australia

Climatic evidence Climatic evidence ice
ice flow
● Glacial deposits and rocks scratched Africa
by stones in moving ice show that ice Australia
covered huge tracts of the southern
continents 300 million years ago.
This suggests that these
landmasses once lay in polar
regions.

● The glacial deposits form
boulder beds called tillites.
These overlie pavements of
solid rock scored by parallel
scratches, or striations.

South America India
Antarctica
© Diagram Visual Information Ltd.

Continental drift: 89
polar paths
EARTH’S ROCKS
Path of the North Pole Key words

continent
pole

North Pole

● Further proof of continental drift lies
in paleomagnetic evidence.

● Some rocks contain magnetic grains
aligned with the magnetic poles’
positions when those rocks were
formed.

● Studies of such alignments show the
north magnetic pole apparently
wandering across the North Pacific
Ocean over the last 2,250 million
years.

● Tests have shown that in fact it is the
continents that have wandered.

South Pole

● Paleomagnetic studies in different
continents reveal two apparent polar
wander paths, not one.

● South American rocks show a different
polar wander path from that shown by
the rocks of Africa.

● If both continents were joined, a
single wander path appears, indicating
that both continents were once joined
but have since moved apart.

Path of the South Pole

The South Pole’s apparent a b million The same path if the million
path between 400 and years ago continents were joined. years ago
250 million years ago, if
the continents were in 400 ab 400
their present positions.

350 350

300 300 © Diagram Visual Information Ltd.
250
250
relative to South America
relative to Africa

90 Wegener’s theory shallow seas

EARTH’S ROCKS Late Carboniferous
Key words

continental drift

Three world maps 300 million years ago

● Maps show the arrangements of Eocene
landmasses at three stages in the past
as envisaged in a 1915 publication by 55–34 million years ago
Alfred Wegener, a German
meteorologist now famous as an early Early Pleistocene
proponent of the theory of
continental drift.

● Wegener showed Africa where it is
today as a point of reference.

● Stippled areas indicate the presence
of shallow seas.

Late Carboniferous

● Late in the Carboniferous period
there was only a single landmass,
known as Pangaea (Greek for “all
land”). One ocean, the Panthalassa
(Greek for “all sea”), covered most of
the rest of the world.

Eocene epoch

● Wegener’s map shows Pangaea
beginning to break up in the Eocene
epoch, early in the Cenozoic era. In
fact break-up began much sooner, in
the Triassic period.

Early Pleistocene epoch

● By early in the Pleistocene epoch,
the major continental landmasses of
North and South America, Africa,
Asia, Australia, and Antarctica are
shown as having drifted apart. In fact
they had virtually assumed their
present positions well before this.

© Diagram Visual Information Ltd. 2 million years ago

Continents: 91
250 million years ago
EARTH’S ROCKS
Key words

continent
period

250 million years ago (Permian period)

true continental rim modern continents

Land through time

● Geologists today can reconstruct the
configuration of the world’s

Asia landmasses by continental drift at
intervals from 250 million years ago.
A
Europe E Permian period
A
North G PANTHALASSA ● In Permian times (about 290–248
equator America N million years ago) continents had
A fused into the single mighty landmass
SouthP Africa of Pangaea, surrounded by the
America immense Panthalassa Ocean.

India Australia ● The continent of Europe collided with
Antarctica Siberia to form the Ural mountains,
and other fusing lithospheric plates
had formed most of the rest of Asia.

● Africa’s (or South America’s) collisions
with Europe and North America
pushed up Europe’s Hercynian
mountains and the Appalachians.

Triassic Period

● Throughout the Triassic period

220 million years ago (Triassic period) (248–206 million years ago) the

Pangaean landmass drifted north, but

parts of Europe

and North

Asia America still lay

E A inside the
A
tropics.

G ● Ice sheets that once

N covered the southern
Europe
A continents had melted,

P world climates ranged from

North warm to mild, and deserts
America
equator were extensive.

● Pangaea now showed signs

PANTHALASSA of breaking up. Where rising

South Africa plumes of magma domed
America
India and split Earth’s crust, rifts
Australia appeared in North America,

Northwest Africa, and © Diagram Visual Information Ltd.

Antarctica Western Europe.

92 Continents:
180 million years ago
EARTH’S ROCKS
180 million years ago (Jurassic period)
Key words
true continental rim continental seas
batholith modern continents
Gondwana
Laurasia LAURASIA Asia
period
North Europe
Jurassic period America

● In the Jurassic period (about 206–144 equator
million years ago) Pangaea began
breaking up into the continents we South Africa
know today. America

● The first split opened up what would G India
become the North Atlantic Ocean. O
N
● The resulting northern supercontinent DW
is known as Laurasia, and the A Australia
southern supercontinent as
Gondwana. Antarctica N A

● Rifting began separating Africa/South
America from Antarctica/Australia but
the Indian subcontinent was probably
still stuck to East Africa.

● Mountains rose in western North
America as the continent moved west
and overrode an oceanic plate.

● Africa pushed against southern
Europe, shedding minicontinents later
tacked onto lands as far apart as Spain
and the Arabian peninsula.

100 million years ago (Cretaceous period)

Cretaceous period North

● During the Cretaceous period Pangaea America
continued splitting into Laurasia and LAURASIA
Gondwana, while both of these Europe
supercontinents were also
fragmenting. Asia

● Dramatic changes added land and equator GONDWANA
mountains to North America. The
overriding oceanic plates of western South Africa
North America continued spawning a America
great island arc of batholiths and
Andean-type volcanoes. India

© Diagram Visual Information Ltd. Australia

Antarctica

Continents: 93
60 million years ago
EARTH’S ROCKS
60 million years ago (Paleocene epoch)
Key words

continent
epoch
period

true continental rim continental seas
modern continents

North Asia Paleocene epoch
America
● The Paleocene epoch
Europe (65–55 million years ago)
saw continents taking on
equator their present shapes and
locations.
South Africa
America ● Shallow seas that had
India invaded parts of North
America, Africa, and
Australia Australia in the Cretaceous
period now drained away.
Antarctica
● North America was still
Today (Holocene epoch) linked to Asia and Europe.

● South America was an
island cut off from North
America.

● The subcontinent of India
was heading toward Asia.

● Everywhere, mammals
filled ecological niches left
vacant when all non-bird
dinosaurs had died out at
the end of the Cretaceous
period.

equator North Europe Asia Holocene epoch
America Africa India
● In the Holocene epoch
South Antarctica Australia (present day), North
America America and Europe are
separated by an ever-
widening North Atlantic © Diagram Visual Information Ltd.
Ocean.

● South America and Africa
are separated by an ever-
widening South Atlantic
Ocean.

● Greenland has been
isolated from North
America and Europe.

● India has impacted with
Asia creating the vast
Himalayan mountain
range.

● North and South America
have joined.

● Australia has separated
from Antarctica and
migrated northward.

94 Lithospheric plates

EARTH’S ROCKS How lithospheric plates are moving
Key words

asthenosphere
plate tectonics

Lithospheric plate
movement

● Earth’s lithospheric plates float on the
asthenosphere (the semifluid upper
mantle). Their movement across
Earth’s surface is believed to be driven
by convection currents in the mantle.

convergent plate boundary (where transform fault boundary (where
adjacent plates come together) adjacent plates move past each
other)
divergent plate boundary (where
adjacent plates move apart) direction of plate movement

Lithospheric plate names North American plate Eurasian plate
Arabian plate
Eurasian plate Iranian plate

Juan de Fuca plate African
plate
Philippine plate Caribbean
plate
Caroline plate Cocos plate
Bismarck plate South
American
Nazca
plate plate

Indo-Australian Pacific plate
plate

Scotia plate

Antarctic plate

© Diagram Visual Information Ltd. Antarctic plate

convergent plate boundary divergent plate boundary transform fault boundary

Plate tectonics 95

Spreading ridge Subduction zone EARTH’S ROCKS
Oceanic crust of plates a and b is At the convergence of an oceanic
spreading apart allowing magma from crustal plate (b) and a continental Key words
the mantle to escape onto the Earth’s crustal plate (c) an oceanic trench
surface to form constructive margins. creates a destructive margin. asthenosphere
lithosphere
plate tectonics

ab bc Earth’s shifting surface © Diagram Visual Information Ltd.

Tectonic Convection within the mantle ● Plate tectonics is the study of
plates The directions of the oceanic and Earth’s restless jigsaw of
continental plates follow the abutting, diverging, and colliding
continental direction of the upper movements lithospheric plates.
crust of magma within the mantle.
oceanic crust ● A section through Earth reveals the
asthenosphere likely mechanisms that move
mantle lithospheric plates around, and
mantle currents balance the creation of new crust with
moving plates the destruction of old crust drawn
down into the mantle.
Conservative margin
Where two lithospheric plates slide past Convection currents
each other lithosphere is neither made
nor lost. ● Each plate involves a slab of oceanic
crust, continental crust, or both,
coupled to a slab of rigid upper
mantle. Collectively these plates
make up the lithosphere.

● This rides upon the asthenosphere, a
dense, plastic layer of the mantle.

● Heat rising through this layer from
Earth’s molten core and lower mantle
seemingly produces convection
currents that drive the plates above.

Plate margins

● Plate movements produce several
kinds of plate margins. Constructive
margins are suboceanic spreading
ridges where new lithosphere is
formed between two separating
oceanic plates.

● Oceanic trenches mark destructive
margins—subduction zones where
oceanic plates colliding with
continental plates are pushed
down below these.
● At conservative margins, plates
slide past each other and
lithosphere is neither made
nor lost.
● At active margins oceanic and
continental plate collisions spark off
volcanoes and earthquakes.

● Passive margins are tectonically quiet
boundaries between continental and
oceanic crust.

96 Crust and lithosphere

EARTH’S ROCKS

Key words Lithosphere

asthenosphere (detail from left section)
lithosphere (densities in g/cm3)
mantle

Earth in section oceanic crust 2.9
continental
● A segment cut through Earth from crust 2.7

crust to core (below) would show the The lithosphere
contains
lithosphere then asthenosphere and oceanic crust,
continental
lower mantle, followed by the dense miles km crust, and
00 rigid upper
molten outer core, then the inner 3.7 6 mantle—
9.6 15 to a depth of
core—immensely hot but kept solid by 62 miles
20 32 (100 km).
tremendous

pressure from Section through Earth
above.
(with densities in g/cm3)

Lithosphere miles km lithosphere
0 0 asthenosphere
● A section 3.4 (weak upper
through the 62 100 mantle zone)
upper Earth
lower mantle
shows 5.7

thicknesses of 435 700 outer core
oceanic crust, 10.75

continental crust,

and rigid upper

mantle—layers

that form the

lithosphere.

● Below this lies a

dense, plastic

layer of the

mantle—the

asthenosphere.

● The relatively 1,800 2,900
lightweight rocks

of oceanic and

continental crust

float upon the

mantle.

62 100 asthenosphere
3.4

3,100 4,980

© Diagram Visual Information Ltd. inner core
15

3,960 6,370

97

Oceanic crust EARTH’S ROCKS

Deep ocean sediments Key words silica
sima
silica carbonates sand and mud basalt spreading ridge
gabbro
mantle
oceanic crust
sediment

Section through oceanic crust Composition of basalt Oceanic crust

silicon 49% ● Most oceanic crust is less than six
miles (about 10 km) thick.
aluminum 16%
● Its rocks are richer in aluminum and
water calcium than the mantle, and their
high silica and magnesium content
sediments iron and has earned oceanic crust the name © Diagram Visual Information Ltd.
0–2.5 miles magnesium 18% of sima.
(4 km)
calcium 8% ● Oceanic crust has three layers:
basalt sodium and sediments overlying two layers of
1.2 miles potassium 5% igneous rocks, the lower resting on
(2 km) the rigid upper mantle.
other elements 4%
gabbro Sediments
3.1 miles
(5 km) ● Muds, sands, and other debris
washed off continents lie thickly
rigid upper on continental shelves and nearby
mantle ocean floor.

● The open ocean’s bed oozes rich in
silica and carbonates (largely the
remains of dead microorganisms from
surface waters), clays, and, in places,
nodules containing substances that
include manganese.

Basalt

● The second layer of oceanic crust is
chiefly basalt, derived from the mantle
and released at spreading ridges as
rounded lumps of pillow lava.

● Analysis of the composition of basalt
shows that it consists largely of the
elements silicon and aluminum.

Gabbro

● The third and lowest layer of oceanic
crust is largely made of gabbro, a
coarse-grained rock equivalent to the
fine-grained basalt of the second layer.

Rigid mantle

● Below and coupled to the bottom
layer of the oceanic crust is the rigid
upper mantle. This may consist largely
of the dense igneous rock peridotite.

98 Hawaiian Islands

EARTH’S ROCKS Asia North America

Key words

hot spot
seamount
volcano

The Hawaiian Islands and a
Emperor seamount chain b

● For at least 70 million years a hot spot c
has been creating islands in the North
Pacific, beginning with the The Hawaiian Islands
construction of the Emperor chain (a). and Emperor seamount chain

● About 25 million years ago the Ages of the Hawaiian Islands
lithospheric plate apparently changed
direction (b) and construction of the Kauai: volcanically inactive island for 3.8 million years
Hawaiian Islands chain began (c).
Oahu: volcanically inactive island for 2.2 million years
● Today Hawaii is a volcanically active Molokai: volcanically inactive island for
island, while to its southeast Loihi has 1.3 million years
yet to rise above sea level. As the
Pacific lithospheric plate moves west it Maui: volcanically inactive island for
subsides so that islands are submerged 0.8 million years
and become seamounts.
Altitude in feet (meters) Hawaii: a volcanically active
Volcanic island formation 0–3,280 feet (0–1,000 m) island for 0.8 million years
3,280–6,560 feet (1,000–2,000 m)
1 The lithospheric plate passes over a 6,560 feet (2,000 m) Loihi: a volcanically
hot spot deep in the mantle. active seamount

2 Magma finds its way up through the Volcanic island formation
seafloor and erupts as a volcano.

3 The active volcano builds a
seamount. The lithospheric plate
continues to move and the
seamount breaks the surface of the
sea to become an island.

4 The island moves away from the hot
spot with the lithospheric plate, and
its volcanic activity ceases. Magma
forces its way through the seabed
again and begins to build another
seamount. Over millions of years, a
chain of islands is built.

1 lithospheric 2 volcano 3 island 4 seamount volcanic
plate island

© Diagram Visual Information Ltd. movement of
lithospheric plate

mantle hot spot

Dating the seafloor 99

Magnetic reversals spreading ridge EARTH’S ROCKS
old seafloor
normal polarity Key words
reversed polarity new
rock basalt
Fossil magnetization pattern rising magnetometer
Upwelling basalt forms new seafloor ocean
at the central ridge. As the basalt
solidifies it takes on the polarity of Striped seafloor
Earth’s magnetic field at that time.
● Great submarine ridges run across the
old sea floor spreading ridge floors of oceans.
old seafloor
● Magnetometer surveys across these
Every few hundred thousand years new rock rising ridges reveal patterns of magnetized
Earth’s magnetic field switches polarity. striping in the basalt rock.
spreading ridge
old sea floor old seafloor Fossil magnetization

new rock rising ● Magnetized striping in seabed rocks
preserves a record of past changes in
Basalt seafloor formed during a period of reversed polarity retains that polarity. Earth’s magnetic field.
Basalt seafloor formed during a period when polarity was the same as today also
retains its polarity. ● Basalt rocks flanking an oceanic ridge
contain particles aligned with Earth’s
Banded polarity upwelling basalt spreading ridge “normal” geomagnetic field, the field
that operates today. Thus the basalt
million years ago 4 3 2 1 0 0 1 2 must have solidified when Earth’s © Diagram Visual Information Ltd.
magnetic field was as it is now.
Bands of reversed and normal polarity in the seafloor record polarity reversals
over millions of years, and indicate rates of seafloor spread. ● On each side of the oceanic ridge lies
a band of rocks containing particles
with a reversed magnetic alignment.
This matching pair of bands must have
formed when Earth’s geomagnetic
field had switched around.

● Beyond this pair of rock bands comes
a succession of more matched pairs,
with alternating “normal” and
“reversed” polarity.

Dating the ocean floor

● Magnetically striped seabed rocks
show that new ocean floor appears at
spreading ridges,
then moves away
from these on either
side. So the oldest
seafloor is that
furthest from a
spreading ridge.
● By dating reversals of
Earth’s magnetic field,
geophysicists have
shown that virtually
no ocean floor is
more than 2,000
million years old.

34