10.2 The Nature of Volcanic Eruptions 1 FOCUS

Section 100.02 10.2 The Nature of
Volcanic Eruptions
1 FOCUS

Section Objectives

10.3 Explain the factors that Key Concepts Vocabulary Reading Strategy
10.4 determine the type of volcanic
10.5 eruptions that occur. What determines the type ◆ viscosity Previewing Copy the table. Before you read
10.6 Describe the various types of of volcanic eruption? ◆ vent the section, rewrite the green topic headings
volcanic materials that are What materials are ejected ◆ pyroclastic material as questions. As you read, write the answers to
ejected from volcanoes. from volcanoes? ◆ volcano the questions.
List the three main types of What are the three main ◆ crater
volcanoes. types of volcanoes? ◆ shield volcano The Nature of Volcanic Eruptions
Distinguish how the different What other landforms are ◆ cinder cone
types of volcanic landforms associated with volcanic ◆ composite cone What factors affect a. ?
form. eruptions? ◆ caldera an eruption?
◆ volcanic neck
Figure 6 A Mount St. Helens ◆ lava plateau
before the May 18, 1980, ◆ lahar
eruption. B After the eruption,
Reading Focus Spirit Lake filled with debris.

Build Vocabulary L2 0n May 18, 1980, Mount St. Helens erupted with tremendous force.

Word Parts Explain to students that The blast blew out the entire north side of the volcano. The eruption
the prefix pyro- is Latin and Greek for ejected nearly a cubic kilometer of ash and other debris. Why do volca-
“fire” or “heat.” Clastic means “made noes like Mount St. Helens erupt explosively, while others like Kilauea in
from fragments of preexisting rocks.” Hawaii are relatively quiet?
Pyroclastic materials are hot fragments
of preexisting rocks that are blown from Factors Affecting Eruptions
the vent of a volcano.
The primary factors that determine whether a volcano erupts
Reading Strategy L2 explosively or quietly include characteristics of the magma and the
amount of dissolved gases in the magma. Magma that has reached
a. viscosity and dissolved gases the surface is called lava. Lava cools and hardens to form solid rock.
b. What are the types of volcanic
materials? lava flows; pyroclastic
material, such as ash; volcanic gases
c. What are the types of volcanoes?
shield volcanoes, cinder cones,
composite cones
d. What are some other volcanic
landforms? calderas, pipes, lava plateaus

2 INSTRUCT

Use Visuals L1

Figure 6 During the eruption of Mount A B
St. Helens, the height of the volcano 286 Chapter 10
was lowered by 400 meters. Ask: What
would have caused this damage? Facts and Figures
(Force built up within the volcano and
blew the top off.) Infer where the debris Volcanoseismology Seismometers tell us a many thousands of small earthquakes that
from this blast went. (Some of the fine great deal about the process of volcanism. resulted. Ironically, though earthquakes
debris particles remained in the air for a Magma has to crack its way through the crust themselves cannot be predicted at this time,
time before settling; some of the material to get to the surface, and this creates huge earthquakes beneath volcanoes can be used to
flowed down the side of the volcano in the numbers of small earthquakes along the way. help predict volcanic eruptions, reducing the
form of mud; some of the material simply When Mt. St. Helen had an episode of risks of living near volcanoes.
tumbled down the side of the volcano.) volcanism in 2005, the network of
Verbal seismometers around the volcano could track
the movement of magma by locating the

286 Chapter 10

Table 1 Magma Composition Factors Affecting
Eruptions
Tendency to Form

Composition Silica Content Viscosity Gas Content Pyroclastics Volcanic Landform
Basaltic
Least (ejected rock fragments)
Andesitic (about 50%)
Granitic Least Least (1–2%) Least Shield volcanoes
Intermediate Basalt plateaus
(about 60%) Cinder cones
Most
(about 70%) Intermediate Intermediate Intermediate Composite cones Why are some L2
(3–4%) volcanoes explosive?

Greatest Most (4–6%) Greatest Pyroclastic flows Objective
Volcanic domes After completing this activity, students
will be able to explain why trapped
Viscosity Magma’s viscosity—whether the magma is thick and gases cause explosive reactions in
sticky or thin and runny—affects the type of eruption that occurs. volcanoes.
Viscosity is a substance’s resistance to flow. For example, maple syrup
is more viscous than water, so it flows more slowly. Magma from an Skills Focus Observing, Inferring,
explosive eruption may be thousands of times more viscous than Predicting
magma that erupts quietly. The temperature and chemical composi-
tion of magma determine the magma’s viscosity. Why are some Prep Time 5 minutes
volcanoes
The effect of temperature on viscosity is easy to see. If you heat explosive? Materials 2 bottles of noncarbonated
maple syrup, it becomes more fluid and less viscous. In the same way, water, 2 bottles of club soda, paper
the viscosity of lava is strongly affected by temperature. As a lava flow Procedure towels
cools and begins to harden, its viscosity increases. The lava flow slows
down and eventually halts. 1. Obtain two bottles of Class Time 20 minutes
noncarbonated water and
The chemical composition of magma has a more important effect two bottles of club soda. Safety Be sure that students point the
on the type of eruption. In general, the more silica in magma, the open bottles away from everyone.
greater its viscosity. Because of their high silica content, rhyolitic lavas 2. Open one bottle of the
are very viscous and erupt explosively. Basaltic lavas, which contain noncarbonated water and Teaching Tip Have paper towels
less silica, are less viscous and tend to erupt quietly. one bottle of the club available for students to use to clean
soda. Record your up after the lab.
Dissolved Gases During explosive eruptions, the gases trapped in observations.
magma provide the force to propel molten rock out of the vent, an open- Expected Outcome Students will
ing to the surface. These gases are mostly water vapor and carbon 3. Gently shake each of the observe dissolved gases and fluid
dioxide. As magma moves nearer the surface, the pressure in the upper remaining unopened “explode” from the bottle of
part of the magma is greatly reduced. The reduced pressure allows dis- bottles. CAUTION: Wear carbonated liquid.
solved gases to be released suddenly. safety goggles and point Kinesthetic, Logical
the bottles away from
Very fluid basaltic magma allows the expanding gases to bubble everyone. Analyze and Conclude
upward and escape relatively easily. Therefore, eruptions of fluid 1. Answers may vary but should state
basaltic lava, such as those that occur in Hawaii, are relatively quiet. 4. Carefully open each that the bottles with non-carbonated
But highly viscous magma slows the upward movement of expanding bottle over a sink or water opened without any escaping
gases. The gases collect in bubbles and pockets that increase in size outside. Record your gases or fizzing. The bottles of club
until they eject the magma from the volcano in an explosive eruption. observations. soda, when opened, fizzed with the
escaping carbon dioxide. After the club
What causes the dissolved gases in magma to be Analyze and soda was shaken, the gases escaped
released? Conclude more explosively.
2. the shaken bottle of club soda
1. Observing What
happened when the For Enrichment L3
bottles were opened?

2. Inferring Which bottle
represents lava with the
most dissolved gas?

Volcanoes and Other Igneous Activity 287 Have students research the violent
eruption of Krakatau in 1883. Have
Customize for English Language Learners students prepare a newspaper article
detailing the events surrounding the
eruption of this volcano. The article
should be written as if the volcano had
erupted recently.

Have students work in pairs to make a chart Students may want to illustrate their facts with Answer to . . .
showing the facts about factors affecting drawings to further their understanding of the
eruptions, volcanic material, types of concepts. Students can use this chart as a Reduced pressure as
volcanoes, and other volcanic landforms. study aid for quizzes and tests. magma nears the
surface allows the gases to bubble out.

Volcanoes and Other Igneous Activity 287

Section 100.20 (continued)
Volcanic Material

L2 A B

Students may have the misconception Figure 7 Lava Flows, Mount Volcanic Material
that earthquakes shaking the region Kilauea, Hawaii Pahoehoe (A)
around the volcano are the only reason is hotter and faster moving than Lava may appear to be the main material produced by a volcano, but
for volcanic eruptions. Earthquakes are aa. (B) this is not always the case. Just as often, explosive eruptions eject huge
common triggers of volcanic eruptions, Drawing Conclusions Which quantities of broken rock, lava bombs, fine ash, and dust.
but are not the only factors involved. type of lava is more viscous? Depending on the type of eruption, volcanoes may produce lava flows
Explain to students that volcanoes can or eject pyroclastic materials, or both. All volcanic eruptions also
erupt whenever magma builds up For: Links on volcanic eruptions emit large amounts of gases.
enough force to erupt from underground Visit: www.SciLinks.org
to the surface. The factors that determine Web Code: cjn-3101 Lava Flows Silica content and temperature affect the characteris-
the violence of the eruption are magma tics of lava flows. Hot basaltic lavas are usually very fluid because of
composition, magma temperature, and 288 Chapter 10 their low silica content. Flow rates of 10 to 300 meters per hour are
the amount of dissolved gases the common. In contrast, the movement of silica-rich, rhyolitic lava is
magma contains. often too slow to be visible.
Verbal
Temperature differences produce two types of basaltic lava: pahoe-
Use Visuals L1 hoe and aa. Pahoehoe (pah HOH ee hoh ee) is hotter, fast-moving
basaltic lava. Pahoehoe forms a relatively smooth skin that wrinkles as
Figure 7 Have students look closely at the still-molten subsurface lava continues to flow. Pahoehoe resembles
these photographs. Ask: How can you the braids in twisted ropes, as shown in Figure 7. Aa (AH ah) is cooler,
tell the aa flow is slow moving? (It is slower-moving basaltic lava. Aa forms a surface of rough, jagged blocks
rough and jagged rather than smooth.) with sharp, spiny projections.
Visual
Gases Magmas contain varied amounts of dissolved gases held
Observing Viscosity L2 under pressure in the molten rock, just as carbon dioxide is held in
soft drinks. As with soft drinks, as soon as the pressure is reduced, the
Purpose Students will observe fluids gases begin to escape. The gaseous portion of most magmas is only
that have different viscosities. about 1 to 6 percent of the total weight. The percentage may be small,
but the actual quantity of emitted gas can exceed thousands of tons
Materials 2 large beakers, hot plate, each day. Samples taken during a Hawaiian eruption consisted of about
water, 2 large test tubes, test-tube 70 percent water vapor, 15 percent carbon dioxide, 5 percent nitrogen,
clamp, ice, corn syrup 5 percent sulfur, and lesser amounts of chlorine, hydrogen, and argon.
Sulfur compounds are easily recognized because they smell like rotten
Procedure Pour corn syrup into the eggs and readily form sulfuric acid, a natural source of air pollution.
two large test tubes in advance. Put one
test tube into a large beaker filled with
ice. Put the other test tube into a large
beaker half filled with water on a hot
plate. Heat the syrup in a hot-water bath
until it is very hot. Boiling the syrup is
not necessary. Slowly pour the contents
of each test tube into another beaker
one at a time to demonstrate the nature
of fluids with differing viscosities.

Expected Outcome Students will Download a worksheet on volcanic
observe that the cold syrup is very eruptions for students to complete,
viscous and flows very slowly—similar and find additional teacher support
to silica-rich lava. The hot syrup is not from NSTA SciLinks.
viscous and flows very fast—similar to
silica-poor lava.
Visual, Verbal

288 Chapter 10

Pyroclastic Materials Particles produced in volcanic eruptions Types of Volcanoes
are called pyroclastic materials. When basaltic lava is extruded, dis-
solved gases propel blobs of lava to great heights. Some of this ejected Build Reading Literacy L1
material may land near the vent and build a cone-shaped structure. The
wind will carry smaller particles great distances. Viscous rhyolitic Refer to p. 278D in this chapter, which
magmas are highly charged with gases. As the gases expand, pulverized provides the guidelines for identifying
rock and lava fragments are blown from the vent. main ideas and details.

The fragments ejected during eruptions range in size from very fine Identify Main Idea/Details Have
dust and volcanic ash (less than 2 millimeters) to pieces that weigh sev- students read Types of Volcanoes on
eral tons. Particles that range in size from small beads to walnuts (2–64 pp. 289–291. Ask them to identify the
millimeters) are called lapilli, or cinders. Particles larger than 64 mil- main idea of each paragraph. Point out
limeters in diameter are called blocks when they are made of hardened that the main idea is usually within the
lava and bombs when they are ejected as glowing lava. first or second sentence of a paragraph.
Encourage students to include this
What is a volcanic bomb? exercise in the notes they use to study.
Verbal

Types of Volcanoes Build Science Skills L2

Volcanic landforms come in a wide variety of shapes and sizes. Each For: Composite volcano eruption Interpreting Diagram Have students
structure has a unique eruptive history. The three main volcanic activity study Figure 8. Ask: What part of a
types are shield volcanoes, cinder cones, and composite cones. volcano connects the magma
Visit: PHSchool.com chamber with the surface? (the
Anatomy of a Volcano Volcanic activity often begins when a Web Code: czp-3102 conduit, or pipe)
fissure, or crack, develops in the crust as magma is forced toward the Visual, Logical
surface. The magma collects in a pocket beneath the surface called the Figure 8 Anatomy of a
magma chamber. The gas-rich magma rises from the magma chamber, “Typical” Volcano This
travels through a circular pipe, and reaches the surface at a vent, as cross section shows a typical
shown in Figure 8. Repeated eruptions of lava or pyroclastic material composite cone.
eventually build a mountain called a volcano. Located at the summit Interpreting Diagrams How
of many volcanoes is a steep-walled depression called a crater. was the volcano in the diagram
formed?
The form of a volcano is largely determined by the composition
of the magma. As you will see, fluid lavas tend to produce broad struc-
tures with gentle slopes. More viscous, silica-rich lavas produce cones
with moderate to steep slopes.

Crater Vent
Conduit
Lava (pipe) Pyroclastic
material

Magma For: Composite volcano eruption
chamber activity

Volcanoes and Other Igneous Activity 289 Visit: PHSchool.com
Web Code: czp-3102
Students interact with a simulated
volcanic eruption online.

FCaucsttsomanizdeFfiogruErens

Predicting Pinatubo The largest volcano to measured the bulging of the interior of the Answer to . . .
erupt in the past 50 years is Mt. Pinatubo, in volcanic dome. Instruments measured the
the Philippines. The eruption did a temperature and composition of the gasses Figure 7 The lava in the aa lava flow
tremendous amount of damage, destroying that were issuing from the summit. is more viscous.
villages and even a United States military base. Seismometers measured the cracking of the
Fortunately, the volcano was successfully crust as magma pushed its way to the surface. Figure 8 The volcano was formed as
predicted. Volcanologists working near The result was a successful prediction that layers of pyroclastic material and lava
Pinatubo used several means to determine saved many human lives. flows were built up around the vent.
that the volcano was ready to erupt. Tiltmeters
a large chunk of
pyroclastic material that
is larger than 64 mm in diameter

Volcanoes and Other Igneous Activity 289

Section 100.20 (continued)

Integrate Physics L2

Geothermal Energy Hot magma near Summit caldera
the surface of Earth can be beneficial. Flank eruption
Geothermal energy takes advantage of
Earth’s internal energy and uses it as an Central vent
energy source. Have students research Magma chamber
this renewable source of energy. Students Oceanic crust
should prepare a short report about this
natural energy source. The report should Figure 9 Shield Volcanoes Shield Volcanoes Shield volcanoes are produced by the accu-
include an illustration showing an Shield volcanoes like Mauna Loa mulation of fluid basaltic lavas. Shield volcanoes have the shape of a
example of how geothermal energy and Mauna Kea in Hawaii are broad, slightly domed structure that resembles a warrior’s shield, as
can be used in a specific application. built mainly of fluid basaltic lava shown in Figure 9. Most shield volcanoes have grown up from the
Verbal flows. These broad, slightly deep-ocean floor to form islands. Examples of shield volcanoes include
domed structures are the largest those in the Hawaiian Islands and Iceland.
Use Visuals L1 volcanoes on Earth.
Cinder Cones Ejected lava fragments that harden in the air build
Figure 9 Have students compare a cinder cone. The fragments range in size from fine ash to bombs but
the photograph to the profile of a shield consist mostly of lapilli, or cinders. Cinder cones are usually a product
volcano in Figure 11. Ask: Why might of relatively gas-rich basaltic or rhyolitic magma. Occasionally, cinder
photographs of shield volcanoes cones produce lava flows.
make them look not as tall as they
really are? (Because shield volcanoes are The shape of a cinder cone is determined by the steep-sided slope
so broad, they often give the impression of that forms as loose pyroclastic material builds up around the vent.
being lower than they are.) How would Cinder cones are usually the product of a single eruption that some-
you describe the viscosity of the lava times lasts only a few weeks and rarely more than a few years. Once
at a shield volcano? (low viscosity) the eruption ends, the magma in the pipe connecting the vent to the
What is the origin of the other islands magma chamber solidifies, and the volcano never erupts again. As
in the diagram? What do you think shown in Figure 11, cinder cones are relatively small.
they would look like under the sea
level? (They are shield volcanoes, or parts Figure 10 Cinder Cones A B
of shield volcanoes. Beneath the surface, A A typical cinder cone has
the rocky formations likely flare outward steep slopes of 30–40 Pyroclastic material Crater
either as individual shield volcanoes or as degrees. B Many cinder
portions of a volcano shared by one cones, like this one near
or more of the other islands.) Flagstaff, Arizona, are
located in volcanic fields.
Others form on the sides of
larger volcanoes.
Inferring What feature is
shown in the lower part of
the photograph?

Central vent filled
with rock fragments

290 Chapter 10

Facts and Figures

Parícutin is an active volcano in Mexico. It is The resulting fire, ash, and lava destroyed
one of the youngest volcanoes on Earth. On two villages. In one of the villages, a local
February 20, 1943, Parícutin began erupting church is still standing at the edge of the lava
from a fissure in a cornfield. By the end of the flow. The top of the church and the bell tower
first year, the cone had reached an elevation of are visible, but the lower portions of the
450 m. Volcanic eruptions finally ended in 1952. church are buried in lava.

290 Chapter 10

Sea level

A Mauna Loa, Hawaii, a large shield volcano 0 10 20 km Observing an L2
B Mount Rainier, Washington, 4 km Explosive Eruption

a large composite cone C Sunset Crater, Arizona, Purpose Students will observe the
explosive nature of gases trapped in
a large cinder cone an enclosed container.

Composite Cones Earth’s most beautiful and potentially dan- Figure 11 Comparing Volcanic Materials 2-L soda bottle, rubber
gerous volcanoes are composite cones, or stratovolcanoes. A composite Landforms Cinder cones and stopper, white vinegar, baking soda,
cone is a large, nearly symmetrical volcanic mountain composed of composite cones are small in paper towel, rubber band, thin bath
layers of both lava and pyroclastic deposits. For the most part, com- comparison with a large shield or kitchen towel, scissors
posite cones are the product of gas-rich magma having an andesitic volcano like Mauna Loa in Hawaii.
composition. The silica-rich magmas typical of composite cones gen- Procedure Pour 150 mL of vinegar
erate viscous lavas that can travel only short distances. Composite into the empty soda bottle. For safety,
cones may generate the most explosive eruptions, ejecting huge quan- fold the towel around the bottle. Secure
tities of pyroclastic material. the towel at the neck of the bottle with
a rubber band. Cut an 8-cm square
Most composite cones are located in a relatively narrow zone that piece of paper towel. Put about 5 mL of
rims the Pacific Ocean, the Ring of Fire. The Ring of Fire includes the baking soda in the center of the paper
large cones of the Andes in South America and the Cascade Range of towel. Fold the paper towel around the
the western United States and Canada. The Cascade Range includes baking soda to make a packet. Drop the
Mount St. Helens, Mount Rainier, and Mount Shasta, shown in Figure packet into the bottle. Put the stopper
12. The most active regions in the Ring of Fire are located along volcanic into the bottle. Do not put the stopper
island arcs next to deep ocean trenches. This nearly continuous chain of in too tight. Have everyone stand a safe
volcanoes stretches from the Aleutian Islands to Japan, the Philippines, distance away from the bottle.
and New Zealand.
Expected Outcome Students will
observe the explosive forces that are
created when trapped gases are released.
Point out to students that this is similar
to gases trapped inside an active volcano.
When enough force is built up, trapped
gases can blow the top off of the volcano.
Gases, magma, and pyroclastic materials
then flow from the volcano through the
new opening.
Visual

Figure 12 Composite Cone
Mount Shasta, California, is one
of the largest composite cones in
the Cascade Range. Shastina is
the smaller cone that formed on
the left flank of Mt. Shasta.

Volcanoes and Other Igneous Activity 291

Customize for English Language Learners

Living in the shadow of a composite cone can the harbor also were spared. Answer to . . .
be dangerous. In 1902, Mount Pelée erupted in Have students use the Internet to research this
a fiery pyroclastic flow that destroyed the port volcanic eruption and prepare an eyewitness Figure 10 a lava flow
town of St. Pierre on the Caribbean island of account of it as seen from one of the ships in
Martinique. The destruction happened in the harbor. The account should describe the
moments. All of the 28,000 inhabitants of the scene before, during, and after the eruption.
town were killed with the exception of one Students should then present their account to
person who was being held in a dungeon on the class.
the outskirts of town. A few people on ships in

Volcanoes and Other Igneous Activity 291

Section 100.02 (continued) Caldera Formation Other Volcanic Landforms

Other Volcanic L1 Eruption of Volcanic mountains are not the only landforms that
Landforms Mount Mazama result from volcanic activity. Volcanic landforms
also include calderas, volcanic necks, and lava
Use Visuals A plateaus. Each of these features forms in a different way.
Long after eruptions have ended, these landforms can
Figure 13 Have students study the Partialy emptied provide evidence of volcanic activity.
diagrams in Figure 13. Tell students that magma chamber
the word caldera means “a cooking Calderas One spectacular reminder of what can
pot.” Ask: Why is an eruption that B happen when a volcano’s activity ends is the caldera. A
empties or partially empties the caldera is a depression in a volcanic mountain. Most
magma chamber an important first Collapse of calderas form in one of two ways: by the collapse of the
step for a caldera to form? (The Mount Mazama top of a composite volcano after an explosive eruption,
magma chamber must be emptied or or from the collapse of the top of a shield volcano after
partially emptied to create a void. Then C the magma chamber is drained.
the volcano collapses into the newly
created void to create a deep depression Formation of Crater Lake and Wizard Island Crater Lake in Oregon occupies a caldera. This
in the landscape.) Why is the name caldera formed about 7000 years ago when a compos-
caldera a good description of this ite cone, Mount Mazama, erupted violently, as shown
type of landform? (When the volcano in Figure 13. The eruption of Mount Mazama partly
collapses, a large well is created that emptied the magma chamber. The roof of the magma
resembles a cooking pot.) chamber collapsed, forming a huge depression that
Verbal, Logical became Crater Lake. A later eruption produced Wizard
Island, the small cinder cone in the lake.

There are several other large calderas in the United
States. In Hawaii, the vast “crater” atop Mount
Haleakala on the island of Maui is, in fact, a caldera. In
the Yellowstone caldera in Wyoming and the Valles
caldera in northern New Mexico, hot springs are evi-
dence of past, and perhaps future, volcanic activity.

D E

Figure 13 Crater Lake in
Oregon occupies a caldera
about 10 kilometers in
diameter that formed
about 7000 years ago.

292 Chapter 10

Facts and Figures in 1902. There were 28,000 deaths, primarily
the result of pyroclastic flows. (4) Nevado del
The five deadliest volcanic eruptions known Ruiz, Colombia, which occurred in 1985.
are (1) Tambora, Indonesia, which occurred There were 25,000 deaths, primarily the result
in 1815. There were 92,000 deaths, primarily of mudflows. (5) Unzen, Japan, which occurred
the result of starvation. (2) Krakatau, Indonesia, in 1792. There were 14,000 deaths, primarily
which occurred in 1883. There were 36,000 the result of a volcano collapse and a tsunami.
deaths, primarily the result of a tsunami.
(3) Mount Pelée, Martinique, which occurred

292 Chapter 10

Volcanic Necks Another volcanic Use Visuals L1

landform that provides evidence of past vol- Figure 14 Have students study Figure
14. Ask: Infer why the volcanic neck is
canic activity is the volcanic neck. A volcanic still in place while the surrounding
terrain has eroded away. (The rock in
neck is a landform made of magma that the volcanic neck is more resistant to
erosion than the surrounding terrain.)
hardened in a volcano’s pipe and later was Verbal, Logical

exposed by erosion. Recall that a volcano’s

pipe connects the magma chamber with the

surface. When a volcano’s activity ends, A
magma remaining in the pipe hardens to
Use Community
form igneous rock. Resources L2

Weathering and erosion act constantly to

wear away volcanoes. For example, cinder Many U.S. Geological Survey (USGS)
offices have educational outreach staff
cones are easily eroded because they are Figure 14 Volcanic Neck Ship and programs. Contact your regional
Rock, New Mexico, is a volcanic office and ask a USGS scientist to speak
made up of loose materials. But the rock in a volcano’s pipe is more neck. Ship Rock consists of to your class about plate tectonics and
igneous rock that crystallized in volcanic activity.
resistant to erosion, so it is left standing above the surrounding land the pipe of a volcano that then Interpersonal
was eroded away.
after most of the cone has been eroded. Ship Rock, shown in Figure
Figure 15 Lava Plateau Lava
14, is a volcanic neck in New Mexico. erupting from a fissure forms
fluid lava flows called flood
Lava Plateaus If you visited the Columbia River gorge in basalts that build up in layers to
Washington state, you would see huge cliffs made up of layers of dark, form a lava plateau. These dark-
volcanic rock. These layers of rock are part of the Columbia plateau, a colored basalt flows are near
huge lava plateau that covers parts of Washington, Oregon, and Idaho. Idaho Falls, Idaho.
A lava plateau is a volcanic landform produced by repeated eruptions
of very fluid, basaltic lava. As shown in Figure 15, the lava that forms
a lava plateau erupts through long cracks called fissures. Instead of
building a cone, the lava spreads out over a wide area.

The Columbia plateau is nearly 1.6 kilometers thick. The plateau
formed over hundreds of thousands of years as a series of lava flows,
some 50 meters thick, buried the landscape. Another major lava
plateau is the Deccan plateau in India.

What is a volcanic neck?

Lava
fountaining

Basaltic
lava flows

Fissure

Volcanoes and Other Igneous Activity 293

FCaucsttsomanidzeFfiogruErens

Floor Basalts and Hot Spots Giant flood basalt in South America and the Answer to . . .
outpourings of basaltic lava known as flood Etandeka flood basalt in Africa, at a time when
basalts seem to be related to hot spots. Many the two continents were connected. The A volcanic neck is the
hot spot tracks trace back to a flood basalt. For Yellowstone hot spot traces back to the remains of a volcanic
instance, the Reunion Island hot spot is the last Columbia River flood basalt. One hypothesis is pipe left standing above the surface
of a series of volcanoes that trace back to the that the head of a rising mantle plume erupts after erosion has removed the softer
Deccan Traps flood basalt in India that erupted as the flood basalt and the magma coming up rock around it.
65 million years ago. The Tristan hot spot in the tail of the plume creates the succeeding
the Atlantic Ocean traces back to the Karoo hot spot volcanoes.

Volcanoes and Other Igneous Activity 293

Section 100.02 (continued) Volcanic Hazards

Volcanic Hazards Throughout history, people often have settled near volcanoes because
rich volcanic soils are good for farming. They may not realize that an
Build Science Skills active or dormant volcano can erupt at any time, even centuries after
the last eruption. Volcano hazards include lava flows, volcanic
Using the Internet Students may ash, pyroclastic flows, and mudflows.
think of volcanoes as a danger that is far
removed from everyday life. Ask: What Lava flows are a major volcanic hazard. For example, the frequent
parts of the U.S. are potentially at risk lava flows from Mount Kilauea in Hawaii sometimes destroy homes
from volcanic eruptions? (Several and other structures in their path.
states, including California, Oregon,
Washington, Alaska, and Hawaii, have Active composite volcanoes, like those in the Cascade Range of the
active volcanoes that are potentially Pacific Northwest, are among the most dangerous volcanoes. A com-
dangerous.) Ask: Is the danger from a posite volcano can eject huge quantities of volcanic ash, burying
volcano limited to the area near the widespread areas under thick ash deposits. Ash from the eruption of
volcano? (No. Lahars and lava flows can Mount Vesuvius completely buried the Roman city of Pompeii.
travel tens of kilometers, and winds can
carry volcanic ash hundreds of kilometers An explosive eruption can also release a pyroclastic flow, a scorch-
through the atmosphere.) The United ing mixture of glowing volcanic particles and gases that sweeps rapidly
States Geologic Survey (USGS) down a volcano’s flanks. In 1902, a pyroclastic flow from Mount Pelée
maintains a number of Web sites that on the the island of Martinique in the Caribbean killed 29,000 people.
describe volcanic activity and hazards in
the United States. Ask students to use Composite volcanoes may also produce mudflows called lahars. A
these Web sites to research volcano lahar occurs when water-soaked volcanic ash and rock slide rapidly
hazards in different parts of the U.S. and downhill. Ice and snow melted by an eruption or by heavy rains can
report their findings to the class. trigger a lahar. In 1985, a lahar caused by the eruption of Nevada del
Ruiz in Colombia killed 23,000 people.
3 ASSESS
Volcanoes usually give some warning that an eruption is near. For
Evaluate L2 example, seismographs can detect the small earthquakes caused by
Understanding movement of magma beneath the surface. Gases and ash released by a
volcano may also signal an approaching eruption.
Have students play a quiz game to
review the material in this section. Ask Section 10.2 Assessment
each student to write three questions on
three separate sheets of paper. Collect Reviewing Concepts 7. Applying Concepts Explain how a caldera
the questions. Divide the class into two forms.
teams. To play the game, alternate 1. What factors determine the type of
giving a member of each team a volcanic eruption? 8. Making Judgments Should a resort hotel be
question from the collected papers. built on the side of an active composite
Give each team a point for each correct 2. List the materials ejected from volcanoes. volcano? Explain.
response. The team with the most 3. Describe the three main types of volcanoes.
points wins the game. 4. List three volcanic landforms. Summary Research a volcanic eruption.
Write a paragraph describing the eruption.
Reteach L1 Critical Thinking Make sure to classify the type of volcano
that erupted.
Set aside any questions that are 5. Relating Cause and Effect What propels
answered incorrectly from the quiz magma out of a volcano during an eruption?
game above. After the game, give each
team the stack of missed questions. Let 6. Comparing and Contrasting Compare
the entire team work together to give and contrast the magma that forms a shield
the correct response to the questions. volcano with the magma that forms a
composite cone.

294 Chapter 10

Answers will vary, but should accurately Section 10.2 Assessment 4. Caldera, volcanic neck, lava plateau
classify the volcano and give a clear 5. Expanding gases dissolved in the magma
description of the eruption. 1. Magma composition, magma temperature, 6. Shield volcanoes have fluid basaltic
and amount of dissolved gases magma. Composite cones generally have
294 Chapter 10 2. Lava, gases, and pyroclastic materials, such more viscous, andesitic magma.
as ash, dust, cinders, volcanic blocks, and vol- 7. A caldera may form when the top of a
canic bomb composite cone shatters in an explosive erup-
3. Cinder cones are small, steep cones, tion or when the magma chamber of a shield
made of loose cinders. Shield volcanoes are volcano empties and collapses.
large, gently sloping volcanoes made of layers 8. No—the hotel would be in danger from
of mainly quiet lava flows. Composite cones volcanic ash, lava flows, and pyroclastic flows
are large, steep cones, made of lava flows and if the volcano became active.
pyroclastic material.