FOCUS ON EARTH SCIENCE (10)

Volcanoes
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Volcanoes are locations
where magma reaches
Earth’s surface. They affect
human and wildlife
habitats.
1.e, 7.b
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Volcanoes and Plate
Boundaries

LESSON
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Most

volcanic
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activity occurs
along plate boundaries
where plates move relative to one another.

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Volcanic

Eruptions
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LESSON

and Features

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sition of magma controls
volcanic eruptions and
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Hazards of Volcanic
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>ˆ˜Ê`i> Volcanic

eruptions can change
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290
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Beautiful, but Dangerous

These tourists watch a fountain of lava
on the coast of Hawaii, at the Kilauea volcano. The Hawaiian Islands are
forming as a result of volcanic activity. The Hawaiian Islands are in the middle
of the Pacific Plate, far from its edges. They sit above a hot spot under the
Pacific Plate. Volcanic eruptions create scenic landscapes as the lava is
ejected from volcanoes.

-Vˆi˜ViÊÊ+PVSOBM Imagine you are among the tourists viewing the volcanic
eruption. Make a list of what you might hear, smell, feel, see, and possibly
taste while watching the eruption.

Start-Up Activities

How did these rocks
form?
Examine three photos of
rocks or rock samples
from your teacher. Each
of these rocks formed
from lava ejected during
a volcanic eruption.

Volcanic Features Make
the following Foldable to
identify volcanic features.
STEP 1 Fold a sheet of paper in half
lengthwise.

Procedure
1. Design a chart to record the color, texture,
and mass of each rock.
2. Estimate the number and size of bubbles
and crystals in each rock.

STEP 2 Cut along every third line of the
top flap to form tabs.

Think About This
• Infer which rock cooled the fastest and
which one cooled most slowly.

• Predict what environmental conditions
might have existed when each rock was
formed.
1.d, 7.e

STEP 3 Label as shown.
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ELA6: R 2.4

Visit ca6.msscience.com to:
▶
▶
▶
▶

view
explore Virtual Labs
access content-related Web links
take the Standards Check

Reviewing
As you read Lesson 2, identify and list the
volcanic features on the tabs. Include a
sketch and information under the tabs.

291

Get Ready to Read
Make Predictions
Learn It!

A prediction is an educated
guess based on what you already know. One way to predict while reading is to guess what you believe the author
will tell you next. As you are reading, each new topic
should make sense because it is related to the previous
paragraph or passage.

Practice It!

Read the excerpt below
from Lesson 2. Based on what you have read, make predictions about what you will read in the rest of the lesson.
After you read Lesson 2, go back to your predictions to
see if they were correct.
Think about how
the composition of
magma could affect
a volcanic eruption.

Predict how the
amount of silica present in magma could
affect the energy of a
volcanic eruption.

The composition of magma is an important
characteristic when attempting to predict a volcanic
eruption. Scientists can predict the energy of a
volcanic eruption based on the percentage of silica

and oxygen that is present in the magma. Silica is
just one of the chemical characteristics that
controls the eruptive behavior of a body of magma.
—from page 302

Determine what other
chemical characteristics
control the eruptive behavior of a body of magma.

Apply It!

Before you read,
skim the questions in the Chapter Assessment.
Choose three questions and predict the
answers.
292


Target Your Reading

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Use this to focus on the main ideas as you read the chapter.
1

Before you read the chapter, respond to the statements
below on your worksheet or on a numbered sheet of paper.
• Write an A if you agree with the statement.
• Write a D if you disagree with the statement.

2

After you read the chapter, look back to this page to see if
you’ve changed your mind about any of the statements.

• If any of your answers changed, explain why.
• Change any false statements into true statements.
• Use your revised statements as a study guide.

Before You Read
A or D

Statement

After You Read
A or D

1 Most volcanic eruptions occur at plate boundaries.
2 Magma rises buoyantly and exerts an upward force
on Earth’s surface.
3 Scientists are able to predict when a volcano will
erupt.
4 Volcanic ash is dangerous in the air and on the ground.
5 Hot spots form where two oceanic plates converge.
6 Lava flows are the most dangerous type of volcanic
hazard.
Print a worksheet of
this page at
ca6.msscience.com .

7 A lava dome is filled with thick, viscous lava.
8 Lava that contains high amounts of silica can be
extremely explosive.
9 Shield volcanoes are the smallest type of volcano.
10 Composite volcanoes are composed of alternating

layers of lava and tephra.
293


LESSON 1
Science Content
Standards
1.d Students know that earthquakes are
sudden motions along breaks in the crust
called faults and that volcanoes and fissures
are locations where magma reaches the
surface.
1.e Students know major geologic events,
such as earthquakes, volcanic eruptions, and
mountain building, result from plate
motions.
7.b Select and use appropriate tools and
technology (including calculators,
computers, balances, spring scales,
microscopes, and binoculars) to perform
tests, collect data, and display data.

Reading Guide
What You’ll Learn
▼

Explain what causes
volcanic activity.

▼

Relate the location of
volcanoes to plate
boundaries.

Why It’s Important
Understanding how
volcanoes form and where
they occur helps scientists
predict volcanic eruptions.

Vocabulary
volcano
hot spot
vent
fissure eruption

Review Vocabulary
lithospheric plate: large,
brittle pieces of Earth’s outer
shell composed of crust and
uppermost mantle (p. 183)

294 Chapter 7 • Volcanoes
Jeremy Bishop/SPL/Photo Researchers

Volcanoes and Plate
Boundaries
>ˆ˜Ê`i> Most volcanic activity occurs along plate boundaries where plates move relative to one another.

Real-World Reading Connection Maybe you’ve been outside when a hot-air balloon flew overhead. When the pilot
turned on the flame under the balloon, the balloon rose
higher in the sky. As temperature increases, particles move
faster,
ˆ} the air to expand and become less dense.
>ˆ˜ causing
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Particles in molten rock also move faster, expand, and
become
less dense as the temperature increases.
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What is a volcano?
A volcano is a land or underwater feature that forms
when magma reaches the surface of Earth. Recall from
Chapter 2 that magma is molten, liquid rock material
found underground and forms igneous rocks. Magma rises
up to Earth’s surface because its density is less than the
rock through which it moves. When magma rises to Earth’s
surface it is called lava.
Volcanoes can cause the eruption of liquid, gas, or solid
materials. Hot gases, magma, and even solid rock particles
can be explosively erupted from openings on Earth. Scientists, as shown in Figure 1, study the composition of lava to
better understand volcanic eruptions.

Figure 1

This photo
shows volcanologists
collecting a sample of
lava from a volcano.


How do volcanoes form?
Recall from Chapter 2 that some of Earth’s internal heat is
left over from when the planet first formed. When rocks
become hot enough, and pressure and other conditions are
right, they can melt.

WORD ORIGIN

What causes rock to melt?

volcano
from Italian vulcano;
means burning mountain

In order for a volcano to form, magma must first reach
Earth’s surface. Magma is less dense then the rock from which
it melted. Therefore, magma tends to rise above the denser
rock. The density of magma depends upon its composition, the
amount of dissolved gas, and its temperature.
Magma also tends to be more buoyant when compared to
the rocks that surround it. Magma is forced to rise toward
Earth’s surface because of the buoyant force pushing up on the
magma.

Where do volcanoes occur?
Volcanoes are not common in all regions of Earth’s crust.
Volcanoes typically occur along divergent and convergent
plate boundaries and hot spots, where the flow of heat from
Earth’s interior is high. Figure 2 shows the locations of active
volcanoes and plate boundaries on Earth. Do you notice a
relationship between the locations of active volcanoes and
plate boundaries?

Figure 2

This world
map shows that most
active volcanoes are
located near the edges
of the plate boundaries.
The Ring of Fire is a belt
of active volcanoes that
circles the Pacific Ocean.
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Lesson 1 • Volcanoes and Plate Boundaries

295


Convergent Plate Boundaries
Recall from Chapter 5 that when two lithospheric plates
move toward each other, a convergent plate boundary forms.
They include areas where an oceanic plate is subducted below
a continental plate or where an oceanic plate is subducted
below another oceanic plate. As the plate subducting below
the other plate gets deep enough, it becomes hot enough to
partially melt and form magma. The magma is then forced
up toward Earth’s surface and forms volcanoes. Volcanoes
that form along convergent plate boundaries tend to erupt
more violently than other volcanoes. Figure 3 shows examples

of convergent plate boundary landforms.
Figure 3 Identify the locations of two convergent
plate boundary landforms.

Volcanic Arcs When an oceanic plate subducts beneath a
continental plate, a volcanic arc is formed. A volcanic arc is a
string of volcanoes that forms on land parallel to the leading
edge of the continent. An example of a volcanic arc is shown
in Figure 3.
Island Arcs When two oceanic plates move toward each
other, an island arc is formed. Island arcs are long, curved
strings of volcanic islands. Figure 3 shows an example of an
island arc volcano.

Figure 3

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Chapter 7 • Volcanoes

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Divergent Plate Boundaries
Imagine a divergent plate boundary as a huge crack, or rift,
in Earth’s surface. The boundary can occur in either the
ocean, as a mid-ocean ridge, or on a continent, as a continental rift. Tension stresses cause the lithospheric plates to be
pulled apart. This pulling action forms rifts that allow
magma to reach Earth’s surface through fissure eruptions.
In the ocean, lava flows from these rifts are cooled quickly
by seawater and form new volcanic rock. As more lava flows
and hardens, it builds up on the seafloor. Sometimes, the volcanoes rise above sea level, forming islands such as Iceland.

SCIENCE USE V. COMMON USE
rift
Science Use fissure, crevasse.
As the lithospheric plates were
pulled apart, a rift valley was
created.
Common Use to burst open,
divide. The argument caused a
rift between the two friends.

How do volcanoes form at divergent plate
boundaries?

Heat Escapes Volcanic eruptions are one of the most noticeable signs that heat is escaping from Earth’s interior. After
many of thousands or even millions of years, magma reaches
Earth’s surface. When this magma erupts from the central,
circular or oval-shaped opening of a volcano, called a vent, a
cone-shaped landform develops. This landform develops
because lava flows out from one source in many directions.
As lava flows out, it cools quickly and becomes solid, forming
layers of igneous rock around the vent. the steep-walled
depression around a volcano’s vent is the crater.

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ACADEMIC VOCABULARY
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We found the source of the loud
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Lesson 1 • Volcanoes and Plate Boundaries

297


Figure 4 This crack formed
because of a fissure eruption
at a mid-ocean ridge. The currents caused by this activity
attract the crabs.

Fissure Eruptions When magma escapes from narrow and
elongated cracks in Earth’s crust, a fissure eruption (FIH
shur · ih RUP shunz) occurs. The magma pushes out along
both sides of the crack and flows smoothly. Long, sheetshaped landforms develop.
Fissure eruptions can occur in the ocean at a divergent
plate boundary. They form mid-ocean ridges and new seafloor, as shown in Figure 4. Fissure eruptions can also occur
on a continent at a divergent plate boundary. They form a rift
and produce new crust on Earth’s surface.

Volcanoes Away from Plate Boundaries
Some volcanoes do not form along plate boundaries. These
volcanoes are known as hot spots, which are localized areas
of high heat in Earth’s interior. Scientists continue to study
how hot spots form and move. Figure 5 shows an example of
a hot spot.

Figure 5 The Hawaiian Islands are actually volcanoes that formed over a
hot spot. They formed over a period of about 5 million years.

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298 Chapter 7 • Volcanoes
(t)Dr. Ken MacDonald/SPL/Photo Researchers, (b)NASA/SPL/Photo Researchers


Formation of Volcanoes
Heat and pressure from Earth’s interior cause rock to melt
and become magma. Because magma is less dense than the
surrounding rock, the buoyant force causes it to rise to
Earth’s surface. Most volcanoes form at divergent or convergent plate boundaries. However, hot spots form away from
plate boundaries. Scientists continue to study hot spots to
better understand how and why they form.

LESSON 1 Review
Standards Check

Summarize
Create your own lesson
summary as you design a
study web.
1. Write the lesson title,
number, and page numbers at the top of a sheet
of paper.
2. Scan the lesson to find
the red main headings.
3. Organize these headings
clockwise on branches
around the lesson title.
4. Review the information
under each red heading

to design a branch for
each blue subheading.
5. List 2–3 details, key terms,
and definitions from each
blue subheading on
branches extending
from the main heading
branches.

ELA6: R 2.4

Using Vocabulary
Complete the sentences using the
correct term.
vent

fissure eruption

1. A cone-shaped landform
develops when magma erupts
from the
forming a
1.d
volcano.
2. Long, sheet-shaped landforms
develop when a(n)
pushes magma out along both
sides of a crack in Earth’s crust.
1.d

Understanding Main Ideas
3. Illustrate and label two types
of openings from which
1.d
magma erupts.
4. Organize Draw a diagram like
the one below. List five settings where volcanoes occur.
1.e
1.
2.
Volcanoes occur 3.
4.
5.

5. Sketch the formation of a volcano. Include the forces at
work in Earth’s interior that
cause a volcano to form and
magma to flow to the surface.
1.d
6. Compare and contrast the
movement of lithosphere at a
mid-ocean ridge and a sub1.e
duction zone.

Applying Science
7. Hypothesize why volcanoes
are not likely where two continental plates converge.
Support your answer with
information from this lesson.
1.e

8. Relate the occurrence of volcanoes along the Pacific Ring
of Fire to the types of plate
boundaries in the region. Consider the direction in which
the boundaries are moving
and any geologic features that
affect the volcanic activity. 1.e

Science

nline

For more practice, visit Standards
Check at ca6.msscience.com .
Lesson 1 • Volcanoes and Plate Boundaries

299


How do volcanoes form?
Buoyant forces push magma up and cause
it to rise to Earth’s surface. This building of
pressure causes volcanic eruptions. Volcanic
eruptions allow heat to escape from Earth’s
interior. Can you model how volcanoes form
and erupt?

Procedure
1. Read and complete a lab safety form.
2. Obtain a beaker with hardened wax in the
bottom from your teacher.

3. Layer 1 cm of sand on top of the wax.
4. Pour 8 cm of cold water onto the sand.
5. Set up a ring stand with wire gauze to hold your beaker over a
Bunsen burner. Light the burner.

6.
7.
8.
9.

Observe the wax as it erupts through the sand and water.
Extinguish the burner. Allow the wax to cool.
Record your observations of the exposed wax formations.
Compare your volcano to your classmates’.

Analysis
1. Sequence how your volcano developed, erupted,
and formed volcanic features.

2. Infer how the density of the wax changed as it was heated.
3. Compare and contrast your model to a real-life volcano.

Science Content Standards
1.d Students know that earthquakes are sudden motions along breaks in the crust called faults
and that volcanoes and fissures are locations where magma reaches the surface.
7.b Select and use appropriate tools and technology (including calculators, computers, balances,
spring scales, microscopes, and binoculars) to perform tests, collect data, and display data.

300

Horizon Companies


LESSON 2
Science Content
Standards
1.d Students know that earthquakes are
sudden motions along breaks in the crust
called faults and that volcanoes and fissures
are locations where magma reaches the
surface.
1.f Students know how to explain major
features of California geology (including
mountains, faults, volcanoes) in terms of
plate tectonics.
7.g Interpret events by sequence and time
from natural phenomena (e.g., the relative
ages of rocks and intrusions).
Also covers: 7.h

Volcanic Eruptions
and Features
>ˆ˜Ê`i> The composition of magma controls volcanic
eruptions and determines the different types of lava flow and
volcanic features.

Real-World Reading Connection Think about squeezing
toothpaste out of the tube. A thick, gooey blob of paste
rolls slowly out the end. Imagine the tube filled with water
instead

The composition of toothpaste and
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>ˆ˜ of toothpaste.
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water
different. How would this difference affect the
manner
in which these materials flow?
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Reading Guide
What You’ll Learn
▼

Relate the composition of
magma to characteristics
of volcanic eruptions.

▼

Compare and contrast
different types of
volcanoes.

▼

Analyze California’s
volcanic activity.

▼

Differentiate between
volcanic features.

What controls volcanic eruptions?
The composition of magma controls how lava flows and
the way a volcano erupts. For example, in 2004, a thick,
sticky mass of lava began flowing from Mount St. Helens’
vent. Scientists have termed the mass the whaleback
because of its unique shape. This blob of lava, shown in
Figure 6, flows the way it does—as a sticky, gooey, lumpy
mass—because of its composition.

Figure 6 A thick, sticky mass of lava is flowing from
the vent of Mount St. Helens.
Identify the volcanic feature in the center of the volcano.

Why It’s Important
Understanding the
composition of magma helps
scientists better predict how
volcanoes erupt.

Vocabulary
viscosity
shield volcano
cinder cone volcano
tephra

composite volcano

Review Vocabulary
landform: feature
sculpted by processes on
Earth’s surface (p. 79)
Lesson 2 • Volcanic Eruptions and Features

301

Charles Rogers/Visuals Unlimited


Composition of Magma

Figure 7

The composition of magma
affects the viscosity. Generally, magma that
has a high percentage of silica has high viscosity. Magma that has a low percentage of
silica has low viscosity.

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302

High Silica In general, magma that contains
a high percentage of silica is thicker and
slower moving. Magma with high silica content tends to be thick and sticky, like honey or
frosting. The viscosity (vihs KAH suh tee) of
a material is a physical property that describes
the material’s resistance to flow. In other
words, if magma has high viscosity, it does
not flow easily.
Low Silica In general, magma that contains
a low percentage of silica and more iron and
magnesium has low viscosity. In other words,
the magma will flow easily. Magma with low
silica content tends to be thin and runny, like
warm syrup. Figure 7 shows how the composition of magma affects viscosity.

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Chapter 7 • Volcanoes

(t)U.S. Geological Survey, (b)CORBIS

The composition of magma is an important characteristic when scientists attempt to
predict a volcanic eruption. Scientists can
predict the energy of a volcanic eruption
based on the percentage of silica and oxygen
that is present in the magma. Silica is just one
of the chemical characteristics that controls
the eruptive behavior of magma.

Dissolved Gases and Temperature
The amount of dissolved gases and the
temperature of magma also affect how a volcano erupts. In general, the higher the temperature of magma, the more easily it flows.
The most common dissolved gas in magma
is water vapor, or H2O. Other important
gases that may be present in magma and
released during volcanic eruptions are carbon
dioxide, sulfur dioxide, and hydrogen sulfide.
Typically, the more gas that is present in
magma, the greater the chance of an explosive eruption, even if the composition of the
magma would suggest that it should have a
quiet eruption.


Types of Magma and Lava
When you classify something, it’s good to keep the main
types in your mind as you compare examples. Try this for
the types of magma and lava. Figure 7 also shows two types
of magma, basaltic (buh SAWL tihk), and granitic

(gra NIH tihk).

Basaltic Magma and Lava
Magma that contains a low percentage of silica is called
basaltic magma. Basaltic magma typically has low viscosity,
meaning it flows freely. It is also a much thinner, more fluid
magma.
Basaltic lava that erupts from a volcano tends to pour
from the vent and run down the sides of the volcano. As this
pahoehoe (pa HOY hoy) lava cools, it develops a smooth skin
and forms ropelike patterns. As stiff, slowly moving aa
(AH ah) lava forms, it flows at a lower temperature. Basaltic
lava that erupts underwater at fissure eruptions forms
bubble-like pillow lava. Basaltic lava tends to flow easily and
produce quiet, nonexplosive eruptions, as shown in Figure 8.

Granitic Magma and Lava
Magma that contains a high percentage of silica is called
granitic magma. When granitic magma emerges on Earth’s
surface, it typically will have high viscosity, meaning it flows
slowly. It will also be more sticky and lumpy. Granitic lava
tends to trap gases, which causes pressure to build up and
produces explosive eruptions, as shown in Figure 8.

ACADEMIC VOCABULARY
emerge (ih MERJ)
(verb) to rise from, to come
out into view
I saw the diver emerge from the
cloudy water.

Figure 8 Compare the explosiveness of the two
volcanic eruptions.

Figure 8

The percentage of silica,
dissolved gases, and
temperature are all
physical characteristics
that affect the eruptive
behavior of magma.

Quiet eruption

Explosive eruption
Lesson 2 • Volcanic Eruptions and Features

303

(l)Stephen & Donna OÕMeara/SPL/Photo Researchers, (r)Bettmann/CORBIS


Types of Volcanoes
There are three types of volcanoes that are common on
Earth’s surface—shield volcanoes, cinder cone volcanoes,
and composite volcanoes. These volcanoes do not represent
all possible types of volcanic eruptions, but they illustrate
many common volcanic features and the processes that
form them.

What are the three types of volcanoes?

Shield Volcanoes
A shield volcano is a huge, gently sloping volcanic landform that is mainly composed of basaltic lava. Basaltic lava
moves freely. Shield volcanoes develop as layer upon layer of
gently flowing basaltic lava piles up. Shield volcanoes, like the
one shown in Table 1, were named because they resemble the
slightly bent shape of a warrior’s shield. The Hawaiian Islands
are examples of shield volcanoes. Types of lava that typically
flow from a shield volcano are shown in Figure 9.

Cinder Cone Volcanoes
WORD ORIGIN
tephra
from Greek; means ashes

A cinder cone volcano is mainly composed of solid fragments. These solid fragments are known as tephra (TEH
fruh) and include fragments of volcanic rock or lava. These
fragments range in size from tiny particles to huge boulders.
When lava erupts from the vent, cools quickly in the air, and
falls to the surface as tephra, it forms a distinctly steep-sided,
cone-shaped landform, as shown in Table 1.
Cinder cone volcanoes typically produce explosive volcanic
eruptions.

Composite Volcanoes
A composite volcano is mainly composed of alternating
layers of lava and tephra. These layers accumulate from the
alternating of quiet and explosive volcanic eruptions. The
quiet and explosive eruptions occur because the composition

of magma associated with composite volcanoes is somewhere
between basaltic and granitic.
What causes the alternating layers of a composite
volcano?

Composite volcanoes often form tall, majestic mountains,
as shown in Table 1. Some volcanoes have reached altitudes of
more than 2,438 meters.

304 Chapter 7 • Volcanoes


Table 1 Types of Volcanoes

Type of Volcano

Interactive Table Organize information about
types of volcanoes at ca6.msscience.com.

Examples of Volcanoes

Shield Volcanoes
• Huge, gently sloping
sides
• Composed of basaltic
lava
• Develop from layers
of basaltic lava
• Quiet eruptions

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• Steep-sided, cone
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• Composed of basaltic
lava
• Develop from layers
of tephra
• Explosive eruptions

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Composite Volcanoes
• Tall, majestic

mountains
• Composed of lava
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granitic
• Develop from
alternating layers of
lava and tephra
• Eruptions are both
quiet and explosive

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Lesson 2 • Volcanic Eruptions and Features

305

(t)Robert A. Jensen/USDA Forest Service, (c)Charles Rogers/Visuals Unlimited, (b)Steve Kaufman/Accent Alaska


Visualizing Lava

Figure 9
Lava rarely travels faster than a few kilometers per hour. Therefore, it poses
little danger to people. However, homes, property, and crops can be damaged. On land, there are two main types of lava flows—aa and pahoehoe.
When lava comes out of cracks in the ocean floor, it forms pillow lava. The
lava cooling here came from a volcanic eruption on the island of Hawaii.

Aa lava flows, like this one on Mount Etna
in Italy, carry sharp, angular chunks of rock.
Aa flows move more slowly than hotter
pahoehoe flows.

Pahoehoe lava flows, like this one near
Kilauea’s Mauna Ulu Crater in Hawaii, are
hotter and more fluid than aa flows. They
develop a smooth skin and form ropelike
patterns as they push forward and then cool.

306 Chapter 7 • Volcanoes
(bkgd)API/Explorer/Photo Researchers, (tr)Krafft/HOA-QUI/Photo Researchers, (bl)Robert Hessler/Planet Earth Pictures, (br)Paul Chesley

Pillow lava occurs where lava oozes out of
fissure eruptions in the ocean floor. It forms
bubble-shaped lumps as it cools. Pillow lava is
the most common type of lava on Earth.
Contributed by National Geographic


Volcanoes in California
As you have read in previous chapters, California has a large
variety of landscapes and landforms. Examples of volcanic

landforms found in California are shown in Figure 10.
These volcanoes form at a convergent plate boundary as
part of a volcanic arc. They form where the Juan de Fuca
Plate subducts beneath the North American Plate at the
southern end of the Cascades Range.

Figure 10 This map shows the variety of active volcanic
landforms along the western coast of the United States.
Identify a California volcano that is part of the volcanic arc.

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Black Butte—
Cinder Cone Volcano

Medicine Lake—
Shield Volcano

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The batholith in

the top photo and the volcanic neck in the bottom
photo are examples of volcanic features.

Most magma never reaches Earth’s surface to form volcanoes or flow from fissure eruptions. This magma slowly cools
underground and produces intrusive features that later could
become exposed by erosion, as shown in Figure 11.

Batholiths
The largest intrusive volcanic features are batholiths.
Batholiths form when magma slowly cools and solidifies
before reaching the surface. As shown in Figure 12, they can
be many hundreds of kilometers in width and length and
several kilometers thick.

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Intrusive Volcanic Features

Figure 11

Dikes and Sills
Magma sometimes squeezes into cracks in rocks below the
surface, as shown in Figure 12. Magma that cuts across rock
layers and hardens is called a dike. Magma that is parallel to
rock layers and hardens is called a sill. Most dikes and sills
run from a few meters to hundreds of meters long.

Figure 12 This diagram shows intrusive and
other features associated with volcanic activity.
Identify which features shown are formed above ground.

Which are formed below ground?

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flowing lava, visit ca6.msscience.com.

308 Chapter 7 • Volcanoes

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Other Volcanic Features
Not all volcanic features form underground from igneous activity. Some volcanic
features form from past and current volcanic
eruptions and lava flows.

Figure 13

Examples of other volcanic
features include volcanic necks, lava
domes, and lava tubes.

Volcanic Neck
When a volcano stops erupting, magma
hardens inside the vent. Erosion, usually by
water and wind, begins to wear away the volcano. The cone is much softer than the igneous rock in the vent, and tends to erode first,
leaving behind the igneous core as a volcanic
neck. Shiprock, located in New Mexico and
shown in Figure 13, is an example of an
eroded volcanic neck.

Volcanic Neck

Which part of a volcano tends to
erode first?

Lava Domes
A lava dome is a rounded volcanic feature
that forms when a mass of highly viscous lava

slowly erupts from the vent. The thick, sticky
granitic lava piles up instead of flowing freely.
The viscosity of the granitic lava does not
allow it to release gases easily. When pressure
builds up within the lava dome, gas, lava, and
solid materials can be violently ejected into
the air.
Lava domes like the one shown in
Figure 13, can become dangerous, explosive
volcanic eruptions.

Lava Dome

Lava Tubes
A lava tube is a hollow tube that forms
when lava flows through a channel, cools, and
hardens on the surface. Lava tubes often form
underground. Swiftly moving magma flows
from magma chambers and branches to the
sea. The lava tube may be over 8 m in diameter and as long as 48 km when the flow of
magma stops and drains. Figure 13 shows a
lava tube that was formed from pahoehoe.

Lava Tube
Lesson 2 • Volcanic Eruptions and Features

309

(t)Ernesto Burciaga/Omni-Photo Communications, (c)David McNew/Newsmakers/Getty Images, (b)Philip Rosenberg/Pacific Stock

Caldera

Figure 14

Calderas form when the top
of a volcano collapses.

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A caldera (cal DAYR uh) is a large, usually
circular depression. A caldera forms when the
top of a volcano collapses and becomes
wedged into the nearly empty magma chamber as shown in Figure 14. The large, sunken
crater can become filled with water, forming
scenic lakes and landscapes. Calderas can be
up to 50 km in diameter. Volcanic eruptions
that form calderas are often the largest eruptions on Earth.
Figure 14 Describe the steps in the
formation of a caldera.

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Long Valley Caldera, located in the Cascade Range is an example of a caldera. It is a
large, oblong caldera on top of an active volcano. It is plugged with a large, bubblelike
lava dome.

Figure 15 shows another example of a
caldera, Crater Lake. Crater Lake Caldera is
597 m deep. The lake partly fills a nearly
1,220 m deep caldera that was formed around
6,900 years ago by the collapse of the volcano
Mount Mazama.

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310

Chapter 7 • Volcanoes

Randy Wells/Stone/Getty Images

Figure 15 Wizard Island in Crater Lake
is a cinder cone volcano that erupted
after the formation of the caldera.


Characteristics of Volcanic Eruptions
The way a volcano erupts is controlled by the composition
of the magma, the amount of dissolved gas, and its temperature. These factors also affect the viscosity and flow of lava,
which result in different volcanic features. Shield volcanoes,
cinder cone volcanoes, and composite volcanoes all are common types of volcanoes. Fissure eruptions also release heat
from Earth’s interior through lava flows. The Cascade Range
located across California, Oregon, and Washington, contains
potentially active volcanoes. Next, you will read about how

volcanic eruptions can produce hazards that change human
and wildlife habitats.

LESSON 2 Review
Standards Check

Summarize
Create your own lesson summary as you write a script for
a television news report.
1. Review the text after the
red main headings and
write one sentence about
each. These are the headlines of your broadcast.
2. Review the text and write
2–3 sentences about each
blue subheading. These
sentences should tell who,
what, when, where, and
why information about
each red heading.
3. Include descriptive details
in your report, such as
names of reporters and
local places and events.
4. Present your news report
to other classmates alone
or with a team.

ELA6: LS 1.4

Using Vocabulary
1. In your own words, write a
1.d
definition for viscosity.
2. What is the difference
between a cinder cone
volcano and a composite
volcano?

5. Compare and contrast the
shape and composition of a
cinder cone volcano with a
1.d
shield volcano.
6. Predict how the viscosity of
lava affects volcanic features.
1.d

1.d

Understanding Main Ideas
3. Identify three common types
1.d
of volcanoes.
4. Organize Draw a diagram like
the one below. List three factors that affect the flow of lava.
1.d

Applying Science
7. Infer why some volcanic eruptions are explosive and some

1.d
eruptions are quiet.
8. Evaluate the conditions you
would expect to find where
the Pacific Plate and the North
American Plate converge. Use
an example of volcanic activity
to support your reasoning. 1.f

Factors
that Affect
Lava Flow

Science

nline

For more practice, visit Standards
Check at ca6.msscience.com .
Lesson 2 • Volcanic Eruptions and Features

311


Model Structures of Volcanoes
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Data
1. Redraw the diagram of the volcano in your notebook using
colored pencils.

2. Create a diagram key to identify the depth of the three lava
flows, magma, magma chamber, caldera, vent, and surrounding
topography. Give your key a title.

Data Analysis
1. Summarize the event that formed the caldera.
2. Suggest three possible processes that might have formed the
caldera fill.

3. Sequence the processes by which a lava dome might be

formed in this volcano.

4. Predict how a batholith, dikes, and sills would become evident
in this landscape.

Science Content Standards

312

MA6: MR 1.1

1.d Students know that earthquakes are sudden motions along breaks in the crust called faults
and that volcanoes and fissures are locations where magma reaches the surface.
7.g Interpret events by sequence and time from natural phenomena (e.g., the relative ages of
rocks and intrusions).
7.h Identify changes in natural phenomena over time without manipulating the phenomena
(e.g., a tree limb, a grove of trees, a stream, a hillslope).


LESSON 3
Science Content
Standards
2.d Students know earthquakes, volcanic
eruptions, landslides, and floods change
human and wildlife habitats.
7.a Develop a hypothesis.
7.b Select and use appropriate tools
and technology (including calculators,
computers, balances, spring scales,
microscopes, and binoculars) to perform

tests, collect data, and display data.
7.d Communicate the steps and results
from an investigation in written reports and
oral presentations.

Hazards of Volcanic
Eruptions
>ˆ˜Ê`i> Volcanic eruptions can change human and wildlife
habitats.

Reading Guide

Real-World Reading Connection You might remember
a day when you had plans to go outside, but couldn’t
because of poor air quality. Solar radiation can cause a
chemical reaction with ozone in the atmosphere and gases
emitted
ˆ} cars and industry. Smog can be produced.
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Volcanic eruptions can cause changes in air quality too.
But, ,i>`ˆ˜}
the effects often last longer and can be more devastat
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ing than

What You’ll Learn

Effects on Habitats

▼

Describe effects of
volcanic eruptions on
human and wildlife
habitats.

▼

Discover geologic events
that scientists observe and
measure to help predict
volcanic eruptions.

Why It’s Important
As scientists become better
able to predict and monitor
volcanic eruptions, more
lives and property are saved.

When a volcano remains quiet for centuries, people
tend to forget about the forces at work in Earth’s interior,
including those that could cause environmental hazards.
One such hazard that affects human and wildlife habitats is
the release of large amounts of fine-grained tephra, called
volcanic ash. Figure 16 shows volcanic ash that was ejected
thousands of meters into the atmosphere from the eruption
of Mount St. Helens in 1980.

Figure 16 Volcanic ash released from Mount St. Helens
spread through the atmosphere and began falling like snow.

Vocabulary
volcanic ash
lahar
pyroclastic flow

Review Vocabulary
seismic waves: wave that
travels through Earth
generated by earthquake
(p. 252)

Lesson 3 • Hazards of Volcanic Eruptions

313

(l)Douglas Kirkland/CORBIS, (c)D.E. Wieprecht/U.S. Geological Survey, (r)RMF/Visuals Unlimited


Figure 17

Examples of volcanic
hazards include volcanic ash,
lahars, gases, pyroclastic flows,
and lava flows. They all cause
change to human and wildlife
habitats.

Volcanic Ash

Lahar

Volcanic Ash
Ash made from burning wood is similar to volcanic ash.
They both contain tiny grains of material. But, unlike ash
made from wood, volcanic ash is made of tiny, sharp mineral
and glasslike particles.
What is the difference between ash made from
burning wood and volcanic ash?

Damage to human and wildlife habitats can be caused by
volcanic ash. When layers of ash build up on rooftops, the
increased weight can cause structural damage. Volcanic ash
can bury plants and animals and their food sources and contaminate the water supply. A cloud of volcanic ash from the
eruption of Mount Vesuvius is shown in Figure 17.

Landslides and Lahars
Scientists refer to a lahar (LAH har) as a rapidly flowing
mixture of volcanic debris and water. An example is shown in
Figure 17. Some of the largest lahars begin as landslides. Landslides can occur from volcanic eruptions, earthquakes, precipitation, or gravity. Volcanic ash, tephra, dirt, rocks, and
even trees can mix with ground water and precipitation and
form a lahar. Rivers of debris can move downhill at rates up
to tens of meters per second. It is not possible for humans to
outrun a swiftly moving lahar.
When human habitats are built in river valleys near
volcanoes, the flow of debris can become directed toward the
town. A fast-moving lahar provides little time for warning.
The effects can be disastrous.

314

Chapter 7 • Volcanoes

(l)Bettmann/CORBIS, (r)Jacques Langevin/CORBIS