22.6 Volcanoes

Section 22.6
1
FOCUS
Objectives
22.6.1 Describe the internal structure
of a volcano and how
volcanoes form.
22.6.2 Relate the type of volcanic
eruption to the characteristics
of magma.
22.6.3 Describe the different types of
volcanoes and where they are
typically located.
22.6.4 Describe several types of
igneous features and how
they are formed.
Reading Focus
Build Vocabulary
Venn Diagram Have students build a
Venn diagram to compare the similarities
and differences between composite
volcanoes, shield volcanoes, and cinder
cones. Ask students to incorporate terms
from the vocabulary list.
Reading Strategy
690 Chapter 22
L2
L2
a. Gases in magma expand rapidly and,
depending on the characteristics of the
magma, there is a quiet or explosive
eruption. b. Lava and other types of
volcanic material cool and solidify,
building up layers of a volcano over time.
22.6 Volcanoes
Key Concepts
How do volcanoes form?
Why are some volcanic
eruptions quiet and
others explosive?
Where are volcanoes
found?
What landforms are
formed from lava
and magma?
690 Chapter 22
Section Resources
Print
• Reading and Study Workbook With
Math Support, Section 22.6
• Transparencies, Section 22.6
Vocabulary
◆ volcano
◆ magma chamber
◆ pipe ◆ vent
◆ crater
Reading Strategy
Sequencing Copy the
flowchart. As you read,
complete it to show how
a volcano forms.
◆ caldera
◆ hot spot
◆ shield volcano
◆ cinder cone
Magma
forms and
rises toward
surface.
◆ composite volcano
◆ batholith
◆ sill ◆ dike
◆ volcanic neck
a. ? b. ?
From northern California through the state of Washington, a chain
of snow-capped, cone-shaped mountains rises majestically above the
surrounding evergreen forests. These peaks form the backbone of the
Cascade Range, which is made up of volcanoes. A volcano is a mountain
that forms when magma reaches the surface.
Most of the time, the volcanoes of the Cascade Range are inactive.
Occasionally, however, one of these volcanoes erupts with great violence.
That’s what happened in May 1980, when
Mount St. Helens in Washington exploded. This
eruption blew away the top of the mountain. It
flattened trees as far as 17 kilometers away, and
spewed a cloud of ash more than 20 kilometers
into the sky, as shown in Figure 38.
Halfway across the Pacific Ocean, on the
island of Hawaii, is another volcano called
Kilauea. Unlike Mount St. Helens, Kilauea is often
active. Lava bubbles out of the ground and quietly
flows down gentle slopes. Although Mount St.
Helens and Kilauea are both volcanoes, they are
quite different from each other. Volcanoes can
result from several different geological processes,
and can take a variety of forms.
Figure 38 When Mount St. Helens erupted, trapped
gases caused the north side of the mountain to explode.
Volcanic ash was ejected high into the atmosphere.
Technology
• Interactive Textbook, Section 22.6
• Presentation Pro CD-ROM, Section 22.6
• Go Online, Planet Diary, Volcano activity

Formation of a Volcano
The process that leads to a volcanic eruption begins deep inside Earth.
Under certain conditions, small amounts of mantle rock can
melt, forming liquid magma. The magma rises upward through the
crust, erupting at the surface as a volcano. Magma rises because it is
less dense than the solid rock around and above it.
How a Volcano Erupts What causes a volcanic eruption? The
process is similar to what happens when you quickly open a bottle of
soda that has been shaken. Like the soda in the bottle, magma is under
pressure and contains dissolved gases. In the case of magma, the gases
include carbon dioxide and water vapor. As magma approaches the
surface, lower pressure allows the gases in magma to expand rapidly.
An eruption occurs when the gases bubble out through a crack in the
crust, propelling magma to the surface.
Structure of a Volcano Before an eruption, magma often collects
in a pocket called a magma chamber, shown in Figure 39. Magma
slowly accumulates in the magma chamber until enough pressure
builds up to start an eruption. Then, magma rises to the surface in a
narrow, vertical channel called a pipe.
An opening in the ground where magma escapes to the surface is
called a vent. Often there is one central vent at the top of a volcano. Sometimes
there are other vents that open along a volcano’s side. At the top of
the central vent in most volcanoes is a bowl-shaped pit called a crater.
After an eruption, a volcano’s magma chamber and main vent may
empty of magma, creating a hollow shell. If this shell collapses inward,
it creates a huge depression, called a caldera, at the top of the volcano.
Crater
Lava
Customize for Inclusion Students
Learning Disabled
For students with difficulty absorbing concepts
by reading, use Figure 39 as a visual aid as you
describe the path of magma through a volcano.
Vent
Pipe
Figure 39 When a volcanic
mountain erupts, magma under
pressure is forced upward from
the magma chamber. It passes
through the pipe, and out the
vent. Magma flows onto the
surface as lava. Predicting What
might happen to a volcano if its
pipe became plugged with
hardened magma?
Magma chamber
691
Be sure students understand which part is which
and the order in which magma travels through
each part.
2
INSTRUCT
Formation of a
Volcano
Build Science Skills
Using an Analogy Have students
expand upon the soda bottle analogy
presented in How a Volcano Erupts.
Review the analogy to be sure students
understand that the water in the
magma is being compared to the
carbon dioxide in the soda. The
presence of water in magma can cause
explosive, steam-filled eruptions. As
magma erupts, the pressure drops. This
drop in pressure allows the hot water to
expand into steam. This expansion can
cause very explosive eruptions. Ask,
How does opening the bottle relate
to magma rising to the surface?
(The pressure drops in both cases.)
Why does the gas expand when the
pressure drops? ( , with
temperature constant, as pressure
decreases, volume increases) What
volcanic rock is similar to the frothy
part of the soda outburst? (Pumice)
Logical
FYI
P1V1
T1
P2V2
T2
L2
There is another similarity between a
steam-filled, explosive volcanic eruption
and a shaken bottle of soda. In both
cases, what comes out is often frothy and
filled with bubbles. When this molten
volcanic froth is ejected, it cools and
solidifies in the air before it lands on the
ground. These bits of solidified froth are
called pyroclastic deposits, and they have
a large range in size and appearance.
The most common pyroclastic deposit is
ash, which has very small grains. Because
ash is so small, it can be thrown high into
the atmosphere, where it can be carried
all around Earth.
In some pyroclastic deposits, like
pumice, the bubbles of the molten froth
are retained as pockets of air called
vesicles. Pumice has so much air trapped
in vesicles that it will float on top of
water. Some pyroclastic deposits contain
a black glass called obsidian. This occurs
when the magma cools so fast that
crystals do not have time to form, and
the atoms solidify in a random,
amorphous structure.
Answer to . . .
Figure 39 Pressure will build up until
the volcano erupts explosively.
Earth’s Interior 691

Section 22.6 (continued)
Quiet and Explosive
Eruptions
Build Reading Literacy
Identify Main Idea/Details Refer to
page 98D in Chapter 4, which provides
the guidelines for identifying main ideas
and details.
Have students read Quiet and Explosive
Eruptions. Ask them to read the key
concept aloud. Then, have students find
the details that support this key concept.
Ask, Which characteristics of the
magma determine how the volcano
erupts? (Temperature, water content, and
silica content)
Verbal
FYI
692 Chapter 22
L1
Magma melts for different reasons in
different regions. Beneath hot spots, the
upwelling rock comes from deep within
Earth and may be a few hundred degrees
hotter than surrounding rock, causing it
to melt more easily. Beneath mid-ocean
ridges, the opening plates pull up rock
from below to fill the void, and the
release of pressure causes melt to begin
to form. The magma trickles upward,
forming small magma chambers directly
beneath the ridge. Subduction zone
volcanoes occur because of water. The
subducting plates carry water down into
the mantle with them. Much of this
water dehydrates off of the slab at a
depth of about 100 km, and rises up into
the overlying wedge of mantle rock. This
water lowers the melting point of the
rock, causing magma to form, which rises
up to form subduction zone volcanoes.
Figure 40 Lava may flow in the
form of rough chunks called aa,
or smooth coils called pahoehoe.
Formulating Hypotheses
Develop a hypothesis to account
for the difference in texture
between pahoehoe and aa.
Figure 41 Mt. Kilauea in Hawaii
erupts quietly, producing lowviscosity
lava flows.
692 Chapter 22
pahoehoe aa
Facts and Figures
Pyroclastic Flows Under certain conditions,
volcanic eruptions can be very dangerous.
One of the most deadly volcanic hazards is a
pyroclastic flow, which is a hot cloud of gas
and dust that rushes down the sides of a
volcano at speeds of up to 150 kilometers per
Quiet and Explosive Eruptions
A volcanic eruption like that of Mount St. Helens has the force
of a powerful explosion. But in other volcanic eruptions, lava
flows gently over the surface. Volcanoes erupt explosively
or quietly, depending on the characteristics of the magma.
Magma can vary in viscosity, the resistance to flow.
Magma with high viscosity is thick and resists flowing.
Magma with low viscosity is thin and flows easily.
There are three main factors that determine the viscosity
of magma: temperature, water content, and silica
content. Higher temperatures lower the viscosity of magma,
so it flows more easily. The presence of water in magma also
helps it flow more easily. Magma that is high in silica has high
viscosity. The silicon and oxygen atoms in silica are held together with
strong bonds. The silica in magma acts like glue, preventing the
magma from flowing easily.
Quiet Eruptions Volcanoes that have very hot, low-silica magma
generally erupt quietly. In a quiet eruption, lava erupts in a stream of
low-viscosity lava, called a lava flow. Lava flows from a quiet eruption
can travel for great distances. Quiet eruptions produce two different
kinds of lava, as shown in Figure 40. Hot, fast-moving lava with a
ropelike surface is called pahoehoe (pah HOH ee hoh ee). Cooler, slowmoving
lava with a chunky, crumbly appearance is called aa (AH ah).
Explosive Eruptions High-silica magma produces explosive
eruptions. Thick magma can clog a volcanic pipe, causing enormous
pressure to build up. Trapped steam inside the volcano adds to the
pressure, as was the case in the eruption of Mount St. Helens. When
the volcano finally explodes, lava and hot gases are hurled outward. The
lava solidifies very quickly and shatters into pieces of different sizes.
These particles range in size from fine dust and ash, to pebble-sized
cinders, to bombs—chunks of lava that can be the size of a small car.
hour. With temperatures of up to 700C, a
pyroclastic flow can burn everything in its
path. One of the most famous pyroclastic
flows destroyed the town of Herculaneum
during the eruption of Mt. Vesuvius in A.D. 79.

Eurasian Plate
Indo-Australian
Plate
KEY
Convergent boundary
Divergent boundary
Transform boundary
Uncertain plate boundary
Volcano
Philippine
Plate
Juan de Fuca Plate
Pacific Plate
Cocos
Plate
North American
Plate
Nazca
Plate
Caribbean
Plate
Location and Types of Volcanoes
Most volcanoes occur along plate boundaries or at hot spots in
the crust. Volcanoes often form along a converging plate boundary
where an oceanic plate is subducted into the mantle. As it sinks
through the mantle, the plate causes melting. Magma forms and rises
to the surface. As Figure 42 shows, many volcanoes have formed along
the trenches that rim the Pacific Ocean. This region is called the “Ring
of Fire.” Volcanoes also form along a diverging plate boundary where
magma rises to fill the gap between two separating plates.
Some volcanoes occur at hot spots, as shown in the Concepts in
Action pages. A hot spot is a region where hot rock extends from deep
within the mantle to the surface.
Different types of volcanic eruptions produce different types of volcanoes.
Each type is named for its shape or interior structure. The
three major types of volcano are shield volcanoes, cinder cones, and
composite volcanoes. A quiet eruption of low-viscosity lava produces
a wide, flat volcano called a shield volcano. If an eruption is entirely
ash and cinders, the result will be a small, steep-sided volcano called a
cinder cone. A volcano that forms from explosive eruptions that produce
a combination of lava and ash is called a composite volcano.
What is a shield volcano?
South American
Plate
Scotia
Plate
Eurasian Plate
Arabian
Plate
African
Plate
Antarctic Plate
Figure 42 Except for hotspot
volcanoes, most of the
world’s volcanoes form near
plate boundaries.
Relating Cause and Effect Why
are there volcanoes in the middle
of the Atlantic Ocean?
For: Activity on volcanoes
Visit: PHSchool.com
Web Code: ccc-3226
Earth’s Interior 693
Location and Types
of Volcanoes
Use Visuals
Figure 42 Point out that most
volcanoes occur at the boundaries
between plates. Mid-ocean ridges are
not marked by individual volcano
symbols on this diagram, but are places
where magma erupts at Earth’s surface,
forming oceanic crust. Ask, Which
volcanoes are formed from melting
slabs of subducting oceanic crust?
(Students should point to volcano symbols
at convergent boundaries.) Where are
volcanoes located in the United
States? (Alaska, Hawaii, the west coast)
Visual
FYI
L1
Most volcanoes are inactive during a
single person’s lifetime. However, most of
these volcanoes are considered dormant,
or sleeping, and not extinct. They will still
erupt again, but not during our lifetimes.
This is another reminder that geologic
events occur over much longer time
scales than humans normally experience.
Find links to additional activities and
have students monitor phenomena
that affect Earth and its residents.
Answer to . . .
Figure 40 The difference in texture
between pahoehoe and aa is mainly due
to the different temperatures of the two
lavas and the speed at which they flow.
Pahoehoe is hotter and faster moving
than aa. Also, escaping gas within aa
helps to produce a rough surface.
Figure 42 Many of these volcanoes
are located along the Mid-Atlantic
ridge, a divergent boundary where
magma is rising from the mantle to
form new oceanic crust.
A wide, flat volcano
produced by quiet
eruptions of low-viscosity lava
Earth’s Interior 693

Types of Volcanoes
Background
Composite and cinder cone volcanoes
form when magma is rich in silicates,
the primary source of which is melting
continental crust. These types of
volcanoes are often found above
subduction zones, where magma from
the melting wedge of mantle above the
subducted slab must travel through a
thick wedge of continental crust to
reach the surface, contact melting
occurs, and silica-rich magma becomes
incorporated in the volcanic arc.
Similarly, when hot spots are located
beneath continental crust, cinder cones
and composite volcanoes again result.
However, hot spots in the ocean melt
only silica-poor oceanic crust. In these
cases, shield volcanoes are produced.
Build Science Skills
694 Chapter 22
L2
L2
Inferring
Purpose Students will
infer the type of volcano
based on the characteristics
of the rock it produced.
Materials several samples of
pumice, obsidian, ropy basalt, and
angular rhyolite per group
Class Time 15–20 minutes
Procedure After reading the Concepts
in Action feature, have students work in
groups to study volcanic rock samples.
For each rock, ask, What type of
volcano do you think this rock most
likely came from? (Basalt is most
associated with shield volcanoes, rhyolite
with cinder cones or composite volcanoes.)
Expected Outcome Using the
information about pumice and obsidian
on p. 695, students can deduce that
these samples came from near the
surface of a lava flow. The contact with
air caused quick cooling in both cases.
Students can infer that the ropy basalt
came from the type of runny basaltic
flow associated with shield volcanoes.
Rhyolite has a higher silica content than
the basalt, and is much lighter in color
and “ashier” in appearance. Many
rhyolite samples also contain vesicles.
The gases trapped in viscous, silica-rich
magma produce the explosive eruptions
associated with composite volcanoes
and cinder cones.
Kinesthetic, Logical
Types of Volcanoes
Volcanoes have different types of magma. The viscosity
of the magma plays a large part in determining a
volcano’s ultimate shape and appearance.
Volcanoes most commonly occur at three major
locations on Earth—mid-ocean ridges,
subduction zones, and hot spots. Each type of
volcanic region produces a different kind of
volcano because of the type of magma
that exists there. The three main types of
volcano are shield volcanoes, composite
volcanoes, and cinder cones. Most
volcanoes that form above subduction
zones, for example, are composite
volcanoes. In contrast, many volcanoes that
form above hot spots are shield volcanoes.
Shapes of Volcanoes
Each type of volcano has a different shape. The
shape is determined by the kind of lava, ash, and
cinders that erupt from the volcano’s vent, and
especially the viscosity of the lava.
Shield volcano
Shield volcanoes are broad,
gently sloping volcanic mountains.
Their eruptions consist of hot,
flowing basaltic lava that travels
a long way before it solidifies.
694 Chapter 22
Facts and Figures
Yellowstone Hot Spot A special case of hot
spot volcanism occurs when the hot spot is
beneath a continent. One such hot spot is
located in Yellowstone National Park. The
continent acts like a thick lid, and it is hard for
the hot spot to break through. When this does
happen, a lot of continental rock gets melted as
well, giving the magma a composition rich in
silica. The result can be very explosive eruptions.
Composite volcano
Composite volcanoes are tall with
steep sides that are built up from
layers of viscous lava, ash, and
volcanic bombs. They often have
secondary vents.
Cinder cone
Cinder cones are small with
steep sides. They form from
ash and cinders that are
ejected into the air and fall
back onto the volcano.
Gigantic eruptions at Yellowstone 2.1, 1.35, and
0.64 million years ago blanketed the western
half of North America with ash. Fortunately,
these giant explosions are very rare, and most
hot spot activity at Yellowstone consists of hot
springs of water. However, Yellowstone is still
geologically active and will likely have another
large explosion in the future.

Volcanic Rocks
The silica and water content of magma
help to determine whether an eruption is
quiet or explosive and how viscous the
lava is when hot. When the lava cools,
different types of rock are formed.
Pumice
Pumice is a light, spongelike
rock usually found at
the surface of a lava flow.
It consists of a mass of gas
bubbles frozen in fragile
volcanic glass and minerals.
Hot-spot Volcanoes
A hot spot is a rising column of hot
rock that extends from deep within
the mantle to the surface. Hot spots
often exist in the middle of plates.
Because the hot spot stays in one
place while the plate moves over it,
a chain of volcanoes is formed over
time.
A very ancient hot spot under
the Pacific plate has, over millions of
years, produced the Hawaiian
Islands.
Kilauea erupting
A shield volcano,
Kilauea is the
youngest volcano
on the island of
Hawaii and one
of the most active
in the world.
Video Field Trip
Mountains of Fire
Extinct volcanoes form
chains of islands
Plate carries active
volcanoes away and they
become extinct as new
island begins to form.
After students have viewed the Video Field Trip,
ask the following questions: Where do the most
violent volcanic eruptions typically occur? (Near
Going Further
Lava bombs
Lava bombs form from a glob
of magma that is blown explosively
into the air. Bombs are
usually at least 10 cm in diameter,
and can be much larger.
Vent
Rising
magma
Obsidian
Obsidian is a dense,
volcanic glass formed
from lava that has
cooled too quickly for
minerals to crystallize.
Volcano erupts
over stationary
hot spot and
creates an island.
■ Use library or Internet resources to research a
volcano located on another planet or moon.
Write a one-page report
describing the volcano that
you select. Include an
explanation of how this
volcano compares with
volcanoes on Earth.
■ Take a Discovery Channel
Video Field Trip by watching
“Mountains of Fire.”
Video Field Trip
Earth’s Interior 695
subduction zones, where Earth’s tectonic plates are
pushed together and one plate slips under another.)
What is a composite volcano? (A volcano with
alternating layers of different materials.) What is a
shield volcano? (A wide, flat volcano that produces
quiet eruptions of low-viscosity lava.) Name a third
major kind of volcano. (Cinder cone volcanoes.)
Going Further
L3
Volcanoes occur on several planets and
moons in the solar system besides Earth,
including Venus, Mars, Io, and possibly
Triton. In addition, there is evidence of
ancient volcanic activity on Mercury and
Earth’s moon. Some of the most wellknown
volcanoes include Olympus
Mons, Arsia Mons, Pavonis Mons,
Ascraeus Mons, and Elysium Mons on
Mars; Theia Mons, Sif Mons, and Maat
Mons on Venus; and Pele, Ra, Loki, and
Prometheus on Io. Have interested
students research volcanoes on other
planets or moons and present photos to
the class.
Visual
Earth’s Interior 695

Section 22.6 (continued)
Other Igneous
Features
Many students have the misconception
that the only way igneous rocks are
formed is when molten lava flows onto
Earth’s surface. After students read Other
Igneous Features, have them work in
groups to create a visual describing five
ways igneous rocks can form. (In lava
flows at the surface, dikes, sills, batholiths,
and volcanic necks)
Visual, Group
ASSESS
3
Evaluate
Understanding
Using Figure 41, ask students what type
of volcano most likely produced this
type of lava. (Shield volcano)
Reteach
Use Figure 39 to explain the main
features of a volcano.
Students have many eruptions to choose
from, including Mt. Mazama (Crater
Lake), Oregon, around 5000 B.C.; Mt.
Vesuvius, Italy, in A.D. 79; Mt. Fuji, Japan,
in 1707; Tambora, Indonesia, in 1815;
Krakatoa, Indonesia, in 1883; Mt. Pelée,
Martinique, in 1902; Mt. Katmai, Alaska,
in 1912; Mt. St. Helens in 1980;
Mt. Pinatubo, Philippines, in 1991;
and Montserrat in 1995.
If your class subscribes to
the Interactive textbook, use it to review
key concepts in Section 22.6.
Section 22.6 Assessment
L2
L2
L1
1. Magma is formed from melted rock within
the mantle. The magma is under pressure and
less dense than the solid rock around it. The
magma is forced upward through the crust,
erupting at the surface.
2. Magma with a low silica content results in
gentle eruptions. Magma with a high silica
content can lead to explosive eruptions.
3. Volcanoes form above converging plate
boundaries (where a plate is subducted into
the mantle) and at diverging boundaries
696 Chapter 22
Figure 43 Ship Rock in New
Mexico is a volcanic neck. It
formed when the soft rock
around a volcano’s pipe wore
away, revealing hard, igneous
rock. The long ridge extending
from the volcanic neck is a dike.
Section 22.6 Assessment
Reviewing Concepts
1. Describe the process that leads up to the
eruption of a volcano.
2. How does the silica content of magma
affect how explosive an eruption is?
3. Explain why volcanoes form in certain
regions of Earth’s surface.
4. Describe four types of intrusive igneous
rock features.
Critical Thinking
5. Relating Cause and Effect What condition
must exist in a volcano for a caldera to form?
696 Chapter 22
(where magma rises to fill the gap left by two
separating plates). Hot spot volcanoes form
from columns of hot rock that have risen up
through the mantle to the surface.
4. Batholiths are large rock masses that form
the core of many mountain ranges. A sill is
hardened lava squeezed into a crack that is
parallel to existing rock layers. A dike is
hardened lava in a crack that cuts across
existing rock layers. A volcanic neck is an
igneous structure formed from lava that
hardened in a volcano’s pipe.
Other Igneous Features
Sometimes magma does not reach the surface, but
cools and hardens in the crust. This magma forms
intrusive igneous rock that may eventually be forced
upward and exposed at Earth’s surface. Igneous
features formed by magma include batholiths,
sills, dikes, and volcanic necks. Lava plateaus are
features formed of extrusive igneous rock.
A batholith is the largest type of intrusive
igneous rock mass. Batholiths often form the core
of a mountain range, such as the Sierra Nevada
range in California. Magma sometimes squeezes into a crack between
layers of rock and then hardens. If the crack is parallel to existing rock
layers, the magma hardens into a structure called a sill. If the crack
cuts across rock layers, the hardened magma forms a dike. When
magma hardens in a volcano’s pipe, a structure called a volcanic neck
may form, as shown in Figure 43.
The largest lava flows don’t come from individual volcanoes.
Rather, large amounts of easily-flowing lava sometimes erupt from a
cluster of long, thin cracks in the crust. This lava may spread out over
an enormous area before solidifying. After many years, layers of hardened
lava may form a high, level area called a lava plateau. An example
is the Columbia Plateau, which has an average thickness of more than
1 kilometer and covers an area of nearly 200,000 square kilometers in
the Pacific Northwest.
6. Predicting What would eventually happen,
as a result of plate movement, to a volcano
that formed over a hot spot?
7. Comparing and Contrasting What is the
difference between a dike and a sill?
Descriptive Paragraph Use library or
Internet resources to research a major volcanic
eruption. Write a paragraph about the
eruption including when and where it took
place and the type of volcano involved.
5. The magma chamber and main vent must
be empty after an eruption, creating an empty
shell that can collapse inward.
6. As the plate moves, the volcano would be
carried away from the hot spot, and the
volcano would no longer be active.
7. Both are structures formed from magma that
hardens in cracks in layers of rock. If the crack is
parallel to existing rock layers, the structure is
called a sill. If the crack cuts across rock layers,
the structure is called a dike.