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Name
CHAPTER 13
Class
Date
Volcanoes
2 Volcanic Eruptions
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• How does the composition of magma affect volcanic
eruptions and lava flow?
•
•
•
•
What are the five major types of pyroclastic material?
What are the three main types of volcanic cones?
How does a caldera form?
What are three events that may signal a volcanic
eruption?
What Are the Two Types of Magma?
When magma cools, it hardens into volcanic rock.
Scientists study the composition of volcanic rocks to
understand the material in Earth’s crust and mantle.
Scientists classify magma based on its composition.
There are two main types of magma: mafic and felsic.
Type of
Magma
Composition
Color
Where It Is
Commonly Found
Mafic
high in magnesium
and iron
generally dark
oceanic crust
Felsic
high in silicate minerals
generally light
continental crust
Felsic magma is also more viscous than mafic magma.
A viscous fluid is thick, so it does not flow easily. Magma’s
viscosity affects what happens when a volcano erupts.
What Are the Two Main Types of Eruptions?
READING TOOLBOX
Summarize When you
finish reading this section,
create a concept map using
the following terms: mafic
lava, volcanic ash, lapilli,
pahoehoe, felsic lava, aa,
volcanic bomb, pyroclastic
material, shield volcano,
volcanic dust, and volcanic
block.
Critical Thinking
1. Contrast How could you
tell the difference between a
mafic rock and a felsic rock
by looking at them?
Volcanic eruptions can be quiet or explosive. Mafic
lava usually causes quiet eruptions. Felsic lava usually
causes explosive eruptions.
QUIET ERUPTIONS
Most eruptions on the ocean floor form from mafic
magma. Therefore, eruptions on the ocean floor are
generally quiet. During a quiet eruption, magma flows
down the sides of the volcano like a river. Geologists
classify the magma from quiet eruptions by the shapes it
forms when it cools. The table on the next page shows a
few of these shapes.
Critical Thinking
2. Explain What is the most
likely reason that felsic
magma usually causes
explosive eruptions?
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Volcanic Eruptions continued
LOOKING CLOSER
3. Describe What does
pahoehoe look like when it
cools?
Type of Lava
What It Forms From
What Happens to It When It Cools
Pahoehoe
hot, fluid, mafic lava
It has a smooth, ropelike texture.
Aa
the same material as
pahoehoe, but with
more gas in it
The crust breaks into sharp pieces.
Blocky lava
mafic lava that is more It breaks into large chunks.
viscous than the lava
that forms pahoehoe
EXPLOSIVE ERUPTIONS
READING CHECK
4. Identify Which type of
lava is pyroclastic material
usually made from?
Explosive eruptions are much less common than quiet
eruptions. However, explosive eruptions can be much
more destructive. During an explosive eruption, hot ash,
gas, and rocks shoot out of the volcano.
The lavas that produce explosive eruptions are
generally felsic lavas. Felsic lavas often contain a lot of
trapped gases, such as water vapor and carbon dioxide.
The gas in the lava creates pressure inside the volcano,
like the bubbles in a can of soda. The pressure can make
the volcano erupt explosively. In an explosive eruption,
chunks of lava fly through the air. These chunks are
called pyroclastic material.
How Do Scientists Classify Pyroclastic
Material?
Some kinds of pyroclastic material form when solid
lava breaks apart and is thrown into the air. Other
kinds of pyroclastic material form when liquid lava flies
through the air and hardens. Scientists classify pyroclastic material based on its size. The table below shows the
five main types of pyroclastic material.
Type of Material
Description
Volcanic dust
Volcanic dust is made of pieces of solid rock that are
less than 0.25 mm in diameter.
Volcanic ash
Volcanic ash is made of pieces of solid rock that are
less than 2 mm in diameter. Volcanic dust is a type of
volcanic ash.
Lapilli
Lapilli are pieces of solid rock that are between 2 mm
and 64 mm in diameter. They generally fall to the
ground near the vent.
Volcanic blocks
Volcanic blocks are pieces of solid rock that are more
than 64 mm in diameter. They are the largest type of
pyroclastic material.
Volcanic bombs
Volcanic bombs form when large chunks of lava fly out
of a volcano and cool in the air. They can have many
different shapes.
Talk About It
Apply Concepts Use a
meterstick to identify how
large the four types of
pyroclastic material are. As a
class, discuss what each of
the materials would look like.
Find objects in the classroom
that are as big as one of the
types of material.
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What Are the Three Main Types of Volcanoes?
Quiet and explosive eruptions produce different types
of volcanoes. During quiet eruptions, lava flows out of
the vent to create a volcanic cone. During explosive
eruptions, pyroclastic material piles up around the
vent. There are three main types of volcanoes: shield
volcanoes, cinder cones, and composite volcanoes.
SHIELD VOLCANOES
Shield volcanoes form from quiet eruptions of hot,
mafic lava. The lava spreads out over a wide area, and
layers of lava build up. Therefore, a shield volcano
has a wide base, and its sides have a very gentle slope.
Although shield volcanoes have a gentle slope, they can
still be very large. The volcanoes in the Hawaiian Islands
are shield volcanoes.
Lava flow
READING CHECK
5. Identify What are
the three main kinds of
volcanoes?
Critical Thinking
6. Describe From where do
you think the shield volcano
got its name?
Quiet eruptions of thin, fluid lava form shield volcanoes.
CINDER CONES
Cinder cones form from small, explosive eruptions.
They have very steep slopes, and they are made of
pyroclastic material. Cinder cones are generally very
small. Parícutin volcano in Mexico is an example of a
cinder cone volcano.
Pyroclastic material
LOOKING CLOSER
7. Explain Which type of
volcanic eruption produces
cinder cone volcanoes?
Pyroclastic material that builds up during explosive eruptions can form a cinder cone.
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COMPOSITE VOLCANOES
Composite volcanoes form when a volcano has both
explosive eruptions and quiet eruptions. During a quiet
eruption, lava flows down the sides of the cone. When an
explosive eruption occurs, pyroclastic material falls all
around the vent. Therefore, composite volcanoes contain
layers of hardened lava and layers of pyroclastic material.
Composite volcanoes generally have a gentle slope
near the base, but a steeper slope near the vent. Mount
St. Helens is an example of a composite volcano.
Pyroclastic material
Lava flow
LOOKING CLOSER
8. Identify What two
materials are composite
volcanoes made of?
Composite volcanoes contain both lava flows and pyroclastic material.
How Are Calderas and Craters Different?
Many explosive volcanoes have circular, funnel-shaped
pits around their vents. These pits are called craters.
They form when pyroclastic material builds up around
the vent. As wind and water wear away the rock around
the edge of the crater, the crater becomes larger.
Some volcanoes contain large, deep depressions called
calderas. A caldera is different from a crater. When the
magma chamber beneath a volcano empties, the volcano
may collapse. A deep pit forms where the volcanic cone
was. This pit is the caldera.
Talk About It
FORMATION OF A CALDERA
Describe Processes Talk
with a partner about the
process shown in the figures.
Together, discuss what is
shown in each part of the
figure.
Ground surface sinks.
Caldera
Magma
Chamber
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Volcanic Eruptions continued
Can Scientists Predict Volcanic Eruptions?
Scientists cannot always predict where or when a
volcano will erupt. However, by studying volcanoes,
scientists have been able to identify some clues about
when an eruption may happen.
One way scientists predict volcanic eruptions is by
studying the earthquakes that happen near a volcano.
Just before an eruption, the earthquake activity around a
volcano may change. This can happen for any of the
following reasons:
• Magma moving upward puts increasing pressure on the
surrounding rock.
• The temperature inside the rocks can change.
• Rocks near the volcano can break as magma moves
through them.
Scientists also study the gases that rise from the
volcano. Just before an eruption, volcanoes often give off
more gas. The composition of the gas may also change
before an eruption.
These scientists are taking
samples of gases
that are coming
from a volcano.
Scientists also study the shape and size of a volcano
to figure out whether it may erupt. Before an eruption,
rising magma can cause the volcano’s sides to bulge. The
movement of magma can also cause the ground to tilt
slightly. Scientists can use special instruments to measure
these changes.
Scientists need to identify patterns in earthquake activity, released gases, and land tilt to be able to make these
predictions. Unfortunately, only a few active volcanoes
have been studied for long enough for these methods to
work. In addition, volcanoes that have been inactive for
years may erupt without warning. Therefore, scientists
generally cannot accurately predict volcanic eruptions.
LOOKING CLOSER
9. Infer Why are these
scientists probably wearing
protective clothing?
READING CHECK
10. Explain Why might the
volcano’s sides bulge before
an eruption?
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Section 2 Review
SECTION VOCABULARY
caldera a large, circular depression that forms
when the magma chamber below a volcano
partially empties and causes the ground above
to sink
felsic describes magma or igneous rock that is
rich in feldspars and silica and that is generally
light in color
mafic describes magma or igneous rock that
is rich in magnesium and iron and that is
generally dark in color
pyroclastic material fragments of rock that form
during a volcanic eruption
1. Explain Why does felsic lava produce explosive eruptions more often than mafic
lava does?
2. Compare Give one similarity and one difference between lapilli and volcanic
blocks.
3. Describe How do calderas form?
4. Infer Which would be more likely to increase the steepness of a volcanic cone—a
quiet eruption or an explosive eruption? Explain your answer.
5. Explain Scientists today are studying many active volcanoes around the world.
They are even studying volcanoes that have not erupted for hundreds or
thousands of years. What is the most likely reason they are studying these
volcanoes?
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