1. science
2. geology
3. volcanology
4. volcanic eruptions

Name
CHAPTER 6
Class
Date
Volcanoes
1 Volcanic Eruptions
SECTION
National Science
Education Standards
BEFORE YOU READ
After you read this section, you should be able to answer
these questions:
ES 1c
• What are two kinds of volcanic eruptions?
• How does the composition of magma affect eruptions?
• What are two ways that magma can erupt from a
volcano?
What Is a Volcano?
When you think of a volcano, what comes into your
mind? Most people think of a steep mountain with smoke
coming out. In fact, a volcano is any place where gases and
magma, or melted rock, come out of the ground. A volcano
can be a tall mountain or a small hole in the ground.
STUDY TIP
Compare After you read this
section, make a chart that
describes the features of
each kind of lava and
pyroclastic material.
THE PARTS OF A VOLCANO
If you could look inside an erupting volcano, it would
look similar to the figure below. Below the volcano is a
body of magma called a magma chamber. The magma
from the magma chamber rises to the surface and erupts
at the volcano. Magma escapes from the volcano through
openings in the Earth’s crust called vents. When magma
flows onto the Earth’s surface, it is called lava.
READING CHECK
1. Define What is a magma
chamber?
Lava runs out of the
vents and down the side
of the volcano. The rivers
of lava are called lava
flows. As they cool and
harden, they make the
volcano bigger.
When the magma reaches
the surface, it erupts out
of vents.
When the magma chamber
is full, magma rises through
the crust and erupts out of
the volcano.
TAKE A LOOK
2. Describe What makes
volcanoes grow larger?
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Textbook
99
Volcanoes
Name
SECTION 1
Class
Date
Volcanic Eruptions continued
What Happens When Volcanoes Erupt?
Many people think that all volcanic eruptions are alike.
However, this is not the case. Scientists put volcanic
eruptions into two groups: nonexplosive eruptions and
explosive eruptions.
NONEXPLOSIVE ERUPTIONS
READING CHECK
3. Identify What is the most
common type of volcanic
eruption?
Critical Thinking
4. Compare How are
nonexplosive eruptions
different from explosive
eruptions? Give two ways.
Nonexplosive volcanic eruptions are the most common
type of eruption. These eruptions produce fairly calm
flows of lava. The lava flows over the Earth’s surface.
Nonexplosive eruptions do not produce very much ash
or dust, but they can release huge amounts of lava.
For example, most of the rock of the ocean floor was
produced by nonexplosive eruptions.
EXPLOSIVE ERUPTIONS
Explosive eruptions are much less common than
nonexplosive eruptions. However, explosive eruptions can
be more destructive than nonexplosive eruptions. During
an explosive eruption, clouds of hot ash, gas, and rock
fragments shoot rapidly out of a volcano.
Most explosive eruptions do not produce lava flows.
Instead of flowing calmly over the Earth’s surface,
magma sprays into the air in tiny droplets. The droplets
harden to form particles called ash. The ash from an
explosive eruption can reach the upper parts of the Earth’s
atmosphere. It can stay there for years, blocking sunlight
and causing the climate to get cooler.
An explosive eruption can blast millions of tons of material
from a volcano. The explosive eruption of Mount St. Helens
in 1980 caused an entire side of a mountain to collapse. The
blast from the eruption flattened 600 km2 of forest.
The eruption of
Mount St. Helens in
1980 was an explosive eruption. It was
very destructive.
Say It
Investigate Find out more
information about the eruptions of Mount St. Helens.
Share your findings with a
small group.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Textbook
100
Volcanoes
Name
SECTION 1
Class
Date
Volcanic Eruptions continued
Why Do Volcanoes Erupt?
By comparing magma from different eruptions,
scientists have been able to figure out why volcanoes
erupt in different ways. The main factor affecting an
eruption is the composition of the magma. The amounts of
water, silica, and gas in the magma determine the type of
eruption.
WATER CONTENT
If magma contains a lot of water, an explosive eruption
is more likely. Beneath the surface, magma is under high
pressure. The high pressure allows water to dissolve
into the magma. If the magma rises quickly, the pressure
suddenly decreases and the water turns to bubbles of gas.
As the gases expand, they cause an explosion.
This is similar to what happens when you shake a
can of soda and open it. When you shake the can, the
gas dissolved in the soda forms bubbles. Pressure builds
up inside the can. When you open the can, the pressure
causes the soda to shoot out.
READING CHECK
5. Identify What is the main
factor that determines how a
volcano erupts?
SILICA AND GAS CONTENT
The amount of silica in magma also affects how
explosive an eruption is. Silica is a compound made of
the elements silicon and oxygen. Magma that contains a
lot of silica is very thick and stiff. It flows slowly and may
harden inside a volcano’s vents, blocking them. As more
magma pushes up from below, the pressure increases. If
enough pressure builds up, the volcano can explode.
Silica-rich magma may be so stiff that water vapor
and other gases cannot move out of the magma. Trapped
bubbles of gas may expand until they explode. When
they explode, the magma shatters and ash is blasted from
the vent. Magma with less silica is thinner and runnier.
Therefore, gases can move out of the magma easily, and
explosive eruptions are less likely.
Material
READING CHECK
6. Describe How can
magma that contains a lot
of silica cause an explosive
eruption?
How it affects eruptions
TAKE A LOOK
Water
7. Identify Relationships Fill
in the blank spaces in the
table.
Silica
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Textbook
101
Volcanoes
Name
SECTION 1
Class
Date
Volcanic Eruptions continued
How Can Magma Erupt from a Volcano?
READING CHECK
8. Define What is pyroclastic
material?
There are two main ways that magma can erupt from
a volcano: as lava or as pyroclastic material. Pyroclastic
material is hardened magma that is blasted into the air.
Nonexplosive eruptions produce mostly lava. Explosive
eruptions produce mostly pyroclastic material.
Most eruptions produce either lava or pyroclastic
material, but not both. However, a single volcano may
erupt many times. It may produce lava during some
eruptions and pyroclastic material during others.
TYPES OF LAVA
Geologists classify lava by the shapes it forms when it
cools. Some kinds of lava form smooth surfaces. Others
form sharp, jagged edges as they cool. The figure below
shows four kinds of lava flows.
TAKE A LOOK
9. Compare How are aa and
blocky lava similar?
Aa is lava that
forms a thick, brittle crust as it cools.
The crust is torn
into sharp pieces
as lava moves
underneath it.
Pahoehoe is lava
that forms a thin,
flexible crust as it
cools. The crust
wrinkles as the
lava moves
underneath it.
Blocky lava is cool,
stiff lava that does
not travel very far
from the volcano.
Blocky lava usually oozes from a
volcano and forms
piles of rocks with
sharp edges.
Pillow lava is lava
that erupts under
water. As it cools,
it forms rounded
lumps that look
like pillows.
TYPES OF PYROCLASTIC MATERIAL
Pyroclastic material forms when magma explodes from
a volcano. The magma solidifies in the air. Pyroclastic
material also forms when powerful eruptions shatter
existing rock.
Geologists classify pyroclastic material by the size of
its pieces. Pieces of pyroclastic material can be the size of
houses or as small as dust particles. The figure on the top
of the next page shows four kinds of pyroclastic materials.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Textbook
102
Volcanoes
Name
SECTION 1
Class
Date
Volcanic Eruptions continued
Volcanic bombs are large blobs of lava that
harden in the air.
Lapilli are small bits of lava that harden
before they hit the ground. Lapilli are
usually about the size of pebbles.”
Volcanic ash forms when gases trapped in magma or lava
form bubbles. When the bubbles explode, they create millions of tiny pieces.
Volcanic blocks are large
pieces of solid rock that come
out of a volcano.
TAKE A LOOK
10. Describe How do lapilli
form?
PYROCLASTIC FLOWS
A pyroclastic flow is a dangerous type of volcanic
flow. Pyroclastic flows form when ash and dust race
down the side of a volcano like a river. Pyroclastic flows
are very dangerous. They can be as hot as 700ºC and can
move at 200 km/h. A pyroclastic flow can bury or destroy
everything in its path. A pyroclastic flow from the eruption
of Mount Pinatubo is shown in the figure below.
Math Focus
11. Convert How fast can
pyroclastic flows move? Give
your answer in miles per
hour.
1 km = 0.62 mi
This pyroclastic flow formed during the 1991 eruption of Mount Pinatubo,
in the Philippines.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Textbook
103
Volcanoes
Name
Class
Section 1 Review
Date
NSES
ES 1c
SECTION VOCABULARY
magma chamber the body of molten rock that
feeds a volcano
vent an opening at the surface of the Earth
through which volcanic material passes
volcano a vent or fissure in the Earth’s surface
through which magma and gases are expelled
1. Compare How is lava different from magma?
2. Identify What are the two kinds of volcanic eruptions?
3. Explain How does the amount of water in magma affect how a volcano erupts?
4. Explain Why is magma that contains little silica less likely to erupt explosively?
5. Compare How is pahoehoe lava different from pillow lava? How are they similar?
6. Describe How do volcanic bombs form?
7. Describe How does volcanic ash form?
8. Define What is a pyroclastic flow?
9. Infer Do pyroclastic flows form during explosive or nonexplosive eruptions?
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Textbook
104
Volcanoes
F
Inside the Restless Earth Answer Key continued
5. Magnitude is the strength of an earthquake,
4. Explosive eruptions are less common.
but intensity is how much damage it causes.
A moderate earthquake can cause a lot of
damage if it happens in a place with weak
soil or non-earthquake-resistant buildings.
5.
6.
SECTION 3 EARTHQUAKES AND SOCIETY
7.
1. Answers will vary.
2. about 50 times more
3. If an active fault hasn’t had a strong earthquake
4.
5.
6.
7.
8.
9.
10.
in a long time, pressure is building up. More
pressure leads to stronger earthquakes.
height, mass, composition, location
A mass damper is placed at the top of a building. An active tendon system is in its basement.
You will know what to do in an earthquake.
Possible answers: doorway, under a desk or
table
any nonperishable food
Possible answers: under the desk, in the
doorway
a parent, teacher, police officer, or firefighter
8.
9.
10.
11.
Review
1. Lava is magma that flows on the Earth’s
2.
3.
4.
Review
1. Earthquakes are more likely to occur in seis-
5.
mic gaps than in other places.
2.
Explosive eruptions produce ash and dust
instead of lava.
magma composition
It can block the volcano’s vents, causing
pressure to build up.
Water: More water makes explosive
eruptions more likely.
Silica: More silica makes explosive
eruptions more likely.
hardened magma that is blasted into the air
They both have sharp edges.
Small pieces of lava harden before they hit
the ground.
about 120 mi/h
If you are...
Then you should...
...inside a
building
...crouch or lie face down under a desk or
table, away from glass or heavy objects;
cover your head with your hands
...outside
...lie face down on the ground with your
hands on your head, far from power
lines or fire hazards
8.
...in a car
or bus
...ask the driver to pull over and stop;
stay inside the car or bus until the
earthquake is over
9.
6.
7.
3. They look at the damage to other buildings
surface.
explosive and nonexplosive
Magma with a lot of water is likely to erupt
explosively.
It is thinner and runnier, so gases can escape
more easily.
Both are lava flows with smooth surfaces.
Pillow lava forms underwater. Pahoehoe
forms on land.
Large blobs of magma harden in the air.
Pressure builds up inside bubbles in magma.
When the pressure gets high enough, the bubbles shatter, producing tiny fragments of rock.
a fast-moving “river” of hot ash and dust
from a volcano
explosive
SECTION 2 EFFECTS OF VOLCANIC
ERUPTIONS
from earthquakes.
4. Small earthquakes occur much more often
than large earthquakes.
5. Possible answers: The car is on fire; the car is
in a dangerous location (e.g., railroad tracks).
1. Ash and dust in the atmosphere block
2.
3.
4.
5.
6.
7.
Chapter 6 Volcanoes
SECTION 1 VOLCANIC ERUPTIONS
1. a body of magma below a volcano
2. hardened lava flows
3. nonexplosive
sunlight, preventing it from reaching
Earth’s surface.
Layers of lava from nonexplosive eruptions
build up into a mountain.
pyroclastic material
explosive eruptions
They are made of layers of different materials.
ash and lava
a funnel-shaped pit around the central vent
of a volcano
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Textbook Answer Key
42
Inside the Restless Earth