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Earthquakes and Volcanoes
section ●
Before You Read
What do you think of when you hear the word volcano? On
the lines below, write words that describe volcanoes.
Read to Learn
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
How do volcanoes form?
Inside Earth is a layer of hot, liquid rock material called
magma. Most of the time magma remains deep inside
Earth. Sometimes, however, it is forced to the surface. Rising
magma can lead to an eruption—an event where magma,
solids, and gas spew onto Earth’s surface. A volcano is a
cone-shaped hill or mountain formed when hot magma,
solids, and gas erupt onto Earth’s surface.
When magma flows to Earth’s surface, it is called lava.
Volcanoes have circular holes called craters. Lava can flow
gently or explode violently out of the crater. Some violent
eruptions throw lava and rock thousands of meters into
the air.
What You’ll Learn
how volcanoes form
how volcanoes affect
■ about three types of
■ ways that volcanoes
add material to Earth’s
Study Coach
Flash Cards As you read the
sections, write important words
on flash cards. On the back of
each flash card, write an
explanation in your own words.
Use the flash cards to review this
What occurs when plates collide?
Earth’s upper mantle and crust are made up of large
plates that move around on a layer of magma. When these
plates collide, volcanoes can form. When two plates collide,
the denser plate sinks below the less dense plate. As the
denser plate sinks, it melts and forms chambers of magma.
This magma is the source of volcanic eruptions that formed
the Caribbean Islands.
Reading Essentials
Picture This
Label where magma and
lava in the figure would be
Forms of Volcanoes
The figure above shows a view inside a volcano forming
where a denser plate sinks under a less dense plate. The
magma is rising toward Earth’s surface. Volcanoes can cause
great destruction. But they also can add new material to
Earth’s surface. The way volcanoes add this new material to
Earth’s surface varies greatly. Different types of eruptions
produce different types of volcanoes.
All volcanoes are a result of magma rising to Earth’s
surface. But some volcanic eruptions are violent, while
others are quiet. Different factors affect volcanic eruptions.
The composition of the magma and the amount of pressure
in the magma determine how a volcano erupts.
How does magma composition
affect eruptions?
Lava contains many different compounds, including silica.
Silica is made of silicon and oxygen. Lava that contains
more silica tends to be thicker and does not flow easily. Lava
containing more iron and magnesium and less silica tends
to flow more easily.
Describe What is lava
like that contains more
Earthquakes and Volcanoes
How does magma pressure affect eruptions?
The amount of pressure built up in the magma also affects
the type of eruption. When you shake a bottle of carbonated
soft drink before opening it, the pressure from the gas builds
up and is released suddenly when the container is opened.
In the same way, steam builds pressure in magma. Lava
containing more silica is thicker, so it tends to trap more
water vapor and other gases. This creates tremendous pressure.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
What determines how a volcano erupts?
How does water vapor affect eruptions?
Water can be carried from the surface of Earth to the
mantle when one plate sinks beneath another. The magma
produced when plates melt tends to contain more silica than
the magma produced deep inside Earth. This silica traps
more water, which turns to steam. Steam produces
tremendous pressure in thick, silica-rich magma. When
enough pressure builds up, an eruption occurs.
How are shield volcanoes formed?
Basaltic lava contains more iron and magnesium than
silica, so it flows in broad, flat layers. The buildup of
basaltic layers forms a large, broad volcano with gently
sloping sides called a shield volcano. A shield volcano is
shown on the left in the figure below. They are the largest
type of volcano. Shield volcanoes occur where plates are
separating, or in other places where magma can be forced
up from deep inside Earth.
Picture This
Explain How does lava
flow from the shield
Sea level
0.3 km
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
9 km
50 km
Shield volcano
0.5 km
Cinder cone volcano
How do cinder cone volcanoes form?
Gases build up in magma as it rises to Earth’s surface.
When the gas builds up enough pressure, the volcano
erupts. The eruption throws ash, cinders, and lava into the
air. The lava cools quickly and particles of solid lava, ash,
and cinders fall to the surface. These particles of solid lava,
ash, and cinders, called tephra, form a small cone of
volcanic material. A cinder cone volcano is a relatively small
volcano formed by moderate to explosive eruptions of
tephra. This type of volcano is shown on the right in the
figure above. Cinder cone volcanoes erupt violently because
the eruption is powered by the high gas content. However,
the eruptions usually do not last very long. After the gas is
released, the force behind the eruption is gone.
B Organize Information
Construct a four-tab Foldable as
shown to record information
about volcanoes.
Cinder cone
Reading Essentials
What is a composite volcano?
Composite volcanoes are steep-sided volcanoes formed by
alternating layers of tephra and lava. This volcano is shown
in the figure below. Sometimes composite volcanoes erupt
violently as in a cinder cone volcano and spew out tephra.
At other times, lava flows out gently as in a shield volcano.
Composite volcanoes form where one plate sinks under
another plate. They are intermediate in size and shape
compared to shield volcanoes and cinder cone volcanoes.
Composite volcano
Infer Why do composite
volcanoes have steep sides?
3 km
6 km
Magma that is highly fluid can ooze through cracks or
fissures in Earth’s surface. This type of magma is usually linked
to fissure eruptions. The lava flows freely across the land to
form flood basalts. When flood basalts are exposed to erosion
over time, they can become large, flat landforms called lava
plateaus. The Columbia River Plateau in the northwestern
United States was formed millions of years ago. Several
fissures erupted and lava flows built up layer by layer. The
map below shows the location of the Columbia River Basalts.
Picture This
Use Models Outline the
Columbia River Basalts on
the map.
Columbia River
Earthquakes and Volcanoes
100 km
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
What is a fissure eruption?
After You Read
Mini Glossary
cinder cone volcano: relatively small volcano formed by
moderate to explosive eruptions of tephra
composite volcano: steep-sided volcano formed with
alternating layers of violent eruptions of tephra and
quieter eruptions of lava
lava: molten rock flowing onto Earth’s surface
shield volcano: large, broad volcano with gently sloping
sides that is formed by the buildup of basaltic layers
volcano: cone-shaped hill or mountain formed when hot
magma, solids, and gases erupt onto Earth’s surface
through a vent
1. Review the terms and their definitions in the Mini Glossary. Write two to three sentences
using at least two terms to describe different volcanic eruptions.
2. Compare shield volcanoes to cinder cone volcanoes, using this Venn diagram. In the
middle, write things that the two types of volcanoes have in common. On each side,
write things that are different.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Cinder cone volcanoes
Shield volcanoes
3. In this section you made flash cards. How did the flash cards help you to learn
about volcanoes?
Visit to access your textbook, interactive
games, and projects to help you learn more about volcanoes.
End of
Reading Essentials