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 section. Study Coach Earthquakes and Volcanoes section ● Volcanoes 2 What You’ll Learn ■ how volcanoes form ■ how volcanoes affect humans ■ about three types of volcanoes ■ ways that volcanoes add material to Earth’s surface Read to Learn Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. Before You Read What do you think of when you hear the word volcano? On the lines below, write words that describe volcanoes. 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 v olcano 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 la v a . 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 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. chapter 3 8 Reading Essentials 109
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Flash Cards As you read thesections, write important wordson flash cards. On the back ofeach flash card, write anexplanation in your own words.Use the flash cards to review thissection.
Study Coach
Earthquakes and Volcanoes
section ● Volcanoes2
What You’ll Learn■ how volcanoes form■ how volcanoes affect
humans■ about three types of
volcanoes■ ways that volcanoes
add material to Earth’ssurface
Read to Learn
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Before You ReadWhat do you think of when you hear the word volcano? Onthe lines below, write words that describe volcanoes.
How do volcanoes form?Inside Earth is a layer of hot, liquid rock material called
magma. Most of the time magma remains deep insideEarth. Sometimes, however, it is forced to the surface. Risingmagma can lead to an eruption—an event where magma,solids, and gas spew onto Earth’s surface. A volcano is acone-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 flowgently or explode violently out of the crater. Some violenteruptions throw lava and rock thousands of meters intothe air.
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 theseplates collide, volcanoes can form. When two plates collide,the denser plate sinks below the less dense plate. As thedenser plate sinks, it melts and forms chambers of magma.This magma is the source of volcanic eruptions that formedthe Caribbean Islands.
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Reading Essentials 109
Picture This1. Label where magma and
lava in the figure would befound.
2. Describe What is lavalike that contains moresilica?
Forms of VolcanoesThe figure above shows a view inside a volcano forming
where a denser plate sinks under a less dense plate. Themagma is rising toward Earth’s surface. Volcanoes can causegreat destruction. But they also can add new material toEarth’s surface. The way volcanoes add this new material toEarth’s surface varies greatly. Different types of eruptionsproduce different types of volcanoes.
What determines how a volcano erupts?All volcanoes are a result of magma rising to Earth’s
surface. But some volcanic eruptions are violent, whileothers are quiet. Different factors affect volcanic eruptions.The composition of the magma and the amount of pressurein the magma determine how a volcano erupts.
How does magma compositionaffect eruptions?
Lava contains many different compounds, including silica.Silica is made of silicon and oxygen. Lava that containsmore silica tends to be thicker and does not flow easily. Lavacontaining more iron and magnesium and less silica tendsto flow more easily.
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 carbonatedsoft drink before opening it, the pressure from the gas buildsup and is released suddenly when the container is opened.In the same way, steam builds pressure in magma. Lavacontaining more silica is thicker, so it tends to trap morewater vapor and other gases. This creates tremendous pressure.
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110 Earthquakes and Volcanoes
Picture This3. Explain How does lava
flow from the shieldvolcano?
●B Organize InformationConstruct a four-tab Foldable asshown to record informationabout volcanoes.
Cinder conevolcanoes
Shieldvolcanoes
Compositevolcanoes
Fissureeruptions
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 magmaproduced when plates melt tends to contain more silica thanthe magma produced deep inside Earth. This silica trapsmore water, which turns to steam. Steam producestremendous pressure in thick, silica-rich magma. Whenenough 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 ofbasaltic layers forms a large, broad volcano with gentlysloping sides called a shield volcano. A shield volcano isshown on the left in the figure below. They are the largesttype of volcano. Shield volcanoes occur where plates areseparating, or in other places where magma can be forcedup from deep inside Earth.
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 volcanoerupts. The eruption throws ash, cinders, and lava into theair. 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 ofvolcanic material. A cinder cone volcano is a relatively smallvolcano formed by moderate to explosive eruptions oftephra. This type of volcano is shown on the right in thefigure above. Cinder cone volcanoes erupt violently becausethe eruption is powered by the high gas content. However,the eruptions usually do not last very long. After the gas isreleased, the force behind the eruption is gone.
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50 km
Sea level9 km
Shield volcano
0.5 km
Crater
0.3 km
Cinder cone volcano
Reading Essentials 111
4. Infer Why do compositevolcanoes have steep sides?
Picture This5. Use Models Outline the
Columbia River Basalts onthe map.
What is a composite volcano?Composite volcanoes are steep-sided volcanoes formed by
alternating layers of tephra and lava. This volcano is shownin the figure below. Sometimes composite volcanoes eruptviolently 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 underanother plate. They are intermediate in size and shapecompared to shield volcanoes and cinder cone volcanoes.
What is a fissure eruption?Magma that is highly fluid can ooze through cracks or
fissures in Earth’s surface. This type of magma is usually linkedto fissure eruptions. The lava flows freely across the land toform flood basalts. When flood basalts are exposed to erosionover time, they can become large, flat landforms called lavaplateaus. The Columbia River Plateau in the northwesternUnited States was formed millions of years ago. Severalfissures erupted and lava flows built up layer by layer. Themap below shows the location of the Columbia River Basalts.
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112 Earthquakes and Volcanoes
6 km
Crater
3 km
WASHINGTON
Columbia RiverBasalts
OREGON
IDAHO
0 100 km
Casc
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Composite volcano
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1. Review the terms and their definitions in the Mini Glossary. Write two to three sentencesusing at least two terms to describe different volcanic eruptions.
2. Compare shield volcanoes to cinder cone volcanoes, using this Venn diagram. In themiddle, write things that the two types of volcanoes have in common. On each side,write things that are different.
3. In this section you made flash cards. How did the flash cards help you to learnabout volcanoes?
After You ReadMini Glossarycinder cone volcano: relatively small volcano formed by
moderate to explosive eruptions of tephracomposite volcano: steep-sided volcano formed with
alternating layers of violent eruptions of tephra andquieter eruptions of lava
lava: molten rock flowing onto Earth’s surface
shield volcano: large, broad volcano with gently slopingsides that is formed by the buildup of basaltic layers
volcano: cone-shaped hill or mountain formed when hotmagma, solids, and gases erupt onto Earth’s surfacethrough a vent
End ofSection
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Reading Essentials 113
Cinder cone volcanoes Shield volcanoes
Both
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