Other Ways To P


Source: Chapter 18, p. 498

Read the chapter title to find out what the topic will be.

Skim the photos, illustrations, captions, graphs, and tables.

Look for vocabulary terms that are boldfaced and highlighted.

Create an outline using section titles and heads.

Volcanic eruptio

As You Read

Within each section you will find a tool to deepen your understanding and a tool to check your understanding.

Section 18


) Describe how plate tectonics influences the formation of volcanoes. ) Locate maior zones of volcanism. ) Identify the parts of a volcano. ) Differentiate between volcanic landforms.

Review Vocabulary convergent: tending to move toward one point or to approach each other

New Vocabulary volcanism hot spot flood basalt conduit caldera shield volcano cinder cone composite volcano

Figure 18.1 Most of Earth's active volcanoes are located along plate boundaries.


MAQX^dea The locations of volcanoes are mostly determined by plate tectonics.

Real-World Reading Link Road crews spread salt on icy winter roads because salt makes the ice melt at a lower temperature. At extremely high temperatures, rocks can melt. Often, if heated rocks are in contact with water, they melt more easily.

Zones of Volcanism

Volcanoes are fueled by magma. Recall from Chapter 5 that magma is a slushy mixture of molten rock, mineral crystals, and gases. As you observed in the Launch Lab, once magma forms, it rises toward Earth's surface because it is less dense than the surrounding mantle and crust. Magma that reaches Earth's surface is called lava. Volcanism describes all the processes associated with the discharge of magma, hot fluids, and gases.

As you read this, approximately 20 volcanoes are erupting. In a given year, volcanoes will erupt in about 60 different places on Earth. The distribution of volcanoes on Earth's surface is not random. A map of active volcanoes, shown in Figure 18.1, reveals striking patterns on Earth's surface. Most volcanoes form at plate boundaries. The majority form at convergent boundaries and divergent boundaries. Along these margins, magma rises toward Earth's surface. Only about 5 percent of magma erupts far from plate boundaries.

Arctic Ocean

Figure 18.1 Most of Earth's active volcanoes are located along plate boundaries.

Arctic Ocean


I Pacific Ocean

I Pacific Ocean

{ The Real-World Reading Link describes how the section's content may relate to you.

Source: Section 18.1, p. 502


Figure 18.3 Eruptions at divergent boundaries tend to be nonexplosive. At the

500 Ch apt« r 18 • Volcani sm

divergent boundary on tfie ocean floor, eruptions often form huge piles of lava called pillow lava.

Source: Section 18.1, p. 500

Interactive Figure To see an animation of divergent plate boundaries, visit

flfReading Checks are questions that assess }

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Science usage v. Common usage


Science usage: an elongated column

Divergent volcanism Recall from Chapter 17 that at divergent plate boundaries tectonic plates move apart and new ocean floor is produced as magma rises to fill the gap. At ocean ridges, this lava takes the form of giant pillows like those in Figure 18.3, and is called pillow lava. Unlike the explosive volcanoes detailed in Figure 18.4, volcanism at divergent boundaries tends to be nonexplosive, with effusions of large amounts of lava. About two-thirds of Earth's volcanism occurs underwater along divergent boundaries at ocean ridges.

0 Reading Check Convert the fraction of volcanism that happens underwater to a percentage.

Hot spots Some volcanoes form far from plate boundaries over hot spots. Scientists hypothesize that hot spots are unusually hot regions of Earth's mantle where high-temperature plumes of magma rise to the surface.

Volcanoes constantly shape Earth's surface.

4845 B.C. Mount Mazama erupts in Oregon. The mountain collapses into a 9-km-wide depression, known today as Crater Lake (topographic map).

4845 B.C. Mount Mazama erupts in Oregon. The mountain collapses into a 9-km-wide depression, known today as Crater Lake (topographic map).

4 1630 B.C. In Greece, Santorini explodes, causing tsunamis 200 m high. Nearby, Minoan civilization on the Isle of Crete disappears.

502 Chapter 18 • Volcanism


Common usage: a large, showy feather of a bird

Figure 18.4

Volcanoes in Focus


Ask yourself: What is the

Think about people, places, and situations that you've encountered. Are there any similarities with those mentioned in this book?

Relate the information in this book to other areas you have studied.

Predict events or outcomes by using clues and information that you already know.

Change your predictions as you read and gather new information.

After You Read

Follow up your reading with a summary and assessment of the material to evaluate if you understood the text.

Each section concludes with an assessment. The assess- ^ ment contains a summary and questions. The summary reviews the section's key concepts while the questions test your understanding.

Types of Volcanoes

The appearance of a volcano depends on two factors: the type of material that forms the volcano and the type of eruptions that occur. Based on these two criteria, three major types of volcanoes have been identified and are shown in Table 18.1. Each differs in size, shape, and composition.

Shield volcanoes A shield volcano is a mountain with broad, gently sloping sides and a nearly circular base. Shield volcanoes form when layers of lava accumulate during nonexplosive eruptions. They are the largest type of volcano. Mauna Loa, which is shown in Table 18.1, is a shield volcano.

Cinder cones When eruptions eject small pieces of magma into the air, cinder cones form as this material, called tephra, falls back to Earth and piles up around the vent. Cinder cones have steep sides and are generally small; most are less than 500 m high. The Lassen Volcanic Park cinder cone shown in Table 18.1 is 700 m high. Cinder cones often form on or very near larger volcanoes. Composite volcanoes Composite volcanoes are formed of layers of hardened chunks of lava from violent eruptions alternating with layers of lava that oozed downslope before solidifying. Composite volcanoes are generally cone-shaped with concave slopes, and are much larger than cinder cones. Because of their explosive nature, they are potentially dangerous to humans and the environment. Some examples of these are Mount Augustine in Alaska, shown in Table 18.1, and several in the Cascade Range of the western United States, such as Mount St. Helens.

Section 18.1 Assessment

|i ill Gddr

IBGfflHS Volcanoes develop from magma moving upward from deep within Earth.

a(p. 505) cone (p. 507) osite volcano (p. 507) it (p. 505) (p. 505) issuie (p. 504) lood basalt (p 504) lot spot (p 502) .hield volcano (p 507) loup. 505)

The locations of volcanoes are mostly determined by plate

• Volcanism includes all the processes in which magma and gases rise to

• Most volcanoes on land are part of two major volcanic chains: tte Circum-Pacific Belt art the Mediterranean Belt.

• Parts of a volcano inclut a vent, magma chamber, crater, and caldra.

• Flood basalts form when lawi flows from fissures to form flat plains or

• There are three major types of vokanoes: shield, composite, and cinder

3 The composition of magma determines the characteristics of a volcanic eruption.

• There are three major types of magma: basaltic, andesitic, and rhyolitic.

• Because of their relative silica contents, basaltic magma is tte bast explosive magma and rhyolitic magma is tte most explosive.

• Temperature, pressure, and tte presence of water are factors that aftect tte formation of magma.

• Rock fragments ejected during eruptions are called tephra.

batholith (p. 515) dke (p. 516) laccolith (p. 515) pluton (p. 514)

] Magma that solidifies below ground forms geologic features difffaent from those formed by magma that cools at tte surface.

• Intrusive igneous rocks are classified according to their size, shape, and relationship to the surrounding rocks.

• Most of Earth's vokanism happens below Earths surface.

• Magma can intrude into rockin different wry^, taking different forms

• Batholiths form the core of many mountain ranges.

Section Summary

| Volcanism includes all the processes in which magma and gases rise to Earth's surface. | Most volcanoes on land are part of two major volcanic chains: the Circum-Pacific Belt and the Mediterranean Belt. t Parts cf a volcano include a vert, magma chamber crater, and caldera. ) Flood basalts form when lava flows from fissures to form flat plains or plateaus. t There are three major types of volcanoes: shield, composite, and cinder cone.

Understand Main Ideas

1. Explain how the location of volcanoes is related to the theory of plate tectonics.

2. Identify two volcanoes in the Mediterranean Belt.

3. Draw a volcano, labeling the parts

4. Propose Yellowstone National Park is an area of previous volcanism. using a map of the united States suggest the type(s) of tectonic processes associated with this area.

Think Critically

5. Evaluate the following statement: Volcanoes are only found along coastlines

6. Decide whether a flood basalt is or is not a volcano.

Earth Science

7. If the Pacific Plate has moved 500 km in the last 4.7 million years, calculate its average velocity in centimeters per year. Refer to the Skillbuilder Handbook for more information.

Self-Check Quiz glencoe.co

Source: Section 18.1, p. 507

^ At the end of each chapter you will find a Study Guide. The chapter's vocabulary words as well as key concepts are listed here. Use this guide for review and to check your comprehension.

Source: Chapter 18, p. 520


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Solar Panel Basics

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