BA 86 Unit: The Changing Earth

VOCABULARYvolcano p. 86lava p. 87pyroclastic flow p. 88

KEY CONCEPT

Volcanoes form asmolten rock erupts.

EXPLORE Eruptions

What happens when a volcano erupts?

PROCEDURE

Add water to an empty film canister until it is three-fourths full.

Drop an antacid tablet in the water and putthe lid on the canister. Observe what happens.

WHAT DO YOU THINK?• What happened to the water and to the

canister lid?• What caused the changes you observed?• How might the events you observed be

similar to the eruption of a volcano?

2

1

MATERIALS• empty film

canister• effervescent

antacid tablet• water

BEFORE, you learned

• Magma is molten rock insideEarth

• Magma forms as a plate sinkingin a subduction zone starts to melt

• Volcanoes can form over hotspots far from plate boundaries

NOW, you will learn

• Where most volcanoes are located

• How volcanoes erupt• What types of volcanoes

there are

VOCABULARYMake a word triangle for volcano in your notebook.

Volcanoes erupt many types of material.Earth’s thin outer layer is made of cool rock, but most of Earth is madeof extremely hot rock and molten metal. Some of the heat inside Earthescapes to the surface through volcanoes. A is an opening in Earth’s crust through which molten rock, rock fragments, and hotgases erupt. A mountain built up from erupted material is also called a volcano.

A volcano may erupt violently or gently. A violent eruption cancause tremendous destruction even if not much molten rock reachesthe surface. For example, a volcano might throw out huge amounts of rock fragments that start fires where they land or fall in thick layerson roofs, causing them to collapse. A volcano can erupt gently yet pourout rivers of molten rock that flow long distances. The violence of aneruption depends mainly on the type of magma feeding the volcano.

volcano

Magma collects in a magma chamber beforeerupting through a volcano.

Where does magmabecome lava?

MagmaA major portion of all magma is silica, which is a compound of siliconand oxygen. Magma also contains gases, which expand as the magmarises. Magma that is high in silica resists flowing, so expanding gasesare trapped in it. Pressure builds up until the gases blast out in a violent,dangerous explosion. Magma that is relatively poor in silica flows easily,so gas bubbles move up through it and escape fairly gently. Though aneruption of silica-poor magma can throw lava high into the air, forminglava fountains, visitors can usually watch safely nearby.

Magma rises toward Earth’s surface as long as it is less dense thanthe surrounding rock. Once magma stops rising, it can collect in areascalled magma chambers. Magma can remain in a chamber until itcools, forming igneous rock, or it can erupt. Volcanic eruptions occurwhen, for example, a chamber is not large enough to hold additionalmagma that pushes in. When magma erupts, it is called lava. ismagma that has reached Earth’s surface.

Lava

Chapter 3: Mountains and Volcanoes 87 BA

rock fragments

lava flow

rising magma

magma chamber

CONTENT FRAMEMake a content frame for volcanic materials.Add categories across the top for what they are made of and howthey are erupted.

Structure of a Volcano

reading tip

The prefix pyro- means“heat,” and clasticmeans “made up of rock fragments.”

Rock FragmentsA great deal of material erupts from volcanoes as rock fragments.The fragments form as

• escaping gas bubbles pop, tearing magma apart

• larger pieces of lava are thrown into the air, cooling and hardening during their flight

• rocks of all sizes rip loose from volcanoes’ walls during eruptions

Tiny rock fragments form volcanic ash, which consists of particlesranging from the size of dust to about the size of rice grains. Volcaniccinders are somewhat larger. The largest fragments are volcanic bombsand blocks. Bombs are molten when they are thrown out and often havestreamlined shapes. Blocks, which can be the size of houses, erupt assolid pieces of rock. Large rock fragments fall quickly, but ash can becarried long distances by winds—even all the way around Earth.

Volcanic GasesWhat looks like smoke rising from a volcano is actually a mixture of ash and gases. The main gases in magma are water vapor and carbon dioxide. Some volcanic gases combine with water in the air toform acids—you will read about these in the next section.

During an eruption, volcanic gases can mix with rock fragmentsand stay near the ground. The mixture forms a (PY-roh-KLAS-tihk), which is a dense cloud of superhot gases and rock fragments that races downhill. Such a flow can be as hot as 800°C (1500°F) and can travel faster than 160 kilometers per hour(100 mi/h). Pyroclastic flows are the most dangerous type of volcaniceruption.

check your reading What are two reasons why pyroclastic flows are dangerous?

pyroclastic flow

88 Unit: The Changing EarthBA

ash cinders

block

Volcanic ash is madeup of rock fragmentsless than 2 millimetersin diameter.

Cinders contain holesand tunnels left byescaping gases.

Large fragmentsare called blocksor bombs.

VISUALIZATIONCLASSZONE.COM

Watch clips of eruptedvolcanic material.

BAChapter 3: Mountains and Volcanoes 89

Most volcanoes form along plateboundaries.

Volcanoes are common along tectonic plate boundaries where oceanicplates sink beneath other plates. As a plate sinks deep into a subductionzone, it heats and begins to melt, forming magma. If the magmareaches the surface it can build tall volcanic mountains.

Volcanoes are also common along tectonic boundaries where platespull apart, allowing magma to rise from the mantle. Some of thesevolcanoes are in Africa’s Great Rift Valley. However, much of Earth’svolcanic activity takes place underwater. Magma erupts along spread-ing centers in the ocean and cools to form new lithosphere.

Less commonly, a volcano can form over a hot spot far from a plateboundary. Heat carried by material rising from deep in the mantlemelts some of the rock in the lithosphere above it. Eruptions over a hotspot built the Hawaiian Islands.

More than 400 volcanoes—about 80 percent of all active volcanoesabove sea level—are along subduction zones in the Pacific Ocean. Anactive volcano is one that is erupting or has erupted in recorded history.The volcanoes around the Pacific Ocean form a belt called the Ring ofFire. Some of these volcanoes are in the western United States.

Equator

AUSTRALIA

ASIANORTH

AMERICA

SOUTHAMERICA

PACIFIC OCEAN ATLANTICOCEAN

PACIFIC OCEAN

INDIANOCEAN Ring of fire

Active volcanoes

Subduction zone

Ring of Fire

What is the relationship between volcanoes and subduction zones in the Ring of Fire?

Two types of material form volcanoes:rock fragments that fall close to the openings they erupted from and lavaflows that have cooled and hardened.

Three Types of Volcanoes

Volcanoes can have many shapes and sizes.Mount St. Helens is a cone-shaped volcano in Washington. Its eruptionin 1980 killed 57 people. One side of the volcano exploded, blasting outa mixture of hot rock, ash, and gases that destroyed trees tens of kilo-meters away. Since 1980, this volcano has had many smaller eruptions.

Volcanoes can have many shapes, including steep cones and nearlyflat land. Most volcanoes erupt from openings in bowl-shaped pitscalled craters. Some volcanoes erupt from long cracks in the ground.The type of magma feeding a volcano determines its shape.

Shield Volcano A shield volcano is shaped like a broad, flat dome.It is built up by many eruptions of lava that is relatively low in silicaand therefore flows easily and spreads out in thin layers. Thelargest volcano on Earth, Mauna Loa (MOW-nuh LOH-uh), is ashield volcano. It makes up much of the island of Hawaii. The totalheight of this volcano is about 17 kilometers (10.5 mi), but onlyabout 4 kilometers (2.5 mi) is above sea level. At the top of MaunaLoa is a crater that is 5 kilometers (3 mi) across at its widest point.Mauna Loa is one of Earth’s most active volcanoes.

Cinder Cone A cinder cone is a steep, cone-shaped hill formed bythe eruption of cinders and other rock fragments that pile up around

2

1

CONTENT FRAMEMake a content frame fortypes of volcanoes. Addcategories for shape, size,makeup, and examples.

Shield Volcano1

A shield volcano is built up of many thin layers ofhardened lava. Rangitoto, a shield volcano in NewZealand, is broad and has gently sloping sides.

90 Unit: The Changing EarthBA

shield volcano

RESOURCE CENTERCLASSZONE.COM

Learn more about historic and current volcanic eruptions.

reading tip

The word composite comesfrom a Latin word meaning“put together.” Somethingthat is composite is madeof distinct parts.

a single crater. Cinders form as gas-rich magma erupts. Escapinggases throw small chunks of lava into the air, where they hardenbefore landing. Cinder cones are tens to hundreds of meters tall.Many of them form on the sides of other types of volcanoes.

Composite Volcano A composite volcano is a cone-shaped volcanobuilt up of layers of lava and layers of rock fragments. Its magma ishigh in silica, and therefore is pasty. A composite volcano is steepnear the top and flattens out toward the bottom. Because hardenedlava flows add strength to the structure of a composite volcano, itcan grow much larger than a cinder cone.

Composite volcanoes have violent eruptions for two reasons. First,expanding gases trapped in rising magma tend to cause explosions.Second, hardened lava from earlier eruptions often plugs openings inthese volcanoes. This rock must be blown out of the way before anymore magma can escape. Mount St. Helens is a composite volcano.Though its 1980 eruption was devastating, many composite volcanoeshave exploded with much greater power.

Check Your Reading List the three main types of volcanoes. What questions do youhave about how they form?

3

Composite Volcano3Cinder Cone 2

A composite volcano is usually cone-shaped and is builtup of layers of hardened lava and of rock fragments.Mount St. Helens is a typical composite volcano.

A cinder cone, like this one in Arizona,has steep sides and is a loose pile of volcanic rock fragments.

Chapter 3: Mountains and Volcanoes 91 BA

composite volcano

cinder cone

Both shield volcanoes and composite volcanoes can form featurescalled calderas (kal-DAIR-uhz). A caldera is a huge crater formed bythe collapse of a volcano when magma rapidly erupts from underneathit. The crater at the top of Mauna Loa in Hawaii is a caldera. CraterLake in Oregon fills a caldera formed by a composite volcano about7700 years ago. A violent eruption emptied much of its magma cham-ber, and the top of the volcano collapsed into it. The caldera nowholds the deepest lake in the United States.

Scientists monitor volcanoes.Before Mount Pinatubo (PIHN-uh-TOO-boh) in the Philippines eruptedin 1991, most people living in the area did not realize that it was acomposite volcano. It had not erupted in about 500 years, and erosionhad changed its shape. Fortunately, scientists in the Philippines knewthat the volcano was becoming active months before it exploded. Theywere able to warn the government and ask people to leave the area.Their efforts probably saved tens of thousands of lives.

92 Unit: The Changing EarthBA

CONTENT FRAMEMake a content frame fortypes of data used to predict eruptions. Includecategories for current activity and history.

Crater Lake fills the caldera of a composite volcano.

Formation of Crater Lake

A huge eruption removed much of themagma from the magma chamber.

The volcano collapsed, creating acaldera 8 kilometers in diameter and1.6 kilometers deep.

New eruptions built a small cone inthe caldera. The caldera filled withwater from rain and snow.

BA

KEY CONCEPTS1. Where are most volcanoes

located, and why are theylocated there?

2. How does the type of materialthat erupts from a volcanodetermine the shape of thevolcano?

3. What conditions do scientistsexamine when they monitorvolcanoes?

CRITICAL THINKING4. Compare and Contrast

How do the three main typesof volcanoes differ?

5. Infer Volcanic ash can bedeposited in areas many kilometers away from the volcano that produced it. Whatare two ways in which the ashcan reach these areas?

CHALLENGE6. Analyze Draw diagrams

showing how a composite volcano might change in shapeby getting larger or smallerwith repeated eruptions.

Chapter 3: Mountains and Volcanoes 93

As the 1991 eruption of Mount Pinatubo shows, volcanoes can go hundreds of years between eruptions. Before Pinatubo’s eruption,scientists noticed warning signs that included the occurrence of manysmall earthquakes followed by explosions of steam near the volcano’stop. Researchers brought in equipment to monitor the volcano’s activity. Although they could not stop the eruption, theywere able to tell when people should leave.

Scientists monitor volcanoes around the world forsigns of eruptions. Indications that magma is movingunderneath a volcano include earthquake activity andchanges in the tilt of the ground. Scientists also monitorthe temperatures at openings, springs, and lakes on volcanoes, as well as the amounts and types of gasesgiven off by the volcanoes. Rising temperatures andchanges in volcanic gases can indicate that fresh magmahas moved into a shallow magma chamber.

Scientists study the ages and types of volcanic rocksaround a volcano to understand the volcano’s history,including how much time has passed between eruptionsand how violent the eruptions have been. This informa-tion gives clues about possible future eruptions.

Even with close monitoring, most property damagefrom volcanic eruptions cannot be prevented. But warningpeople to move away from a volcano that is about toerupt can save lives. Many of the active volcanoes that are closelymonitored are located near major cities. Among these are MountRainier (ruh-NEER), which is near Seattle, Washington, and MountVesuvius (vih-SOO-vee-uhs), which is near Naples (NAY-puhlz), Italy.

check your reading What is the purpose of monitoring volcanoes?

The robot Dante II is about to enter thecrater of Mt. Spurr, Alaska, where it willcollect video data as well as water and gas samples.