FOCUS ON EARTH SCIENCE (10)

>ˆ˜Ê`i> The compo-sition of magma controls volcanic eruptions and determines the different types of lava flow and eruptions can change human and wildlife habitats... Movement along plate

These tourists watch a fountain of lava

on the coast of Hawaii, at the Kilauea volcano The Hawaiian Islands are forming as a result of volcanic activity The Hawaiian Islands are in the middle

of the Pacific Plate, far from its edges They sit above a hot spot under the Pacific Plate Volcanic eruptions create scenic landscapes as the lava is ejected from volcanoes.

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-Vˆi˜ViÊÊ+PVSOBM Imagine you are among the tourists viewing the volcanic eruption Make a list of what you might hear, smell, feel, see, and possibly taste while watching the eruption.

Beautiful, but Dangerous

Volcanoes are locations

where magma reaches

Earth’s surface They affect

human and wildlife

volcanic activity occurs

along plate boundaries

where plates move

rela-tive to one another

>ˆ˜Ê`i> The

compo-sition of magma controls

volcanic eruptions and

determines the different

types of lava flow and

eruptions can change

human and wildlife

habitats

1.d, 1.e, 7.b

1.d, 1.f, 7.g, 7.h

2.d, 7.a, 7.b, 7.d

Visit to:

▶ view

▶ explore Virtual Labs

▶ access content-related Web links

▶ take the Standards Check

Start-Up Activities

How did these rocks

form?

Examine three photos of

rocks or rock samples

from your teacher Each

of these rocks formed

from lava ejected during

a volcanic eruption

Procedure

and mass of each rock

and crystals in each rock

Think About This

• Infer which rock cooled the fastest and

which one cooled most slowly

• Predict what environmental conditions

might have existed when each rock was

formed

291

Volcanic Features Make the following Foldable to identify volcanic features

STEP 1 Fold a sheet of paper in half

lengthwise

STEP 2 Cut along every third line of the

top flap to form tabs

ca6.msscience.com

1.d, 7.e

ELA6: R 2.4

Get Ready to to Read

292

Learn It! A prediction is an educated guess based on what you already know One way to pre-

dict while reading is to guess what you believe the author

will tell you next As you are reading, each new topic

should make sense because it is related to the previous

paragraph or passage.

Practice It! Read the excerpt below from Lesson 2 Based on what you have read, make pre-

dictions about what you will read in the rest of the lesson

After you read Lesson 2, go back to your predictions to

see if they were correct.

The composition of magma is an important characteristic when attempting to predict a volcanic eruption Scientists can predict the energy of a volcanic eruption based on the percentage of silica and oxygen that is present in the magma Silica is just one of the chemical characteristics that controls the eruptive behavior of a body of magma.

Predict how the

amount of silica

pres-ent in magma could

affect the energy of a

volcanic eruption.

Determine what other chemical characteristics control the eruptive behav- ior of a body of magma.

Make Predictions

Apply It! Before you read, skim the questions in the Chapter Assessment Choose three questions and predict the

answers.

Target Your Reading

Use this to focus on the main ideas as you read the chapter.

1 Before you read the chapter, respond to the statements

below on your worksheet or on a numbered sheet of paper

2 After you read the chapter, look back to this page to see if

you’ve changed your mind about any of the statements

1 Most volcanic eruptions occur at plate boundaries

on Earth’s surface

erupt

5 Hot spots form where two oceanic plates converge

hazard

7 A lava dome is filled with thick, viscous lava

extremely explosive

9 Shield volcanoes are the smallest type of volcano

10 Composite volcanoes are composed of alternating layers of lava and tephra

Before You Read

LESSON 1

Figure 1 This photo shows volcanologists collecting a sample of lava from a volcano.

294 Chapter 7 • Volcanoes

Reading Guide

What You’ll Learn

▼Explain what causes

volcanoes form and where

they occur helps scientists

predict volcanic eruptions.

lithospheric plate: large,

brittle pieces of Earth’s outer

shell composed of crust and

out-What is a volcano?

A volcano is a land or underwater feature that forms

when magma reaches the surface of Earth Recall from Chapter 2 that magma is molten, liquid rock material found underground and forms igneous rocks Magma rises

up to Earth’s surface because its density is less than the rock through which it moves When magma rises to Earth’s surface it is called lava

Volcanoes can cause the eruption of liquid, gas, or solid materials Hot gases, magma, and even solid rock particles can be explosively erupted from openings on Earth Scien-

better understand volcanic eruptions

Science Content

Standards

1.d Students know that earthquakes are

sudden motions along breaks in the crust

called faults and that volcanoes and fissures

are locations where magma reaches the

surface

1.e Students know major geologic events,

such as earthquakes, volcanic eruptions, and

mountain building, result from plate

motions

7.b Select and use appropriate tools and

technology (including calculators,

computers, balances, spring scales,

microscopes, and binoculars) to perform

tests, collect data, and display data.

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of the plate boundaries The Ring of Fire is a belt

of active volcanoes that circles the Pacific Ocean.

How do volcanoes form?

Recall from Chapter 2 that some of Earth’s internal heat is

left over from when the planet first formed When rocks

become hot enough, and pressure and other conditions are

right, they can melt

What causes rock to melt?

In order for a volcano to form, magma must first reach

Earth’s surface Magma is less dense then the rock from which

it melted Therefore, magma tends to rise above the denser

rock The density of magma depends upon its composition, the

amount of dissolved gas, and its temperature

Magma also tends to be more buoyant when compared to

the rocks that surround it Magma is forced to rise toward

Earth’s surface because of the buoyant force pushing up on the

magma

Where do volcanoes occur?

Volcanoes are not common in all regions of Earth’s crust

Volcanoes typically occur along divergent and convergent

plate boundaries and hot spots, where the flow of heat from

volcanoes and plate boundaries on Earth Do you notice a

relationship between the locations of active volcanoes and

plate boundaries?

WORD ORIGIN

volcano

from Italian vulcano;

means burning mountain

Lesson 1 • Volcanoes and Plate Boundaries 295

Figure 3 Volcanoes

typically form at five

geologic locations

Movement along plate

boundaries and hot

spots tends to form

volcanic eruptions

with unique

characteristics.

296 Chapter 7 • Volcanoes

Convergent Plate Boundaries

Recall from Chapter 5 that when two lithospheric plates move toward each other, a convergent plate boundary forms They include areas where an oceanic plate is subducted below

a continental plate or where an oceanic plate is subducted below another oceanic plate As the plate subducting below the other plate gets deep enough, it becomes hot enough to partially melt and form magma The magma is then forced

up toward Earth’s surface and forms volcanoes Volcanoes that form along convergent plate boundaries tend to erupt

of convergent plate boundary landforms

plate boundary landforms

Volcanic Arcs When an oceanic plate subducts beneath a

continental plate, a volcanic arc is formed A volcanic arc is a string of volcanoes that forms on land parallel to the leading edge of the continent An example of a volcanic arc is shown

Island Arcs When two oceanic plates move toward each

other, an island arc is formed Island arcs are long, curved

island arc volcano

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Figure 3 Volcanoes typically

form at five geologic locations

Movement along plate

bound-aries and hot spots tends to

form volcanic eruptions with

unique characteristics.

Divergent Plate Boundaries

Imagine a divergent plate boundary as a huge crack, or rift,

in Earth’s surface The boundary can occur in either the

ocean, as a mid-ocean ridge, or on a continent, as a

continen-tal rift Tension stresses cause the lithospheric plates to be

pulled apart This pulling action forms rifts that allow

magma to reach Earth’s surface through fissure eruptions

In the ocean, lava flows from these rifts are cooled quickly

by seawater and form new volcanic rock As more lava flows

and hardens, it builds up on the seafloor Sometimes, the

vol-canoes rise above sea level, forming islands such as Iceland

How do volcanoes form at divergent plate boundaries?

Heat Escapes Volcanic eruptions are one of the most

notice-able signs that heat is escaping from Earth’s interior After

many of thousands or even millions of years, magma reaches

Earth’s surface When this magma erupts from the central,

circular or oval-shaped opening of a volcano, called a vent, a

cone-shaped landform develops This landform develops

because lava flows out from one source in many directions

As lava flows out, it cools quickly and becomes solid, forming

layers of igneous rock around the vent the steep-walled

depression around a volcano’s vent is the crater

SCIENCE USE V COMMON USE

rift

Science Use fissure, crevasse.

As the lithospheric plates were pulled apart, a rift valley was created

Common Use to burst open,

divide The argument caused a rift between the two friends.

ACADEMIC VOCABULARY

source (SORS)

(noun) point of origin

We found the source of the loud music to be a concert.

Lesson 1 • Volcanoes and Plate Boundaries 297

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Figure 4 This crack formed

because of a fissure eruption

at a mid-ocean ridge The

cur-rents caused by this activity

attract the crabs.

298 Chapter 7 • Volcanoes

Fissure Eruptions When magma escapes from narrow and

elongated cracks in Earth’s crust, a fissure eruption (FIH

shur · ih RUP shunz) occurs The magma pushes out along both sides of the crack and flows smoothly Long, sheet-shaped landforms develop

Fissure eruptions can occur in the ocean at a divergent plate boundary They form mid-ocean ridges and new sea-

on a continent at a divergent plate boundary They form a rift and produce new crust on Earth’s surface

Volcanoes Away from Plate Boundaries

Some volcanoes do not form along plate boundaries These

volcanoes are known as hot spots, which are localized areas

of high heat in Earth’s interior Scientists continue to study

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LESSON 1 Review

Lesson 1 • Volcanoes and Plate Boundaries 299

Formation of Volcanoes

Heat and pressure from Earth’s interior cause rock to melt

and become magma Because magma is less dense than the

surrounding rock, the buoyant force causes it to rise to

Earth’s surface Most volcanoes form at divergent or

conver-gent plate boundaries However, hot spots form away from

plate boundaries Scientists continue to study hot spots to

better understand how and why they form

Science nline

For more practice, visit Standards

Check at .

Summarize

Create your own lesson

summary as you design a

study web.

1 Write the lesson title,

number, and page

num-bers at the top of a sheet

of paper

2 Scan the lesson to find

the red main headings.

3 Organize these headings

clockwise on branches

around the lesson title.

4 Review the information

under each red heading

to design a branch for

each blue subheading.

5 List 2–3 details, key terms,

and definitions from each

2 Long, sheet-shaped landforms

develop when a(n) pushes magma out along both sides of a crack in Earth’s crust

1.d

Understanding Main Ideas

3 Illustrate and label two types

of openings from which

4 Organize Draw a diagram like the one below List five set- tings where volcanoes occur

vol-1.d

6 Compare and contrast the movement of lithosphere at a mid-ocean ridge and a sub-

Applying Science

7 Hypothesize why volcanoes are not likely where two conti- nental plates converge

Support your answer with information from this lesson.

ELA6: R 2.4

How do volcanoes form?

Buoyant forces push magma up and cause

it to rise to Earth’s surface This building of

pressure causes volcanic eruptions Volcanic

eruptions allow heat to escape from Earth’s

interior Can you model how volcanoes form

and erupt?

Procedure

1 Read and complete a lab safety form.

2 Obtain a beaker with hardened wax in the

bottom from your teacher

3 Layer 1 cm of sand on top of the wax.

4 Pour 8 cm of cold water onto the sand.

5 Set up a ring stand with wire gauze to hold your beaker over a

Bunsen burner Light the burner.

6 Observe the wax as it erupts through the sand and water.

7 Extinguish the burner Allow the wax to cool

8 Record your observations of the exposed wax formations.

9 Compare your volcano to your classmates’

Analysis

1 Sequence how your volcano developed, erupted,

and formed volcanic features.

2 Infer how the density of the wax changed as it was heated

3 Compare and contrast your model to a real-life volcano.

Science Content Standards

1.d Students know that earthquakes are sudden motions along breaks in the crust called faults

and that volcanoes and fissures are locations where magma reaches the surface.

7.b Select and use appropriate tools and technology (including calculators, computers, balances,

spring scales, microscopes, and binoculars) to perform tests, collect data, and display data.

LESSON 2

Figure 6 A thick, sticky mass of lava is flowing from the vent of Mount St Helens

Identify the volcanic feature in the center of the volcano.

Lesson 2 • Volcanic Eruptions and Features 301

Reading Guide

What You’ll Learn

▼Relate the composition of

composition of magma helps

scientists better predict how

Real-World Reading Connection Think about squeezing toothpaste out of the tube A thick, gooey blob of paste rolls slowly out the end Imagine the tube filled with water instead of toothpaste The composition of toothpaste and water are different How would this difference affect the manner in which these materials flow?

What controls volcanic eruptions?

The composition of magma controls how lava flows and the way a volcano erupts For example, in 2004, a thick, sticky mass of lava began flowing from Mount St Helens’

vent Scientists have termed the mass the whaleback

because of its unique shape This blob of lava, shown in

mass—because of its composition

Science Content

Standards

1.d Students know that earthquakes are

sudden motions along breaks in the crust

called faults and that volcanoes and fissures

are locations where magma reaches the

surface

1.f Students know how to explain major

features of California geology (including

mountains, faults, volcanoes) in terms of

plate tectonics

7.g Interpret events by sequence and time

from natural phenomena (e.g., the relative

ages of rocks and intrusions)

Also covers: 7.h

302 Chapter 7 • Volcanoes

Composition of Magma

The composition of magma is an tant characteristic when scientists attempt to predict a volcanic eruption Scientists can predict the energy of a volcanic eruption based on the percentage of silica and oxygen that is present in the magma Silica is just one

impor-of the chemical characteristics that controls the eruptive behavior of magma

High Silica In general, magma that contains

a high percentage of silica is thicker and slower moving Magma with high silica con-tent tends to be thick and sticky, like honey or

frosting The viscosity (vihs KAH suh tee) of

a material is a physical property that describes the material’s resistance to flow In other words, if magma has high viscosity, it does not flow easily

Low Silica In general, magma that contains

a low percentage of silica and more iron and magnesium has low viscosity In other words, the magma will flow easily Magma with low silica content tends to be thin and runny, like

compo-sition of magma affects viscosity

How does the percentage of silica affect the viscosity of magma?

Dissolved Gases and Temperature

The amount of dissolved gases and the temperature of magma also affect how a vol-cano erupts In general, the higher the tem-perature of magma, the more easily it flows.The most common dissolved gas in magma

gases that may be present in magma and released during volcanic eruptions are carbon dioxide, sulfur dioxide, and hydrogen sulfide Typically, the more gas that is present in magma, the greater the chance of an explo-sive eruption, even if the composition of the magma would suggest that it should have a quiet eruption

Figure 7 The composition of magma

affects the viscosity Generally, magma that

has a high percentage of silica has high

vis-cosity Magma that has a low percentage of

silica has low viscosity.

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Lesson 2 • Volcanic Eruptions and Features 303

Types of Magma and Lava

When you classify something, it’s good to keep the main

types in your mind as you compare examples Try this for

of magma, basaltic (buh SAWL tihk), and granitic

(gra NIH tihk)

Basaltic Magma and Lava

Magma that contains a low percentage of silica is called

basaltic magma Basaltic magma typically has low viscosity,

meaning it flows freely It is also a much thinner, more fluid

magma

Basaltic lava that erupts from a volcano tends to pour

from the vent and run down the sides of the volcano As this

pahoehoe (pa HOY hoy) lava cools, it develops a smooth skin

and forms ropelike patterns As stiff, slowly moving aa

(AH ah) lava forms, it flows at a lower temperature Basaltic

lava that erupts underwater at fissure eruptions forms

bubble-like pillow lava Basaltic lava tends to flow easily and

Granitic Magma and Lava

Magma that contains a high percentage of silica is called

granitic magma When granitic magma emerges on Earth’s

surface, it typically will have high viscosity, meaning it flows

slowly It will also be more sticky and lumpy Granitic lava

tends to trap gases, which causes pressure to build up and

volcanic eruptions

ACADEMIC VOCABULARY

emerge (ih MERJ)

(verb) to rise from, to come

out into view

I saw the diver emerge from the cloudy water.

Figure 8 The centage of silica, dissolved gases, and temperature are all physical characteristics that affect the eruptive behavior of magma

per-Quiet eruption Explosive eruption

304 Chapter 7 • Volcanoes

Types of Volcanoes

There are three types of volcanoes that are common on Earth’s surface—shield volcanoes, cinder cone volcanoes, and composite volcanoes These volcanoes do not represent all possible types of volcanic eruptions, but they illustrate many common volcanic features and the processes that form them

What are the three types of volcanoes?

Shield Volcanoes

A shield volcano is a huge, gently sloping volcanic

land-form that is mainly composed of basaltic lava Basaltic lava moves freely Shield volcanoes develop as layer upon layer of gently flowing basaltic lava piles up Shield volcanoes, like the

slightly bent shape of a warrior’s shield The Hawaiian Islands are examples of shield volcanoes Types of lava that typically

Cinder Cone Volcanoes

A cinder cone volcano is mainly composed of solid ments These solid fragments are known as tephra (TEH

frag-fruh) and include fragments of volcanic rock or lava These fragments range in size from tiny particles to huge boulders When lava erupts from the vent, cools quickly in the air, and falls to the surface as tephra, it forms a distinctly steep-sided,

Cinder cone volcanoes typically produce explosive volcanic eruptions

Composite Volcanoes

A composite volcano is mainly composed of alternating

layers of lava and tephra These layers accumulate from the alternating of quiet and explosive volcanic eruptions The quiet and explosive eruptions occur because the composition

of magma associated with composite volcanoes is somewhere between basaltic and granitic

What causes the alternating layers of a composite volcano?

Composite volcanoes often form tall, majestic mountains,

more than 2,438 meters

WORD ORIGIN

tephra

from Greek; means ashes

Table 1 Types of Volcanoes

Type of Volcano Examples of Volcanoes

lava and tephra

• Eruptions are both

quiet and explosive

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Lesson 2 • Volcanic Eruptions and Features 305

Interactive Table Organize information about types of volcanoes at ca6.msscience.com

306 Chapter 7 • Volcanoes

Aa lava flows, like this one on Mount Etna

in Italy, carry sharp, angular chunks of rock

Aa flows move more slowly than hotter pahoehoe flows.

Visualizing Lava

Pillow lava occurs where lava oozes out of fissure eruptions in the ocean floor It forms bubble-shaped lumps as it cools Pillow lava is the most common type of lava on Earth.

Pahoehoe lava flows, like this one near

Kilauea’s Mauna Ulu Crater in Hawaii, are

hotter and more fluid than aa flows They

develop a smooth skin and form ropelike

patterns as they push forward and then cool.

Figure 9

Lava rarely travels faster than a few kilometers per hour Therefore, it poses

little danger to people However, homes, property, and crops can be

dam-aged On land, there are two main types of lava flows—aa and pahoehoe

When lava comes out of cracks in the ocean floor, it forms pillow lava The

lava cooling here came from a volcanic eruption on the island of Hawaii.

Contributed by National Geographic

Volcanoes in California

As you have read in previous chapters, California has a large

variety of landscapes and landforms Examples of volcanic

These volcanoes form at a convergent plate boundary as

part of a volcanic arc They form where the Juan de Fuca

Plate subducts beneath the North American Plate at the

southern end of the Cascades Range

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Figure 10 This map shows the variety of active volcanic

landforms along the western coast of the United States

Identify a California volcano that is part of the volcanic arc

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Figure 12 This diagram shows intrusive and other features associated with volcanic activity

Identify which features shown are formed above ground

Which are formed below ground?

308 Chapter 7 • Volcanoes

Intrusive Volcanic Features

Most magma never reaches Earth’s surface to form noes or flow from fissure eruptions This magma slowly cools underground and produces intrusive features that later could

Batholiths

The largest intrusive volcanic features are batholiths

Batholiths form when magma slowly cools and solidifies

be many hundreds of kilometers in width and length and several kilometers thick

Dikes and Sills

Magma sometimes squeezes into cracks in rocks below the

layers and hardens is called a dike Magma that is parallel to rock layers and hardens is called a sill Most dikes and sills run from a few meters to hundreds of meters long

Figure 11 The batholith in

the top photo and the

volca-nic neck in the bottom

photo are examples of

vol-canic features.

To see an animation of erupting and

flowing lava, visit ca6.msscience.com

Lesson 2 • Volcanic Eruptions and Features 309

Other Volcanic Features

Not all volcanic features form

under-ground from igneous activity Some volcanic

features form from past and current volcanic

eruptions and lava flows

Volcanic Neck

When a volcano stops erupting, magma

hardens inside the vent Erosion, usually by

water and wind, begins to wear away the

vol-cano The cone is much softer than the

igne-ous rock in the vent, and tends to erode first,

leaving behind the igneous core as a volcanic

neck Shiprock, located in New Mexico and

eroded volcanic neck

Which part of a volcano tends to erode first?

Lava Domes

A lava dome is a rounded volcanic feature

that forms when a mass of highly viscous lava

slowly erupts from the vent The thick, sticky

granitic lava piles up instead of flowing freely

The viscosity of the granitic lava does not

allow it to release gases easily When pressure

builds up within the lava dome, gas, lava, and

solid materials can be violently ejected into

the air

Lava domes like the one shown in

volcanic eruptions

Lava Tubes

A lava tube is a hollow tube that forms

when lava flows through a channel, cools, and

hardens on the surface Lava tubes often form

underground Swiftly moving magma flows

from magma chambers and branches to the

sea The lava tube may be over 8 m in

diame-ter and as long as 48 km when the flow of

lava tube that was formed from pahoehoe

Figure 13 Examples of other volcanic features include volcanic necks, lava domes, and lava tubes.

Volcanic Neck

Lava Dome

Lava Tube