FOCUS ON EARTH SCIENCE (5)

Heat escaping from Earth’s internal layers constantly changes the planet’s inside and outside Earth produce Earth’s diverse >ˆ˜Ê`i> The solid Earth is made of minerals... 2 Earth’s inter

Earth’s Structure

74

Imagine the results of a fender bender between two cars The fenders of each are a crumpled mass of metal When

two continents collide, the results are similar —the rocks become crumpled

and broken The photo shows folded rock layers near Lulworth in the United

Kingdom They are the result of a collision between the African and European

plates hundreds of kilometers away.

Heat escaping from Earth’s

internal layers constantly

changes the planet’s

inside and outside Earth

produce Earth’s diverse

>ˆ˜Ê`i> The solid

Earth is made of minerals

Start-Up Activities

75

How can you model

landscapes?

Imagine you are hiking

through a natural area

such as Yosemite Valley,

California Make a list of

the landscape features

you think you would see

Procedure

1 Identify features on your list that are the

highest and the lowest in elevation

2 What makes each feature unique? Were

some flat, or peaked on the top?

3 Stack several pieces of artfoam in layers,

one on top of another Put your hands on

both ends of the stack, and shape the

layered artfoam into different terrains

Think About This

• Explain What did you do to the artfoam

that might indicate how a landscape

would form in nature?

• Examine the side of the model you made

What might the layers represent?

Visit to:

▶ explore Virtual Labs

▶ access content-related Web links

▶ take the Standards Check

STEP 1 Fold a sheet of paper in half

lengthwise Make the back edge about 2 cm longer than the front edge

STEP 2 Fold into thirds.

STEP 3 Unfold and cut along the folds of

the top flap to make three flaps

STEP 4 Label as shown.

>˜Ìi

following Foldable to show Earth’s layers

Earth’s layers on the tabs Under each tab, explain the features and describe the energy in that layer

1.a, 7.e

ELA6: R 2.4

ca6.msscience.com

Learn It! Main ideas are the most important ideas in a paragraph, a lesson, or a chapter

Supporting details are facts or examples that explain the

main idea Understanding the main idea allows you to

grasp the whole picture.

Practice It! Read the following graph Draw a graphic organizer like the one below to

para-show the main idea and supporting details.

The wearing away of soil and rock is called erosion

Water does most of this work Rivers and streams carry rock fragments as the water flows downhill

Over long periods of time, this action changes the landscape Mountains are worn down to flat plains

As rivers flow toward lakes or oceans, they carve valleys and steep-sided canyons.

—from page 80

Main Idea

Get Ready to to Read

76

Identify the Main Idea

Apply It! Pick a paragraph from another lesson of this chapter and diagram the main idea as you did above.

ELA6: R 2.3

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

• Write an A if you agree with the statement.

• Write a D if you disagree with the statement.

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

you’ve changed your mind about any of the statements

• If any of your answers changed, explain why

• Change any false statements into true statements

• Use your revised statements as a study guide

The ma in idea is

often th e first sen tence in

a parag raph but not always.

1 Energy from the Sun changes Earth’s landscapes

2 Earth’s internal energy pushes up the land; surface processes wear it down

3 Most of Earth, including its interior, is composed

of rock

4 Hardness and color are the two main characteristics

of gems used in jewelry

5 Matter and energy move from Earth’s interior toward the surface

6 Heat is always escaping from Earth’s interior

7 Humans have drilled holes and collected samples to about 500 km deep in Earth

8 There is one type of crust near Earth’s surface, and it

is found on the continents

9 The thickest of Earth’s layers is the core

10 Seismic waves do not penetrate Earth’s layers

Before You Read

LESSON 1

Figure 1 Earth’s landscape is the result of internal and external forces constantly acting upon the surface.

78 Chapter 2 • Earth’s Structure

How do landscapes form?

You live on the surface of Earth Look out the window at this surface, or look at a photograph or drawing of a land-scape Figure 1 is an example There are tall mountains, deep valleys, and flat plains Why does the landscape have different shapes and forms?

An endless interaction of forces reshapes Earth’s raphy The transfer of matter and energy from Earth’s inte-rior builds mountains Forces on the surface continuously wear down the mountains These forces are caused by uneven heating of the surface by the Sun In turn, this energy is transferred to the atmosphere This makes weather that constantly bombards surface material and erodes it away, especially in higher areas Without these competing forces, the planet’s surface would be a flatter and less exciting place to live

topog-What is the source of energy for Earth’s weather?

Why It’s Important

You’ll appreciate landforms

around you as you discover

how they form and change.

temperature, wind speed and

direction, humidity, and air

pressure (Grade 5)

Science Content

Standards

1.e Students know major geologic events,

such as earthquakes, volcanic eruptions,

and mountain building, result from plate

motions.

1.f Students know how to explain major

features of California geology (including

mountains, faults, volcanoes) in terms of

plate tectonics

2.a Students know water running

downhill is the dominant process in shaping

the landscape, including California’s

landscape.

7.c Construct appropriate graphs from

data and develop qualitative statements

about the relationships between variables.

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map shows major landform regions

in the continental United States.

Identify What landform covers much

of California?

Lesson 1 • Landforms 79

Landforms

Features sculpted by processes on Earth’s surface are called

landforms They can cover large regions or be smaller, local

features Figure 2 shows the landform regions of the

continen-tal United States These are large areas with similar

topogra-phy Find your location on the landform map in Figure 2

Three main types of landforms are shown on the landform

map These examples are mountains, plateaus, and plains

Mountains and plateaus are areas with high elevations Plains

are low, flat areas

Landforms Made by Uplift

Uplift is any process that moves the surface of Earth to a

higher elevation Both mountains and plateaus are formed by

uplift If a large flat area is uplifted, a plateau is formed If the

uplifted area is not flat, but has many steep slopes, it is called

a mountain

Earth’s internal energy produces uplift As thermal energy

from Earth’s interior moves toward the surface, it also causes

matter in the interior to move upward An example of a

land-form moved by uplift is shown in Figure 3 Sometimes Earth’s

internal heat energy melts rocks If this melted rock moves to

the surface, a mountain called a volcano can form More

often, the heat does not melt the rocks but makes mountains

by pushing solid rocks upward Scientists call the forces that

can push solid rocks upward plate tectonics, which you will

read about in Chapter 5

Landforms Mountains and

plateaus are made by uplift

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80 Chapter 2 • Earth’s Structure

Landforms Shaped by Surface Processes

While Earth’s internal energy pushes up the land, surface processes wear it down As you read earlier, energy from the Sun drives some of these processes on the surface Water, wind, ice, and gravity break apart the rocks that make up mountains These broken fragments are carried downhill, making the mountains smaller

The wearing away of soil and rock is called erosion Water

does most of this work Rivers and streams carry rock ments as the water flows downhill Over long periods of time, this action changes the landscape Mountains are worn down

frag-to flat plains As rivers flow frag-toward lakes or oceans, they carve valleys and steep-sided canyons Figure 4 shows land-forms that can form as the material is eroded and

transported by rivers

When rivers eventually slow, they deposit some of their load of rock fragments The fragments are distributed by the water to build other landforms, like the beach shown

in Figure 4 Wave action from the ocean moves fragments

of rocks, such as the sand on this beach, along the coastline

valleys, canyons, and beaches are made by

erosion and deposition of rock material

that once was part of uplifted landforms.

Locate areas where eroded fragments

have been deposited.

ACADEMIC VOCABULARY

transport (trans PORT)

(verb) to carry from one place

to another

A large truck was needed to

transport the cargo.

Yosemite Valley

Lassen Peak

Yosemite Valley and Lassen Peak show how diverse the California landscape can be.

Lesson 1 • Landforms 81

California Landforms

California has many types of landforms

Some are so spectacular that they are

pre-served in state or national parks Maybe you

have taken a trip to visit one of these parks

Yosemite Valley

For example, the U-shaped surface of the

valley in California’s Yosemite National Park

is shown in Figure 5 Glaciers carved this

shape into the valley as they moved across its

surface about one million years ago In

con-trast, rivers usually carve sharper, V-shaped

valleys as they cut through and erode rock

How do valleys carved by glaciers differ in shape from valleys carved

by rivers?

Lassen Peak

Another national park with landforms is

Lassen Volcanic National Park It features an

active volcano, which is shown in Figure 5

Lassen Peak is a volcano that is part of the

Cascade Mountain Range A series of violent

volcanic eruptions in 1915 blasted out a new

crater at Lassen Peak’s summit The

explo-sion expelled melted rock, gas, and ash that

dramatically changed the landscape around

the volcano Volcanic ash mixed with snow

and ice This caused a rapid flow of mud

down the sides of Lassen Peak and into river

valleys below Residents living in the vicinity

of the eruptions lost their homes

These California landforms show how

dif-ferent forces can act to change the landscape

External forces that caused precipitation for

glacial ice to accumulate shaped the

land-scape of Yosemite Valley Internal forces

caused volcanic eruptions that altered the

landscape surrounding Lassen Peak

82 Chapter 2 • Earth’s Structure

Mountains

California’s major landforms are shown in Figure 6 This

is a shaded relief map of the state Find the Sierra Nevada and the Coastal Ranges These are examples of mountains formed

by the forces of plate tectonics Solid rock was pushed up, forming high peaks Because the ranges are long and narrow, they sometimes are called mountain belts

Figure 6 Identify two landform regions to the north of the Transverse ranges.

Now find Mount Shasta in Figure 6 It looks different from the other mountains In fact, Mount Shasta looks like a dis-tinct circle on the map Mount Shasta is a volcano It did not form by uplift of solid rock, as did most of the mountains in California Mount Shasta’s cone-shape formed when melted rock poured out from its center onto the land surface

California’s mountains continue to grow upward Most often they grow so slowly you don’t even realize this uplift is happening Other times a volcanic eruption or an earthquake causes sudden uplift

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California Agriculture Statistics

• California has been the top agricultural state for more than 50 years.

• Agriculture generates almost

$26.7 billion per year.

• Almost one-third of California’s land area is used for farming.

• California produces more than 350 crops.

• California grows more than half of the United States’

fruits, vegetables, and nuts.

mountain ranges surrounding the Central

Val-ley have provided the soil base for producing

most of California’s agricultural products.

Lesson 1 • Landforms 83

Valleys

Next to the California mountain ranges are flat, open

val-leys As the mountain peaks rise upward, erosion by water,

wind, ice, and gravity wear them down Water is a powerful

force, capable of carrying loosened rock fragments and soil

particles from the mountains down to the valleys This loose

material helps make the valley’s farmland rich in soil

nutri-ents for growing plants

These fertile valleys make California a top-ranked

agricul-tural producer in the United States Figure 7 shows a farm

located in the Great Central Valley What is being produced

on the farm shown here?

California also has many deep, narrow valleys Rivers carve

these valleys as they flow from the mountains toward the

Pacific Ocean The water carries loosened rock fragments

from the west side of the Sierra Nevada, down toward the

Central Valley, and eventually to the Pacific coast

Beaches

Sand-sized grains of rock loosened from mountains toward

the east provide material for beaches along the Pacific coast

Beaches are temporary features that must have sediment

added constantly in order to exist This is because sand is

constantly washed away by ocean currents moving parallel

to the shore Without rivers continuously adding more sand,

beaches would disappear Material that has been transported

by a creek and deposited along the Pacific shore is shown in

Figure 8

Source: USDA Agriculture in the Classroom

slows down, its sediment is deposited in sandbars and

on the beaches.

land-Science nline

For more practice, visit Standards

Check at .

Summarize

Create your own lesson

summary as you design a

visual aid.

1 Write the lesson title,

number, and page

num-bers at the top of your

poster

2 Scan the lesson to find

the red main headings

Organize these headings

on your poster, leaving

space between each.

3 Design an information

box beneath each red

heading In the box, list

2–3 details, key terms,

and definitions from each

1 A glacier scraping sediment

and rock from the sides of a mountain is an example of

2 In your own words, write a

definition for landform 1.e

Understanding Main Ideas

3 How did the landform shown

above most likely form? 1.e

A when a block of rock

uplifted

B when sediment was piled

up by a river

C when a volcano erupted

D when a glacier passed over

Applying Science

7 Predict what would happen

to Earth’s surface if all of Earth’s internal heat

Many different types of landforms make up California’s landscape Mountains

are especially prominent throughout the state Explore how to determine the

differences among them and if these differences are clues to how the

mountains formed.

Data Collection

1 Visit ca6.msscience.com to examine some bird’s-eye view images to find

different types of mountains in different regions of California.

2 Make a table of observations like the sample data table below Use the

menu along the margin of the Web site to observe the mountains listed in

the data table Explain any differences you observe Draw some

outstand-ing features for later comparisons

Data Analysis

1 Identify a mountain range that was formed by volcanic eruptions.

2 Compare and contrast characteristics of the mountains you studied.

3 Graph Make a bar graph that includes the names of the mountains and

plateaus and their elevations Use the following data: Mt Shasta (4,317 m),

Mt Eddy (2,751 m), Mt Diablo (1,173 m), Mt Whitney (4,417 m).

Science Content Standards

7.c Construct appropriate graphs from data and develop qualitative statements about the relationships between

variables.

LESSON 2

Figure 9 Identify items in this picture that you

think were made from minerals or rocks.

86 Chapter 2 • Earth’s Structure

Minerals and Rocks

>ˆ˜Ê`i> The solid Earth is made of minerals and rocks

Real-World Reading Connection You stand on the bank

of a creek and throw rocks in the water Rocks seem to be everywhere But in your yard there are hardly any rocks What are rocks? What are they made from? Where do they come from?

What is Earth made of ?

The solid part of Earth is made up of minerals and rocks People use them to build homes and roads Minerals and rocks break down to form the soil in which farmers grow food Some rocks and minerals are even used as jew-elry because they are so beautiful Minerals and rocks are such a common part of the environment that you might not realize they are all around you Figure 9 shows some common items made from mineral and rock resources.Minerals are the substances that make up rocks Scien-tists have identified about 3,800 distinct minerals, but most

of these are rare There are only about 30 common als Minerals form when crystals grow in nature For exam-ple, they can grow in melted rock material or from material dissolved in water

miner-Reading Guide

What You’ll Learn

▼Identify minerals by

observing their properties.

▼Explain the value of

minerals in your life.

▼Classify rocks according to

how they form.

▼Illustrate how the rock

cycle continuously recycles

Earth materials.

Why It’s Important

The majority of Earth

materials, even those in the

deep interior, are solid rock.

igneous rock: rock that

forms from magma or lava

(Grade 4)

Science Content

Standards

2.c Students know beaches are dynamic

systems in which the sand is supplied by

rivers and moved along the coast by the

action of waves.

6.b Students know different natural

energy and material resources, including air,

soil, rocks, minerals, petroleum, fresh water,

wildlife, and forests, and know how to

classify them as renewable or nonrenewable.

6.c Students know the natural origin of

the materials used to make common objects.

7.e Recognize whether evidence is

consistent with a proposed explanation.

Lesson 2 • Minerals and Rocks 87

What is a mineral?

The word mineral has several common meanings You

might drink mineral water, or someone might tell you to eat

healthful food, so that you get all the vitamins and minerals

that you need to be healthy In Earth science, the word

min-eral has a specific definition A minmin-eral is a naturally

occur-ring, generally inorganic solid that has a crystal structure and

a definite chemical composition How can you tell if

some-thing you are looking at is a mineral? Materials classified as

minerals have the following properties

Naturally Occurring To be considered a mineral, a

sub-stance must be found in the natural world Anything

manu-factured by people, such as one of the gemstones in Figure 10,

are not minerals For example, diamonds mined from Earth

are minerals, but synthetic diamonds made in laboratories

are not

Generally Inorganic Most minerals are formed by processes

that do not involve living things But, there are some

miner-als made by living things The mineral aragonite is found in

pearls, which are made by oysters, and the mineral apatite is

found in your bones and teeth

Solid Substances that are liquids or gases are not considered

minerals Therefore, natural emeralds like the ones shown in

Figure 10 are minerals, but the liquid that would form if they

were to melt is not a mineral

Synthetic Emerald Natural Emerald

of the mineral called beryl

Compare and contrast the appearances of the

synthetic and the natural emeralds.

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WORD ORIGIN mineral

minera- Latin; means mine or

ore

mineralis- Latin; means of or

from the mine

organic compound made by humans

made in laboratories, hard

to distinguish from natural rubies

gemstone ranging

in color from clear

or green to violet and blue black

88 Chapter 2 • Earth’s Structure

Crystal Structure The atoms in a mineral are arranged in

orderly, repeating patterns This regular atomic pattern is called a crystal structure The smooth flat surfaces on a crys-tal represent a well-organized, internal structure of atoms Observe the crystal structure of the mineral halite shown in

Figure 11 Notice that the outer, smooth faces of the halite crystal make the same shape as its internal atomic structure

Definite Composition A mineral is made of specific

ele-ments Not only must a mineral have certain elements, but the elements also must be in definite proportions A common example is the mineral quartz It is made of the elements sili-con (Si) and oxygen (O) The chemical formula for quartz is SiO2 The formula tells you there are two oxygen atoms for every silicon atom in quartz The chemical formula shows both the elements and their proportions

nature of the halite crystal

is one property used to

identify it.

Interactive Table Organize information about minerals at ca6.msscience.com.

Table 2 Mohs Hardness Scale

Mineral Hardness Common

copper coin deeply

hardness as glass

glass

glass and feldspar

quartz

most minerals

common materials

Physical Properties

of Minerals

You can tell one mineral from another by

its physical properties Physical properties are

characteristices that can be observed or

mea-sured without changing the identity of the

mineral If you learn how to test a mineral for

these properties, you will be able to use the

tests to identify many minerals Some of the

more common physical properties you can

use to identify minerals are described next

Hardness

You can test the hardness of a mineral by

observing how easily it is scratched Any

mineral can be scratched by another mineral

that is harder In the early 1800s, Austrian

scientist Friedrich Mohs developed a

hard-ness scale with 10 minerals On this scale, the

hardest mineral, diamond, has a hardness of

10 The softest mineral, talc, has a hardness

of 1 Table 2 shows the Mohs’ hardness scale

Quartz, feldspar, and calcite are on the scale,

and they all are common minerals

Table 2 Which minerals can be scratched by feldspar?

Color

A mineral’s color can sometimes help you

identify it The mineral malachite, for

exam-ple, always has a distinctive green color

because it contains the metal copper Most

minerals do not have a single distinctive

color, as shown by the many colors of quartz

Figure 13 Constant Streak Although the colors

of hematite can be different, the streak is always reddish-brown

Infer which is harder—the porcelain tile or the hematite.

90 Chapter 2 • Earth’s Structure

Streak and Luster

Streak is the color of powder from a mineral You can look

at the powder by scratching the mineral across a tile made of unglazed porcelain Some minerals that vary in color have distinct streak colors For example, the color of the mineral hematite can be silver, black, brown, or red But, notice in

Figure 13 that the two different-colored hematite samples both show a reddish-brown streak

Luster is the way a mineral’s surface reflects light gists use several common words to describe mineral luster Two of these are shown in Figure 14 Galena has a shiny metallic luster Quartz has a glassy luster Other terms used

Geolo-to describe luster are greasy, silky, and earthy Look again at

Figure 13 and try to use these terms to describe the luster

of the hematite samples Do both hematite samples have the same luster?

Crystal Shape

Every mineral has a unique crystal shape A crystal that forms on Earth’s surface will be small, because the erupting lava flow cools rapidly Crystals are large and perfect when they form underground where Earth’s heat is maintained and the magma source cools slowly As Figure 14 illustrates, each crystal has a distinct shape, which sometimes is referred to

as crystal habit

Glassy Luster

quartz have distinctive

crystal shapes and lusters.

Metallic Luster

Figure 15 The way a mineral breaks into pieces can help with identifica- tion Striking a piece of calcite with a hammer causes it to break along flat cleavage planes Quartz mineral (inset) breaks on curved fracture surfaces

Lesson 2 • Minerals and Rocks 91

Cleavage and Fracture

Cleavage and fracture describe the way a mineral breaks If

it breaks along smooth, flat surfaces, it has cleavage A eral can have one or more distinct cleavage directions If a mineral breaks along rough or irregular surfaces, it displays fracture Figure 15 shows examples of both cleavage and frac-ture The calcite has three distinct cleavage directions This makes it break into blocks Quartz does not have cleavage It breaks along curved surfaces, so it displays fracture

min-How many directions of cleavage does calcite have?

Density

Density is the amount of matter an object has per unit of

volume Some minerals are denser than others If you pick up

a piece of galena and a piece of quartz, and both are about the same size, you can feel that the galena is much heavier

This is because galena is denser than quartz

Most metals have high densities compared to nonmetals

Minerals with atoms packed closely together also tend to have higher densities Quartz and feldspar are common min-erals with relatively low densities Olivine, with a closely packed structure of atoms and some iron in its structure, has

a relatively high density When a mineral has an especially high or low density, its density can be used to identify it

Quartz

Calcite

92 Chapter 2 • Earth’s Structure

properties that help identify them

Explain how the property of magnetism could help physically separate minerals.

Other Properties

Some minerals have properties that make them easy to identify For example, magnetite is magnetic Figure 16 shows how magnetite attracts a magnet Calcite reacts chemically to acids If you place a drop of acid on calcite, it fizzes

Calcite also shows an interesting property that occurs when light interacts with it If you look at an object through a clear calcite crystal, you can see two images of the object, as shown

in Figure 16 This is called double refraction, and it occurs when light splits into two separate rays, each forming its own distinct image of the object

What property of calcite produces double images of objects viewed through it?

Many properties of minerals make them ideal to use in industry For example, quartz can produce an electric current when pressure is applied to it Graphite can be used to mark

on paper Copper is used in electronic wiring because it is a good conductor of electricity

Every mineral has properties that can be observed to help identify it But remember that many minerals have similar properties You need to test for a combination of properties to find those that are unique to a particular mineral It can be a challenge to find an unfamiliar mineral and try to figure out what it is

Double Refraction Magnetism

Other Properties

It can be challenging to identify a mineral correctly, because many of them have lar properties But, with a few simple tools, you can observe a set of characteristic

simi-physical properties for an unknown mineral This can help you determine what it is.

Procedure

1 Complete a safety worksheet.

2 Obtain three or four unknown numbered mineral samples from your teacher.

3 Use a field guide for rocks and minerals, a magnifying glass, a streak plate, a

copper coin, a glass plate, a magnet, a graduated cylinder, and a triple-beam ance to help you determine the physical properties of each sample.

bal-4 For each sample, observe and record the physical properties, color, streak, luster,

hardness, and cleavage or fracture using information in Lesson 2.

5 To determine the density of a sample, place it on the triple-beam balance and sure the mass in grams Then tie a string around the sample and carefully lower it into the graduated cylinder that has a recorded volume of water in it Subtract the original volume from the new volume of water Divide the mass by the volume.

mea-Analysis

1 Compare your results to the information in the field guide.

2 Identify each mineral using your observations and the guide.

3 Evaluate which properties were most helpful for you to identify a mineral

Describe any properties that could help you identify a mineral without testing

other properties.

Mineral Identification

by Property

Science Content Standards

7.e Recognize whether evidence is consistent with proposed explanation.

Figure 17 The clear

diamond, ruby, blue

sapphire, and ruby

are cut and polished

materi-appreciate some minerals solely for their beauty.

Metallic Ores

Rich deposits of valuable minerals are called ores The als you use every day come from these ores.The minerals chalcopyrite and malachite are examples of copper ores Cop-per is a common metal used in wires to conduct electricity.Iron used to make steel comes from hematite and magne-tite Steel is used to manufacture cars, bridges, skyscrapers, and many other things you use every day Galena is the major ore for producing lead Most lead is used to manufacture auto-mobile batteries The minerals gold and silver are considered precious metals They are used in industry and also in jewelry

met-What is the major ore used for producing lead?

Gemstones

People have been collecting minerals for their beauty for thousands of years These minerals are called gems Many gems have intense colors, a glassy luster, and are 7 or more on the Mohs hardness scale Diamonds, rubies, sapphires, and emeralds are among the most valuable gemstones When these rare minerals are cut and polished, their value can last for hundreds of years Figure 17 shows the difference between these minerals before and after they are cut and polished

Cut ruby on uncut matrix

Cut sapphire Uncut sapphire

Uncut diamond

Cut diamond

ACADEMIC VOCABULARY

appreciate (uh PRE

shee ayt)

(verb) to grasp the nature,

quality, worth or significance

of

It is difficult for most people to

appreciate patience.

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Lesson 2 • Minerals and Rocks 95

Rocks

A rock is a natural, solid mixture of particles These

parti-cles are made mainly of individual mineral crystals, broken

bits of minerals, or rock fragments Sometimes rocks contain

the remains of organisms or are made of volcanic glass

Geol-ogists call the particles that make up a rock grains

Most of Earth is made of rocks Mountains, valleys, and

even the seafloor under the oceans are made of rocks You

might not always notice the rocks under your feet Figure 18

shows an example of how rocks and soil are present beneath

a landscape’s surface

Rocks are classified, or placed into groups, based on

the way they form There are three major groups of rocks:

igneous rocks, metamorphic rocks, and sedimentary rocks

Figure 18 What happens to particles eroded from the mountains?

of the crust move up or down

along the faults, producing

mountains, hills, and valleys.

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Cool-ing Rates The grain

size of an igneous

rock depends in part

on how quickly the

magma cools.

96 Chapter 2 • Earth’s Structure

Igneous Rocks

Igneous rocks are formed from molten, or liquid, rock

material called magma As the temperature of magma drops,

tiny crystals of minerals begin to form These tiny crystals become the grains in an igneous rock

Located at Earth’s surface, magma, now called lava, cools

quickly The crystals in lava do not have much time to grow,

so they are small Volcanic glass forms when lava cools so idly that atoms do not form well-organized crystal structures.Deep within Earth, magma cools slowly because thick lay-ers of rock surround it There is more time for larger crystals

rap-to grow Figure 19 shows a cross-section, or slice, through Earth Notice that the igneous rock called granite in

Figure 19 has larger mineral grains than the igneous rock called basalt This is because granite cools much more slowly than basalt does

Why does magma cool slowly?

Like the word mineral, texture is a common word But in

Earth science it has a specific definition The grain size and the way grains fit together in a rock are called texture

Because granite and basalt have different-sized grains, they have different textures Granite’s texture is coarse grained and basalt’s texture is fine grained Figure 20 shows El Capi-tan, which is a huge mountain of granite now exposed at the surface by uplift

The igneous rocks granite and basalt do not differ only in texture They also differ in mineral composition Granite contains low-density minerals such as quartz and feldspar Basalt is made of higher-density minerals than granite, such

as olivine and magnetite