California science grade 5 (11)

• room-temperature water • plastic water bottle When pressure is increased on a volume of air, its temperature increases.. Air always moves from an area of high pressure to an area of l

Shaping Earth’s Surface

What effects do weathering, erosion, and deposition have on Earth’s landforms?

338

L e s s o n 1

Atmospheric Pressure, Temperature, and

Weather

L e s s o n 4

Beaches and Wave Erosion

L e s s o n 2

Weathering, Erosion, and Deposition

6 ES 2 Topography is reshaped by the weathering

of rock and soil and by the transportation and deposition

of sediment.

339

ELA R 6.2.4 Clarify an

understanding of texts by creating outlines, logical notes, summaries, or reports.• ELA W 6.2.5 Write persuasive

compositions.

Literature

MAGAZINE ARTICLE

340

Write About It

Response to Literature This article outlines the problem of beach erosion Geologists and engineers are working on solutions to the problem Do you think they should take the concerns of surfers or sunbathers into account? Write a persuasive essay explaining your point of view.

- Journal Write about it online

by Kim Y Masibay

Surfing buffs have a word for the perfect wave—

“tubular.” A wave curls into a tube and surfers ride inside it toward shore But the endless pounding

of surf can really grind down a beach “About 86 percent of U.S shoreline is eroding [washing away],” says Chad Nelson, environmental director of the Surfrider Foundation

The reasons are many: Damming rivers and

building coastal structures cause some harm And the U.S Environmental Protection Agency thinks higher concentrations of greenhouse gases in the atmosphere [are] causing Earth’s temperature to warm, which

in turn could raise sea level a few feet in the next century Bye-bye beaches?

Not if geologists or engineers can help it But beach lovers are divided over how to best halt coastal erosion Why? Because tactics that keep sand where sunbathers want it can also wipe out the wave action in the surf zone, the area between the outermost breakers and the shore Can human ingenuity save the day?

from SCIENCE WORLD

341

Lesson 1

Atmospheric Pressure,

Temperature, and Weather

A television meteorologist wraps up the weather forecast:

“Today was warm and humid Temperatures are expected

to cool rapidly tonight, bringing fog during the early

morning Skies will clear by mid-morning as temperatures

rise again.” What causes fog? Where is fog likely to form?

coastal fog, Mount Tamalpais State Park, California

6 ES 4.e Students know differences in pressure, heat, air movement, and humidity result in changes of weather.

342

ENGAGE

• room-temperature water

• plastic water bottle

When pressure is increased on a volume of air, its

temperature increases When pressure is decreased,

the volume of air expands and cools How do

temperature and humidity affect the formation of

fog? Write your answer as a hypothesis in the form

“If moist air in a bottle is cooled, then ”

Test Your Hypothesis

Put a small amount of room-temperature water

in a plastic water bottle.

Have your teacher add smoke by lighting a

match, blowing it out, and holding the smoking

match inside the bottle.

After a few seconds, have your teacher take

the match out and screw on the bottle’s cap.

Experiment Squeeze the bottle to increase

the pressure on the air inside Release the

pressure on the bottle.

Draw Conclusions

Analyze What happened when you released

the pressure on the air in the bottle? How is this

related to the temperature of the air in the bottle?

Infer How does moist air’s temperature affect

a change from water vapor to water droplets?

How is this similar to the formation of fog?

What would you see if you did this experiment first

with warm water and then with cold water? Form

hypotheses and then test them with your teacher

or another adult Analyze and present your results.

Explore More

6 IE 7.a Develop a hypothesis • 6 IE 7.d Communicate the steps and

results from an investigation in written reports and oral presentations.

343

EXPLORE

What is the difference between climate and weather?

An area’s climate (KLIGH•muht) is the long-term average weather pattern of that region It includes atmospheric conditions such

as precipitation, temperature, and wind Many areas of the United States have climates with warm summers and cold winters Weather

refers to the day-to-day conditions in an area Even though the climate in many states in the United States include hot summers, the weather

on any single summer day may be cool

For example, the southern United States has a warmer climate than the northern United States Winter temperatures in Southern California and Florida rarely fall below freezing, and snow rarely falls there Winter temperatures in North Dakota

Differences in air pressure

and temperature cause

winds, storms, and other

weather patterns An area’s

climate is the long-term

average of its weather.

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This satellite photo of

Earth shows how the

ocean temperatures

around the world vary

Yellow areas are the

warmest.

Explore weather patterns with

a meteorologist.

344

EXPLAIN

and Montana, on the other hand,

are often below freezing, and heavy

snowfalls are frequent

Altitude also affects climate The

higher up you go on a mountain, the

cooler you will find the temperature

to be Even at the same latitude, a

mountain peak may have snow in the

spring and summer while a nearby

valley is warm enough for swimming

and other summer sports Radiation

from the Sun first heats the land Then

heat from Earth’s surface radiates into

the atmosphere and warms it The

thinner air at higher altitudes contains

fewer molecules to absorb heat, so it

remains cooler You may have noticed

this change in temperature if you have

traveled from the warm coast to the

cooler inland mountains

Changes in Weather

Many factors make up daily

weather, including temperature, air

pressure, and humidity (hew•MID•i•tee)

Humidity is the amount of water vapor in the air Precipitation, cloud cover, ocean temperature, and wind patterns also affect weather A change

in any of these factors alters the daily weather in a particular location

Weather conditions averaged over a long period of time determine climate Constant high temperatures and rain characterize tropical climates A place that is cold all year has a polar climate

climate in Alaska different from the climate in California?

difference between climate and weather?

345 EXPLAIN

What is air pressure?

Air pressure is the force caused by

the weight of air above Even though

air molecules cannot be seen, they

take up space and have weight Air

pressure depends on temperature and

gravity When the temperature rises, air

molecules move apart This hotter, less

dense air exerts less pressure When

the temperature is cooler, air molecules

slow down, and the cooler, denser air

exerts more pressure

The force of gravity attracts air

particles and pulls them toward

Earth, increasing the air pressure

At higher altitudes, fewer air particles

are attracted toward Earth, and air

pressure falls

If you take a trip to the top of

a mountain, you may experience

a “popping” in your ears The air

pressure around you decreases as you

climb the mountain Soon, the pressure inside your ear is no longer in balance with the pressure outside your ear The popping is your body’s attempt to equalize the pressure Also, during your trip up the mountain, you might notice that you have to breathe more rapidly to get enough oxygen as you climb Fewer molecules of oxygen exist at higher altitudes, so you have to take more-rapid breaths to get the same number

of molecules of oxygen into your lungs.The table shows the change in air pressure with altitude For example, 4,418 m (14,495 ft) is the height of Mount Whitney, the highest point in the 48 adjoined United States, and about 80.5 km (100 mi) is considered

by some to be the point at which space begins Air pressure is recorded in Pascals, a unit of measure that equals about 0.009 kg (0.02 pounds) per 0.09 square meter (0.96 square foot)

Air Pressure and Altitude Height

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States, for example, a winter pressure system can bring frigid Arctic air south across the country

high-A low-pressure system often brings cloudy weather The warm air in the center rises, cools, and condenses, and this process creates clouds In winter,

a low-pressure system can bring heavy snowstorms or ice storms in some parts of the country During the warmer months, low-pressure systems often bring thunderstorms or other severe weather

High- and Low-Pressure Systems

How does air pressure affect

weather? Solar radiation does not heat

all areas of Earth in the same way

Temperature affects air pressure

Because temperature and pressure are

related to each other in the atmosphere,

differences in air pressure around the

world exist because of differences in the

amount of solar radiation Differences

in air pressure create winds that blow

around the globe The direction of the

wind is always from an area of high

pressure to an area of low pressure

The cool air in the center of a

high-pressure system sinks toward Earth

and spreads out along the ground

Clouds rarely form when air sinks

A high-pressure system, then, often

means fair weather A cool

high-pressure system typically brings clear

skies and pleasant weather In the

winter, though, a high-pressure system

can bring a cold wave In the United

Quick Check

pressure does outer space have?

air become thinner as you climb

to the top of a mountain?

U S Weather Map

347 EXPLAIN

What causes rain?

As energy from the Sun heats Earth,

the water in oceans, lakes, and rivers,

as well as groundwater, evaporates

and turns into water vapor, a gas

Evaporation (ee•vap•uh•RAY•shuhn)

is the changing of a liquid into a gas

Humid air is less dense than drier air,

and therefore, it rises As the air rises,

its air pressure and temperature fall

Condensation (con•den•SAY•shuhn)

occurs when a gas changes into

a liquid Water vapor in the air

condenses into droplets, which then

collide and form larger droplets

When the droplets become too

large and heavy to stay in the air,

they fall to Earth as precipitation

(pree•sip•i•TAY•shuhn) Precipitation

includes all forms of water that fall to

the ground The type of precipitation

is a result of changes in air pressure

and temperature

Dew Point

As the temperature falls, less water vapor can stay in the air When the air cannot hold any more water, the

relative humidity is 100% Relative humidity measures the amount of

water vapor in the air compared

to the amount that will saturate the air At the saturation point, the vapor condenses into a liquid The temperature at which this condensation occurs is called the dew point When humid air near the ground cools to its dew point during the night, that condensation results in the water droplets—the dew—that can often

be seen early in the morning on blades of grass

▲ These low clouds formed from water droplets The arrows show the path water on the surface takes as it evaporates and then condenses and forms clouds.

348

EXPLAIN

Types of Precipitation

Temperature differences in the

atmosphere determine the type of

precipitation that falls The four

main types of precipitation are rain,

sleet, snow, and hail All forms of

precipitation begin as water droplets

or as ice crystals in clouds The

temperature of the lower atmosphere

that the precipitation passes through as

it falls to Earth determines which form

it will take

Measuring Humidity

Make a map of the inside and the outside of your school Choose places to measure the humidity

in the air.

Obtain cobalt chloride strips from your teacher These strips measure humidity

in the air by turning pink.

Tape each cobalt chloride strip to a piece of white paper.

Observe Hang the papers in the chosen locations Wait at least

30 minutes Then collect the papers and examine the colors

of the strips.

Which location had the highest humidity? Which one had the lowest? Explain your results.

Quick Check

determines whether precipitation

falls as rain or snow?

warm air hold more water vapor

than cooler air?

Precipitation

Rain falls when the air temperatures are above freezing.

Sleet occurs when raindrops fall through freezing air near

Earth’s surface and turn into pellets of ice.

Snow falls when water vapor passes through air that is

so cold that the water vapor crystallizes and builds into snowflakes.

Hail forms when thunderstorm winds push water back up

into the atmosphere, where the water turns into ice, is coated with more water, and pushed up to freeze again This process repeats until the hailstone is heavy enough to fall.

349 EXPLAIN

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More solar radiation reaches the

equator than reaches regions north or

south of the equator The warmer air at

the equator rises and moves toward the

poles, creating air currents that move

across the world These currents are

pushed to the west or east by Earth’s

rotation For example, in Mexico and

the Caribbean, the winds usually blow

from the northeast to the southwest

These are called trade winds In the

United States, the wind usually comes

from the west or the southwest

The Coriolis Effect

The atmosphere around Earth moves constantly, with warm air rising and cold air falling Winds, powered by temperature differences caused by solar radiation, blow from areas of higher pressure to areas of lower pressure However, the winds

do not move in straight lines They are pushed to the right or the left by Earth’s rotation This shift in wind direction is called the Coriolis effect

(kaw•ree•OH•luhs), after the French scientist and engineer who described it

Describe how prevailing winds differ north and south

of the equator.

Clue: Note the directions of the arrows on each part of the diagram.

Reading Diagrams Prevailing Winds

350

EXPLAIN

As the diagram on the previous page

shows, the Coriolis effect causes winds

that blow south from the North Pole to

rotate to the right in northern latitudes

but change direction near the equator in

a clockwise direction In the Southern

Hemisphere, winds rotate to the left,

or in a counterclockwise direction The

Coriolis effect is more powerful on

stronger winds than on weaker ones and

more powerful on winds closer to the

poles than on those closer to the equator

Sea and Land Breezes

If you have ever been at the beach

during a summer day, you may have

noticed that the wind blows in from

the ocean to the land At night the

wind reverses and blows from the

land to the ocean What causes this

change in the wind’s direction?

On a warm summer day, the

Sun’s energy heats the land to a higher

temperature than the nearby water

The warmer air above the surface

of the land heats, expands, and rises This creates an area of low pressure Cooler, high-pressure air over the water moves in to replace the rising warm air above the land This flow of air from

water to land is called a sea breeze.

At night the reverse may take place The land cools more quickly than the nearby water, so the cool, high-pressure air is above the land, and the warm, low-pressure air is above the water Air always moves from an area of high pressure to an area of low pressure,

and a land breeze develops as the wind

blows from the land toward the water

◀ During the day, winds usually blow from the water toward the land.

▶ At night, the wind

changes direction

and blows from

the land toward

the water.

Quick Check

winds help early explorers sail around the globe?

sea breezes?

351 EXPLAIN

How else does wind

affect weather?

Sometimes smaller currents of air

spin off from the main currents of air

that flow around Earth and rotate in

the opposite direction from the main

currents These currents are called

eddies An eddy is a small, spinning

air current that often develops when

wind flows over buildings, mountains,

or other obstructions Some eddies

form on the downwind, or sheltered,

side of a mountain or a building The

size of the eddy depends on how big

the obstruction is and how quickly

the wind is blowing

Cyclones

A cyclone is a huge mass of

spinning air that forms when an area

of low pressure is surrounded by high

pressure on all sides The high-pressure

air flows inward and is deflected to the left or the right by the Coriolis effect This effect means that in the Northern Hemisphere, the winds in a cyclone blow inward and counterclockwise.Because they are low-pressure systems, cyclones often bring thunderstorms, tornadoes, and other stormy weather Under the right conditions, cyclones can turn into

hurricanes The powerful winds and

heavy rains that accompany hurricanes can cause great destruction An

anticyclone is an area of high pressure

that usually brings fair weather

Quick Check

direction would the winds

Writing Link Social Studies Link

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Air pressure is the

force that is exerted by the weight of air above the surface.

(pp 346–347)

The Coriolis effect

results from Earth’s rotation and affects the direction of airflow and winds.

(pp 350–351)

Think, Talk, and Write

average of an area’s weather called?

water vapor turns into liquid form

is known as

were 60°F (16°C), what type of precipitation would fall?

California to Australia What information could help you determine the weather along your route?

movement of cooler air from the ocean to the warmer land.

A sea breeze

B westerly wind

C trade wind

D land breeze

air molecules exert on the objects beneath them?

A the Coriolis effect

B the dew point

C air pressure

D low pressure

Write a School Newspaper Article

Write a newspaper article about climbing

a mountain Include descriptions of how

climbers deal with the effects of changes

in air pressure and altitude.

Compare Weather Around the World

Study the weather in one country in the Northern Hemisphere and one in the Southern Hemisphere, both at about the same distance from the equator What similarities and differences are there?

Make a

Study Guide

Make a three-tab book

(see pp 487–490) Use

the titles shown On

the inside of each tab,

make inferences about

how each relates to

Earth’s weather.

Summarize the Main Idea

- Review Summaries and quizzes online @www.macmillanmh.com 353

EVALUATE

Analyze Data

Scientists study weather maps

and analyze data from them They

look at temperature patterns in

order to predict future weather in an

area They also look for fronts where

cold and warm air meet Then they

analyze the data to draw conclusions

and explain why things happen.

Learn It

When you analyze data, you use

information that has been gathered to find patterns,

answer questions, or solve problems It is usually easier to

analyze the data if it has been organized and placed on a chart

or a graph Then you can see at a glance any patterns and any

extreme changes in the data.

Try It

▶ Look at the map It shows high and low temperatures for six

cities for one day in July Then look at the chart It lists the

average high and low temperatures and the rainfall for these

U.S cities during July in past years Analyze data from the

map and the chart to help you answer the questions.

Average July Temperatures and Precipitation by City

New York Miami Chicago Phoenix Dallas Bakersfield High

temperature 80.8°F 88.5°F 84.4°F 109.0°F 95.2°F 98.4°FLow

temperature 65.7°F 74.1°F 65.7°F 75.9°F 72.0°F 69.4°FPrecipitation 3.3 in 8.1 in 4.0 in 0.6 in 2.4 in 0.0 in.

▶ How did the high and low temperatures for Bakersfield shown

on the map, differ from its average high and low temperatures?

▶ Which city had high and low temperatures closest to its

average high and low temperatures?

▶ Which city had temperatures cooler than its average high and

low temperatures?

354

EXTEND

Apply It

▶ Now use data from the chart to create bar graphs, like the ones

started here, to compare the high temperatures or the amount

of precipitation.

▶ Finally, look at a weather map from your local newspaper

Compile data from the map to make two bar graphs Include

temperatures and precipitation for cities in California Analyze

data in your graph to predict what the weather may be in your

area tomorrow Share your findings with your classmates.

6 IE 7.c Construct appropriate graphs from data and develop qualitative statements about

the relationships between variables • 6 IE 7.f Read a topographic map and a geologic

map for evidence provided on the maps and construct and interpret a simple scale map. 355

EXTEND

Lesson 2

Weathering, Erosion, and Deposition

Flowing water can be a beautiful and powerful force It

can move soil, cut through rock, and change the shape of

the land How does flowing water change the landscape?

What other forces shape the land?

6 ES 2.a Students know water running downhill is the dominant process in shaping the landscape, including California’s landscape.

356

ENGAGE

How does the steepness of

a slope affect stream erosion?

Form a Hypothesis

A stream causes erosion by carrying sediment

and other materials away Do you think a stream

in a steep streambed will cause more erosion

than a stream in a level streambed? Write your

answer as a hypothesis in the form “If a streambed

is steeper, then ”

Test Your Hypothesis

Make a Model Fill the dishpan or bin with the

mixture of sand, gravel, and pebbles to model

a streambed.

Use the watering can to pour a thin stream

of water down the middle of the model

What happens?

Smooth out the streambed, and use a book or

wood block to prop up the dishpan or bin and

make the streambed slightly steeper Pour water

down the middle of the model What happens

now? Stick about two dozen toothpicks into

the slope to represent trees Pour water again

Do the trees make a difference?

Draw Conclusions

Analyze Make a chart to organize your data

Did your observations support your hypothesis?

Infer What do you think would happen if there

were many plants or trees along a stream’s banks?

• dishpan or plastic bin

• mixture of sand, gravel, and pebbles

• small watering can

• books or wood blocks

• toothpicks

What would happen if you used two books or wood

blocks to make the streambed steeper? Form a hypothesis

and test it Then analyze and present your results.

Explore More

Step

6 IE 7.a Develop a hypothesis • 6 IE 7.h Identify changes in

natural phenomena over time without manipulating the phenomena

(e.g., a tree limb, a grove of trees, a stream, a hillslope).

357

EXPLORE

▶ Main Idea 6 ES 2.a

Water, wind, and glaciers

are forces that wear down

Earth’s surface and deposit

soil and rocks in new

or broken down is through weathering

Weathering is the gradual breakdown of rock into smaller pieces by natural processes such as precipitation, wind, plant growth, and temperature change

▼ Weathering can eventually create

a dome, such as Half Dome in Yosemite National Park.

358

EXPLAIN

Types of Weathering

When a concrete sidewalk is first

poured, it looks smooth and fresh

Over time the sidewalk starts to chip,

crack, and become rough If a tree

is growing nearby, the sidewalk may

begin to lift as the tree’s roots grow

underneath it The same natural

processes that slowly break apart a

sidewalk also act to break apart rock

Rock can undergo two kinds

of weathering: physical and

chemical Physical weathering breaks

rock apart into smaller and smaller

pieces that retain the characteristics of

the original rock Physical weathering is

also known as mechanical weathering.

One way that physical weathering

happens is through the freezing and

thawing of water When water freezes,

it expands Water works its way into

cracks in rock On a cold day, the water

freezes and expands, pushing the rock

apart and making it crack and break

After many freeze-thaw cycles, the rock

is broken into pieces Root pressure

from plants can also crack rocks

Another kind of physical weathering

takes place when tiny rock fragments

carried by wind and water wear down

landforms through a process called

abrasion Abrasion occurs when sand,

stones, and pebbles move and scrape

across Earth’s surface, acting like

sandpaper to pit and polish the surface

A different type of weathering is

chemical weathering Unlike physical

weathering, chemical weathering

changes the composition of the rock

The new minerals have properties

different from those of the original

rock Chemical weathering happens when minerals in rock interact with chemicals found in water and air These chemicals cause rocks and minerals to break down For example, the acids in rain eat away limestone Both physical weathering and chemical weathering tend to round off corners, smoothing away a rock’s sharp edges

Most landscapes are a product

of both types of weathering A cliff may break apart as a result of physical weathering at the same time as it

is altered by chemical weathering

▲ Tree roots can penetrate cracks in rock

In time the rock may break apart.

Quick Check

physical weathering change rock?

chemical weathering to physical weathering

359 EXPLAIN

▲ a waterfall in Yellowstone National Park

How is Earth’s surface

reshaped?

Through the process of weathering, rock is

broken into smaller pieces These pieces are then

carried away by wind and water to be deposited in

new places, so Earth’s surface is reshaped Erosion

is the wearing away of Earth’s surface by the

breakdown and transportation of rock and soil

Water, wind, gravity, and ice loosen and carry

away bits of rock Deposition is the process by

which the eroded soil and rock are put down in

new places, reshaping the landscape

Water is the primary agent in shaping the

landscape in both California and the rest of the

world After falling on the land as precipitation,

water flows downhill because of the pull of

gravity The flowing water erodes the landscape

by washing away sand and soil, moving boulders,

and cutting through rock It then deposits these

materials in new places One of the most important

causes of change to Earth’s surface is the flowing

water in streams and rivers Over millions of years,

these moving waters can carve canyons through

360

EXPLAIN

Weathered rock may also be

carried away by blowing wind This

is especially true in the desert, where

there is little vegetation to hold down

the sand particles and prevent wind

erosion The wind blows the smallest

particles of sand across the surface of

the desert and deposits them, forming

mounds called sand dunes The wind

can pile sand into dunes hundreds of

meters high Away from the desert,

erosion is usually slowed by the

presence of plants and trees The roots

that have grown into the ground help

hold soil particles in place However,

large, open spaces such as barren

farmland are exposed to erosion by

wind and water

Weathered rock carried away by the

wind is eventually deposited Most of

the rock particles, called sediment ,

are deposited in river flood plains,

desert basins, swamps, dunes, and

oceans Over a long period of time,

layers of sediment may build up and

be pressed together by the layers

above them With heat and time, the

sediment eventually changes back

into rock again

Quick Check

else can erode the landscape

the way wind and water do?

how the landscape is

changed through deposition

Layers of Sediment

Make a Model You can use

a sediment jar to model how sediment is deposited in layers Pour 1 cup of water into

a widemouthed jar Put another cup of water into another jar along with 21 cup each

of flour, dry rice, and dry red beans Close the lid of the second jar, and shake the mixture you made.

Observe Pour the mixture into the water in the first jar Record what you see You may wish to draw a diagram Continue recording what you see every 20 minutes for

1 hour and again in 24 hours.

Infer How does particle size affect the order in which particles settle? Why?

▶ Red Rock Canyon State Park,

near the northwestern edge of

the Mojave Desert, is famous for

the colors of its rock formations.

361

EXPLAIN

What are landslides?

The rate at which materials erode is affected by the speed

and force of water or wind You can see this for yourself if

you turn water from a hose onto sand or dirt A fast, hard

stream of water will carry away much more soil than a soft,

slow stream of water

Soil and rock can move downhill without being carried

by water This type of erosion is also affected by the slope

of the land Earth’s surface is seldom perfectly flat

It consists of slopes that range from steep cliffs and

mountains to rolling hills and meadows The slope of

the land affects how fast rock and soil move downhill

A steep slope erodes more quickly than a gentle slope

At one extreme, erosion can be so slow that it is barely

noticeable At the other extreme is the rapid, downslope

movement of a mass of rock, soil, and debris known as

a landslide Landslides can be very dangerous, because they

happen so suddenly and often involve large amounts of soil

and rock Each year, landslides in the United States cause

25 to 50 deaths and more than $1.5 billion in losses

Why might the house have moved to cause the damage seen in this photograph?

Clue: What features in this photo suggest movement of land?

Reading Photos Landslide

Laguna Beach

362

EXPLAIN

Factors in Landslides

Gravity is the main force in

landslides, but other factors play

roles as well One of these factors

is water Water seeps into sediment,

filling spaces The water allows

sand and rock particles to slide

by one another so that they can

move downhill Water also makes

soil heavier The added weight of

water alone can be enough to cause

sediment to slide down a slope This

explains why landslides in California

often follow heavy rains

Another factor in landslides is

slopes that are extremely steep As the

angle of a slope increases, the slope

becomes too steep to hold rock and

soil The rocks and soil fall downhill

until the slope begins to even out and

becomes more gradual again

What causes a slope to become

too steep? One cause can be a

stream flowing through a valley

The force of the water can cut away

rock and soil until the valley wall

becomes too steep Another cause is

waves pounding against the base of

a cliff Earthquakes can also cause

slopes to become too steep

Very loud noises can also trigger

landslides Such loud noises include

vibrations from traffic or machinery,

blasting, and even thunder

Quick Check

landslides?

landslides are so destructive

▲ Gravity and a steep slope caused this landslide near

La Conchita, California.

363 EXPLAIN

How do glaciers reshape

Earth’s surface?

Did you know that almost 10% of

Earth’s surface is covered with ice?

Most of the ice is in glaciers , large

masses of moving ice that form over

hundreds or thousands of years In

fact, glaciers hold most of Earth’s fresh

water Although a glacier moves very

slowly, it can erode the land just as

liquid water and wind do One way to

think of a glacier is as a slow-moving

river of ice

Many small glaciers are found in

high, mountainous areas, where they

usually follow valleys that were carved

out by streams Unlike a stream, a

glacier typically moves very slowly,

perhaps only a few centimeters per day

As a glacier advances, the entire ice

mass slips along the ground, dragging

loose rocks and soil along with it The

rocks and pebbles act like sandpaper,

eroding the land beneath and depositing

it downslope to reshape Earth’s surface

Other glaciers exist as huge sheets of

ice These enormous glaciers flow out in

all directions from one or more centers

Earth today has only two ice sheets,

although during past ice ages, much of

the land’s surface was covered by ice

As a glacier flows, the ice scrapes

and tears rocks and soil from the

surface of Earth A glacier can carry

huge boulders many kilometers,

until the ice melts and the rocks are

deposited In mountainous areas,

erosion by valley glaciers often carves

bowl-shaped valleys, sharp ridges, and

other beautiful features

Because glaciers pick up and transport huge amounts of debris, they also change the landscape through deposition In areas of North America that were once covered by ice sheets, glaciers have deposited layers

of soil and rock that are tens or even hundreds of meters thick, creating small hills and flat fields The rocky pastures of New England, the rolling farmlands of the Midwest, and the wide fields of the Dakotas were all created by glacial deposits

Quick Check

glaciers shaped Earth?

glacier reshapes Earth differently from the way a stream does

Alaskan glacier

364

EXPLAIN

Writing Link Math Link

Summarize the Main Idea

Erosion and deposition

reshape Earth’s surface.

Think, Talk, and Write

process that shapes Earth’s landscapes?

ice is a

gravity play in landslides?

often trigger landslides?

processes scrapes Earth’s surface, acting like sandpaper to pit and polish it?

A induction

B deposition

C abrasion

D subduction

rapid, downslope movement of a mass of rock, soil, and debris?

Research a landslide that has occurred

in California If possible, read

first-person accounts of the event Write an

article explaining what people did to

clean up after the landslide.

Calculate Sediment

A landslide displaces 5.5 tons of sediment down the side of a mountain How many pounds of sediment is this? (1 ton = 2,000 pounds)

Make a

Study Guide

Make a three-tab folded book (see pp

487–490) Use the titles shown On the

inside of each tab, list the causes of

each event and

Beaches in California are eroding They are losing sand

and becoming narrower However, before my grandparents’ home crashed into the sea, I thought that erosion had no real effect on my life

After many years of working hard and saving their money, my grandparents bought their dream house on the beach I used to love to visit them there Their house stood high up on stilts for protection from the water I would sit in the front room facing the ocean and listen to the rhythm of the pounding waves The sights and sounds were thrilling.One afternoon, my family received a devastating phone call My grandparents’ house had collapsed Over time,

the rolling waves had pulled sand back into the ocean

The seawater had slowly weakened the stilts on which the

house stood

Now my grandparents’ dream is just a memory Yet the warning remains: the sea can be a mighty foe It can erode the beach It can also destroy homes—and dreams

ELA W 6.2.1 Write narratives: a Establish and develop a plot and setting and present a point of view that is appropriate to the stories b Include sensory details and concrete language to develop plot and character

c Use a range of narrative devices (e.g., dialogue, suspense).

366

EXTEND

Write About It

Narrative Writing Tell a personal story

about the effects of beach erosion and

protecting beaches Use descriptive details,

and retell events in a logical order Use the

first-person point of view, and use dialogue,

if appropriate Using print and online

research, include information about why

beaches are important.

- Journal Write about it online

A good personal narrative

▶ tells a story from personal experience.

▶ uses descriptive language.

▶ has an interesting beginning, a middle, and an end.

▶ shares events in

a sequence that makes sense.

▲ Stilts elevate these houses above the surf and shifting sand.

367

EXTEND

Lesson 3

Rivers and

Streams

A fast-flowing river has the power to wear down

a mountain or cut through a cliff A slow-flowing

river may flow in gentle curves across a plain What

affects the flow of rivers and streams? How does

flowing water change Earth’s surface?

6 ES 2.b Students know rivers and streams are dynamic systems that erode, transport sediment, change course, and flood their banks in natural and recurring patterns.

368

ENGAGE

• piece of cardboard the same size as the map

What information will you need to map a watershed?

Write your answer in the form “If I want to map my

area’s watershed, then I will need ”

Procedure

Choose a site to map: a stream, pond, or other

small body of water Place the plastic sheet

over the topographic map, and tack both to the

cardboard Mark your site on the topographic

map, using the road map to help you.

Mark the bodies of water near your site on the

topographic map Use the map’s contour lines to

find the highest and lowest points around your

site Mark each hilltop with an x.

Use x’s to label the directions in which the local

ditches and streams flow On your map find the

highest ground between two bodies of water

Draw a line to connect the highest points (the

x’s) around your site, including its mouth—the

place where it drains into another body of water.

Draw Conclusions

Analyze What can you tell about how water

flows in the watershed? Use the topographic

map to explain.

How could this watershed map be used to serve your

community? What information does it provide? Make

a prediction and test it Then analyze and present your

results.

Explore More

Step

Materials

6 IE 7.e Recognize whether evidence is consistent with a

proposed explanation • 6 IE 7.f Read a topographic map

and a geologic map for evidence provided on the maps and

construct and interpret a simple scale map.

369

EXPLORE

▼ After a heavy rain, the amount of water in a river increases, so the river flows at a greater speed.

What affects how a river

or stream flows?

If you live near a river or a stream, you may have noticed that on some days, the water flows fast and high On other days, the water flows more slowly and may be reduced to just a trickle

or a narrow stream

Rainfall can affect the speed of flowing water When rain falls, water falls both on the land and into rivers and streams Some of the water that falls on the land seeps into the soil, but if the rain is heavy, much of the water flows over the land surface into nearby rivers and streams Adding more water to a body of moving water increases the moving water’s energy Because rivers and streams normally flow downhill, the water in them is already moving quickly As more water enters a river or a stream, its water level rises, and its speed increases

Rivers and streams change

Earth’s surface through

weathering, erosion, and

deposition of rocks and

[SO\RS`W\Uab`SO[

Flowing Water

A bank is the rising ground that

borders a river or stream When a

river overflows its banks, it covers

its flood plain , the flat area of land

on both sides of the river A flood plain

forms from layers of sediment—silt,

sand, and gravel—left behind each

time the river overflows its banks

Because this sediment is rich in

nutrients, it makes good farmland

Flowing water is a powerful force

that changes the shape of Earth’s

surface The faster the water moves,

the bigger and denser the particles of

rock the water can carry as it flows

downhill Most large particles roll,

slide, or bounce along the bottom of a

river or stream Smaller particles swirl

in the water or are dissolved in it

The bits and pieces carried by

moving water act like tiny drills They

slam into rocks and chip away at

them They erode the banks, and they

carve out or cut into the riverbed, or

bottom of the river This scratching

and scraping, or abrasion, is greater

in a fast-moving river than in a moving one

slow-What can cause a river or stream

to slow down? An obstacle, such as a large rock or a tree trunk, might block the flow of water A steep river might flow onto a flat plain The river or the stream might flow into a large body of standing water, such as a pond, a lake,

or an ocean Whenever the water slows down, it drops some of the particles

it has been carrying These particles,

or sediment, are deposited and form mounds or layers on the riverbed or the floor of a pond, a lake, or an ocean

Flood Plain

Quick Check

when heavy rains increase the volume of water in a river?

speed of water flowing in a river

or a stream affect the landscape?

▲ Because of sediments deposited by rivers and streams, a flood plain usually has rich soil that can produce good crops.

371 EXPLAIN

What are the stages of

stream development?

Over time streams can carve out

deep, V-shaped channels that have

steep sides However, streams come in

a variety of forms Some are narrow

and swift-moving, while others are

wide and slow-moving Some streams

have so much water added to them

that they become rivers Some streams

also empty into rivers Streams differ

because they are in different stages of

development A stream’s stage depends

on the slope of the ground over which

it flows No two streams are exactly alike, but as they flow from higher ground to lower ground, all go through three main stages: young (upper), mature (middle), and old (lower)

A young stream moves swiftly down steep slopes While young it may have white-water rapids and waterfalls On nearly level ground, the stream matures and flows smoothly through its valley Most of the rocks in the bed of the

A Typical River

The Life of a Stream

A river usually has more

water than a stream

However, scientists consider

all rivers to be streams, and

they believe that all streams

go through similar stages of

development.

The source of a river or

a stream often occurs in

a mountainous area Here

the force of gravity causes

water to flow quickly Some

streams flow from springs,

lakes, or the ends of

glaciers.

The fast-flowing stream

can carry many sediments

and often carves out

V-shaped valleys Fast-moving

streams are often used for

rafting.

Smaller streams add

water, increasing the size

of the stream and often

forming a young river.

stream are eroded by flowing water

and the sediment it carries Streams

in the mature stage develop broad

curves called meanders (mee•AN•durz)

The flat valley floor formed by a

meandering stream or river is the flood

plain When a stream or river floods, it

covers all or part of the flood plain

The third stage in the development

of a stream is the old stage An old

stream flows slowly through a broad,

flat flood plain Near its mouth the

stream or river begins to deposit its load

of sediment This forms a delta, an area

of flat, fertile land that extends into the ocean or other large body of water

Quick Check

white-water rafting take place on young rivers?

characteristics of an old-stage or lower river?

As the river reaches flatter land, it slows down

As it slows it deposits some

of the sediments it carried.

The slow-moving river flows across flat land, forming large curves called

meanders Water flows

faster along the outside

of a meander, eroding or wearing away the bank.

When a river empties into a larger body of water such as an ocean or a bay,

it deposits the remaining sediments it has carried These deposits can form

a delta that extends from the river’s mouth.

4

5

6

▼ A slow, meandering river may flow

at a rate of less than 1 kilometer (0.6 miles) per hour.

Describe how water flows from a river’s source to its mouth.

Clue: Follow the course of the river, from its source to its mouth, in the diagram.

Reading Diagrams

Watch the life of

a stream @ www.macmillanmh.com 373

EXPLAIN