Name
CHAPTER 14
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Date
Weathering and Erosion
4 Erosion
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
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What is erosion, and what are four agents of erosion?
What are four farming methods that conserve soil?
How does gravity contribute to erosion?
What are the three major landforms shaped by weathering and erosion?
What Is Erosion?
Weathering causes rock particles to break away from
rock. However, the rock particles do not always stay near
the parent rock. Forces in the environment may move the
particles to other places. This process, in which Earth
materials are moved from one place to another, is called
erosion. The most common agents of erosion are
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READING TOOLBOX
Determine Cause and
Effect As you read, make
a chart showing the causes
and effects of erosion. When
you are finished, compare
your chart with a partner’s.
gravity
wind
Critical Thinking
glaciers
1. Identify Relationships
What is the relationship
between weathering and
erosion?
water
How Does Erosion Affect Soil?
Tiny particles of rock that form during weathering mix
with water, air, and humus to form soil. Soil erodes constantly, but soil erosion is normally a slow process. New
soil forms fast enough to replace eroding soil. However,
living and nonliving factors can upset this natural balance. When this happens, soil erodes quickly before new
soil can replace it.
Some scientists think that soil erosion is the greatest
environmental problem today. Erosion removes fertile
topsoil and makes it harder to grow crops.
Certain farming techniques can accelerate, or speed
up, soil erosion. For example, some farmers plow long,
narrow rows called furrows. Furrows on slopes channel
water from rainfall, washing away the soil. Over time,
the furrows become larger. They form deep ditches,
or gullies. This type of soil erosion is called gullying.
Gullying can ruin farmland.
Critical Thinking
2. Make Connections What
is the relationship between
soil erosion and hunger?
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Erosion continued
SHEET EROSION
READING CHECK
3. Describe What kind of
conditions can lead to sheet
erosion?
In the process of sheet erosion, wind and water
remove layers of topsoil. Over time, erosion may expose
the surface of the subsoil.
Sheet erosion may happen in places where heavy rains
wash away layers of topsoil. During dry periods, wind
can also cause sheet erosion. Wind carries the loose, dry
soil away in clouds of dust and sand. These soil particles
may form large dust storms.
RESULTS OF SOIL EROSION
Constant erosion makes it harder for soil to support
life. The flowchart below shows how erosion affects the
fertility of soil.
Erosion removes the A horizon, which contains fertile humus.
LOOKING CLOSER
The B horizon is less fertile because it does not have much humus. The
B horizon is difficult to farm.
4. Describe What makes the
B horizon at risk for erosion?
The B horizon has no plant life to protect it from more erosion.
Erosion continues to remove layers of soil. Over time, erosion may
remove all the soil.
EFFECTS OF HUMAN ACTIVITIES ON EROSION RATES
Human activities affect how quickly soil erodes.
Farming techniques and construction projects can
increase the rate of erosion. For example, people remove
plants to build houses and roads. Without plants for
protection, the topsoil erodes more quickly. New soil
cannot form fast enough. In some areas, it may take
thousands of years for new topsoil to form.
Human actions can also reduce or prevent soil erosion.
Soil conservation involves taking steps to prevent soil
erosion.
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Erosion continued
What Are Some Methods of Soil
Conservation?
People have begun to understand how human activities affect the environment. Now, many people are trying
to protect soil through soil conservation. For example,
some land development projects are leaving trees and
plants in place when possible. Other projects are adding
plants to the soil to prevent erosion. Farmers also use
soil conservation methods to prevent erosion. Some of
these methods are
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Talk About It
Discuss With a partner, talk
about land use in your community. What activities may
be damaging the soil? What
activities may be helping the
soil?
contour plowing
strip-cropping
terracing
crop rotation
CONTOUR PLOWING
Farmers around the world use different planting
methods to reduce soil erosion. In the contour plowing
method, farmers plow in curved bands. These bands
follow the contour, or shape, of the land. Contour
plowing prevents water from flowing straight down
slopes. Therefore, this method helps prevents gullying.
STRIP-CROPPING
Farmers may also use strip-cropping to prevent
erosion. In this method, farmers plant crops in alternating
bands, as shown below. For example, a farmer may plant
rows of corn in one band. Then the farmer plants a cover
crop, such as alfalfa, next to the corn. A cover crop fully
covers the surface of the land. This cover crop slows the
runoff of rainwater and protects the soil.
READING CHECK
5. Describe How does
contour plowing help
conserve soil?
READING CHECK
6. Identify Give an example
of a cover crop.
Contour Plowing
Strip-Cropping
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Erosion continued
TERRACING
READING CHECK
Normally, the soil on hillsides is thin because of
erosion. However, farmers can use terracing to farm on
slopes. In this method, farmers build steplike ridges, or
terraces, that follow the shape of a sloped field. Terraces
slow the movement of water down the slope. Slowing
the water prevents soil from washing away quickly. Many
farmers in Asia use terracing to grow rice.
7. Explain What is one
advantage of terracing?
Terraced fields such as these help
slow runoff and prevent rapid soil
erosion.
CROP ROTATION
READING CHECK
8. Identify What are two
functions of cover crops?
In crop rotation, farmers change the type of crop they
plant each year. One year, a farmer may plant a crop that
will be harvested, or picked. The next year, the farmer
may plant a cover crop. The cover crop is not harvested.
Instead, it helps to slow runoff and hold the soil in place.
Other types of crop rotation help keep the soil fertile.
What Is the Role of Gravity in Erosion?
Gravity causes rock fragments to move down a slope.
This movement of fragments down a slope is called mass
movement. Some mass movements happen quickly.
Others happen very slowly. The most destructive mass
movements happen quickly.
Types of Mass Movement
Fast Mass Movements
Slow Mass Movements
• rockfalls
• solifluction
• landslides
• creep
• mudflows
• slumps
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ROCKFALLS AND LANDSLIDES
The fastest kind of mass movement is a rockfall. In
a rockfall, rock falls from a steep cliff. The rocks in
rockfalls can be tiny fragments or giant boulders.
In a landslide, masses of loose rock and soil suddenly
fall down a slope. In large landslides, loose blocks of bedrock fall. This happens on very steep slopes. Small landslides often happen on the steep hills next to highways.
Heavy rainfall, volcanic eruptions, and earthquakes can
cause landslides.
Critical Thinking
9. Compare What is the
difference between a rockfall
and a landslide?
An earthquake in El
Salvador caused this
landslide.
MUDFLOWS AND SLUMPS
The quick movement of a large amount of mud forms a
mudflow. Mudflows happen in mountain regions because
of volcanic eruptions or heavy rainfall. Mudflows move
down slopes and valleys. The mud typically spreads out
in a fan shape at the base of the slope.
Sometimes, a large block of soil and rock falls downhill in one piece. The block then slides along the slope
of the surface. This movement is a slump. Slumping
happens on very steep slopes. Water and loss of friction
cause the soil and rock to slip downhill.
SOLIFLUCTION
Many slopes do not seem to change. However, such
slopes often experience slow mass movements. Slow
mass movements actually move more rock material
overall than landslides. One form of slow mass movement
is solifluction. In solifluction, soil that is soaked with
water flows over hard or frozen layers of soil.
Solifluction happens in arctic and mountain climates,
where the subsoil is always frozen. Water from the topsoil cannot flow into the frozen subsoil. As a result, the
surface layer becomes muddy and flows slowly downhill.
Critical Thinking
10. Infer Solifluction can
also happen in warmer areas
where the subsoil is made of
hard clay. Why would subsoil
with hard clay likely lead to
solifluction?
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Erosion continued
CREEP
READING CHECK
Another form of slow mass movement is creep. Creep
is the slow downhill movement of weathered rock
material. Soil creep moves the most soil of all types of
mass movements. Factors such as water between rock
particles, growing plants, burrowing animals, and freezing
and thawing cause soil creep. All of these factors help
loosen particles, which allows gravity to pull the particles
downhill.
The rock fragments form piles as they collect at
the base of a slope. These piles are called talus. Talus
weathers into smaller fragments that move farther down
the slope. The fragments wash into gullies and streams.
Eventually, the gullies and streams carry the particles
into rivers.
11. Define What is talus?
The movement
of rock fragments
downhill caused
these talus cones
to form at the base
of the Canadian
Rockies.
Talk About It
Discuss Use an
encyclopedia or the Internet
to find images of each major
landform. Describe each
landform to a partner. Work
with your partner to locate
an example of each landform
on a map.
READING CHECK
12. Identify What are two
forces that affect mountains?
How Does Erosion Affect Landforms?
The physical features on Earth’s surface are called
landforms. Weathering and erosion help shape three
major landforms: mountains, plains, and plateaus. They
also shape minor landforms such as hills, valleys, and
dunes.
EROSION OF MOUNTAINS
Mountains go through different stages over time. First,
tectonic forces form mountains by lifting Earth’s crust.
The forces cause mountains to rise. The mountains rise
faster than they are eroded. The mountains are generally
rugged and have sharp peaks.
Over time, forces stop uplifting the mountains.
Weathering and erosion wear down the rugged peaks.
The mountains become rounded and have gentle slopes.
Eventually mountains become low hills. Such features
are called peneplains, which means “almost flat.”
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Erosion continued
These mountains in the Andes are still being uplifted.
LOOKING CLOSER
13. Apply Reasoning Which
of these mountain ranges is
probably older—the Andes
or the Appalachians? Explain
your reasoning.
The Appalachian Mountains have been
eroded over millions of years.
EROSION OF PLAINS AND PLATEAUS
A plain is a flat landform near sea level. A plateau is
a wide, flat landform that has a high elevation. A plateau
experiences much more erosion than a plain. Young
plateaus typically have deep valleys that separate the
wide, flat regions.
Plateaus weather in different ways. In dry climates,
rocks that are resistant to weathering form plateaus
with flat tops. Over time, erosion may split the plateaus
into smaller areas called mesas. Mesas erode into even
smaller formations called buttes. In dry areas, buttes and
mesas have steep walls and flat tops. In wet climates,
humidity and precipitation weather landforms into round
shapes.
READING CHECK
14. Sequence Which landform develops first—a mesa
or a butte? Explain.
Ancient rivers carved plateaus into mesas. The mesas eventually eroded into the
buttes of Monument Valley in Arizona.
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Section 4 Review
SECTION VOCABULARY
creep the slow downhill movement of weathered rock material
erosion a process in which the materials of
Earth’s surface are loosened, dissolved, or
worn away and transported from one place
to another by a natural agent, such as wind,
water, ice, or gravity
landform a physical feature of Earth’s surface
mass movement the movement of a large mass
of sediment or a section of land down a slope
sheet erosion the process by which water flows
over a layer of soil and removes the topsoil
solifluction the slow, downslope flow of soil
saturated with water in areas surrounding
glaciers at high elevations
1. Define In your own words, define erosion.
2. Organize Complete the spider map below to identify and describe four methods of
soil conservation. Be sure to include how each method protects soil.
Soil conservation
methods
3. Identify What are three major landforms shaped by weathering and erosion?
4. Explain How does gravity cause erosion? Give two examples.
5. Apply Ideas Imagine that you wanted to grow grapes on a hillside. What farming
method or methods would you use? Explain your answer.
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