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CHAPTER 18
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Erosion by Wind and Waves
1 Wind Erosion
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• What are two ways that wind erodes land?
• How do the two types of wind deposits compare?
How Does Wind Shape Land?
Wind has energy. This energy can move a sailboat or
turn a windmill. It can also carry sand or dust particles
from one place to another. These particles shape the land
through erosion and deposition.
How Does Wind Move Sand and Dust?
Sand and dust are different types of particles. Sand is
fragments of weathered rock and minerals. Most grains of
sand are made of quartz. Other common minerals in sand
are mica, feldspar, and magnetite.
Dust is made up of tiny particles that are smaller than
sand grains. Most dust particles are tiny fragments of
rocks and minerals. Other sources of dust are plants,
animals, bacteria, and pollution.
READING TOOLBOX
Take Notes As you read this
section, use a four-corner
fold to compare the different
types of sand dunes. Label
the outer flaps with the types
of dunes. Describe each
dune underneath the correct
flap.
READING CHECK
1. Compare How are sand
and dust different?
SAND MOVEMENT
Sand particles can be heavy. Therefore, wind cannot
keep sand in the air. Instead, wind rolls sand grains along
the ground.
Sand can also move by jumps and bounces called
saltation. Saltation happens when rolling sand grains
collide and some bounce into the air. The sand grains in
the air move a short distance and then fall. The falling
sand grains hit other sand grains. During saltation, sand
grains move in the same direction that the wind blows.
Wind direction
Smaller particles are lifted
and carried by the wind.
LOOKING CLOSER
Most sand grains rise
less than 1 m above the
ground during saltation.
2. Describe How does wind
move sand by saltation?
Larger particles bounce and skip
along the ground.
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Erosion by Wind and Waves
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Wind Erosion continued
DUST MOVEMENT
READING CHECK
3. Identify Name two
natural events that can add
dust to the air.
READING CHECK
4. Identify What is one
feature that forms from wind
erosion?
Talk About It
Use Root Words With a
partner, use a dictionary to
find two other words based
on the Latin word ventus.
How can this root help you
remember the meaning of
the words?
Dust particles are very small and light. Therefore,
wind can carry dust particles through the air. Dust from
volcanic eruptions can stay in the air for many years. In
some areas, strong winds lift large amounts of dust and
create dust storms.
What Are the Effects of Wind Erosion?
Wind erosion happens all over the world. However,
wind erosion has the greatest effect on deserts and coastlines. These areas have fewer plants to hold the soil in
place. Also, deserts have thin, dry soil. The wind can
easily blow the soil away. Moist soil is heavier and sticks
together. As a result, moist soil is more difficult to move.
Wind erosion causes many changes at Earth’s surface.
Three features that form from wind erosion are
• desert pavement
• deflation hollows
• ventifacts
DEFLATION
One common form of wind erosion is deflation. In the
process of deflation, wind removes the top layer of fine,
dry soil. The wind leaves behind larger rock particles.
Deflation forms a surface of small rocks called desert
pavement or stone pavement. Desert pavement protects
underlying land from erosion.
Deflation is a serious problem for farmers because it
blows away the best soil for growing crops. As the topsoil
blows away, a shallow pit may form. This pit is called
a deflation hollow. A deflation hollow may grow to be
several kilometers wide and up to 20 m deep.
VENTIFACTS
Small rocks on deserts and beaches are often exposed
to wind abrasion. Wind abrasion can weather the rocks
until they become flat and smooth. Rocks that are smooth
from wind abrasion are called ventifacts. The word
ventifact comes from the Latin word ventus, for “wind.”
Ventifacts can show the direction of the wind in an
area. If the wind usually blows in one direction, the
smooth surfaces of ventifacts will face in that direction.
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Wind Erosion continued
LIMITS OF WIND EROSION
Most wind erosion happens near the ground, where
saltation occurs. Wind erosion also happens very slowly.
As a result, wind erosion has a limited effect on large
masses of rock.
Critical Thinking
5. Apply Ideas Why might
rocks on a beach erode more
quickly than the top of a sea
cliff?
What Are the Effects of Wind Deposition?
The wind deposits, or drops, particles when it cannot
carry them anymore. Over time, more particles drop and
cover the deposited particles. Pressure and chemical
weathering can bind these particles together. This
compaction and cementation is one way sedimentary
rocks can form.
Wind deposition can also affect the landforms on
Earth’s surface. Two landforms that form through wind
deposition are dunes and loess deposits.
DUNES
Wind deposits can create piles of sand called dunes.
Dunes form in places with dry, sandy soil and strong
winds. Dunes commonly form in deserts and along the
shores of oceans and lakes.
A dune forms in stages, as shown below. First, a
barrier slows the speed of the wind. This causes sand
to collect on both sides of the barrier. Over time, the
wind deposits more sand. The dune grows and buries the
original barrier.
READING CHECK
6. Identify What are two
landforms that can form
through wind deposition?
LOOKING CLOSER
Barrier
7. Summarize How do sand
dunes form?
Sand buildup
Dune
Direction of wind
Original
barrier
Dune formation begins with a barrier, such as a rock, a fence, or some grass. Over
time, sand builds up and covers the barrier.
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Wind Erosion continued
TYPES OF DUNES
READING CHECK
8. Explain Which side of a
sand dune generally has the
steeper slope? Why?
Wind affects the shape of sand dunes. In general, the
blowing wind forms a gentle slope on the side of the
dune facing the wind. This side of the dune is called the
windward side. As wind blows sand over the top of the
dune, the sand collects on the other side. The side of the
dune away from the wind is the slipface. The buildup of
sand on the slipface creates a steep slope.
Wind can form different types of sand dunes, as shown
below. In some cases, wind blows around the ends of
the dune and forms two long points. This gives the dune
a crescent, or moon, shape. Both barchan dunes and
parabolic dunes have a crescent shape. A barchan dune
has its open side away from the wind. A parabolic dune
has its open side facing the wind.
In areas with large amounts of sand, the wind may
form long ridges of sand. These ridges are called
transverse dunes. Transverse dunes form at right angles
to the wind direction. Longitudinal dunes also form in
ridges. However, longitudinal dunes are parallel to the
wind direction.
Barchan dunes
In a barchan dune, the open
side faces away from the wind.
Transverse dunes
Transverse dunes are
perpendicular to the direction
the wind blows.
Parabolic dunes
In a parabolic dune, the open
side faces the wind.
Longitudinal dunes
LOOKING CLOSER
Longitudunal dunes are
parallel to the direction
the wind blows.
9. Describe Draw an arrow
on each picture to show the
direction the wind is blowing.
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Wind Erosion continued
DUNE MIGRATION
The movement of dunes is called dune migration. If
the wind generally blows in the same direction, dunes
will move in the direction of the wind.
Dune migration happens when wind blows sand over
the top of the dune. The sand builds up on the slipface
of the dune. Over time, the sand continues to move from
one side of the dune to the other.
In flat areas, dunes migrate until they reach a barrier.
People often build fences or plant trees to stop dunes
from covering highways and farmland.
Critical Thinking
10. Analyze Processes Why
does planting grass and trees
prevent dunes from covering
roads?
LOESS
Wind carries dust higher and farther than it carries
sand. Wind may deposit dust in very thin layers. Finegrained sediment that is deposited by the wind is called
loess. Loess is made of fine sediments of quartz, feldspar,
clay, and other minerals. Most loess is deposited in thin
layers. However, very thick layers of loess called loess
deposits also form in some areas.
Loess is soft and erodes easily. It sometimes forms
steep bluffs, as shown below. Loess deposits are very
fertile. They can form good soil for growing crops.
READING CHECK
11. Describe What is loess?
These loess deposits are in Vicksburg, Mississippi.
A deep layer of loess covers areas in northern China.
The material in this deposit came from the Gobi Desert,
in Mongolia. Deposits of loess also exist in central
Europe. North America has loess deposits in Oregon,
Washington, and many midwestern states. These deposits
probably formed from the dust from dried glacial lakes.
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Section 1 Review
SECTION VOCABULARY
deflation a form of wind erosion in which fine,
dry soil particles are blown away
dune a mound of wind-deposited sand that
moves as a result of the action of wind
loess fine-grained sediments of quartz, feldspar,
hornblende, mica, and clay deposited by the
wind
saltation the movement of sand or other sediments by short jumps and bounces that is
caused by wind or water
ventifact any rock that is pitted, grooved, or
polished by wind abrasion
1. Organize Complete the concept map to show how wind affects Earth’s surface.
Use the terms dunes, deposition, ventifacts, erosion, loess, and desert pavement.
Wind shapes Earth’s surface
through processes of
which forms
which forms
deflation hollows
2. Compare How are dunes and loess deposits similar? How are they different?
3. Explain Why does wind erosion have a major effect on deserts?
4. Evaluate Methods Suppose you visited a desert and marked the location of a
sand dune. If you came back the next year, would the dune be in the same place?
Explain your answer.
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Erosion by Wind and Waves