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Wind Erosion and Deposition

What You Will Learn

• Explain why some areas are more affected by wind erosion than other areas are. • Describe the process of saltation. • Identify three landforms that result from wind erosion and deposition. • Explain how dunes move. Have you ever been working outside and had a gusty wind blow an important stack of papers all over the place? Do you remember how fast and far the papers traveled and how long it took to pick them up? Every time you caught up with them, they were on the move again. If this has happened to you, then you have seen how wind erosion works. As an agent of erosion, the wind removes soil, sand, and rock particles and transports them from one place to another. Certain locations are more vulnerable to wind erosion than others. An area with little plant cover can be severely affected by wind erosion because plant roots anchor sand and soil in place. Deserts and coastlines that are made of fine, loose rock material and have little plant cover are shaped most dramatically by the wind.

The Process of Wind Erosion

Wind moves material in different ways. In areas where strong winds occur, material is moved by saltation.

Saltation

is the skipping and bouncing movement of sand-sized particles in the direction the wind is blowing. As you can see in

Figure 1,

the wind causes the particles to bounce. When moving sand grains knock into one another, some grains bounce up in the air, fall forward, and strike other sand grains. These impacts cause other grains to roll and bounce forward.

Figure 1

The wind causes sand grains to move by saltation.

Deflation

The removal of fine sediment by wind is called

deflation .

During deflation, wind removes the top layer of fine sediment or soil and leaves behind rock fragments that are too heavy to be lifted by the wind. Deflation may cause

desert pavement

, which is a surface consisting of pebbles and small broken rocks

.

An example of desert pavement is shown in

Figure 2. Figure 2

Desert pavement, such as that found in the Painted Desert in Arizona, forms when wind removes all the fine materials. Have you ever blown on a layer of dust while cleaning off a dresser? If you have, you may have noticed that in addition to your face getting dirty, a little scooped-out depression formed in the dust. Similarly, in areas where there is little vegetation, the wind may scoop out depressions in the landscape. These depressions are called

deflation hollows

. Where do deflation hollows form?

Abrasion

The grinding and wearing down of rock surfaces by other rock or sand particles is called

abrasion .

Abrasion commonly happens in areas where there are strong winds, loose sand, and soft rocks. The blowing of millions of sharp sand grains creates a sandblasting effect. This effect helps to erode, smooth, and polish rocks.

Wind-Deposited Materials

Much like rivers, the wind also carries sediment. And just as rivers deposit their loads, the wind eventually drops all the material it carries. The amount and the size of particles the wind can carry depend on the wind speed. The faster the wind blows, the more material and the heavier the particles it can carry. As wind speed slows, heavier particles are deposited first.

Loess

Wind can deposit extremely fine material. Thick deposits of this windblown, fine-grained sediment are known as

loess

(LOH ES). Loess feels like the talcum powder a person may use after a shower.

Because wind carries fine-grained material much higher and farther than it carries sand, loess deposits are sometimes found far away from their source. Many loess deposits came from glacial sources during the last Ice Age. In the United States, loess is present in the Midwest, along the eastern edge of the Mississippi Valley, and in eastern Oregon and Washington.

Dunes

When the wind hits an obstacle, such as a plant or a rock, the wind slows down. As it slows, the wind deposits, or drops, the heavier material. The material collects, which creates an additional obstacle. This obstacle causes even more material to be deposited, forming a mound. Eventually, the original obstacle becomes buried. The mounds of wind-deposited sand are called

dunes .

Dunes are common in sandy deserts and along the sandy shores of lakes and oceans.

Figure 3

shows a large dune in a desert area. Figure 3 Dunes migrate in the direction of the wind.

The Movement of Dunes

Dunes tend to move in the direction of strong winds. Different wind conditions produce dunes in various shapes and sizes. A dune usually has a gently sloped side and a steeply sloped side, or

slip face,

as shown in

Figure 4.

In most cases, the gently sloped side faces the wind. The wind is constantly transporting material up this side of the dune. As sand moves over the crest, or peak, of the dune, it slides down the slip face, creating a steep slope.

Figure 4

Dunes are formed from material deposited by wind.

In what direction do dunes move? Section Summary

Areas with little plant cover and desert areas covered with fine rock material are more vulnerable than other areas to wind erosion.

Saltation is the process in which sand-sized particles move in the direction of the wind.

Three landforms that are created by wind erosion and deposition are desert pavement, deflation hollows, and dunes.

Dunes move in the direction of the wind.

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