10.3 Erosion and Deposition by Wind

Lesson Objectives

• Explain how wind causes erosion.
• Describe sediments deposited by wind.
• Identify ways to prevent wind erosion.

Vocabulary

• loess
• sand dune

Introduction

Wind is only air moving over Earth’s surface, but it can cause a lot of erosion. Look at Figure below. It will give you an idea of just how much erosion wind can cause. The dust storm in the photo occurred in Arizona. All that dust in the air was picked up and carried by the wind. The wind may carry the dust for hundreds of kilometers before depositing it.

Dust storm over Arizona desert. Have you ever experienced a dust storm like this one?

Wind Erosion

Dust storms like the one in Figure above are more common in dry climates. The soil is dried out and dusty. Plants may be few and far between. Dry, bare soil is more easily blown away by the wind than wetter soil or soil held in place by plant roots.

How the Wind Moves Particles

Like flowing water, wind picks up and transports particles. Wind carries particles of different sizes in the same ways that water carries them. You can see this in Figure below.

• Tiny particles, such as clay and silt, move by suspension. They hang in the air, sometimes for days. They may be carried great distances and rise high above the ground.
• Larger particles, such as sand, move by saltation. The wind blows them in short hops. They stay close to the ground.
• Particles larger than sand move by traction. The wind rolls or pushes them over the surface. They stay on the ground.

Wind transports particles in different ways depending on their size (left). A dust storm in the Middle East (right).

Abrasion

Did you ever see workers sandblasting a building to clean it? Sand is blown onto the surface to scour away dirt and debris. Wind-blown sand has the same effect. It scours and polishes rocks and other surfaces. Wind-blown sand may carve rocks into interesting shapes. You can see an example in Figure below. This form of erosion is called abrasion. It occurs any time rough sediments are blown or dragged over surfaces. Can you think of other ways abrasion might occur?

Sand blown by fierce winds have carved this rock in to an interesting shape.

Wind Deposition

Like water, when wind slows down it drops the sediment it's carrying. This often happens when the wind has to move over or around an obstacle. A rock or tree may cause wind to slow down. As the wind slows, it deposits the largest particles first. Different types of deposits form depending on the size of the particles deposited.

Deposition of Sand

When the wind deposits sand, it forms small hills of sand. These hills are called sand dunes. For sand dunes to form, there must be plenty of sand and wind. Sand dunes are found mainly in deserts and on beaches. You can see examples of sand dunes in Figure below.

Sand dunes form where the wind deposits sand. (A) Desert sand dunes. (B) Sand dunes line many beaches like this one in Australia.

How Sand Dunes Form

What causes a sand dune to form? It starts with an obstacle, such as a rock. The obstacle causes the wind to slow down. The wind then drops some of its sand. As more sand is deposited, the dune gets bigger. The dune becomes the obstacle that slows the wind and causes it to drop its sand. The hill takes on the typical shape of a sand dune, shown in Figure below.

A sand dune has a gentle slope on the side the wind blows from. The opposite side has a steep slope. This side is called the slip face.

Migration of Sand Dunes

Once a sand dune forms, it may slowly migrate over the land. The wind moves grains of sand up the gently sloping side of the dune. This is done by saltation. When the sand grains reach the top of the dune, they slip down the steeper side. The grains are pulled by gravity. The constant movement of sand up and over the dune causes the dune to move along the ground. It always moves in the same direction that the wind usually blows. Can you explain why?

Loess

When the wind drops fine particles of silt and clay, it forms deposits called loess. Loess deposits form vertical cliffs. Loess can become a thick, rich soil. That’s why loess deposits are used for farming in many parts of the world. You can see an example of loess in Figure below.

Loess cliffs in Mississippi.

Preventing Wind Erosion

It’s very important to control wind erosion of soil. Good soil is a precious resource that takes a long time to form. Covering soil with plants is one way to reduce wind erosion. Plants and their roots help hold the soil in place. They also help the soil retain water so it is less likely to blow away.

Planting rows of trees around fields is another way to reduce wind erosion. The trees slow down the wind, so it doesn’t cause as much erosion. Fences like the one in Figure below serve the same purpose. The fence in the figure is preventing erosion and migration of sand dunes on a beach.

Protecting Sand Dunes from Wind Erosion. Many beaches uses fences like this one to reduce wind erosion of sand. If plants start growing on the dunes, they help hold the sand in place.

Lesson Summary

• Dry, bare soil is more likely to be eroded by the wind than moist soil or soil covered with plants. How wind carries particles depends on their size. The sediment in wind causes erosion by abrasion.
• Sand dunes form when the wind deposits sand. Loess form when the wind deposits clay and silt.
• Wind erosion can be prevented by keeping the ground covered with plants. They help hold the soil in place. Rows of trees and fences can help by slowing the wind.

Lesson Review Questions

Recall

1. How does the wind carry particles of sand?

2. What is abrasion?

3. What are sand dunes? Where are they found?

4. Describe loess.

5. Identify two ways to reduce wind erosion.

Apply Concepts

6. Wind-blown snow forms drifts that are similar to sand dunes. Apply lesson concepts to infer how you could reduce snowdrifts in a driveway.

Think Critically

7. Compare and contrast how the wind transports clay, sand, and pebbles.

8. Explain why a sand dune migrates.

Points to Consider

Abrasion is the main way that wind causes erosion. The next lesson explains how glaciers cause erosion.

• How do you think glaciers cause erosion?
• Do you think glaciers might erode by abrasion, like the wind?