Eolian Systems

You may recall, from the Weathering and Soils Unit of this course, that deserts can also grow larger due to human-made causes. If human activity degrades the soil in areas adjacent to deserts, then the land erodes more easily, which increases deflation and abrasion and encourages the desert to expand. Once non-desert areas become barren, dry lands. Practices that lead to desertification include:

-poor cultivation practices -overgrazing of livestock -deforestation -compaction of soil

Rain Shadow Deserts

A third place where eolian landscapes are found is the rain shadow deserts of the world. These are located behind mountain ranges. The mountains intercept warm moist air, so that the backside of the mountain is dominated by dry and cool air that descends down the mountain and makes the other side a desert.

Questions 1. True or False: Desertification can be a natural process. 2. True or False: Deforestation is a leading cause of desertification. 3. True or False: Grazing livestock protect the soil from erosion and help prevent desertification. 4. True or False: Soil compaction and other poor cultivation practices can accelerate desertification. 5. True or False: Lake Chad in the Sahara Desert is growing larger because the desert is shrinking there. 6. True or False: Deserts can naturally expand or shrink due to natural changes in climate.

Answers 1. True Feedback: Desertification can be natural or human-caused. 2. True Feedback: Deforestation is a leading cause of desertification because it removes vegetation and makes the soil more vulnerable. 3. False Feedback: Grazing animals remove vegetation, thus increasing susceptibility to desertification. 4. True Feedback: These can expose the soil to more erosion and make it susceptible to desertification. 5. False Feedback: Lake Chad has shrunk 97% because the desert is growing and expanding its borders. 6. True Feedback: Deserts may expand or shrink when natural climate changes shift the wind or alter the amount of rainfall, etc.

Words 1. These are long linear ridges formed by abrasion. 2. These are pebbles that have bee n shaped and polished by the wind. 3. This is the removal of sediment by wind erosion. 4. This is a type of desert that forms on the back si de of mountains, after warm moist air is deflected by the mountain, leaving only cool dry air on the other side. 5. This is the process of wind erosion most like sandblasting. 6. This is a broad shallow depression in the ground, left behind after wind has removed the surface sediment. 7. These are the large sediments left behind a fter wind erosion carries away sand and dust. 8. This forms when lag deposits harden and form a protective layer of the soil.

Answers 1. yardangs Feedback: Yardangs are long linear ridges shaped by abrasive wind erosion. 2. ventifacts Feedback: Pebbles that are shaped and polis hed by the wind are called ventifacts. 3. deflation Feedback: The removal of sediment by wind erosion is called deflation. 4. rain shadow desert Feedback: Rain shadow deserts form on the back sides of mountains. 5. abrasion Feedback: Abrasion is like sandblasting. 6. deflation basin Feedback: Deflation basins are left behind a fter the wind removes the surface sediment 7. lag deposits Feedback: The large deposits left behind are the lag deposits. 8. desert pavement Feedback: Desert pavement is made of lag deposits that harden together over time.

Desertification

As you have read, sand dunes migrate on their downwind side. They can swallow up anything in their path as they do. This movement of sand dunes is natural and takes place on the immediate edges of existing deserts. Changes in climate, such as differences in the distribution of rain, can make a desert shrink or grow larger, depending on how those changes affect sand dune migration. Desertification is the process where deserts expand their borders as a result of dune migration.

Weathering is a necessary first step before eolian landscapes can form. Why?

Before wind can erode away soil and sand particles, weathering has to first create those particles. Small unconsolidated particles are produced by weathering. They are then further eroded away by the wind.

Subtropical Deserts

Eolian landscapes are most common in the subtropical desert areas of the world; those parts of the world where vegetation is sparse and there is an abundant supply of loose, unconsolidated sediment. In these subtropical regions, dry descending air makes for dry deserts on the ground. The low moisture level means that the soil is easily blown away by wind.

Eolian landforms are also common in dry areas with very little humidity. How does the air's moisture level affect wind erosion?

Humid climates discourage wind erosion. High humidity means more rainfall, more vegetation, and more moist soil. Moist, vegetated soil is much harder for wind to erode away than dry, bare soil. Water holds soil particles together and gives them greater cohesion.

Wind Erosion

Imagine trying to walk in a wind storm like this one, in which storm-force winds swirl thousands of sand grains around you. What might it feel like to have grains of sand blasted against your skin? It would hurt because of the abrasive power of fast-moving sand striking you. This is the process of abrasion, or the sandblasting action of wind-blown sand. Both sediment removal and abrasion work to erode eolian environments. Click through the tabs to learn more about sediment removal and abrasion.

Sand blows in the wind until some obstacle stops it. Where wind velocity decreases, sand grains are deposited to form a dune. Dunes migrate downwind and can completely obliterate anything in their paths. They can cover forests, block streams, and bury villages.

Sand Dunes As sand is blown up and around the obstruction, sand grains accumulate on the downwind face. The downwind face, called the lee slope, is usually very steep. Notice how the dunes grow from the downwind side. Note too how the internal st ructure of the migrati ng dune consists of cross- beds, which are layers of sand put down at different times.

Based on the sand activity, what is necessary for a sand dune to form?

Sand dunes need three things to form: an adequate supply of loose sand wind to blow the sand something that causes the sand to settle like a rock or clump of grass

Why does dust travel out from the deserts, but sand is deposited within the deserts?

Sand is heavier and it settles out first. The dust is finer and much lighter. It can travel in the wind for hundreds of kilometers before it finally settles out.

The picture shows sandstone, the sedimentary rock that forms when sand particles are lithified together over time. What caused the banding t hat you see in some of the rock layers?

The bands are cross-beds. They represent different stages of sand deposition in a sand dune

Eolian Systems: Introduction

The famous explorer, Marco Polo, first heard this unusual sound in the Gobi Desert and thought it was the voice of spirits. Ancient Chinese literature says the sound has divine power. A British explorer, R. A. Bagnold, heard the sound in 1941 in the Saharan Desert and called it the "toiling of underground bells in sand-engulfed monasteries." The source of the sound is not completely understood even by scientists, but we do know that at only about 35 locations worldwide, huge desert sand dunes "sing." Called "singing sand" or "booming dunes,"" these unusual sand dunes produce a natural sound phenomenon, like a loud, low-pitch rumble or prolonged musical note. In this section, we will not figure out why the singing sands sing (that has been a mystery for centuries), but you will learn about some of the more explainable features of sand dunes. You will learn how wind creates these spectacular landscape features and about other issues related to wind-dominated environments.

Avatar: Deflation Basins

The sand that is removed during a storm or other wind event carries sand away from an area. Listen to this presentation to find out what is left behind after the sand is removed. Select the play button to begin the avatar video, and then use the navigation buttons to pause/stop, continue, or reset the avatar. View the presentation as often as you would like, and take notes as you follow along. Be sure to set your volume at a reasonable level before you begin

What can landscape features like this tell us about past climate?

They show us that the area in the picture was once dominated by eolian processes. By dating the rock, we can know when that was. Also, by studying the shape of the cross beds, we can know which direction the wind was blowing.

Deflation Basins

To deflate a balloon, you must let the air out. The end result is a much smaller balloon. Likewise, deflation is the removal of loose particles of sand from a particular area. It is the most significant type of wind erosion. It occurs only where loose material is exposed at the surface. The feature left behind after the removal of sand is called a deflation basin. These landscape features can cover several hundred kilometers in area and are associated with many of the world's deserts.

Eolian landforms are common in deserts, where vegetation is sparse. What effect does vegetation have on the wind's ability to shape the landscape?

Vegetation holds the soil down, acting like an anchor that keeps it from blowing away in the wind. If there is no vegetation at all, such as in extreme desert environments, the soil easily blows away.

Sand Dunes

What image comes to mind when you think about the world's great deserts? In movies and music videos, sand dunes are often used to represent desert environments. There is a reason deserts are so commonly dotted with sand dunes. As you noticed in the previous activity, wind blows the sand into piles to make dunes, especially in deserts.

Coastal Environments

While eolian landscapes are most common in the subtropical deserts, they are also found in coastal environments, where winds are high and there is an abundant supply of loose sand on the ground

Abrasion

Wind abrasion is like sandblasting. Wind-blown grains of sand impact rock surfaces and small particles are knocked off the rock. Pebbles that have been shaped and polished by the wind are called ventifacts. The abrasive action of sand can also leave grooves and polished surfaces on desert pavement. Though less common, wind abrasion does produce larger landforms in some desert regions linear ridges, called yardangs. Yardangs, like the ones shown here, resemble the hull of a boat. Facing the wind is a steep blunt face that gradually gets lower and narrower through abrasive erosion. Yardangs form only in the driest areas of deserts.

Eolian Systems

Wind is second only to running water as a powerful agent of erosion. It both erodes material away and deposits it somewhere else. Thus, wind has great potential to shape the landscape. In Greek mythology, Eolius was the ruler of the winds. Likewise, we call environments shaped by wind, eolian systems. Because they are driven by wind, eolian systems derive all their energy from the Sun—since incoming solar radiation affects all of Earth's winds.

Loess: Wind-Blown Dust

Wind not only deposits sand in sand dunes; it also deposits accumulations of silt and clay (dust) called loess. Loess blankets many regions throughout the world. It is composed mostly of silt-sized grains, but clay particles may also be present. Loess deposits are commonly found away from deserts, because the loess is blown away from them.

Lag Deposits

Wind usually removes only sand and dust-sized particles. Larger sediment is left behind; this is called the lag deposit. Lag deposits harden together to form desert pavement, like you see in this image. Desert pavement acts like a protective covering that keeps the soil from further deflation.

The Kelso Sand Dunes in Mojave Desert, California.

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