Lesson Objectives
  • Describe the ways materials are carried by wind.
  • Discuss several ways that wind erosion changes land surfaces.
  • Describe how sand dunes form.
  • Describe the type of deposits formed by windborne silts and clays.
Introduction
The power of wind to erode depends on particle size, wind strength, and whether the particles are able to be picked up. Wind is a more important erosional force in arid than humid regions.
Transport of Particles by Wind
Wind transports small particles, such as silt and clay, over great distances, even halfway across a continent or an entire ocean basin. Particles may be suspended for days. Wind more easily picks up particles on ground that has been disturbed, such as a construction site or a sand dune. Just like flowing water, wind transports particles as both bed load and suspended load. For wind, bed load is made of sand-sized particles, many of which move by saltation (Figure below). The suspended load is very small particles of silt and clay.
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Wind transport is by suspension, saltation and creep (bed load).
In a sandstorm, sand is usually within a meter of the ground. A dust storm's smaller particles can travel higher (Figure below).
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A dust storm as it approaches Al Asad, Iraq.
Wind Erosion
Wind is a stronger erosional force in arid regions than it is in humid regions because winds are stronger. In humid areas, water and vegetation bind the soil so it is harder to pick up. In arid regions, small particles are selectively picked up and transported. As they are removed, the ground surface gets lower and rockier, causing deflation. What is left is desert pavement (Figure below), a surface covered by gravel sized particles that are not easily moved by wind.
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This desert pavement formed in the Mojave Desert as a result of deflation.
Particles moved by wind do the work of abrasion. As a grain strikes another grain or surface it erodes that surface. Abrasion by wind may polish natural (Figure below) or human-made surfaces, such as buildings.
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As wind blows from different direction, polished flat surfaces create a
Exposed rocks in desert areas often develop a dark brown to black coating called desert varnish. Wind transports clay-sized particles that chemically react with other substances at high temperatures. The coating is formed of iron and manganese oxides (Figure below).
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Ancient people carved these
Wind Deposition
Deserts and seashores sometimes have sand dunes (Figure below). Beach dunes have different compositions depending on their location. Beach dunes are usually quartz because in humid areas other minerals weather into clays. In the tropics, sand dunes may be composed of calcium carbonate, which is common. In deserts, sand dunes may be composed of a variety of minerals. There is little weathering and so less stable minerals are left behind.
Dune sands are usually very uniform in size and shape. Particles are sand-sized, because larger particles are too heavy for the wind to transport by suspension. Particles are rounded, since rounded grains roll more easily than angular grains.
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This sand dune in Morocco shows secondary sand ripples along its slip face.
For sand dunes to form there must be an abundant supply of sand and steady winds. A strong wind slows down, often over some type of obstacle, such as a rock or some vegetation and drops its sand. As the wind moves up and over the obstacle, it increases in speed. It carries the sand grains up the gently sloping, upwind side of the dune by saltation. As the wind passes over the dune, its speed decreases. Sand cascades down the crest, forming the slip face of the dune. The slip face is steep because it is at the angle of repose for dry sand, about 34o (Figure below).
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Sand dunes slope gently in the upwind direction. Downwind, a steeper slip face forms.
Wind deposits dune sands layer by layer. If the wind changes directions, cross beds form. Cross beds are named for the way each layer is formed at an angle to the ground (Figure below).
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The cross-bedded sandstones in Escalante Canyons, Utah, are ancient sand dunes.
The types of sand dune that forms depends on the amount of sand available, the character and direction of the wind, and the type of ground the sand is moving over. Some dune types are shown below.
Linear dunes form long straight lines parallel to the wind direction. They form in areas with low sand and winds coming together from different directions (Figure below).
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Linear dunes have straight parallel lines.
Crescent shaped barchan dunes need adequate an amount of sand, winds consistent in one direction and hard ground. The crescent shape curves in the direction the wind blows (Figure below).
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Barchan dunes blend together into large scale sand ripples called
Star-shaped dunes form in areas of constantly changing wind direction (Figure below).
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Star-shaped dunes have several ridges of sand radiating from a central point.
Parabolic dunes form a U-shape that curves into the wind direction (Figure below).
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Some type of vegetation at least partly covers the sand in a parabolic dune.
Loess
Windblown silt and clay deposited layer on layer over a large area are loess, which comes from the German word loose (Figure below). Loess deposits form downwind of glacial outwash or desert, where fine particles are available. Loess deposits make very fertile soils in many regions of the world.
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Loess deposits form nearly vertical cliffs, without grains sliding down the face.
Fine-grained mud in the deep ocean is formed from silts and clays brought from the land by wind. The particles are deposited on the sea surface, then slowly settle to the deep ocean floor, forming brown, greenish, or reddish clays. Volcanic ash may also settle on the seafloor.
Lesson Summary
  • Wind can carry small particles such as sand, silt, and clay.
  • Wind erosion abrades surfaces and makes desert pavement, ventifacts, and desert varnish.
  • Sand dunes are common wind deposits that come in different shapes, depending on winds and sand availability.
  • Loess is a very fine grained, wind-borne deposit that can be important to soil formation.
Review Questions
  1. Discuss suspended load and bed load transport by wind.
  2. Describe how desert pavement forms.
  3. Discuss the factors necessary for sand dunes to form.
  4. Name four types of sand dunes that form in desert areas.
  5. Name one type of wind deposition.
  6. Why is wind erosion more important in arid regions than humid areas?
Vocabulary
ventifacts Polished, faceted stones formed by abrasion by sand particles. slip face Steeper, downwind side of a dune where sand grains fall down from the crest. sand dune Sand deposit formed in regions of abundant sand and constant winds. loess Extremely fine-grained, windborne deposit of silts and clays; forms nearly vertical cliffs. desert varnish Dark mineral coating that forms on exposed rock surfaces as windborne clays are deposited. desert pavement Rocky, pebbled surface created as finer silts and clays are removed by wind. deflation Wind removes finer grains of silt and clay, causing the ground surface to subside. barchan dune Crescent shaped dune that forms with ample sand, constant winds, and hard ground.
Points to Consider
  • Would hurricane-force winds along a coastline produce wind related erosion?
  • What would be needed to convert a desert area back to a productive region for farming?
  • Do you think wind could sculpt exposed rocks? Explain how this might happen.