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Wind Erosion

From the sand seas of the Sahara to the loess plateau of China, to the yardang fields of Iraq and the dune fields of the Lençóis Maranhenses in Brazil, wind erosion has led to the development of distinctive and often dramatic landscapes. Many of the most recognizable of these landscapes, the sand seas and dunes, for example, are the product of the deposition of wind-eroded material. Wind erosion has also shaped landforms on Mars. Although wind does not have the erosive power of flowing water or ice, its pervasiveness compensates for that lack of force. It has been estimated that as much as a third of the land area of the Earth is susceptible to wind erosion (with the continents of Asia and Africa most at risk). Catastrophic wind erosion of soils in the Great Plains of the United States during the Dust Bowl era of the 1930s caused the infamous “black blizzard” dust storms and also led to the creation of the U.S. Department of Agriculture and the (then) Soil Conservation Service. This discussion of wind erosion is organized around four central themes: (1) erosion processes, (2) transport processes, (3) erosional landforms, and (4) wind erosion hazards.

Erosion Processes

Wind causes erosion mainly through two mechanisms: deflation and abrasion. Deflation is the direct removal of unconsolidated sediments from a surface. Abrasion occurs when the wind is carrying a sediment load that can work on and reshape an otherwise resistant surface. Once materials have been eroded, they are then subject to transportation by the wind and deposition in another location.

Deflation

The wind is able to deflate materials when the force it exerts on a surface exceeds the resisting forces acting to stabilize the particles. For a dry sand surface, there is a physics-based relationship that balances the resisting force of gravity against the driving force of the wind. The state where the resisting and driving forces are in balance is termed the threshold condition. Quantification of the threshold state is fundamental to predicting wind erosion and is simplest when sediments are cohesion-less. When they are not, the process becomes much more complicated. If the sand is wet, for example, then there is an added resisting force caused by surface tension binding the grains. The presence of salt crusts or algae will also increase the necessary threshold wind speed.

For dust particles (i.e., silts and clays), establishing the threshold condition is more complicated. Such particles may be bound to each other by electrostatic forces, for example. For soils, the initiation of motion also depends on the characteristics of clods, such as size, organic content, and moisture content. For agricultural soils, there are also larger-scale controls such as cultivation practices or the presence of wind shelter belts. The Wind Erosion Prediction System is a model developed to account for these and other factors, based on simulation of field conditions such as particle size, clod processes, moisture, and organic content; wind speed and threshold conditions; and management practices, among other things.

Abrasion

Aeolian abrasion is caused by the impact of wind-borne sand or dust particles on a lithified or cohesive surface. Abrasion occurs when the resisting force, in this case, the strength of the surface material, is less than that imparted by the impact of grains. Saltating sand grains may affect igneous or metamorphic rocks with enough force to generate microscopic fractures so that small pieces of the parent rock become susceptible to chipping away—a natural sand-blasting process. Abrasion of sedimentary rocks is more apt to occur as a result of wearing away of the cementing materials that bond individual grains, thus releasing those grains from their matrix. The rate of abrasion is a function of the cube of wind speed and the size and availability of grains for transport. Abrasion is most common in dry windy climates, where there is minimal vegetation to stabilize surfaces and reduce surface wind speeds. For these reasons, abrasion is an exceptionally active process on Mars, where wind speeds are quite high and there is an abundance of unconsolidated sediment and no vegetation.

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