Aeolian processes

Aeolian processes are those processes of erosion, transport, and deposition of sediments that are caused by wind at or near the surface of the earth. Aeolian processes are most important in areas where there is little or no vegetation. The lee (downwind) side of river valleys in semiarid regions are often blanketed with sand and sand dunes. Examples in North America include the Platte, Arkansas, and Missouri Rivers. ==Wind erosion==

of the Mojave Desert, California Wind erodes the Earth's surface by deflation (the removal of loose, fine-grained particles by the turbulent action of the wind) and by abrasion (the wearing down of surfaces by the grinding action and sandblasting by windborne particles). Once entrained in the wind, collisions between particles further break them down, a process called attrition. Worldwide, erosion by water is more important than erosion by wind, but wind erosion is important in semiarid and arid regions. Wind erosion is increased by some human activities, such as the use of 4x4 vehicles. Deflation Deflation is the lifting and removal of loose material from the surface by wind turbulence. It takes place by three mechanisms: traction/surface creep, saltation, and suspension. Traction or surface creep is a process of larger grains sliding or rolling across the surface. Saltation refers to particles bouncing across the surface for short distances. Suspended particles are fully entrained in the wind, which carries them for long distances. Saltation likely accounts for 50–70 % of deflation, while suspension accounts for 30–40 % and surface creep accounts for 5–25 %. Regions which experience intense and sustained erosion are called deflation zones. Most aeolian deflation zones are composed of desert pavement, a sheet-like surface of rock fragments that remains after wind and water have removed the fine particles. The rock mantle in desert pavements protects the underlying material from further deflation. Areas of desert pavement form the regs or stony deserts of the Sahara. These are further divided into rocky areas called hamadas and areas of small rocks and gravel called serirs. Desert pavement is extremely common in desert environments. Blowouts are hollows formed by wind deflation. Blowouts are generally small, but may be up to several kilometers in diameter. The smallest are mere dimples deep and in diameter. The largest include the blowout hollows of Mongolia, which can be across and deep. Big Hollow in Wyoming, US, extends and is up to deep. Abrasion Desert, Qinghai Province, China Abrasion (also sometimes called corrasion) is the process of wind-driven grains knocking or wearing material off of landforms. It was once considered a major contributor to desert erosion, but by the mid-20th Century, it had come to be considered much less important. Wind can normally lift sand only a short distance, with most windborne sand remaining within of the surface and practically none normally being carried above . Many desert features once attributed to wind abrasion, including wind caves, mushroom rocks, and the honeycomb weathering called tafoni, are now attributed to differential weathering, rainwash, deflation rather than abrasion, or other processes. Yardangs are one kind of desert feature that is widely attributed to wind abrasion. These are rock ridges, up to tens of meters high and kilometers long, that have been streamlined by desert winds. Yardangs characteristically show elongated furrows or grooves aligned with the prevailing wind. They form mostly in softer material such as silts. Abrasion produces polishing and pitting, grooving, shaping, and faceting of exposed surfaces. These are widespread in arid environments but geologically insignificant. Polished or faceted surfaces called ventifacts are rare, requiring abundant sand, powerful winds, and a lack of vegetation for their formation. In parts of Antarctica wind-blown snowflakes that are technically sediments have also caused abrasion of exposed rocks. Attrition Attrition is the wearing down by collisions of particles entrained in a moving fluid. Collisions between windborne particles is a major source of dust in the size range of 2-5 microns. Most of this is produced by the removal of a weathered clay coating from the grains. ==Transport==

approaching Spearman, Texas, 14 April 1935 in Amarillo, Texas. FSA photo by Arthur Rothstein (1936) cloud is about to envelop a military camp as it rolls over Al Asad, Iraq, just before nightfall on 27 April 2005. Wind dominates the transport of sand and finer sediments in arid environments. Wind transport is also important in periglacial areas, on river flood plains, and in coastal areas. Coastal winds transport significant amounts of siliciclastic and carbonate sediments inland, while wind storms and dust storms can carry clay and silt particles great distances. Wind transports much of the sediments deposited in deep ocean basins. Small particles may be held in the atmosphere in suspension. Turbulent air motion supports the weight of suspended particles and allows them to be transported for great distances. Wind is particularly effective at separating sediment grains under 0.05 mm in size from coarser grains as suspended particles. The size of shore dunes is limited mostly by the amount of open space between vegetated areas. Saharan dust is also responsible for forming red clay soils in southern Europe. Dust storms Dust storms are wind storms that have entrained enough dust to reduce visibility to less than . Most occur on the synoptic (regional) scale, due to strong winds along weather fronts, or locally from downbursts from thunderstorms. Crops, people, and possibly even climates are affected by dust storms. On Earth, dust can cross entire oceans, as occurs with dust from the Sahara that reaches the Amazon Basin. When the Mariner 9 spacecraft entered its orbit around Mars in 1971, a dust storm lasting one month covered the entire planet, thus delaying the task of photo-mapping the planet's surface. Most of the dust carried by dust storms is in the form of silt-size particles. Deposits of this windblown silt are known as loess. The thickest known deposit of loess, up to , is on the Loess Plateau in China. This very same Asian dust is blown for thousands of miles, forming deep beds in places as far away as Hawaii. The Peoria Loess of North America is up to thick in parts of western Iowa. The soils developed on loess are generally highly productive for agriculture. Small whirlwinds, called dust devils, are common in arid lands and are thought to be related to very intense local heating of the air that results in instabilities of the air mass. Dust devils may be as much as one kilometer high. Dust devils on Mars have been observed as high as , though this is uncommon. ==Deposition==

looking toward the Cottonwood Mountains from the north west arm of Star Dune (2003) , Ethiopia (2019) Wind is very effective at separating sand from silt and clay. As a result, there are distinct sandy (erg) and silty (loess) aeolian deposits, with only limited interbedding between the two. Loess deposits are found further from the original source of sediments than ergs. An example of this is the Sand Hills of Nebraska, US. Here vegetation-stabilized sand dunes are found to the west and loess deposits to the east, further from the original sediment source in the Ogallala Formation at the feet of the Rocky Mountains. He recognized two basic dune types, the crescentic dune, which he called "barchan", and the linear dune, which he called longitudinal or "seif" (Arabic for "sword"). Bagnold developed a classification scheme that included small-scale ripples and sand sheets as well as various types of dunes. The discovery of dunes on Mars reinvigorated aeolian process research, which increasingly makes use of computer simulation. Wind-deposited materials hold clues to past as well as to present wind directions and intensities. These features help us understand the present climate and the forces that molded it. Dunes to the east of Liwa Oasis, United Arab Emirates A dune is an accumulations of sediment blown by the wind into a mound or ridge. They differ from sand shadows or sand drifts in that they are independent of any topographic obstacle. ==Aeolian desert systems==

near Hunyuan, Shanxi Deserts cover 20 to 25 percent of the modern land surface of the earth, mostly between the latitudes of 10 to 30 degrees north or south. Here the descending part of the tropical atmospheric circulation (the Hadley cell) produces high atmospheric pressure and suppresses precipitation. Large areas of this desert is floored with windblown sand. Such areas are called ergs when they exceed about in area or dune fields when smaller. Ergs and dune fields make up about 20% of modern deserts or about 6% of the Earth's total land surface. The sandy areas of today's world are somewhat anomalous. Deserts, in both the present day and in the geological record, are usually dominated by alluvial fans rather than dune fields. The present relative abundance of sandy areas may reflect reworking of Tertiary sediments following the Last Glacial Maximum. Most modern deserts have experienced extreme Quaternary climate change, and the sediments that are now being churned by wind systems were generated in upland areas during previous pluvial (moist) periods and transported to depositional basins by stream flow. The sediments, already sorted during their initial fluvial transport, were further sorted by wind, which also sculpted the sediments into eolian landforms. Flowlines can be traced from erg to erg, demonstrating very long transport downwind. Satellite observations show yardangs aligned with the sandflow lines. All flowlines arise in the desert itself, and show indications of clockwise circulation roughly like high pressure cells. The greatest deflation occurs in dried lake beds where trade winds form a low-level jet between the Tibesti Mountains and the Ennedi Plateau. The flowlines eventually reach the, sea creating great plume of Saharan dust extending thousands of kilometers into the Atlantic Ocean. This creates a steady rain of silt into the ocean. It is estimated that 260 million tons of sediments are transported through this system each year, but the amount was much greater during the Last Glacial Maximum, based on deep-sea cores. Mineral dust of 0.1–1 microns in size is a good shortwave radiation scatterer and has a cooling effect on climate. Another example of an aeolian system is the arid interior of Australia. With few topographic barriers to sand movement, an anticlockwise wind system is traced by systems of longitudinal dunes. The Namib and Oman ergs are fed by coastal sediments. The Namib receives its sediments from the south through narrow deflation corridors from coast that cross more than of bedrock to the erg. The Oman was created by deflation of marine shelf carbonates during the last Pleistocene lowstand of the sea. The Loess Plateau of China has been a long-term sink for sediments during the Quaternary ice age. It provides a record of glaciation, in the form of glacial loess layers separated by paleosols (fossil soils). The loess layers were desposited by a strong northwest winter monsoon, while the paleosols record the influence of a moist southeast monsoon. The African savannah is mostly ergs deposited during the Last Glacial Maximum that are now stabilized by vegetation. Examples Major global aeolian systems thought to be linked with weather and climate variation: • An average of 132 million tons of dust from the Sahara (primarily the Sahel and Bodélé Depression) across the Atlantic each year. • Harmattan winter dust storms in West Africa also occur blowing dust to the ocean. • Asian dust originates in the Gobi Desert and reaches Korea, Japan, Taiwan (at times) and even the western US. • The 2018 Indian dust storms transported dust from the Thar Desert towards Delhi, Uttar Pradesh, and the Indo-Gangetic Plain. • Shamal June–July winds blowing dust in primarily north to south in Saudi Arabia, Iran, Iraq, UAE, and parts of Pakistan. • Haboob dust storms in Sudan, Australia, Arizona associated with monsoon. • Khamsin dust from Libya, Egypt and Levant in Spring associated with extratropical cyclones. • Dust Bowl event in US, carried sand eastward. 5500 tons were deposited in Chicago area. • Sirocco sandy winds from Africa/Sahara blowing north into South Europe. • Kalahari Desert blowing sand/dust east across southern Africa toward Indian Ocean. • Mars in the arid conditions, many aeolian processes have been discovered. In the geologic record near Mt. Carmel road, Zion Canyon Aeolian processes can be discerned at work in the geologic record as long ago as the Precambrian. Aeolian formations are prominent in the Paleozoic and Mesozoic of the western US. Other examples include the Permian Rotliegendes of northwestern Europe; the Jurassic–Cretaceous Botucatu Formation of the Parana Basin of Brazil; the Permian Lower Bunter Sandstone of Britain; the Permian-Triassic Corrie Sandstone and Hopeman Sandstone of Scotland; and the Proterozoic sandstones of India and northwest Africa. Perhaps the best examples of aeolian processes in the geologic record are the Jurassic ergs of the western US. These include the Wingate Sandstone, the Navajo Sandstone, and the Page Sandstone. Individual formations are separated by regional unconformities indicate erg stabilization. The ergs interfingered with adjacent river systems, as with the Wingate Sandstone interfingering with the Moenave Formation and the Navajo Sandstone with the Kayenta Formation. The Navajo and Nugget Sandstones were part of the largest erg deposit in the geologic record. These formations are up to thick and are exposed over . Their original extent was likely 2.5 times the present outcrop area. Though once thought to possibly be marine in origin, they are now all but universally regarded as aeolian deposits. They are made up mostly of fine- to medium-sized quartz grains that are well-rounded and frosted, both indications of aeolian transport. The Navajo contains huge tabular crossbed sets with sweeping foresets. Individual crossbed sets dip at an angle of more than 20 degrees and are from thick. The formation contains freshwater invertebrate fossils and vertebrate tracks. Slump structures (contorted bedding) are present that resemble those in modern wetted dunes. Successive migrating dunes deposited a vertical stacking of eolian beds between interdune bounding surfaces and regional supersurfaces. The Permian Rotliegend Group of the North Sea and north Europe contains sediments from adjacent uplands. Erg sand bodies within the group are up to thick. Study of the crossbedding shows that sediments were deposited by a clockwise atmospheric cell. Drilling core show dry and wet interdune surfaces and regional supersurfaces, and provide evidence of five or more cycles of erg expansion and contraction. A global rise in sea level finally drowned the erg and deposited the beds of the Weissliegend. The Cedar Mesa Sandstone in Utah was contemporary with the Rogliegend. This formation records at least 12 erg sequences bounded by regional deflation supersurfaces. Aeolian landforms preserved in the formation range from damp sandsheet and lake paleosol (fossil soil) beds to thin, chaotically arranged dune sets to equilibrium erg construction, with dunes wide migrating over still larger draas. The draas survived individual climate cycles, and their interdunes were sites of barchan nucleation during arid portions of the climate cycles. ==See also==