Aeolian processes
Aeolian processes, also spelled eoulian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth. Winds may erode, transport, and deposit materials. They are effective agents in regions with sparse vegetation, a lack of soil moisture and a large supply of unconsolidated sediments. Although water is a much more powerful eroding force than wind, aeolian processes are important in arid environments such as deserts.
The term is derived from the name of the Greek god Aeolus, the keeper of the winds.
Definition and setting
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. Sediment deposits produced by the action of wind and the sedimentary structures characteristic of these deposits are also described as aeolian.Aeolian processes are most important in areas where there is little or no vegetation. However, aeolian deposits are not restricted to arid climates. They are also seen along shorelines; along stream courses in semiarid climates; in areas of ample sand weathered from weakly cemented sandstone outcrops; and in areas of glacial outwash.
Loess, which is silt deposited by wind, is common in humid to subhumid climates. Much of North America and Europe are underlain by sand and loess of Pleistocene age originating from glacial outwash.
The lee 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
Wind erodes the Earth's surface by deflation and by abrasion. 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
Abrasion 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. It is effective at rounding sand grains and at giving them a distinctive frosted surface texture.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
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. In ergs, wind is very effective at transporting grains of sand size and smaller.Particles are transported by winds through suspension, saltation and creeping along the ground. The minimum wind velocity to initiate transport is called the fluid threshold or static threshold and is the wind velocity required to begin dislodging grains from the surface. Once transport is initiated, there is a cascade effect from grains tearing loose other grains, so that transport continues until the wind velocity drops below the dynamic threshold or impact threshold, which is usually less than the fluid threshold. In other words, there is hysteresis in the wind transport system.
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.
Saltation is downwind movement of particles in a series of jumps or skips. Saltation is most important for grains of up to 2 mm in size. A saltating grain may hit other grains that jump up to continue the saltation. The grain may also hit larger grains that are too heavy to hop, but that slowly creep forward as they are pushed by saltating grains. Surface creep accounts for as much as 25 percent of grain movement in a desert.
Vegetation is effective at suppressing aeolian transport. Vegetation cover of as little as 15% is sufficient to eliminate most sand transport. The size of shore dunes is limited mostly by the amount of open space between vegetated areas.
Aeolian transport from deserts plays an important role in ecosystems globally. For example, wind transports minerals from the Sahara to the Amazon basin. 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 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. Dust storms on Mars periodically engulf the entire planet. 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
Wind is very effective at separating sand from silt and clay. As a result, there are distinct sandy and silty 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.Some of the most significant experimental measurements on aeolian landforms were performed by Ralph Alger Bagnold, a British army engineer who worked in Egypt prior to World War II. Bagnold investigated the physics of particles moving through the atmosphere and deposited by wind. He recognized two basic dune types, the crescentic dune, which he called "barchan", and the linear dune, which he called longitudinal or "seif". Bagnold developed a classification scheme that included small-scale ripples and sand sheets as well as various types of dunes.
Bagnold's classification is most applicable in areas devoid of vegetation. In 1941, John Tilton Hack added parabolic dunes, which are strongly influenced by vegetation, to the list of dune types. 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. For example, vast inactive ergs in much of the modern world attest to late Pleistocene trade wind belts being much expanded during the Last Glacial Maximum. Ice cores show a tenfold increase in non-volcanic dust during glacial maxima. The highest dust peak in the Vostok ice cores dates to 20 to 21 thousand years ago. The abundant dust is attributed to a vigorous low-latitude wind system plus more exposed continental shelf due to low sea levels.
Wind-deposited sand bodies occur as ripples and other small-scale features, sand sheets, and dunes.