Aeolian processes, also spelled eolian or æolian, 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 and 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. Wind erosion
Wind erodes the Earth's surface by deflation and by abrasion.
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. Almost half of Earth's desert surfaces are stony
deflation zones. The rock mantle in desert pavements protects the underlying
material from deflation. A dark, shiny stain, called desert
varnish or rock varnish, is often found on the surfaces of some desert rocks
that have been exposed at the surface for a long period of time. Manganese,
iron oxides, hydroxides, and clay minerals form most varnishes and provide
the shine. Deflation basins, called blowouts, are
hollows formed by the removal of particles by wind. Blowouts are
generally small, but may be up to several kilometers in diameter.
Wind-driven grains abrade landforms. In parts of Antarctica wind-blown
snowflakes that are technically sediments have also caused abrasion of
exposed rocks. Grinding by particles carried in the wind creates grooves or
small depressions. Ventifacts are rocks which have been cut, and sometimes
polished, by the abrasive action of wind.
Sculpted landforms, called yardangs, are up to tens of meters high and kilometers
long and are forms that have been streamlined by desert winds. The famous
Great Sphinx of Giza in Egypt may be a modified yardang.
Transport Particles are transported by winds
through suspension, saltation and creeping along the ground.
Small particles may be held in the atmosphere in suspension. Upward
currents of air support the weight of suspended particles and hold them
indefinitely in the surrounding air. Typical winds near Earth's surface
suspend particles less than 0.2 millimeters in diameter and scatter them
aloft as dust or haze. Saltation is downwind movement of
particles in a series of jumps or skips. Saltation normally lifts sand-size
particles no more than one centimeter above the ground and proceeds at
one-half to one-third the speed of the wind. 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. Aeolian turbidity currents are better
known as dust storms. Air over deserts is cooled significantly when rain passes
through it. This cooler and denser air sinks toward the desert surface. When it
reaches the ground, the air is deflected forward and sweeps up surface debris in
its turbulence as a dust storm. Crops, people, villages, and possibly
even climates are affected by dust storms. Some dust storms are
intercontinental, a few may circle the globe, and occasionally they may engulf
entire planets. 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, 335 meters, 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. In Europe and in the Americas, accumulations of loess are
generally from 20 to 30 meters thick. Aeolian transport from deserts plays an
important role in ecosystems globally, e.g. by transport of minerals from the
Sahara to Amazonia. Saharan dust is also responsible for forming red clay soils
in southern Europe. Aeolian processes are affected by human activity, such as
the use of 4x4 vehicles. 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.
Deposition 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.
Wind-deposited sand bodies occur as sand sheets, ripples, and dunes.
Sand sheets are flat, gently undulating sandy plots of sand surfaced by grains
that may be too large for saltation. They form approximately 40 percent of
aeolian depositional surfaces. The Selima Sand Sheet in the eastern Sahara
Desert, which occupies 60,000 square kilometers in southern Egypt and
northern Sudan, is one of the Earth's largest sand sheets. The Selima is
absolutely flat in a few places; in others, active dunes move over its
surface. Wind blowing on a sand surface ripples
the surface into crests and troughs whose long axes are perpendicular to the
wind direction. The average length of jumps during saltation corresponds to
the wavelength, or distance between adjacent crests, of the ripples. In
ripples, the coarsest materials collect at the crests causing inverse grading.
This distinguishes small ripples from dunes, where the coarsest materials are
generally in the troughs. This is also a distinguishing feature between water
laid ripples and aeolian ripples. Wind-blown sand moves up the gentle
upwind side of the dune by saltation or creep. Sand accumulates at the brink,
the top of the slipface. When the buildup of sand at the brink exceeds the
angle of repose, a small avalanche of grains slides down the slipface. Grain
by grain, the dune moves downwind. Accumulations of sediment blown by the
wind into a mound or ridge, dunes have gentle upwind slopes on the windward
side. The downwind portion of the dune, the lee slope, is commonly a steep
avalanche slope referred to as a slipface. Dunes may have more than one
slipface. The minimum height of a slipface is about 30 centimeters.
Some of the most significant experimental measurements on aeolian
sand movement were performed by Ralph Alger Bagnold, a British 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".
See also References
"Eolian Processes". USGS. 1997. Archived from the original on 1 December 2006.
Retrieved 2006-11-01. External links
The Bibliography of Aeolian Research Facts about wind erosion and dust storms
on the Great Plains hosted by the UNT Government Documents Department
Aeolian Research the International Society for Aeolian
Research, ISAR