Aeolian (or
Eolian or
Æolian) processes pertain to the activity of the
winds and more specifically, to the winds'
ability to shape the surface of the
Earth and
other
planets. Winds may erode, transport,
and deposit materials, and are effective agents in regions with
sparse
vegetation and a large supply of
unconsolidated
sediments. Although water is
much more powerful than wind, aeolian processes are important in
arid environments such as
deserts.
The term is derived from the name of the Greek god,
Æolus, the keeper of the
winds.
Wind erosion
Wind
erodes the Earth's surface by deflation
(the removal of loose, fine-grained particles), by the
turbulent eddy action of the wind and by
abrasion (the wearing down of surfaces by
the grinding action and
sandblasting of
windborne particles).
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
blowout,
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.
Grinding by particles carried in the wind creates grooves or small
depression.
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 sphinx at
Giza
in Egypt may be a modified yardang.
Image:Ventifact_1871_USGS.jpg|Rock carved by drifting sand below
Fortification Rock in Arizona (Photo by
Timothy H. O'Sullivan, USGS, 1871)
Image:WindErosionTimna.JPG|Structures created
by wind erosion, which has differentially removed rock according to
hardness and the height of wind-blown sand; Timna, Negev
,
Israel.Image:KelsoSand.JPG|Sand blowing off a crest
in the Kelso
Dunes
of the Mojave Desert,
California.
Transport
Particles are transported by winds through suspension,
saltation, and
creep.
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
. In
Europe and in the
Americas, accumulations of loess are generally from
20 to 30 meters thick.
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.
Image:DustStormInSpearmanTexas19350414.jpg|Dust
storm approaching Spearman, Texas
April 14, 1935.Image:Dust_storm_in_Amarillo,_Texas.gif|Dust
storm in Amarillo,
Texas
. FSA photo by
Arthur Rothstein (1936)
Image:Sandstorm.jpg|A
massive sand storm cloud is close to
enveloping a military camp as it rolls over Al Asad
, Iraq, just before nightfall on April 27,
2005.
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, 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 some 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. This distinguishes small ripples from dunes, where
the coarsest materials are generally in the troughs.
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 wind-facing 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 "sief" (Arabic
for "sword").
Image:Grant_1929_crossbedding.png|Crossbedding of
sandstone near Mt.
Carmel road, Zion
Canyon
, indicating wind action and sand dune formation prior to formation of rock (NPS photo by
George A. Grant, 1929)
Image:Mesquite Sand Dunes.JPG|Mesquite
Flat Dunes in Death
Valley
looking toward the Cottonwood Mountains from the north
west arm of Star Dune (2003)Image:EolianiteLongIsland.JPG|Holocene
eolianite deposit on Long Island, The Bahamas
. This unit is formed of wind-blown carbonate
grains. (2007)
References
See also
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