Meaning “of the wind” (after Aeolus, Greek god of
the winds), eolian refers to sediments deposited by wind.
Loess is fine-grained windblown silt and dust that covers surfaces
and forms thick deposits in some parts of the world,
such as Shanxi Province in China. Sand dunes and other
forms are moved by the wind and form extensive dune terrains,
sand sheets, and sand seas in parts of many deserts in
the world.
When wind blows across a surface it creates turbulence
that exerts a lifting force on loose, unconsolidated sediment.
With increasing wind strengths, the air currents are able to lift
and transport larger sedimentary grains, which then bump into
and dislodge other grains, causing large-scale movement of
sediment by the wind in a process called saltation. When these
particles hit surfaces they may abrade or deflate the surface.
Wind plays a significant role in the evolution of desert
landscapes. Wind erodes in two basic ways. Deflation is a
process whereby wind picks up and removes material from
an area, resulting in a reduction in the land surface. Abrasion
is a different process that occurs when sand and other sizes of
particles impact each other. Exposed surfaces in deserts are
subjected to frequent abrasion, which is akin to sandblasting.
Yardangs are elongate streamlined wind-eroded ridges,
which resemble an overturned ship’s hull sticking out of the
water. These unusual features are formed by abrasion, by the
long-term sandblasting along specific corridors. The sandblasting
leaves erosionally resistant ridges but removes the
softer material, which itself will contribute to sandblasting in
the downwind direction and eventually contribute to the formation
of sand, silt, and dust deposits.
Deflation is important on a large scale in places where
there is no vegetation, and in some places the wind has excavated
large basins known as deflation basins. Deflation basins
are common in the United States from Texas to Canada as
elongate (several-kilometer long) depressions, typically only
3–10 feet (1–3 m) deep. However, in places like the Sahara,
deflation basins may be as much as several hundred feet deep.
Deflation by wind can only move small particles away
from the source, since the size of the particle that can be lifted
is limited by the strength of the wind, which rarely exceeds
a few tens of miles per hour. Deflation therefore leaves boulders,
cobbles, and other large particles behind. These get concentrated
on the surface of deflation basins and other
surfaces in deserts, leaving a surface concentrated in boulders
known as desert pavement. Desert pavements represent a
long-term stable desert surface, and they are not particularly
hazardous. However, when the desert pavement is broken,
for instance, by being driven across the coarse cobbles, and
pebbles get pushed beneath the surface, the underlying sands
get exposed to wind action again. Driving across a desert
pavement can raise a considerable amount of sand and dust,
and if many vehicles drive across the surface then it can be
destroyed, and the whole surface becomes active.
Wind moves sand by saltation, in arched paths in a series
of bounces or jumps. Wind typically sorts different sizes of
sedimentary particles, forming elongate small ridges known
as sand ripples, very similar to ripples found in streams. Sand
dunes are larger than ripples, up to 1,500 feet (457 m) high,
composed of mounds or ridges of sand deposited by wind.
Sand dunes are locally very important in deserts, and wind is
one of the most important processes in shaping deserts
worldwide. Shifting sands are one of the most severe geologic
hazards of deserts. In many deserts and desert border areas,
the sands are moving into inhabited areas, covering farmlands,
villages, and other useful land with thick accumulations
of sand. This is a global problem, as deserts are
currently expanding worldwide.
See also DESERT; LOESS; SAND DUNES.
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