adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
Wind as a Geologic Agent Wind is common in arid desert regions because:
Air near the surface is heated and rises, cooler air comes in to
replace hot rising air and this movement of air results in winds.
Arid regions have little or no soil moisture to hold rock and
Wind has the ability to transport, erode, and deposit sediment.
Sediment Transportation by Wind
Wind transports sediment near the surface by saltation. Just as in the bed
load of streams, saltation refers to short jumps of grains dislodged from
the surface and jumping a short distance. As the grains fall back to the
surface they may dislodge other grains that then get carried by wind until
they collide with ground to dislodge other particles. Smaller particles
can become suspended in the wind and may travel for longer distances.
Sand Ripples - Occur as a result of larger grains accumulating as smaller
grains are transported away. Ripples form in lines perpendicular to wind
direction. Wind blown dust - Sand sized particles generally do not travel
very far in the wind, but smaller sized fragments can be suspended in the
wind for much larger distances.
Wind Erosion Wind
can be effective agent of erosion anywhere that it is strong enough to
act. Wind can erode by deflation and abrasion. Deflation is the lowering
of the land surface due to removal of fine-grained particles by the wind.
Deflation concentrates the coarser grained particles at the surface,
eventually resulting in a surface composed only of the coarser grained
fragments that cannot be transported by the wind. Such a surface is called
Ventifacts are any bedrock surface or stone that has been abraded or
shaped by wind-blown sediment in a process similar to sand blasting.
Yardangs are streamlined wind-eroded ridges commonly found in deserts
Wind can deposit sediment when its velocity decreases to the point where
the particles can no longer be transported. This can happen when
topographic barriers slow the wind velocity on the downwind side of the
barrier. As the air moves over the top of the barrier, streamlines
converge and the velocity increases. After passing over the barrier, the
streamlines diverge and the velocity decreases. As the velocity decreases,
some of the sediment in suspension can no longer be held in suspension,
and thus drops out to form a deposit. Topographic barriers can be such
things as rocks, vegetation, and human made structures that protrude above
the land surface.
Sand Dunes - Sand dunes form when there is (1) a ready supply of sand, (2)
a steady wind, and (3) some kind of obstacle such as vegetation, rocks, or
fences, to trap some of the sand. Sand dunes form when moving air slows
down on the downwind side of an obstacle. The sand grains drop out and
form a mound that becomes a dune.
Sand dunes are asymmetrical mounds with a gentle slope in the upwind
direction and steep slope called a slip face on the downwind side. Dunes
migrate by erosion of sand by wind (saltation) on the gentle upwind slope,
and deposition and sliding on the slip face, and thus are cross-bedded
Dunes may cover large areas and reach heights up to 500m.
Barchan Dunes - are crescent-shaped dunes with the points of the crescents
pointing in the downwind direction, and a curved slip face on the downwind
side of the dune. They form in areas where there is a hard ground surface,
a moderate supply of sand, and a constant wind direction.
Transverse dunes - are large fields of dunes that resemble sand ripples on
a large scale. They consist of ridges of sand with a steep face in the
downwind side, and form in areas where there is abundant supply of sand
and a constant wind direction. Barchan dunes merge into transverse dunes
if the supply of sand increases.
Linear Dunes - are long straight dunes that form in areas with a limited
sand supply and converging wind directions.
Parabolic (Blowout) Dunes
Parabolic (also called blowout) Dunes - are "U" shaped dunes with an open
end facing upwind. They are usually stabilized by vegetation, and occur
where there is abundant vegetation, a constant wind direction, and an
abundant sand supply. They are common in coastal areas.
Star Dunes - are dunes with several arms and variable slip face directions
that form in areas where there is abundant sand and variable wind
Wind Blown Dust - Dust consists of silt and clay sized particles
that are often packed together with smooth surface. Such packed dust
is difficult to remove by wind erosion alone, unless the surface is
very dry or is disturbed. When dust it is disturbed, dust storms may
develop, and dust may be transported by the wind over large distances.
Most soil contains some silt and clay particles deposited by the wind.
A large deposits of wind deposited dust is called loess. Much loess
was derived from debris left by glacial erosion.
Dust in Ocean Sediments and Glacial Ice. - Dust can be
transported by the wind and by glacial ice onto the surface of the
oceans. As a result, much of the fine grained continent-derived
sediment that reaches the abyssal plains of the oceans was originally
transported by winds or icebergs.
Volcanic Ash - During explosive volcanic eruptions, large
quantities of dust-sized tephra can be ejected into the atmosphere. If
ejected high enough, such ash can become suspended in the wind and
carried for long distances. Eventually it will settle out to become
Deserts are areas where rainfall is less than 250 mm (10 in.)/year, or
where evaporation exceeds precipitation. Thus, deserts are areas that we
think of as arid. Origin of Deserts Deserts originate by several different
mechanisms that result in several different types of deserts.
Subtropical Deserts - the general atmospheric circulation brings
dry, subtropical air into mid-latitudes. Examples: Sahara of Northern
Africa, Kalhari of Southern Africa, and the Great Australian Desert.
Continental Deserts - Areas in the continental interiors, far from
source of moisture where hot summers and cold winters prevail.
Examples: Gobi, Takla Makan
Rainshadow Deserts - Areas where mountainous regions cause air to
rise and condense, dropping its moisture as it passes over the
mountains. Examples: Deserts east of the Sierra Nevada Mountains,
California & Nevada, East of the Cascades of Oregon and
Washington, and East of the Andes Mountains in South America.
Coastal Deserts - Areas where cold upwelling seawater cools the air
and decreases its ability to hold moisture. Examples : Atacama Desert
of coastal Peru, Namib Desert of coastal South Africa.
Polar Deserts - Cold polar regions where cold dry air prevails and
moisture available remains frozen throughout the entire year.
Examples: Northern Greenland, and ice-free areas of Antarctica.
Surface Processes in Deserts The same geologic processes operate in
deserts as in other more humid climates. The difference is the intensity
to which the processes act.
Weathering and Mass Wasting
Deserts have little soil because moisture is so low and the rate of
chemical weathering is slow.
Little plant life because of lack of soils and water. Plants tend to
hold soil and fine-grained rock fragments in place.
The desert surface is dominated by mechanical weathering processes.
If we compare the surface features of deserts with those in humid
regions, we find that:
deserts are dominated by rock falls, rock slides, and the
accumulation of coarse grained material, and generally have steeper
humid regions have soil and fine-grained regolith covering slopes,
with creep being the dominant mass-wasting process, resulting in
curved gentle slopes.
Streams and Fluvial Landforms
Surface waters are rare in deserts. Streams that do flow in deserts
usually originate at higher elevations and supply enough water for the
stream to pass through the desert region. Streams in deserts tend to be
intermittent, that is they flow only during rains. For this reason, flash
floods and braided streams are common.
Alluvial Fans and Bajadas - An alluvial fan forms where a mountain
stream enters a broad flat valley and deposits sediment as its
velocity decreases on entering the flatter valley (see chapter 9).
When a linear mountain range has several closely spaced valleys, the
alluvial fans may coalesce to form a gentle undulated slope on the
sides of the bounding lowlands. Such coalesced alluvial fans are known
Pediments - A pediment is broad bedrock surface with a gentle slope
away from highlands. With distance away from the highlands the
pediment passes beneath a thin cover of alluvial sediment derived from
erosion of the pediment. The highlands remain as residual hills as the
Playa Lakes - Standing bodies of water like lakes are rare in desert
regions because rainfall and input from streams occurs only
intermittently. Lakes that do form during the rare periods of
rainfall, quickly evaporate, leaving a dry lake bed behind. Playa
Lakes (also called dry lakes) are formed in basins of internal
drainage. The lake beds often consist of salts (evaporites) that were
carried in by streams and precipitated during evaporation. These
precipitated salts give the dry lake bed a white color resembling a
beach (playa means beach in Spanish).
Inselbergs - The word inselberg means island mountain in German.
Inselbergs are steep sided hills that rise above a surrounding relatively
flat plain. They appear to form because the rock making up the inselberg
is more resistant to erosion than the rocks that once made up the
surrounding plain. Once an inselberg forms, it sheds water due to its
steep slopes, and its steep slopes tend to not develop soil. The
surrounding less resistant rock collects this water and is subjected to
more rapid rates of chemical weathering. Thus as the surrounding plain is
reduced by stream erosion and weathering faster than the more resistant
rock. Inselbergs are common in desert regions, although they can also
occur in other areas where differential erosion takes place.
Desertification Desertification occurs as a result of climatic changes,
such as changing positions of the continents, or changes in ocean and air
circulation patterns. Human impacts, such as overgrazing, draining of
land, and lowering of the groundwater table, can also contribute to
desertification. As vegetation dies out, the soil is more easily eroded
and may be lost so that other vegetation becomes destabilized. Since soil
can hold moisture, if the soil erodes, the area may become arid, and the