Abrasion occurs when wind carrying sand particles has a sandblasting effect. Material like granite which has hard and soft material will develop a spongy surface because the softer parts erode away more quickly than the harder parts. Wind abrasion can produce interesting ventifacts (Latin for ‘wind-made’). These ventifacts have two characteristics they are faceted and polished. When a rock is embedded in the ground with only a part of it exposed the windward part of the rock will be abraded into a facet. After time the rock may turn and a new face will be sticking out which will again abrade a new facet. Up to several facets can be developed on a ventifact. A rock or stone scoured on three facets is called a dreikanter. Wind abrasion only erodes 1 meter from the ground and therefore large rocks are only eroded at their bases and eventually from mushroom rocks. This happens because the wind swirls around the rock and attacks it at all angles.
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Deflation is the lowering of the land surface by the removal of small particles by the wind eventually resulting in a surface having only the coarser grained particles that cannot be transported by the wind and a surface like this is called desert pavement.
Wind Deposition
Wind can deposit sediment when its speed decreases so much so that the particles can no longer be transported. This can happen when barriers or obstacles slow the wind speed down. As the air moves over the top of the barrier, wind speed increases. After passing over the barrier the velocity decreases. As the velocity decreases, some of the sediment in suspension can no longer be held, and drops out and gets deposited.
Barriers can be such things like rocks, vegetation, and human made structures that jut out above the land surface.
Saltation
Wind transports sediment near the surface by saltation. Saltation is the short jumps of grains that were loosened from the surface and move in short distances in a jumping motion. As the grains fall back to the surface they may loosen other grains that then get carried by wind until they collide with the ground to loosen other particles. Smaller particles can become suspended in the wind for longer and therefore can travel for longer distances.
Saltation moves small particles in the direction of the wind in a series of short hops or skips.
Sand Dunes
Sand dunes form when there is
(1) A lot of sand,
(2) Constant wind
(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 e.g. rocks. The sand grains drop out and form a pile that becomes a dune. Sand dunes are uneven mounds with a gentle slope in the upwind direction and steep slope called a slip face on the downwind side. Dunes move around by erosion of sand on the upwind-slope which is caused by the wind and deposition and sliding on the slip face.
TYPES OF SAND DUNES
Barchan Dunes - are crescent-shaped dunes with the points of the crescents pointing in the downwind direction. 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. They consist of folds of sand with a steep face in the downwind side, and form in areas where there is lots of sand and a constant wind direction. Barchan dunes merge into transverse dunes if the amount of sand increases.
Linear Dunes - are long straight dunes that form in areas with a small sand supply and converging wind directions.
Parabolic (also called blowout) Dunes - are "U" shaped dunes with an open end facing upwind. They are usually fixed by vegetation, and occur where there is plentiful vegetation, a constant wind direction, and a plentiful sand supply. They are common in coastal areas.
Star Dunes - are dunes with several arms and many different slip face directions and this form in areas where there is lots of sand and different wind directions.
Wind Blown Dust - Dust consists of small sized particles that are often packed together with a smooth surface. The packed dust is difficult to remove by wind erosion, unless the surface is very dry or is disturbed. When dust is disturbed, dust storms can be formed, and dust may be transported by the wind over large distances.
