deserts and winds

Deserts and Winds


Contents of Entire Course


Wind as a Geologic Agent
Sediment Transportation by Wind
Wind Erosion
Wind Deposits:

Sand Dunes:

Deserts
Surface Processes in Deserts
Desertification

adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane University


 
 
 
 
 
 
 


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 mineral fragments.
  • 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.

    saltationwind.jpg

    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 desert pavement

    deflation.jpg


    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

    ventifact.gif
    Wind Deposits

    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.

    winddepos.gif


    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 deposits.

    dune.gif
    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.

    barchan.gif


    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.


     transverse.gif


    Linear Dunes - are long straight dunes that form in areas with a limited sand supply and converging wind directions.
    lindunes.gif


    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.
    parabdun.gif


    Star Dunes - are dunes with several arms and variable slip face directions that form in areas where there is abundant sand and variable wind directions.

    stardun.gif

    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 wind-deposited sediment.


    Deserts

    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.

    winds.gif


    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.

    free hit counter