Chapter 4 Lecture Notes-Deserts

Chapter 4 Lecture Notes-Deserts - Prof. Jerry W. Kousen...

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Prof. Jerry W. Kousen ERTH100 Vincennes University Intro to Earth Science Wind Action and Deserts Wind as a Geologic Agent Wind is common in arid desert regions because: 1. 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. 2. Arid regions have little or no soil moisture to hold rock and mineral fragments. Wind has the ability to transport, erode, and deposit sediment. In this lecture we will discuss each of these aspects of the wind. 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
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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 . 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 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.
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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
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This note was uploaded on 04/03/2011 for the course PHYS 114 taught by Professor Comptonw during the Summer '10 term at Purdue University-West Lafayette.

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Chapter 4 Lecture Notes-Deserts - Prof. Jerry W. Kousen...

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