Saturday, January 1, 2011

Landscaping by wind

On its own, wind has little effect on rocks. But let it transport sand particles and it can blast a desert landscape. Deserts tend to for m under consistently high pressure weather systems, or close to cold ocean currents which prevent evaporation into rain clouds. Clear skies expose rocks to intense heat by day and allow them to cool by night. The constant heating and cooling expands and contracts the surfaces of rocks, powering them to dust or making sheets flake off, producing smooth, rounded hills or inselbergs such as Ayers Rock in Australia.

Weathering processes produce a mixture of sand and rock and, although the popular image of a desert is of an endless expanse of sand, sandy deserts make up only 20 percent of the total. One reason is because wind whips up sand and dust and blows it away, leaving a layer of  heavier pebbles as a protective crust. The pebbles receive an intense blasting of sand which wears away the wind ward side. If the prevailing wind changes or the pebble overbalances, another side is presented and the resulting pebble has several flat faces and is known as a dreikanter.

 Where sand does dominate, it does not lie flat but builds into dunes such as barchans and seif dunes. If there is an obstacle such as a rock out crop or a bush, sand can build up in front and behind it, producing a long tail of sand in the lee of the obstacle.

Although it seldom rains in deserts, when it does occur the storms can be heavy. With little or no vegetation to retain it, the water produces flash floods which races down steep sided water courses, called wadis, scouring sand and rock as they go. Eventually this leads to a highly eroded landscape, a good example being the badlands of Arizona.

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