Wind is an important agent of transport and deposition - Geography Essay Example
A desert can be defined as any region that receives less than 25cm of rain per year where evapo-transpiration equals or exceeds precipitation or in other terms where there is a permanent moisture deficit.
Arid areas cover one third of the earth’s land surface and there is an enormous amount of variety within in them - Wind is an important agent of transport and deposition introduction. Most of these deserts lie in the tropical and subtropical belts between 20o and 30o north and south of the Equator.
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Wind, as in other physical systems, can erode, transport and deposit but as stated its role as an eroder remains open to discussion.
Transport in deserts is carried out in three processes, being suspension, through the air less than 25 metres above ground level; saltation, where grains bounce a few cm above the ground; and surface creep.
In suspension very fine material is picked up by the wind, raised to considerable heights and carried great distances. The finer materials are taken away leaving behind a lag deposit. Transport is effective and important here in that in 1km of air, 900 tonnes of sand can be entrained. In some case red dust from the Sahara has been carried northwards and deposited as ‘red rain’ over parts of Britain and has been found in places as far away as Hawaii.
Dust Storms such as the above have increased as drought has intensified and satellite pictures have shown the considerable extent to which they have developed. The vast sand seas are obviously attributable to wind.
For saltation to occur, wind speeds must exceed the threshold velocity (the speed required to initiate grain movement) where fine and coarse-grained sand particles are lifted. They may rise for several centimetres before returning to the ground in a relatively flat trajectory of less than 12o. As the wind continues to blow, the sand particles bounce along, leapfrogging over one another.
Surface creep occurs where a sand particle transported by saltation dislodges and pushes forward a larger particle (more than 0.25 mm in diameter) which is too heavy to be uplifted. The constant bombardment gradually moves small stones and pebbles over the desert surface.
It is clear then that wind is an important agent of transportation.
Deposition of transported sand grains is also an important process in deserts and wind plays the major role here. Deposition comes in three main forms, ripples, dunes and draa where dunes will be concentrated on.
Dunes develop when sand grains, moved by saltation and surface creep, are deposited.
The first here is the Barchan Dune, a crescent shaped dune up to 30m high, aligned importantly 90o to the wind. They start life as a mound of sand and windward grains are moved by saltation over the apex to accumulate on the lee side. Horns develop on either side as these grains move faster than the heavier middle:
Secondly Seif dunes, much larger than Barchan dunes are created by a combination of deposition and transportation. There are two theories as to how they are created. On one hand they are though to be simply an extension of Barchan dunes where one horn elongates and migrates (Bagnold). On the other hand wind is though to act in a ‘vortex flow’ that whips up the sand:
But in any case the winds are steady and persistent with slight diurnal changes in direction.
Lastly obstacle dunes arise where transportation is interrupted by some impediment. For example wind shadow may result where a hill blocks the wind. This lack of wind is therefore responsible for the subsequent falling dune that develops.
So again it is unquestionable that wind is an important agent here contributing to the development of the preceding features.
It is in erosion that the winds impact is uncertain.
Two major processes of erosion are deflation and abrasion.
Over much of the Sahara vast areas of flat land created by an earlier wetter climate are vulnerable to the wind and the lighter particles are removed leaving the remaining bare interlocking pebbles like cobblestones.
The Oattara depression in Egypt 3000km squared of material has been removed:
However equifinality of form may be evident as deflation hollows could be produced by faulting under the depression or by chemical collapse of the underlying limestone strata. Pavements may have the fine material removed by water in a process called winnowing, where the smaller material is easier to transport by surface water.
Abrasion is the process of sandblasting close to ground level. The strength, frequency and direction of the wind are important influences. Features of such can be seen in the form of Mushroom rocks:
However, even though it definitely occurs it is limited and added to this mushroom shaped rocks may have resulted largely from capillary action and weathering
In addition erosional landforms are minimal in terms of occurrence. The desert can often be covered by protective duricrusts (particles cemented together). Records also show that strong long-term winds do not occur and abrasion only occurs 1.5m above the land surface.
Some support, though is given for winds effectiveness as an eroder with satellite photos showing hard bedrock features from Iran, Peru and the Sahara have the same alignment as the prevailing wind.
It seems that in extremely dry deserts such as the Namib that wind is a major process and the formation of isolated mountains (Inselbergs) is attributed to wind planation. This is termed as the phase of ‘extravagant aeolation’.
Whereas in the ‘wetter’ deserts of North America, wind is less effective, as mentioned above, where wind eroded features are only minor, and wind erosion is shown to have a limited height range.
In conclusion it is safe to say that wind is an important agent of deposition and transportation and still contributes to erosion. Its role as an eroder is not as effective as the other two processes, as other factors contribute creating equifinality of form, but it is nonetheless still a significant factor.