Wind is the moving air. Wind blowing over the solid surface of the lands is also an active agent of landform development. Its activity is particularly intensive in the deserts and semi deserts which constitute about 20% of the surface of continents.
The geological action of wind is particularly effective in areas that lack plant cover, have a considerable diurnal and seasonal temperature variation, and low precipitation.
The geological action of wind can conveniently be divided in to three stages viz. Erosion, Transportation and Deposition. As a whole, the geological action of wind is largely governed by its velocity. But wind alone has little influence on shaping the surface of the ground, because it is only able to move small dry particles.
In humid climatic regions, the surface of the earth is protected by a solid cover of vegetation and also by the cohesive effects of moisture in the soil from sharp temperature fluctuations causing physical weathering and the deflation work of the wind.
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Erosion
Wind erosion manifests itself in three forms viz. (i) deflation, (ii) abrasion or corrasion and (iii) attrition. Wind uses sand as the agent of erosion. Wind and running water are in many respect similar in the ways in which they erode and transport sediment particles.
1. Deflation
A strong wind can transport very coarse sand, lifting it from the ground and carry it for great distances. This process of removal of loose soil of rock particles, along the course of the blowing wind is known as ‘deflation’ (from the Latin de flare=to blow off).
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The wind picks up and removes loose particles from the earth’s surface, and thus helps to lower the general level. This process operates well in dry regions with little or no rainfall. The rate of deflation depends on the force of the wind, the nature of the rock and the degree of weathering it has suffered etc.
Features Produced by Deflation
(i) Hamada
When the loose particles are swept away the hard mantle left behind is known as ‘hamada’. The term has been applied to the stone-strewn surface in the Sahara desert, left after the finer materials are removed by wind. This is a form of lag-deposits.
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(ii) Blow-outs or deflation-hollows
Deflation sometimes leads to the formation of depression or hollows on the land surface. At few places, deflation may continue to deepen a blow-out in fine-grained sediment until it reaches the water-table.
These depressions may range from a few metres to a kilometre or more in diameter, but it is usually only a few metres deep.
Such depressions, when deepen until ‘.he water-table and gets filled with water, create shallow ponds or lakes known as ‘Oases’. The position of the oasis is quickly stabilized by the growth of vegetation-commonly palm trees.
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Some oases are very small with only a few trees, whilst others are large enough to support moderate-sized townships surrounded by gardens and date palms. The pans of South Africa, the so-called lakes of west and central Australia etc. are probably the results of long-continued deflation.
(iii) Lag deposits
Sometimes a layer of residual pebbles and cobbles are strewn upon the surface while intervening finer particles have been removed as a result of deflation. These accumulations of pebbles and boulders have been designated by the general term lag-deposits.
By rolling or jostling about, as the finer particles are removed, the pebbles become closely fitted together forming what is known as a desert pavement. This layer protects the underlying sediment from further deflation. Its widespread occurrence is emphasized by the variety of names applied to it: reg in Algeria; rig in Iran, serir in Libya; the gibbers in Australia.
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2. Abrasion
The loose particles that are blown away by the wind serve as tools of destruction, wearing away the surface with which it comes in contact. This process is also known as corrosion. Abrasion is mainly effective as part of saltation (a mode of wind transport) and can operate only near the ground because of the inability of wind to lift sand more than a few feet.
Its main effect is mostly seen in under cutting and fluting at the base of upstanding rock masses. Depending on the hardness of the rock and the character of the material bome by the wind, the surface of rocks is polished, covered with striations, furrows or grooves, and so on.
For effective abrasion, sand-blasting must continue for a long time and the wind must have a long fetch across a source area of suitably-sized particles.
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Features Produced by Abrasion
(i) Yardang
It is a grooved or furrowed topographic form produced by wind abrasion. The grooves are elongated in the direction of prevailing winds and are separated by sharp ridges.
The yardangs commonly develop, where the exposed rocks have vertical layers, consisting of alternations of hard and soft strata, and when the winds are steady and blow in one direction, the softer strata are scoured away more rapidly than the hard and resistant strata.
Thus, there develops a topographic feature consisting of elongated ridges and furrows, depending on the original rock characteristics. These are also usually under-cut. These are common in parts of the Asiatic deserts. –
(ii) Ventifacts
These are the pebbles faceted by the abrasive effects of wind-blown sand. These are developed when sand has been blown over pebbles for a longtime, so that they become worn from the repeated abrasion and smooth polished surfaces result.
Ventifacts with one smooth surface is called Einkanters, with two abraded surfaces as Zweikanter and with three smooth faces as Dreikanters.
(iii) Pedestal rock
It is a wide rock-cap standing on a slender rock column, produced because of wind abrasion.
As we know, the sand-blast action is most effective just above the surface of the ground where the drift is thickest and it decreases rapidly upwards as a result of which rocks which projects upwards are under-cut.
When soft rocks capped with harder and resistant rocks are exposed to wind abrasion, the softer rocks being more deeply worn, produce a mushroom-shaped form in which the upper widened part of the rock rests upon a relatively thin and short rock-column.
(iv) Zeugen
These are tabular masses of more resistant rock resting on under-cut pillars of softer material and are very often elongated in the direction of prevailing wind; besides the strata are horizontal.
3. Attrition
While on transit, wind born particles often collide with one another and such mutual collision brings about some degree of grinding of the particles.
Thus rounding of grains become perfect to a great extent and the grains are reduced to smaller dimensions. The more the length of transit and velocity, the greater is the degree of rounding.
Feautures Produced by Attrition
Millel-seed sand
These are rounded deserts and grains, produced through the process of attrition and have resemblance with millet-seed grains. Sands of this type are seen sometimes in ancient formations giving indication about the former presence of deserts.