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In this article we will discuss about the denudation and depositional work of wind.
Denudation Work of Wind:
Air moves because of atmospheric pressure and this moving air is known as wind. Winds blows because of variations in air pressure. Wind always blows from high pressure to low pressure. The direction from which wind is coming gives it the name of same direction. Winds perform denudation activity also but their erosion and transportation capacity is low as compared to water.
They work well in dry and desert areas. Desert areas receive rainfall less than 25 centimeters annually and have high temperature. Because of these conditions these areas do not have any vegetation and moreover there are not many obstacles in the flow of wind. Most of the deserts of the world are situated on the western side of continents (20° to 30° N & S latitudes). The south western semi-arid part of Punjab also resembles to desert region.
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Winds help in the process of denudation in desert areas generally because of following reasons:
(i) Deserts are completely different to moist regions. Chemical weathering is common in moist or wet areas but in dry areas only regional weathering is possible. ‘Salt wedging’ is one of its example.
(ii) Desert regions which receive low rainfall and have low vegetation, help winds to carry out denudation process easily.
(iii) Large regions in deserts are impermeable because of which the underneath layer of Earth has no moisture.
(iv) Deserts which have high soil content, expand because of heavy winds and scanty rain. Their size increases continuously.
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(v) Most of the rivers of such regions are seasonal or ephemeral. They flow during rainfall sessions and for short span of time. So, because of less rainfall, vegetation in desert areas is very less and winds feel free play important role in denudation process. Only thorns and shrubs are found in desert areas.
On the basis of erosional and depositional works by winds, deserts may be classified into three types:
i. Soil Desert:
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Sand in the major constituent in such deserts. Wind easily transports sand particles. In Sahara, these are known as ‘Ergs’. In Turkmenistan, these are known as ‘Koun’. The biggest erg is situated in Khalis, Saudi Arabia. Its area is 5,60,000 sq. km.
ii. Stony Desert:
These are formed by rocks, stones etc. Reg in Algeria is its finest example.
iii. Rocky Desert:
These are barren regions, where the upper soft layer vanishes, leaving behind only rock heights/rocks and barren land. Hammada (Barren Bed Rock) in Sahara is its example. Its name ‘Hammada’ also originates from Sahara.
Sahara is the largest desert of world which is situated in Africa. Thar desert of India is a hot desert which includes parts of Rajasthan, Gujarat, South West Punjab and Pakistan (Punjab and Sindh). Some cold deserts are also found in Central Asia. Atacama desert of South Africa is the driest desert of world, it receives annual rainfall less than 1mm.
Wind Erosion:
Like other means of denudation, winds also perform erosion and friction activity. Winds carry rock and soil particles from one place and when their speed reduces they deposit those particles.
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Erosion process may be divided into three stages:
i. Deflation:
High speed winds pick up soil and rock particles, resulting in the decrease or shrinking of upper layers. This process is known as deflation. Sometimes due to this process hollows are formed, which are mostly small in size but their diameter may extend from 1 to 15 kms.
ii. Abrasion:
High speed winds carry soil and rock particles, small pieces of rocks etc. This debris act as ‘sand paper’ and performs erosion and friction activity on other rocks which is known as abrasion. These are also known as tools of wind.
iii. Attrition:
Sand particles carried by winds, start friction process with in itself and because of this their size reduces. This is known as attrition. Erosion process of high speed winds is also fast. Soft rocks break down easily but on the other hand erosional process is long in case of hard rocks. Small particles are transported upto long distances but big rocks and stones (of 5 to 8 centimeter radius) cover only small distance.
Wind erosion results in the formation of various land forms, which are as follows:
i. Oasis:
During deflation the upper layers of stones are eroded by high speed winds and rocks having water appear on the surface. Because of this underground water oozes out (comes out) which is known as Oasis. Any type of vegetation and human life is possible around oasis. These landforms are found in desert regions of Algeria, Libya and Thar in India.
ii. Needles:
Complete to erosion of soft rocks by high speed winds allows steep gradient rocks stand uneroded and still. They look like needles and therefore known as rocky Needles.
iii. Mushroom or Pedestal rocks:
Wind erosion takes place at the average height of 1 meter from the Earth’s surface. While above height of average 2 meters, erosional process is again very low. Resultantly middle portion of vertical rocks is eroded by high speed winds and after erosion rocks look like mushrooms. In Sahara desert such land forms are known as ‘Gaur’ and in Germany these are known as ‘Pitzfelsen’. In India, these are found in ‘North-West’ Jaisalmer (Rajasthan) generally.
iv. Zeugen:
‘Zeugen’ is a word from German language which means ‘Like Table’. When soft rocks covered by hard rocks are eroded by winds, hard rocks left behind looks like table and known as ‘Zeugen’. Their length may vary from 1 meter to 30 meters. Along with winds, rainfall and weathering also help in formation of ‘Zeugen’.
v. Yardangs:
Winds blowing continuously in one direction result in the erosion of zeugen in one direction/side only. Zeugens are eroded much from windward side and less from leeward side. This process forms a very queer structure of these rocks. The ratio of these structures (length and breadth) varies from 3:1 and 4:1 and average height is around 8 meters. In India such landforms are found in Jaisalmer (Rajasthan).
vi. Stone Lattice:
At times rocks are formed by the combination of both soft and hard types of stones. Soft rocks and soft parts of such formations get eroded by winds and only hard parts are left behind which make formations look like ‘net’. These are known as Stone Lattice.
vii. Driekanter:
The direction of winds is never fixed and in the absence of vegetation in a desert, various rocks get eroded continuously in the direction of wind. With such continuous and all directional erosion, rocks attain a triangular shape and these are known as Driekanter.
viii. Window & Bridge:
Continuous erosion by high velocity winds forms holes in the rocks. Such holes are called Wind Windows. Further, the combined action of deflation and abrasion makes the wind windows larger and wider which assumes an arch like shape with solid roof over them. Such land forms are called Wind Bridges.
ix. Demoislle:
Some hard rocks are wrapped all round by soft rocks. With the continuous wind erosion soft rocks get vanished leaving behind the hard rock, which looks like a pillar. This pillar formation is known as Dermoislle.
x. Lag Deposits:
Fast blowing wind carries lighter particles like sand, small pebbles and stones with it. While heavier stones and other big particles lag behind. These particles look like a layer of heavy stones and rubble. Such layers are very common in desert regions. In Sahara desert such lag deposits are known as ‘Hamada’ in local language.
xi. Inselberg:
Wind erosion makes desert appear/look like plain but at some places some small mountains of solid rocks are found. These mountains are known as Inselbergs. Mt. Aabu (Graynite Inselberg) and Sendra near Pali are the finest examples.
Ayers Rock Australia
Transportation by Winds:
Wind transports pieces of rocks, soil, stones etc., like rivers and glaciers, from one place to another. Although winds erosion is less effective as compare to that by rivers but it may be seen clearly in deserts.
Transportation of particles takes place in different ways. High velocity winds can transport heavy debris from one place to another but on the other hand slow speed winds transport light weight particles only. Sometimes heavy and large pieces of rocks are not picked by winds but they roll on ground along the wind direction.
A dust storm having diameter of 500 km can pick up 100 million (10 Crore) tons of soil. It has a capacity of forming a 30 meters high mountain with a base of 3 kilometers. Delhi experienced a dust storm on November 14, 2014. Its speed was 90mph and resulted in loss of life and property.
Thick dust storms in Thar sometimes reduce the visibility to hardly three feet and the particles being transported by such storms cause heavy losses also. The size of particles moving with high velocity winds decreases because of friction produced in itself. When the speed of wind decreases, debris starts depositing.
Shifting of Sand Dunes:
Direction of wind is not fixed because of this sand dunes are not stable, they shift according to the direction of wind. Shifting sand dunes is harmful for fertile plains. Fast growing and deep root plants are planted in desert regions to control this process. They may shift from 5 to 30 meters per year.
Depositional Work of Wind:
Deposition work of wind may be divided into two parts namely:
i. Sand Deposition:
Following landforms come up under this head:-
a. Ripples:
Low speed winds deposit the particles in the shape of waves, which are known as layers of sands or sand ripples. The inter-difference between these waves may vary from few centimeters to few meters. The windward side of Ripples is generally angular at 8° to 10° while leeward side is angular at 20° to 30°. Their height rises to a few centimeters only.
b. Sand Dunes:
Velocity of wind carrying sand decreases when it faces some obstacle and therefore wind it starts depositing the sand particles on the spot of obstacle only. Resulting in the formation of sand dunes. Any shrub, big rock, skeleton of an animal, upland area can act as such obstacle. The height of sand dunes may vary from some meters to 150 meters and their length may vary between 3 kilometers to 150 kilometers.
Sand dunes may be of many types:
a. Barkhan:
Half-moon and crescent shaped sand dunes are known as Barkhans. These are convex on windward side and steeper and concave on leeward side. They might be high upto 30 meters and their length varies from 150 meters to 200 meters.
b. Self or Longitudinal Sand Dunes:
Self is an Arabic word which means ‘Sword’. These sand dunes are generally oriented in a direction parallel to prevailing wind but when the dunes blow out, sand gets deposited in parallel forms.
These may raise to 100 meter high and their breadth varies from 500 to 600 meters. These are not shifting sand dunes. These are found in areas where high velocity winds blow. These are found in Sahara Desert (Africa), Thar Desert (India).
c. Coastal Dunes:
High velocity winds blow in coastal areas because of this, waves deposit sand on the coasts of oceans. Blowing wind gives it a form of ‘Sand Dune’. With the growth of vegetation in these areas, curved sand dunes are formed and sometimes they also look like Barkhans.
These are found in Atlantic coastal regions. In Southern France, there is 240 kilometers long dune along the coast covering 3 to 10 kilometers in inland areas. On the Western coast of India, sand dunes (coastal areas) of Goa are famous for their beauty.
ii. Loess Plains:
Winds deposit light and soft soil over a large area like a blanket, these are known as plains of Loess. ‘Loess’ is a word of German language which means yellow colour, porous soil with very soft particles. Generally, these particles are of same size. This soil does not have layers and it is friable. When we press it crumbles easily.
During rainfall it becomes very sticky, on the other hand in summer it becomes very dry. Loess is found in China, Europe, North America, South America and Africa. The name “Yellow river” in China is also given on the basis of their soil because when it mixes with river water, water appears to be yellow in colour.
Wheat and Maize is grown in Loess plains and more over soil erosion is also very (low/less) in these plains. In India this soil is found in Kashmir and in Pakistan it is found in Potwar plateau.