ACTION OF WIND IN DESERTS
The action of wind is dominant in deserts and other semi-arid regions of the world. Deserts which are associated with aridity (dryness) are caused by low rainfall, high temperature, cold currents and high evaporation rate. About 1/5 of the world’s land is made up of deserts which are confined within 15 degrees to 30 degrees north and south of the equator. These deserts are blown by cold currents.
EXAMPLES OF DESERTS
- Sahara desert (West Africa) – The largest desert.
- Kalahari and Namib deserts (South Africa).
- Arabian, Iranian and Thar deserts (Middle East).
- Australian desert (Australia).
- Atacama Desert (South America).
- Mohave Desert (U.S.A).
CHARACTERISTICS OF DESERTS
- Deserts do experience extreme high temperatures during the day and extreme low temperature at night.
- Deserts do experience low rainfall due to cold currents blowing them.
- There is lack of dense vegetation cover in deserts because of low rainfall.
- As a result of high temperature, there is high rate of evaporation.
- The action of wind tends to be dominant in deserts.
- Cold currents are known to be responsible for the causes of deserts.
REASONS FOR THE PREDOMINANCE OF WIND EROSION IN ARID AREA THAN IN HUMID REGIONS
- Absence of vegetation cover encourages free wind action in arid regions whereas, in humid regions, vegetation impedes or prevents wind speed.
- In arid region, soil is friable and light due to lack of moisture and therefore, prone to wind erosion whereas, in humid regions moisture helps to bind the soil and thus reduce soil erosion.
- Absence of rainfall encourages soil exposure in arid regions whereas; rainfall encourages soil protection by vegetation in humid regions.
TYPES OF DESERTS
There are five distinct kinds of desert landscape. These are:
- Erg or Sandy deserts: These are made up of sand and sand dunes or sheets.
- Hamada or rocky deserts: These are made up of mainly rocks.
- Reg or stony deserts: These are made up of mainly pebbles and gravels.
- Badlands: These are deserts highly eroded by wind.
- Mountain deserts: These deserts are associated with highland areas.
In deserts, mechanical (exfoliation) and chemical weathering are more active in breaking rocks to sand than wind erosion. However, wind transportation and deposition are more active in deserts.
ACTION OF WIND EROSION
Wind erosion is carried out in the following ways:
- Deflation: This is the lifting and blowing away of loose sand and pebbles by wind. Deflation results in the lowering of the land surface to form large depression called Deflation hollows.
- Abrasion: This is the process whereby sand particles carried by wind are used to blast or wear away rock surfaces as in Rock pedestal, Zeugen, and Yandang.
- Attrition: This is the process whereby wind borne particles collide with one another, resulting in the wearing away of each other. This results in reduced and rounded sizes of the particles.
DIFFERENCES BETWEEN DEFLATION AND ABRASION
- Deflation involves blowing while abrasion involves hauling rock against rock surface by wind.
- Deflation involves rolling of loose materials along the ground whereas in abrasion, rock surfaces are polished, scratched and worn away.
- Deflation usually results in lowering of land surface while abrasion is most effective at the base of rocks.
- Deflation is associated with wind while abrasion can be caused by wind, water and wave.
FEATURES OF WIND EROSION IN THE DESERT
- Rock Pedestals: Rock pedestals are irregular in shape e.g. mushroom shape with alternate horizontal layers of hard and soft rocks. They are like pillars in structure, being narrow at the base and broader at the top. They are desert land forms which range from 10 – 15 meters in height and are found in deserts e.g. Sahara deserts.
Rock pedestals are formed by wind abrasion on alternate horizontal layers of hard and soft rocks. The soft rocks are eaten into faster than the hard rocks. Abrasion is greater at ground level, resulting in serious under cutting, to produce irregular shaped feature called rock pedestal or gour in deserts.
- Zeugen: These are tabular masses with a layer of soft rocks lying beneath a surface layer of hard rock. It has a long ridge and furrow landscape with a length of 10 – 30 meters. They are also formed by wind abrasion in deserts.
Zeugen is formed when a tabular mass of rock which has a layer of soft rocks lying under a layer of a more resistant hard rock is lying horizontally to the direction of wind. The mass of rock is then attacked by wind abrasion. This wind abrasion wears the mass into a ridge and furrow landscape, leading to the formation of Zeugen. Mechanical weathering starts the formation by opening up joints of the surface of hard rocks.
- Yardangs: Yardangs have vertical bands of hard and soft rocks, ridge and furrow of landscape of about 10 – 15 meters long. They are formed in the deserts e.g Atacama deserts.
Yardangs are formed when hard and soft rocks in vertical bands are aligned in the direction of prevailing winds. Wind abrasion wears off the softer rocks into long narrow corridors which separate the steep-sided ridges of the hard rocks. These hard rocks are called Yardangs.
Differences between Zeugen and Yardangs
- In Zeugen, the hard and soft rocks are arranged in horizontal bands while that of Yardangs are arranged in vertical bands.
- In Zeugen, there must be lines of fissure in order for wind abrasion to perform its work but in Yardangs, there will be no lines of fissure.
- Yardangs are easier to form than Zeugen.
- Mesas and Buttes: Mesa is a flat, table-like landmass. It has resistant horizontal top layers with steep sides and; is made up of soft and hard layers.
As a result of the action of denudation, the hard top layer of rocks resists agents of denudation and protects the softer layers of rocks below from being eroded.
At times, mesas may be formed in canyon regions. Canyon develops in the space between mesas and butte. Denudation, sometimes may reduce mesas in areas to become isolated flat-topped hills called buttes.
- Inselberg: Inselberg is an arid landform. It is an isolated rocky out crop having steep sides, round top and composed of granite. It may rise up to a height of over 500 metres and may occur singly or in group. Examples are found in Northern Nigeria, Kalahari Desert and Western Australia.
Inselberg is formed from the existence of extensive old plateau. It is caused as a result of weathering and removal of weathered materials by water and wind. It is an exposure of rock out crop.
Importance of Inselberg
- Inselberg serves as tourist centres (tourism).
- It is also a site for quarrying.
- Ventifacts and Dreikanters: These are pebbles sharpened or faceted by sand blasting. Wind abrasion shapes and polishes the pebbles; a new facets develop when wind direction changes. Ventifacts with three wind faceted surfaces are called Dreikanters.
- Deflation Hollows or Depression: They are formed by wind deflation. It is a desert landform with varying depths. There may be presence of sand dunes on the leeward side. It is a basin and has saucer shape. It is very extensive and can form oasis in deserts e.g. Fayumi Quattara, Dust bowl.
As a result of wind deflation, large depressions or hollows are produced by the scooping away of loose sandy materials by wind, sometimes, below the water table. When this happens, water seeps out and, Oasis or swamps are formed.
Features of wind deposition in the Deserts
- Dunes: Dunes are hills or ridges of sand formed by the piling up of sand into hill-shaped by the action of wind. They are initiated when an obstacle of some kind prevents free movement of the wind, thus, creating certain shapes against the obstacle until it covers the latter and falls over to the other side. Dunes could be active or inactive with various shapes. There are:
i. Barchan: A barchan is a cresentic or moon – shaped structure. They may occur in groups or singly. A barchans has a convex shape on windward side and a concave shape on the leeward side with horns of 15 – 30m long. It is formed by wind deposition in deserts. They are desert landform e.g. Turkestan desert, Sahara desert.
A barchans is formed when an obstacle like rock impedes or prevents the movement of winds, resulting in the accumulation of sand materials. Later, the sand begins to accumulate on the other side of the obstacle, leading to a cresentric or moon – shaped structure with horns called Barchan. The windward side of a barchan is convex and gentle, while the leeward side, being sheltered is concave and steep.
ii. Seifs or Longitudinal dunes: Seifs are sword – shaped. They have long and narrow ridges of sand usually of hundreds of kilometres. They are formed by wind deposition in deserts e.g. Sahara deserts.
Seif is formed during wind deposition. They lie parallel to the prevailing winds which clear the corridors between dunes of sand. Eddies blow towards the sides of the corridors and build up Seif dunes.
- Loess: These are loose, fine particles which are carried by wind. They are deposited outside the desert as loess. Loess is fine loam, very fertile and porous. Deposit of loess is found in North – West China.