THE MECHANISM OF ARID EROSION

Arid landforms are as a result of many combined factors, one reacting upon the other. Insufficient rainfall, often less than 127 mm (5 inches), coming at most irregular periods, coupled with very high temperatures (31°C/87°F, is the average) and a rapid rate of evaporation, are the chief causes of aridity. Sub-aerial denudation through the processes of weathering (mechanical and chemical), wind action and the work of water have combined to produce a desert landscape that is varied and distinctive.

Weathering: This is the most potent factor in reducing rocks to sand in arid regions, even though the amount of rain that falls in the desert is small, some manages to penetrate into the rocks and set up chemical reactions in the various minerals. Intense heating during the day and rapid cooling at night by radiation, set up stresses in the already weakened rocks so that they eventually crack. As heat penetrates rocks slowly when the outer surface of rock is being heated by the hot sun, the inner rocks remain quite cool. The heating of the rocks causes the outer surface to expand and so prises itself of from the interior rocks, so that it peels off in very thin successive layers. Such as onion peeling process of mechanical weathering is called exfoliation. Angular rock debris is found in abundance as screes at the foot of under standing rocks. Similarly, when water gets into the cracks and joints of rocks and the temperature at night suddenly drops to below freezing point, the water freezes and therefore expands by 10 per cent of its volume. Successive freezing will prise off fragments of rock which accumulate as screes. These rock fragments become the ‘teeth’ or tools of wind erosion.

The wind, though not the most effective agent of erosion, transportation and deposition, is more efficient in arid than in humid regions. Since there is little vegetation or moisture to bind the loose surface materials in deserts, the effects of wind erosion are almost unrestrained. The sands in deserts are derived from lacustrine and alluvial deposits, and form weathered regolith. They range in grain size from very coarse sands (1 – 2 mm in diameter) to very fine sands (0.1 – 0.5 mm in a diameter).

There are three types of movement or transportation of sand by wind. The first, suspension is the movement of very fine particles with a diameter less than 0.2 mm, these fine particles are light enough to be carried aloft by the wind; so they float in the air. This mode of transport exerts little or no erosive effect as the dust-laden wind is diverted round obstacles. The second mode of transport is by saltation, bouncing or jumping movement. Turbulent flow of air near the surface lifts sand grains; as the grains fall, they bounce off horizontally in the general wind direction. The height of saltation grain depends on wind velocity and on a rocky surface rather than on a sandy surface. Saltation rarely reaches more than 1.5 m above the surface, so the erosive effects is limited in vertical extent.

The third type of movement is by surface creep or the movement of grains on the surface. The impact of falling saltation grain could trigger creep movement. Creep varies with wind velocity. Its erosive impact is limited to an extremely short vertical extent. The overall amount of sand transported by the three modes of transport depends on wind velocity, the moisture status of the sand, and the amount of sand available. In some areas sand supply is limited.

 

Action of Winds in Arid (Desert) and Semi-Arid Regions

Wind erosion is carried out in the following ways:

1) Deflation: This involves the lifting and blowing away of loose materials from the ground and consequently the lowering of the land surface. Such unconsolidated sands and pebbles may be carried in the air or rolled along the ground depending on the grain size. The finer dust and sands may be kilometres away from their place of origin, and deposited even outside the desert margins. Deflation results in the lowering of the land surface to form both small and large depression called deflation hollows.

The main factors of wind deflation are the dry soil, little vegetation and strong to medium wind speed. For example, the speed of a hurricane is up to 126 to 158km/h. Deflation produces deflation hollows, and the large hollows are called depressions, some of which may be oases or swamps. In the oases and swamps the depression reaches the water table. The Faiyum Depression in Egypt lies 40m below the sea-level.

Large areas in the western U.S.A ., stripped of their natural vegetation for farming, were completely deflated when strong winds, moved material as dust-storms, laying waste crops and creating what is now known as the Great Dust Bowl. In a dust-storm, winds may lift dust hundreds of metres high and carry it thousands of kilometres away.

The result of deflation, especially over regions where unconsolidated clays and friable shales are exposed (e.g. in the Sahara, Kalahari and Mongolian Deserts), is the production of wide plains and basin-like depressions. Once the desert floor has been lowered to the level of the ground water, the wind can no longer pick up the moistened particles. Watered by the escaping ground water, vegetation flourishes in these depressions known as oases. The great oases of the world have originated mainly as a result of wind deflation. The Quattara Depression in Egypt which lies almost 135 m below sea level is an example of an oasis. The other well-known oases of Egypt are Bahana, Farafra, Dakhla and El Kharga.

 

2) Abrasion: The sand- blasting of rock surfaces by winds when they hurl sand particles against them is called abrasion. The impact of such blasting results in rock surfaces being scratched, polished and worn away. Abrasion is most effective at or near the base of rocks, where the amount of material the wind is able to carry is greatest. This explains why telegraph poles in the desert are protected by a covering of a metre or two above the ground. A great variety of desert features are produced by abrasion.

 

3) Attrition: When wind-borne particles roll against one another in collision they wear each other away so that their sizes are greatly reduced and grains are rounded into millet seed sand. This process is called attrition.