March 29, 2024

About a fifth of the world’s land is made up of deserts, some rocky, others stony and the rest sandy. Deserts that are absolutely barren and where nothing grows at all are rare and they are better known as ‘true deserts’.

If you look at the world map carefully, you will find that there is a certain definite pattern to the location of the world’s deserts. You will realize that almost all the deserts are confined within the 15° to 30° parallels of latitude north and south of the equator. They lie in the trade wind belt on the western parts of the continents where Trade Winds are off-shore. They are bathed by cold currents which produce a ‘desiccating effect’ so that moisture is not easily condensed into precipitation. Dryness or aridity is the key-note. Such deserts are tropical hot deserts or ‘Trade Wind deserts.’ They include the great Sahara Desert; Arabian, Iranian and Thar Deserts; Kalahari, Namib and Atacama Deserts; the Great Australian Desert and the deserts of south-west U.S.A. and northern Mexico. In the continental interiors of the mid-latitudes, the deserts such as the Gobi and Turkestan are characterized by extremes of temperatures. Because there is relatively little vegetation cover, the land surface is exposed to denudation. The bare rock surfaces are heated by the direct rays of the sun during the day, and because of the rapid radiation at night the rocks cool quickly. Thus the rapid expansion and contraction of the rocks, caused by the high diurnal range of temperature, leads to extensive mechanical weathering. In addition to this, chemical weathering is fairly active, partly because of the presence of small quantities of water in the air. The weathered material may then be subjected to wind and water erosion. The importance of running water as an agent of desert denudation must be stressed, for although the rainfall is extremely low in desert areas, its intensity when it does occur leads to extensive denudation.

The work of winds and water in eroding elevated uplands, transporting the worn-off materials and depositing them elsewhere, has given rise to five distinct kinds of desert landscape.

1) Hamada or rocky desert: This consists of large stretches of bare rocks, swept clear of sand and dust by the wind. The exposed rocks are thoroughly smoothed and polished. The region is bare and sterile. The best known rocky deserts are those of the Sahara Desert, e.g. the Hamada el Homra, in Libya, which covers an area of almost 52 000 sq km (20 000 square miles).

 

2) Reg or stony desert: This is composed of extensive sheets of angular pebbles and gravels which the winds are not able to blow off. Such stony deserts are much more accessible than the sandy deserts, and large herds of camels are kept there. In Libya and Egypt the term serir is used; elsewhere in Africa, stony deserts are called reg.

 

3) Erg or sandy desert: This is a sea of sand which typifies the popular idea of desert scenery. Winds deposit vast stretches of undulating sand-dunes in the heart of the deserts. The intricate patterns of ripples on the dune surfaces indicate the direction of the winds. The Calanscio Sand Sea in Libya is characteristic of a sandy desert. In Turkestan, sandy deserts are also known as koum.

 

4) Badlands: The term ‘badlands’, was first given to an arid area in South Dakota, U.S.A ., where the hills were badly eroded by occasional rain-storms into gullies and ravines. The extent of water action on hill slopes and rock surfaces was so great that the entire region was abandoned by the inhabitants. Deserts with similar features are now referred to as badlands,e.g. the Painted Desert of Arizona , which lies south-east of the Grand Canyon of the Colorado River.

 

5) Mountain deserts: Some deserts are found on highlands such as plateaux and mountain ranges. Erosion has dissected the desert highlands into harsh, serrated outlines of chaotic peaks and craggy ranges. Their steep slopes are cut by wadis (steep-sided, often dry, valleys) and the action of frost has carved out sharp irregular edges. In the Sahara Desert, the Ahaggar Mountains and the Tibesti Mountains are good examples of desert mountains. Many deserts exhibit several of these different types in a small area and it is tempting to think that there may be a cycle of development from the hammada desert to the erg desert as a result of long periods of continuous denudation. Opinions differ in this point and a further doubt about the desert lands arises from the belief, by some geographers, that the climate has changed in relatively recent times. Certainly there is evidence to suggest that the Sahara Desert lands experienced higher rainfall about 2000 years ago. Consequently it is likely that all landscape features were not formed under the present climatic conditions, and may therefore have been formed during a period of higher rainfall.

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