Causes deserts: an overview of some geological explanations

         

            Desert landscapes are the least understood among terrain types of the Earth. To this end, El-baz (1988) argues that the dearth of the basic information on the arid lands has resulted in the misconception that the desert is man-made and that, therefore, we can ‘fix it’.  Scientific evidence indicates that deserts are part of the natural feature of the Earth and Mars. They form as a result of the lack or scarcity of rain, which may be cyclical.

            Most deserts are hot. One geological study explains that within the latitudes of the Tropics of Cancer and Capricorn, the sun appears directly overhead at noon on at least one day of the year. To the north or south of them, the sun is never directly overhead, and not as concentrated as the beam that reaches the tropical areas.

            It is not uneasy to see that deserts are not scattered haphazardly over the earth; the pattern of their occurrence is perhaps shaped by the factors that produced them. The major world deserts occur in two discontinuous belts ringing the globe. Because more land surface occurs in the Northern Hemisphere in the latitudes described, more deserts occur in the Northern than the Southern Hemisphere.

            Not all deserts are hot. Some of these dry lands, like the Atacama of Chile, the Namib and Kalahari of southern Africa, and the western Australian desert are not typically hot.

It is important to understand that warm air holds more moisture than cold air. With this in mind, I shall move on to briefly discuss four main natural causes of deserts: the Earth’s air circulation patterns could potentially cause deserts. Warm air rises at the equator, and cools. Since cool air holds less water, precipitation is common at the equator. When dry air at  moves north and south. At ±30 degrees, the dry air sinks and warms. The warmer air can hold more water resulting in evaporation of water and the formation of deserts. The equatorial region is tropical and ± 30 degrees is desert. 
     Rain shadow effect is also a factor that potentially causes deserts.This occurs when moving moisture in the air encounters a mountain and is moved upward. The ascending air is cooled and releases moisture on the windward side of the range. Once over the summit, the air descends the lee side of the range, warming as it does so, and hence increasing its evaporative power. The windward side of a range may support a heavy well-watered forest, while the leeward side and the area far below it, robbed of moisture, and is occupied by a desert or steppe plant.

            Distance from the ocean is another factor that causes deserts. Most water in atmosphere is evaporated from the sea, and this water eventually precipitates on land. Land closer to the sea generally receives much of this moisture.  As air moves inland, it gets depleted of moisture and precipitation drops.  Areas lying deep within a continent may become desert simply because air currents reaching them have already traversed vast land distances; by the time they arrive over the deserts, these currents have already lost the moisture they once carried. Research has shown that this is true of some of the Asian deserts: the Gobi and Takla-Makan Deserts.

           Finally, deserts could naturally be caused by what is termed as coastal cooling. This occurs when warn, moist air is cooled,and causes excess water to condense and fall as precipitation. If it is subsequently re-warmed, it will be drier than it was previously.

            In conclusion, all these climatic, desert-producing factors descending, drying air currents; mountain-produced rain shadows; distance from oceanic moisture sources; and cold ocean currents are instrumental, sometimes singly, more often in combination, as primary forces producing arid lands.

Reference:

El-baz F., 1988. Origin and Evolution of Deserts, Center for Remote sensing, Boston University,    Boston Massachusetts, USA, Interdisciplinary Science reviews vol. 13, No. 4