How did large areas of the world's deserts form? Scientists have pondered on this problem for more than 100 years, but there is no single explanation till now. They have put forward the theories of sea genesis, river genesis, continent genesis, and hypothesis of climate genesis, etc.

Theory of sea genesis: In 1886 the Russian mining engineer A.M Kang Son carried out an investigation in the outer part of Caspian lowland, and put forth his idea that the desert in KaraKum (Figure4-2) was a large basin which had been occupied by marine in Pliocene, after the marine regression, the sea bed exposed to the ground, and became desert under the effect of wind agency. The later researchers developed his theory into “Theory of sea caused desert”.

Theory of river genesis: Russian scholar B.A. Obruchev did the same investigation with A.M Kang Son in the study field, and raised a theory different from sea genesis after he had compared the sediments in the ancient river bed of Oxus and the sand layer in KaraKum. He believed that the great desert in KaraKum was result in the erosion and deposition of ancient Oxus river. From 1920s to 1930s, this theory became the scientific explanation for the cause of desert.

Figure 4-2 The roughty Karakum Desert, which lies in the Turkmenistan and Kazakhstan

Theory of continent genesis: Russian scholar B.A. Obruchev investigated the deserts distributing in the central part of Asia like China's Xin Jiang, and he believed that Central Asia is the center of the Eurasian Continent, surrounded by high mountains, and humid air current could hardly reach the inner part, while the dry and cold air flow could easily find its way, and led to the dry windy climate to form of desert.

Climate cycle hypothesis: Ameriacan scholar I.G Wilson believed that deserts experience three stages “humid—drying—further drying—humid again”; in the first stage the sediments carried by the river had traveled to plain: in the drying stage, the wind blow off the fine grained alluvial sediments, and accumulated in a certain distance, forming sand dunes. During the period of further drying the sand dunes had developed into desert.

However, the form of desert is not the result of a single factor, but concerted action of manifold factors like weather conditions, material sources and human activities (Figure 4-3). Drought and wind conditions are the essential factor for the forming of deserts, and the areas with such condition are mostly distributed between the north and south latitude 15 ° -35 °; this condition was mainly controlled by sub-tropical high pressure zone, moreover, these areas are far away from ocean and separated by mountains, such as the change of Qinghai-Tibet Plateau, and atmospheric circulation which result in the formation of desert in northern China. The loose sediments caused by alluvial, glaciers agency since 2.5 million years ago had provided the material basis for the development of desert. In addition, human activities put a bad impact on the environment and accelerated the formation of the desert.

Figure 4-3 Illstration of the genesis for deserts

(After Yao Yunfeng, 1998)