10 largest deserts in the world game: music Geographonic:https://soundcloud.com/wouter-bruys Desertification is a type of land degradation in which a relatively dry land region becomes increasingly arid, typically losing its bodies of water as well as vegetation and wildlife.[2] It is caused by a variety of factors, such as climate change and human activities. Desertification is a significant global ecological and environmental problem.[3] Not to be confused with dessert. This article is about dry terrain. For the act of abandoning or withdrawing support, see desertion. For other uses, see Desert (disambiguation). Page semi-protected Traditional concept: Sand dunes in the Rub' al Khali ("Empty quarter") of Saudi Arabia, but most deserts are rocky, not sandy. A desert is part of the terrestrial surface of the globe where little rain falls and in which few plants or animals are able to exist. The lack of vegetation exposes the unprotected surface of the ground to the processes of denudation. About one third of the land surface of the world are arid or semi-arid areas and the polar regions, which also have little precipitation, are also sometimes called "cold deserts". There are a number of ways of classifying deserts including by the amount of precipitation that falls, by the temperature that prevails, by the causes of desertification or by their geographical locations. Deserts are formed by weathering processes as large variations in temperature between day and night put strains on the rocks which consequently break in pieces. Although rain seldom occurs in deserts, there are occasional heavy downpours. Rain falling on hot rocks can cause them to shatter and dry creeks can become raging torrents with little warning. The rock and rubble strewn over the desert floor is further eroded by the wind. This picks up particles of sand and dust and wafts them aloft in sand or dust storms. Wind-blown sand grains striking any solid object in their path can abrade the surface. Rocks are smoothed down and the sand grains break in pieces until they are a uniform size. The grains end up as level sheets of sand or are piled high in billowing sand dunes. Other deserts are flat, stony plains where all the fine material has been blown away and the surface consists of a mosaic of smooth stones. These areas are known as desert pavements and little further erosion takes place. Other desert features include rock outcrops, exposed bedrock and clays once deposited by flowing water. Temporary lakes may form and salt pans may be left when waters evaporate. There may be underground sources of water in the form of springs and seepages from aquifers. Where these are found, oases can occur. Plants and animals living in the desert need special adaptations to enable them to survive in the harsh environment. Plants tend to be tough and wiry with small or no leaves, water-resistant cuticles and often spines to deter herbivory. Some annual plants germinate, bloom and die in the course of a few weeks after rainfall while other long-lived plants survive for years and have deep root systems able to tap underground moisture. Animals need to keep cool and find enough food and water to survive. Many are nocturnal and stay in the shade or underground during the heat of the day. They tend to be efficient at conserving water, extracting most of their needs from their food and concentrating their urine. Some animals remain in a state of dormancy for long periods, ready to become active again when the rare rains fall. They then reproduce rapidly while conditions are favourable before returning to dormancy. People have struggled to live in deserts and the surrounding semi-arid lands for millennia. Nomads have moved their flocks and herds to wherever grazing is available and oases have provided opportunities for a more settled way of life. The cultivation of semi-arid regions encourages erosion of soil and is one of the causes of increased desertification. Many trade routes have been forged across deserts, especially across the Sahara, and traditionally were used by caravans of camels carrying salt, gold, ivory and other goods. Large numbers of slaves were also taken northwards across the Sahara. Some mineral extraction also takes place in deserts and the uninterrupted sunlight gives potential for the capture of large quantities of solar energy.

• published: 11 Sep 2013
• views: 16