Published online by Cambridge University Press: 28 March 2008
THE EVOLUTION OF THE CONTINENT
Geological structure and Precambrian history
The continent of Africa, second only to Asia in size, has an unusual combination of features and environments, including many extreme contrasts. It lies astride the Equator but, unlike South America, has northern and southern limits at almost identical latitudes (37°N and 35°S). However, this symmetry does not apply to area, for the northern portion is twice as large as the southern. Much of the interior of Africa, especially in the south, is an elevated plateau, capped in the east by the great piles of volcanic material that form the highlands of Ethiopia and East Africa. Cutting through these highlands are the trenches of the Great Rift Valley, occupied in part by important lakes, both deep and shallow. Volcanic cones form spectacular mountains with peaks rising almost to 6000 m above sea level and snow-clad despite their equatorial situation. In contrast there are areas in Egypt and Ethiopia that are as much as 120 m below sea level – Afar in the Rift system and the wind-scooped Qattara and other depressions in Egypt. In addition to the volcanic cones, there are other mountainous areas, but they are largely uplifted erosional remnants; true mountain chains are confined to the northern extremity (Atlas Mountains) and to the southernmost margin (Cape Ranges). The largest dry, hot desert in the world, the Sahara, stretches from the Atlantic to the Red Sea, forming a significant continent-wide barrier between the narrow Mediterranean littoral belt and the rest of the continent, often conveniently termed ‘sub-Saharan Africa’.
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