Geology and Geography
Most of Africa is a series of stable, ancient plateau surfaces, low in the north and west and higher (rising to more than 6,000 ft/1,830 m) in the south and east. The plateau is composed mainly of metamorphic rock that has been overlaid in places by sedimentary rock. The escarpment of the plateau is often in close proximity to the coast, thus leaving the continent with a generally narrow coastal plain; in addition, the escarpment forms barriers of falls and rapids in the lower courses of rivers that impede their use as transportation routes into the interior. Northern Africa is underlain by folded sedimentary rock and is, geologically, more closely related to Europe than to the rest of the continent of Africa; the Atlas Mts., which occupy most of the region, are a part of the Alpine mountain system of southern Europe. The entire African continent is surrounded by a narrow continental shelf. The lowest point on the continent is 509 ft (155 m) below sea level in Lake Assal in Djibouti; the highest point is Mt. Uhuru (Kibo; 19,340 ft/5,895 m), a peak of Kilimanjaro in NE Tanzania. From north to south the principal mountain ranges of Africa are the Atlas Mts. (rising to more than 13,000 ft/3,960 m), the Ethiopian Highlands (rising to more than 15,000 ft/4,570 m), the Ruwenzori Mts. (rising to more than 16,000 ft/4,880 m), and the Drakensberg Range (rising to more than 11,000 ft/3,350 m).
The continent's largest rivers are the Nile (the world's longest river), the Congo, the Niger, the Zambezi, the Orange, the Limpopo, and the Senegal. The largest lakes are Victoria (the world's second largest freshwater lake), Tanganyika, Albert, Turkana, and Nyasa (or Malawi), all in E Africa; shallow Lake Chad, the largest in W Africa, shrinks considerably during dry periods. The lakes and major rivers (most of which are navigable in stretches above the escarpment of the plateau) form an important inland transportation system.
Geologically, recent major earth disturbances have been confined to areas of NW and E Africa. Geologists have long noted the excellent fit (in shape and geology) between the coast of Africa at the Gulf of Guinea and the Brazilian coast of South America, and they have evidence that Africa formed the center of a large ancestral supercontinent known as Pangaea. Pangaea began to break apart in the Jurassic period to form Gondwanaland, which included Africa, the other southern continents, and India. South America was separated from Africa c.76 million years ago, when the floor of the S Atlantic Ocean was opened up by seafloor spreading; Madagascar was separated from it c.65 million years ago; and Arabia was separated from it c.20 million years ago, when the Red Sea was formed. There is also evidence of one-time connections between NW Africa and E North America, N Africa and Europe, Madagascar and India, and SE Africa and Antarctica.
Similar large-scale earth movements (see plate tectonics) are also believed responsible for the formation of the Great Rift Valley of E Africa, which is the continent's most spectacular land feature. From c.40 to c.60 mi (60–100 km) wide, it extends in Africa c.1,800 mi (2,900 km), from the northern end of the Jordan Rift Valley in SW Asia to near the mouth of the Zambezi River; the eastern branch of the rift valley is occupied in sections by Lakes Nyasa and Turkana, and the western branch, curving N from Lake Nyasa, is occupied by Lakes Tanganyika, Kivu, Edward, and Albert. The lava flows of the recent and subrecent epochs in the Ethiopian Highlands, and volcanoes farther south, are associated with the rift; among the principal volcanoes are Kilimanjaro, Kenya, Elgon, Meru, and the Virunga range with Mt. Karisimbi, Nyiragongo, and Nyamuragira (Nyamulagira). A less spectacular rift, the Cameroon Rift, is associated with volcanic activity in W Africa and trends NE from St. Helena Island to São Tomé, Príncipe, and Bioko to near the Tibesti Massif in the Sahara.
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