Africa

A5

d

Coastal Processes

Coastal deposition (accumulation of sediment) occurs along much of the African coastline, particularly along the Mediterranean coast, along the Atlantic coast from Liberia to South Africa, and along the Indian Ocean coast of South Africa and southern Mozambique. Where there are strong winds parallel to the coast, waves and currents move sand along the coastline, in the process creating large sand spits and blocking harbors. At the mouths of the Niger and Nile rivers, large fan-shaped deltas have been created through the deposition of vast amounts of sediment carried downstream by these rivers. Few good harbors are found in areas where there are high levels of coastal deposition.

A

6

Economic Geology

Africa is rich in mineral resources, particularly in the south. In South Africa, in the Witwatersrand region and the province of Free State, gold is extracted from rich reefs. These reefs consist of layers of old metamorphosed sediments that have been tilted upwards. The same formations also include several other minerals, such as copper, platinum, chromium, iron ore, and coal.

Some mineral deposits were created when magma poured into older geological formations and cooled slowly, allowing the minerals to concentrate. This process produced the rich chromium deposits of the Great Dyke, a 520-km-long (320-mi-long) ridge bisecting Zimbabwe from northeast to southwest.

A similar process produced diamond-bearing kimberlite deposits in countries such as the DRC, Botswana, South Africa, and Angola. Elsewhere—including Sierra Leone and other parts of the DRC—old streambeds contain diamonds that have been washed downstream.

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Guinea has large reserves of bauxite, the commercial source of aluminum. Here, processes of chemical weathering and leaching formed deposits with very high concentrations of aluminum oxide. Several of Africa’s major sources of iron ore were similarly created as residual deposits associated with chemical weathering.

Africa has significant reserves of petroleum and natural gas, concentrated in two main areas. One is in North Africa, especially in Libya and Algeria, where deposits are found in sedimentary basins south and east of the Atlas Mountains. The other major area of production is along the Atlantic coast between Côte d’Ivoire and Angola, where wells are situated both onshore and offshore. This region’s most important center of production is the Niger River Delta. The central Nile Valley in southern Sudan is emerging as a new center of petroleum production, the first significant one in the African interior.

B

Rivers, Lakes, and Wetlands

The water systems of Africa are extremely diverse, a reflection of the continent’s great range of climate and physical geography. These systems vary from region to region, and from season to season and year to year as well.

B

1

Major River Systems

Africa has several of the world’s greatest rivers. The Congo, which alone accounts for some 38 percent of the continent’s discharge into the ocean, drains an area of more than 4.1 million sq km (1.6 million sq mi), ranking second only to South America’s Amazon River in terms of discharge and size of drainage basin. The Nile, which extends for 6,695 km (4,160 mi), is the world’s longest river; it occupies the fourth largest drainage basin. Other important rivers include the Niger in West Africa and the Zambezi in southern Africa.

The seasonal occurrence of rainfall in most of Africa often results in highly variable river flows. A river may have virtually no discharge in the late dry season followed by severe flooding in the rainy season or early dry season. Few of Africa’s rivers have the relatively constant, year-round discharge of the Congo. Great seasonal fluctuations in discharge create challenges, both in limiting flood damage and in using rivers for irrigation, hydroelectric power generation, and navigation. Several major dams and reservoirs have been constructed, especially during the 1960s and 1970s, to address these problems. Today, dams regulate a greater proportion of total river discharge in Africa than on any other continent.

Africa’s river systems reflect the continent’s unique physical geography. One-third of its area consists of inland basins, such as the Lake Chad and Kalahari (Okavango) basins, where rivers and streams never reach the ocean. Other major river systems, notably the Nile, Niger, and Congo rivers, have large inland deltas in midcourse, indicating that the upper portions of these rivers’ drainage basins were also landlocked at some point. Several major rivers, including the Congo, Zambezi, and Orange, pass through narrow valleys and drop sharply as they cross escarpments fringing the continent. In its lower course, the Congo drops 270 m (886 ft) through a series of 30 rapids and waterfalls. River courses such as this provide ideal conditions for hydroelectric power generation. Africa has about 40 percent of the world’s hydroelectric potential, but only a small proportion has been developed.

B

2

Lakes

Africa’s many lakes have diverse characteristics. They include deep lakes of tectonic origin (such as Lakes Malawi and Tanganyika in East Africa) and shallow lakes located at the center of drainage basins (including Lake Chad in West Africa). Lake Victoria, in East Africa, is the world’s third largest lake by area, while Lake Tanganyika is the world’s second deepest lake and third largest by volume of water. Africa’s natural lakes have quite distinct ecologies: Lakes located close to each other often vary significantly in both abundance and types of fish and plant species. Most lakes contribute significantly to their regional economies, particularly as a source of fish.

In drier regions, several lakes that lack natural outlets have high concentrations of mineral salts, or are actually saltpans that only contain water for part of the year. Some of these salt deposits are mined, among them the commercial soda works of Lake Magadi in Kenya and the centuries-old salt and natron extraction sites in pans in the Sahara.

In addition to natural lakes, Africa has several large artificial lakes that were created by damming major rivers. These reservoirs include Lake Volta on the Volta River, Kainji Lake on the Niger, Lake Kariba on the Zambezi, and Lake Nasser on the Nile. Most of these projects were designed to generate hydroelectricity and, in some cases, to promote irrigated agriculture. A significant fishing industry has developed around some of the artificial lakes, especially Lake Volta. The new lakes flooded settlements and farmland—as well as vital floodplain ecosystems—in valley bottoms. The loss of annual floods due to damming has had a variety of ecological and economic impacts. Annual summer flooding of the Nile once deposited rich sediment along its banks, creating fertile farmland in Egypt for millennia. With the construction of the Aswān High Dam in the 1960s, most silt was deposited in the newly created Lake Nasser, drastically reducing sedimentation and soil fertility downstream.