Africa

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3

Geological Structure

The geological structure of Africa is very complex, reflecting many stages and types of development over a period of 3.5 billion years. Most of the continent consists of rock dating from the Precambrian Period (more than 570 million years ago). These rocks are either igneous rocks such as granite or metamorphic rocks such as schist, gneiss, and quartzite. These ancient rocks—along with some slightly younger sedimentary rocks such as sandstone and limestone—make up what is called the basement complex of the African continent.

In much of the continent, younger deposits of igneous and sedimentary rock were laid down on top of the basement complex. The largest of the sedimentary deposits formed in northern and western Africa during the Paleozoic Era (between 570 million and 240 million years ago). Later in the Paleozoic, sediments were deposited in parts of present-day South Africa. In the Mesozoic Era (between 240 million and 65 million years ago) this area was also covered with igneous basalt from major lava flows. Sedimentary limestone was deposited during the Mesozoic on Africa’s northern edge. More recent sedimentary deposits dating from the Cenozoic Era (from 65 million years ago to the present) occupy the bottoms of the continent’s large, shallow interior basins and some coastal areas.

A

4

Geological Evolution

Africa contains three major cratons, or areas of basement-complex rock that have been geologically stable for hundreds of millions of years. The Kalahari craton is located in southern Africa, the Congo craton is in Central Africa, and the northwest African craton, forming the core of West Africa, is centered in the western Sahara. Areas between the cratons contain somewhat younger rocks. These areas have undergone more extensive and continuing geological change since the late Precambrian Period, caused by processes such as faulting, volcanism, folding, and crustal displacement.

A4

a

Faulting and Rift Valleys

Faulting, meaning the cracking of the Earth’s crust, continues to break apart the African continent. Faults occur between two parts of the crust that are moving slowly and sporadically in relation to each other—either moving away from each other or sliding up, down, or side to side. When two pieces of land are being pulled apart, numerous parallel faults develop between them as the edges cleave off and are displaced downward. The resulting formation is known as a rift valley, with a steadily lowering valley floor bounded by steep cliffs known as rift scarps. The Great Rift Valley system of East Africa traces sets of parallel faults in the African Plate that run from the Afar Depression in Eritrea and Djibouti to southern Mozambique. Millions of years from now, as the Great Rift Valley continues to widen and deepen, East Africa will likely split off from the rest of the continent.

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The Great Rift Valley is not uniform: Different segments are distinct in appearance and are affected by different geological activities. The triangular Afar Depression (also known as the Afar Triangle), a very low area fringed by rift scarps, is geologically unstable. The depression is widening and deepening by several centimeters per year, with accompanying volcanic activity and frequent earthquakes. In the part of the Western Rift where Lake Tanganyika is located, there has been a vertical displacement (the distance between corresponding rock strata in the land above the rift and in the lowering rift valley bottom below) of up to 6,000 m (20,000 ft). Some parts of the rift system (for example, the northern part of the Western Rift) are associated with very extensive volcanic activity, while in other areas (such as the Lake Tanganyika sector), volcanic activity is absent.

Rift systems occur elsewhere in Africa, most notably in the valley of the Benue and lower Niger rivers in Nigeria. Also in West Africa, volcanic activity and tectonic movement occurs along a major fault line that extends inland from the offshore island of Bioko through Cameroon Mountain to beyond Lake Chad. This line has been interpreted as the early stage of a rift system that could eventually result in the separation of West Africa.

A4

b

Volcanism

Volcanism has contributed significantly to the shaping of the African continent since ancient Precambrian times. Considerable volcanic activity accompanied the breakup of Gondwanaland, notably creating extensive lava deposits in southern Africa and covering the Ethiopian Plateau with massive deposits of basalt. Elsewhere in Africa, volcanism is associated with hot spots, areas located directly above focused plumes of magma rising from the Earth’s interior. The Tibesti and Ahaggar mountain ranges of the central Sahara, both volcanically active regions, sit over hot spots. Other hot spots lie under Cameroon Mountain, the Western Rift Valley, and several offshore locations such as Comoros and Réunion in the Indian Ocean and Ascension and Saint Helena in the South Atlantic Ocean.

The most spectacular products of volcanism are several major peaks associated with the Great Rift Valley system in East Africa. These now-dormant peaks include Kilimanjaro, Mount Kenya, Mount Meru, and Mount Elgon. In contrast, Nyiragongo and Nyamulagira in the Virunga Mountains—along the border between Rwanda and the DRC—and Cameroon Mountain are active volcanoes.

Africa’s extensive lava plateaus, though less spectacular than the volcanic peaks, are nonetheless important to the continent’s development. The weathering of these volcanic deposits has provided some of Africa’s most productive soils. Rwanda and Burundi are examples of regions of volcanic origin that support very productive agriculture and high population densities.

A4

c

Folding

Differences in pressure in the Earth’s crust cause it to buckle upwards, or fold. Folded mountains are less prominent in Africa than in other continents, a reflection of the geological stability of its basement-complex rocks. The Atlas Mountains in northwestern Africa and the Cape ranges—including the Swartberg and Langeberg mountain ranges—in South Africa are the only examples of folded mountains on the continent.

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