GEOLOGY OF AFRICA

Formation of Africa

Scientists use the theory of plate tectonics to explain the formation of Africa and the other continents. According to this theory, the crust of Earth’s surface consists of a collection of 14 rigid plates floating on an underlying mantle. These plates are in constant motion—moving apart, colliding, and thrusting beneath one another. Africa sits at the center of the African Plate, one of the largest of Earth’s plates. For much of Earth’s history, the land made up one vast supercontinent known as Pangaea. About 220 million years ago, tectonic activity broke Pangaea apart into the supercontinents of Gondwanaland and Laurasia. Gondwanaland subsequently broke apart as well: First Antarctica, Australia, Madagascar, and the Indian subcontinent broke away, followed by South America. Africa, at the core of Gondwanaland, assumed roughly its present-day shape about 15 million years ago when the formation of the Red Sea split off the Arabian Peninsula.

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.

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.

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.

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.

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.

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.

Crustal Warping and Uplift

Over the last 500 million years, Africa has experienced many sequences of surface warping. In this process, crustal pressure bends the Earth’s surface without creating folds or faults. Downwarping created the continent’s major basins, while upwarping produced upland regions such as the Guinea Highlands and Ethiopian Highlands. These uplands fringe the basins, and divide them from one another. For example, the Ahaggar, Tibesti, Ennedi, and Mambila mountains, together with the Jos Plateau, surround the Chad Basin.

Much of the African continent lifted up after it separated from the other continents due to isostatic adjustment (the tendency for Earth’s crust to seek gravitational equilibrium). This uplift took place over a prolonged period, and was especially significant in the south, where it gave rise to the Great Escarpment along the fringe of the continent. In East Africa, the tectonic processes that created the rift valleys simultaneously created upwarped areas and uplifted mountain ranges, the largest being the Ruwenzori Range along the Uganda-DRC border.

Weathering and Erosion

The surface of Africa, like all continents, is affected by weathering and erosion. Weathering refers to the processes of physical disintegration and chemical decomposition of solid rock materials at or near the Earth’s surface, while erosion refers to the removal of weathered rock and soil material by natural processes such as running water, glaciers, waves, and wind. The general flatness of much of the African landscape is the result of deep chemical weathering of bedrock, together with prolonged erosion that has smoothed the surface over many millions of years.

Processes in Humid Tropical Regions

The year-round rainfall and high temperatures that prevail in the humid tropics are ideal for chemical weathering. Chemical weathering involves the decay and disintegration of rock through chemical alteration of the minerals that make up the rock. In tropical forest environments, water filters through decaying vegetable matter on the ground and becomes acidic, helping it break down rock. Such is the effectiveness of chemical weathering that it is common to find 15 m (50 ft) or more of weathered material overlying solid rock in the tropical environments of Africa. Chemical weathering is important, but somewhat less effective, in savanna regions where rainfall is seasonal.

As weathering forms soil in the humid tropics, iron and aluminum oxides filter downward, often resulting in a well-defined, cementlike layer of ferricrete or plinthite meters below the Earth’s surface. When overlying sediments are eroded away, these layers form a rock-hard crust. These crusts—typically 1 to 10 m (3 to 30 ft) thick—form broad pavements, ledges, and flat cap rocks on mesas.

Chemical weathering in the humid tropics and moister savannas creates isolated, domed rock outcroppings called inselbergs. Inselbergs are made of hard masses of crystalline rock that resist chemical weathering. When surrounding, weathered materials have been eroded away, the inselberg is exposed. The typical domed shape of many inselbergs is created through the successive peeling away (or exfoliation) of surface layers of rock.

Processes in Arid Regions

In deserts, wind erodes and scours the landscape, creating weirdly shaped pinnacles, grooves, and canyons, both in lowland areas and in upland massifs such as the Tibesti and Ahaggar in the Sahara. Sediments carried from rock and gravel desert areas help to build ergs (stretches of sand dunes), including the immense Grand Erg Oriental that covers almost 200,000 sq km (80,000 sq mi). Ergs contain many types of dunes: crescent-shaped barchans, linear seif dunes up to 80 km (50 mi) long, and massive sand ridges known as draas. The shape and orientation of dunes in a particular area reflects several factors such as local wind patterns and variations in the amount of sand. Some ergs have several subregions in which dunes have different orientations. Past and present water action also affects the formation of desert landscapes. In the Namib Desert in southern Africa, salt, fog, and dew carried from the ocean contribute to weathering processes.

Glaciation

Although Africa is now generally warm and tropical, glaciation played a significant role in the continent’s development millions of years ago during the ice ages. When it was still a part of Gondwanaland, areas of Africa were covered in massive continental glaciers. They left behind deep deposits of tillite (rock formed from sediments deposited by glaciers) in southern Africa. Other tillite deposits are found in the Congo River Basin and in the northwestern Sahara. Glaciers are still found at the summits of some of Africa’s highest peaks, including Mount Kenya and Kilimanjaro. These glaciers are all above 4,500 m (14,800 ft) today, but extended as low as 3,000 m (10,000 ft) during the most recent ice age.

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.

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.

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.