Africa
Africa, second largest of Earth’s seven continents, covering 23 percent of the world’s total land area and containing 13 percent of the world’s population. Africa straddles the equator and most of its area lies within the tropics. It is bounded by the Atlantic Ocean on the west, the Indian Ocean and Red Sea on the east, and the Mediterranean Sea on the north. In the northeastern corner of the continent, Africa is connected with Asia by the Sinai Peninsula.
Africa is a land of great diversity. If you were to trek across the continent, you would pass through lush, green forests and wander vast, grassy plains. You would cross barren deserts, climb tall mountains, and ford some of the mightiest rivers on Earth. You would meet diverse people with a wide range of cultures and backgrounds and hear hundreds of different languages. You would pass through small villages where daily life remains largely the same as it has been for hundreds of years, as well as sprawling cities with skyscrapers, modern economies, and a mix of international cultural influences.
Africa is the birthplace of the human race. Here, early humans evolved from apes between 8 million and 5 million years ago. Modern human beings evolved between 130,000 and 90,000 years ago, and subsequently spread out of Africa. Ancient Egypt, one of the world’s first great civilizations, arose in northeastern Africa more than 5,000 years ago. Over time many other cultures and states rose and fell in Africa, and by 500 years ago there were prosperous cities, markets, and centers of learning scattered across the continent.
During the last 500 years, however, Africa became increasingly dominated by European traders and colonizers. European traders sent millions of Africans to work as slaves on colonial plantations in North America, South America, and the Caribbean. Europeans also sought Africa’s wealth of raw materials to fuel their industries. In the late 19th century, European powers seized and colonized virtually all of Africa.
Through slow reform or violent struggle, most of Africa won independence in the 1950s and 1960s. Independent Africa inherited from colonization a weak position in the global economy, underdeveloped communication and transportation systems, and arbitrarily drawn national boundaries. The citizens of these new nations generally had little in terms of history or culture to bind them together.
There are 53 different African countries, including the 47 nations of the mainland and the 6 surrounding island nations. The continent is commonly divided along the lines of the Sahara, the world’s largest desert, which cuts a huge swath through the northern half of the continent. The countries north of the Sahara make up the region of North Africa, while the region south of the desert is known as sub-Saharan Africa. Sub-Saharan Africa is sometimes referred to as “Black Africa,” but this designation is not very helpful, given the ethnic diversity of the entire continent. North Africa consists of the countries of Algeria, Egypt, Libya, Morocco, Sudan, and Tunisia. Sub-Saharan Africa is generally subdivided into the regions of West Africa, East Africa, Central Africa, and southern Africa. For the purposes of this article, West Africa consists of Benin, Burkina Faso, Cameroon, Chad, Côte d’Ivoire, Ghana, Guinea, Guinea-Bissau, Liberia, Mali, Mauritania, Niger, Nigeria, Senegal, Sierra Leone, The Gambia, and Togo. East Africa consists of Burundi, Djibouti, Eritrea, Ethiopia, Kenya, Malawi, Mozambique, Rwanda, Somalia, Tanzania, and Uganda. Central Africa consists of Angola, Central African Republic, Democratic Republic of the Congo, Equatorial Guinea, Gabon, Republic of the Congo, and Zambia. Southern Africa consists of Botswana, Lesotho, Namibia, South Africa, Swaziland, and Zimbabwe. The island nations located off the coast of Africa are Cape Verde and São Tomé and Príncipe in the Atlantic Ocean; and Comoros, Madagascar, Mauritius, and Seychelles in the Indian Ocean.
The great diversity of the African environment makes it difficult to generalize about the continent. While much of the continent consists of vast plains with little relief, there are also towering volcanic peaks and the largest rift valley system in the world. The climate ranges from the year-round heat and humidity of equatorial regions to the dryness of the world’s largest desert to mountaintop conditions cold enough to support glaciers. It contains regions of biological significance due to their biodiversity and huge numbers of species found nowhere else.
The African environment has long been mistakenly seen as hostile, foreboding, and tragically in decline. Popular descriptions of Africa such as “the dark continent,” images of untamed wilderness in nature publications, and sensationalized press coverage of disasters such as droughts and famines have shaped these perceptions of Africa. Geographers’ accounts of Africa used to attribute the underdevelopment of the continent to its unfavorable environment—its oppressive climate, infertile soil, polluted water, and exotic diseases.
These days have begun to wane. Increased scientific research on the African environment has done much to dispel old misconceptions and to provide insights into the physical processes that give shape to the landscape. The relationship of African societies to the environment is also much better understood. Yet much remains to be done before this huge and complex continent is well known and appreciated, especially by the general public.
The African continent covers 30 million sq km (12 million sq mi), including its adjacent islands. It stretches 8,000 km (5,000 mi) from its northernmost point, Ra’s al Abyaḑ in Tunisia, to its southernmost tip, Cape Agulhas in South Africa. The maximum width of the continent, measured from the tip of Cap Vert in Senegal, in the west, to Raas Xaafuun (Ras Hafun) in Somalia, in the east, is 7,500 km (4,700 mi). The highest point on the continent is the perpetually snowcapped Kilimanjaro (5,895 m/19,341 ft) in Tanzania, and the lowest is Lake ‘Asal (153 m/502 ft below sea level) in Djibouti.
Africa is surrounded by oceans and seas: the Atlantic Ocean on the west, the Indian Ocean on the east, the Red Sea on the northeast, and the Mediterranean Sea on the north. Madagascar, the fourth largest island in the world, lies off the southeastern coast. Other offshore islands include the Madeira Islands, Canary Islands, Cape Verde Islands, São Tomé, Príncipe, and Bioko, off the western coast; and the Comoros Islands, Seychelles, Mascarene Islands, and Socotra, off the eastern coast.
Africa generally consists of a series of flat and gently undulating plateaus occurring at different levels, broken by a few mountainous areas and by the rift valleys of East Africa. With a mean elevation of approximately 650 m (2,100 ft) above sea level, Africa is high compared to other continents. The southern and eastern section of the continent, often called High Africa, consists primarily of a high plateau with elevations between 1,000 and 2,000 m (3,000 and 7,000 ft) above sea level. Northern and western Africa, widely known as Low Africa, has much lower mean elevations. Most of the continent’s surface has been warped into a series of large, saucer-like basins separated by highlands. The major basins of Africa are El Djouf, now occupied by the Niger River Basin in West Africa; the Chad Basin, surrounding Lake Chad in west central Africa; the Sudan (or Nile River) Basin in northeast Africa; the Congo River Basin of Central Africa; and the Kalahari (or Okavango) Basin of southern Africa.
The highest elevations in Africa are found in the various ranges of East Africa. After Kilimanjaro, the next highest peaks are Mount Kenya (5,199 m/17,057 ft), north of Kilimanjaro in central Kenya; Margherita Peak (5,109 m/ 16,762 ft) in the Ruwenzori Range on the border between Uganda and the Democratic Republic of the Congo (DRC); Ras Dashen (4,620 m/ 15,157 ft) in the Ethiopian Highlands of northern Ethiopia; Mount Meru (4,565 m/ 14,977 ft), close to Kilimanjaro in Tanzania; and Mount Elgon (4,321 m/ 14,177 ft) on the Uganda-Kenya border.
Africa’s other major mountainous regions occur at the northern and southern fringes of the continent. The Atlas Mountains, a system of high ranges, extend for 2,200 km (1,400 mi) across Morocco, Algeria, and Tunisia, roughly parallel to the northern coast. These ranges enclose a number of broad inland basins and plateaus. In the west, the High (or Grand) Atlas contains Toubkal (4,165 m/ 13,665 ft), the highest peak of the system. Toward the east, the Atlas consists of two parallel ranges: the Tell Atlas to the north and the Saharan Atlas to the south.
In southern Africa, the U-shaped Great Escarpment extends 5,000 km (3,000 mi) along the coast from Angola to Mozambique (an escarpment is a ridge that is steep on one side and slopes down gently on the other). The Drakensberg Mountains form the most pronounced relief of the Great Escarpment, rising to 3,482 m (11,424 ft) at Thabana Ntlenyana in Lesotho.
Cameroon Mountain is the highest peak in West Africa at 4,095 m (13,435 ft). To the north, isolated highlands occur in the desert land of the Sahara, including the Ahaggar Mountains in southern Algeria and the Tibesti in northern Chad.
The Great Rift Valley is one of the most distinctive features of African topography. Formed where Earth’s crust is being pulled apart by the action of convection currents beneath the surface, rift valleys are long, deep valleys bounded by parallel faults, or fractures, in Earth’s crust. The Great Rift Valley system begins in Syria, in the Middle East, and extends southward, down the length of the Red Sea. It enters Africa at the Afar Depression on the coast of Eritrea and Djibouti, and winds some 5,600 km (3,500 mi) to the coast of southern Mozambique. In its middle section, it breaks into two major branches, the Eastern Rift Valley and the Western Rift Valley. The rift valley is flanked by towering escarpments of up to 1,000 m (3,000 ft) in southern Ethiopia, 1,500 m (4,900 ft) along the Eastern Rift in central Kenya, and 1,300 m (4,300 ft) in the northern part of the Western Rift, along the DRC’s border with Uganda, Rwanda, and Burundi. The southern extremities of the rift system are much less spectacular in size and appearance. For more information, see the Faulting and Rift Valleys section of this article.
Several major lakes, typically long and narrow, are located on the floors of the Western and Eastern rift valleys. The Western Rift contains Lake Albert, Lake Edward, and Lake Kivu to the north, Lake Tanganyika in the middle, and Lake Malawi (Lake Nyasa) to the south. The lakes of the Eastern Rift tend to be smaller and include Lake Naivasha, Lake Natron, and the southern part of Lake Turkana.
The Sahara is the world’s largest desert. It stretches from the Atlantic Ocean to the Red Sea, and from the Mediterranean Sea and Atlas Mountains southward for 2,000 km (1,000 mi) until it merges imperceptibly into the semidesert Sahel region. Most of the desert consists of extensive plains covered with loose gravel and boulders, called reg. The rest of the desert is made up of areas of shifting sand dunes, called erg, interspersed with stretches of bare, rocky areas called hamada.
The Namib and Kalahari deserts of southern Africa are much smaller than the Sahara. The Namib Desert stretches along the Atlantic coast for 1,500 km (930 mi) from southern Angola along the entire length of Namibia, and into western South Africa. The nearby Kalahari Desert, in Botswana, Namibia, and South Africa, is a semiarid region in the center of the Kalahari Basin.
Generally, Africa’s coastline is very even, with few good natural harbors. The coastal plain is narrow around much of the continent, particularly in the south and east. Major escarpments run parallel to the coast in several areas. Most of the Red Sea and Indian Ocean coastline is fringed by coral reefs, which are an obstacle to ships. On the Atlantic coast, waters are generally too cold for coral development. Africa’s best natural harbors are found in the many deep coastal inlets between Senegal and Liberia, especially at the mouths of rivers. Lagoon coasts, with a coastal barrier beach backed by lagoons, are common between Liberia and Nigeria.
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.
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.
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.
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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 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.
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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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.
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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.
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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.
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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.
Although
