Africa.

Africa. I INTRODUCTION 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. II NATURAL ENVIRONMENT 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. A The African Landscape 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 Abya? 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. A1 Surface Features 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. A1a Highlands 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. A1b Great Rift Valley 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. A1c Deserts 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. A1d Coastline 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. A2 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. A3 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. A4 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. A4a 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. A4b 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. A4c 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. A4d 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. A5 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. A5a 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. A5b 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. A5c 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. A5d 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. A6 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. 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. B1 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. B2 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 Asw?n High Dam in the 1960s, most silt was deposited in the newly created Lake Nasser, drastically reducing sedimentation and soil fertility downstream. B3 Wetlands Africa's freshwater wetlands come in several forms. They are often located in floodplains, low-lying areas adjacent to rivers that are subject to annual or periodic flooding. Freshwater marshes, such as the huge As Sudd in Sudan, are usually associated with riverside locations, and are dominated by herbaceous species such as papyrus. Swamp forests occur in floodplains or lakeside environments, most notably in the Middle Congo River Basin. This area contains about 8,000 sq km (3,000 sq mi) of permanently or seasonally flooded swamp forest. Wetland areas play a vital hydrological and ecological role in Africa. They trap and slow seasonal floods, dampening the magnitude of floods downstream and spreading out peak flows over several weeks or months. The delay and extension of flood peaks can facilitate downstream fishing and irrigation, especially in areas with an extended dry season. Wetlands also provide habitat for numerous species of animals and plants, many of them unique to these ecosystems. Wetlands near the edge of the Sahara provide vital staging grounds for migratory birds preparing to cross the desert. Wetlands also trap and hold silt carried by rivers, creating fertile alluvial soils that may be used to grow crops such as rice, cotton, and vegetables. However, agricultural development schemes, taking advantage of the presence of both fertile soil and water, pose a threat to many wetlands. Botswana's Okavango Delta is one of Africa's largest and most unique wetlands. The Okavango River, with its source in the highlands of Angola, forms a huge, swampy inland delta as it approaches the Kalahari Desert. During the annual floods, the swamp doubles in size. Although it has several outlets, virtually all of the water entering the Okavango Delta evaporates or is absorbed into the sandy subsurface. The Okavango supports a rich indigenous flora and fauna, and attracts huge numbers of migratory wildlife during the dry season. Like many others, this vital and sensitive wetland ecosystem is threatened by the growth of ranching and tourism, and by proposals to divert water for irrigation and other uses. C Climate Lying between latitudes 37° north and 35° south, Africa has virtually the same climatic zones in the Northern Hemisphere as in the Southern Hemisphere, and they are arranged symmetrically on either side of the equator. The zones are determined mainly by latitude, except in the east where highlands greatly modify the climate. Africa is the most tropical of the continents: Only its northern and southern extremes are directly influenced by mid-latitude westerly winds and are considered to have temperate climates. C1 Climatic Zones Most of Africa lies between the Tropic of Cancer (in the north) and the Tropic of Capricorn (in the south) and has high temperatures throughout the year. The amount, duration, and seasonal distribution of rainfall is therefore the most important factor differentiating its climates. Africa has six types of climatic zones: tropical wet, tropical summer rainfall, semiarid, arid, highland, and Mediterranean. C1a Tropical Wet Tropical wet climates, also called equatorial climates, occur close to the equator in West and Central Africa, and in eastern Madagascar. Rainfall is high, typically exceeding 1,500 mm (60 in) per year and 3,200 mm (130 in) in some places. Rainfall occurs in every month, and many areas experience especially rainy periods in the spring and in the fall. Temperatures remain high throughout the year, averaging more than 27°C (81°F) annually, and rarely falling below 21°C (70°F). C1b Tropical Summer Rainfall Tropical summer rainfall climates, also known as tropical savanna climates, occur north and south of the tropical wet zone, in much of West Africa and southern Africa and most of Madagascar. This climatic zone is marked by a well-defined dry season of three to eight months. Annual rainfall is usually between 500 and 1,500 mm (20 and 60 in), although limited areas have considerably more--for example, Freetown, Sierra Leone, averages 3,800 mm (150 in) per year. The tropical summer rainfall zone is a transitional zone between tropical wet and semiarid zones, so there is a progressive decline, moving poleward, in total rainfall and the duration of rainfall. Areas with a longer rainy season tend to have two rainy periods separated by a short dry spell, while areas with a shorter rainy season have a single rainy period. Temperature ranges in the tropical summer rainfall zone are slightly higher than in the tropical wet zone, and increase with distance from the equator. In the northern section of this zone, daily high temperatures average more than 30°C (90°F) over the course of the year. Temperatures in the southern and eastern sections of this zone tend to be cooler because of higher altitudes. C1c Semiarid and Arid Surrounding the tropical summer rainfall zone are areas of semiarid and then arid climates in north central Africa, east central Africa, and southern Africa. The semiarid, or hot steppe, zone has a short rainy season of up to three months with about 250 to 500 mm (10 to 20 in) of rain per year. Precipitation is unreliable and scarce, creating difficult conditions for plant growth. Temperatures vary in the semiarid zone, with average daily highs ranging from 25° to 36°C (77° to 97°F). Africa's arid desert regions receive little rainfall. Although classified as hot deserts, these regions have significant annual variations in temperature, and extreme fluctuations in temperature over the course of a day. In the Sahara, daytime summer temperatures can exceed 50°C (120°F), and winter nighttime temperatures can drop below freezing. C1d Highland and Mediterranean Tropical highland climates are common in much of East Africa. Temperatures in the highlands of Ethiopia and Kenya average 16° to 21°C (60° to 70°F), on average about 5 Celsius degrees (9 Fahrenheit degrees) cooler than the lower plateau areas of Kenya, Uganda, and Tanzania. In most parts of the world, higher elevations receive higher levels of precipitation, but the highlands of East Africa are an exception, experiencing rather low levels of rainfall. However, the highest mountains and the southeastern flank of the Ethiopian plateau receive greater precipitation on their windward slopes. The coastlands of the Cape region of South Africa and the North African coast from Morocco to Tunisia have Mediterranean climates. These areas have mild, rainy winters followed by a prolonged summer when conditions are warm and dry. They receive between 250 to 1,000 mm (10 to 40 in) of rainfall per year. C2 Major Climate Controls Several factors influence the climate of Africa, determining the continent's climatic regions, creating seasonal variations, and altering day-to-day weather. The most important climate control is related to atmospheric conditions and wind patterns; other major controls include topography, ocean currents, and clouds and other airborne material. C2a Seasonal Shifts of Winds, Atmospheric Pressure, and Air Masses Air masses moving over Africa have different characteristics depending on their areas of origin. Air originating over the tropical oceans, known as maritime tropical air, is warm, moist, and unstable. When this moisture-laden air is forced to rise and become cooler, condensation and precipitation often occur. Conversely, the hot, dry air that originates over the African continent (continental tropical air) is incapable of producing rain, even if it is forced to rise. Over most of Africa, air moves toward the equator. Northern Hemisphere winds from the northeast and Southern Hemisphere winds from the southeast converge at the intertropical convergence zone (ITCZ), a low-pressure zone centered on the equator. Air rises at the ITCZ, spreads out, and descends in the north and south, in subtropical high-pressure zones centered on the Tropic of Cancer and Tropic of Capricorn. After descending, the air flows outward, either poleward into temperate regions or back toward the equator. The ITCZ constantly shifts its position, moving north of the equator during the Northern Hemisphere's summer, and south of the equator during the Northern Hemisphere's winter. This shift brings about seasonal changes, notably in the tropical summer rainfall zone. During the summer season (July to September in the Northern Hemisphere and December to February in the Southern Hemisphere), moist maritime air is drawn in toward the ITCZ and produces rainfall when it is forced to rise. During the winter season (December to February in the Northern Hemisphere and July to September in the Southern Hemisphere), when the ITCZ is centered in the opposite hemisphere, dry conditions prevail due to the dominance of hot, dry winds coming from the subtropical high-pressure zone. These global, seasonal shifts in wind and pressure zones also bring about the seasonal changes that characterize the Mediterranean climates of Africa's northern and southern margins. These seasonal shifts have a comparatively minor impact on rainfall patterns in equatorial and desert regions. The mid-Sahara receives little rain, even in summer, because the ITCZ seldom advances that far north. On the other hand, equatorial regions lack a well-defined dry season because they continue to benefit from moist, maritime air throughout the year. Their position near the equator also ensures that temperatures remain uniformly high year-round. In contrast, Africa's desert regions experience significant temperature shifts seasonally, a reflection of their higher latitudes. Most of the continent receives moisture from air originating over the Atlantic Ocean. In eastern Africa, however, rainfall south of the equator comes from large tropical cyclones originating over the Indian Ocean during the Southern Hemisphere summer. Rainfall from these cyclones is particularly high in eastern Madagascar and the coastal mainland between South Africa and southern Tanzania. North of the ITCZ--centered during January over Tanzania--East Africa receives little rain because its air originates in dry, mainland Asia. C2b Altitude and Relief Africa, unlike other continents, has no great mountain ranges to impede or channel atmospheric circulation. However, altitude is an important influence on temperatures, notably in the high plateaus and highlands of southern and East Africa. Highland regions have lower temperatures throughout the year, compared to lower-elevation areas at the same latitude. Highlands also influence rainfall patterns: Moisture-laden air forced to rise over a mountain barrier produces more rainfall on the windward side than on the leeward side. C2c Ocean Currents Ocean currents affect the climate of the Atlantic Coast from South Africa to southern Angola, and from southern Morocco to Mauritania. The former is affected by the Benguela Current, while the latter is influenced by the Canaries Current. These cold ocean currents result in somewhat cooler temperatures, very low rainfall, and the frequent occurrence of fog near the coast. Elsewhere, an area of surprisingly low rainfall along the coast near Accra, Ghana, has been attributed to localized upwelling of cold water. C2d Clouds and Atmospheric Dust Clouds help to reduce daily temperature fluctuations by reflecting solar radiation away during the day, and by slowing the loss of heat at night. The lack of cloud cover in Africa's desert regions--and during the dry season in other climate regions--results in substantial day-to-night fluctuations of temperature. The Sahara is estimated to generate 300 million metric tons of airborne dust each year, 60 percent of the worldwide total. During the dry season wide areas south of the Sahara are affected by the harmattan, dust-laden winds originating in the desert. Typical episodes last for three to five days, with a dusty haze obliterating the Sun, lowering temperatures, and sometimes reducing visibility to a kilometer or less. The frequency and intensity of the harmattan varies; regions near the desert margins are often affected for 20 to 30 days per year. Dust originating over the Sahara also affects North Africa, southern Europe, and the Arabian Peninsula. The hot, dustladen winds that occur in North Africa between February and June are known collectively as sirocco, and locally by a variety of names (for example, khamsin in Egypt). C3 Climate Change The Sahel--the semidesert transition zone between the Sahara and the wetter tropical areas to the south--suffered from a severe drought from the late 1960s to the early 1980s. Scientists initially interpreted the drought as a new phenomenon in which reduced rainfall and inappropriate human use of the delicate environment were causing the desert to expand relentlessly. After greater research, however, many scientists now believe that the Sahelian drought is part of a long-standing cycle of change. The study of environmental change in the Sahel and in adjacent regions shows that the margins of the Sahara have shifted during the last 20,000 years. Some 18,000 years ago, glaciers covered much of northern Europe, and global climate zones shifted to the south. The boundary of the Sahara was far to the south of its present location, passing through southern Senegal and central Nigeria. Large areas of sand dunes in the Sahel demonstrate that the climate was formerly much drier than today. With the melting of the glaciers in Europe, the Sahara's southern margins shifted to the north as the climate became warmer and moister. About 9,000 years ago the desert had shifted far north of its current margin. Evidence of this shift includes sedimentary deposits indicating that Lake Chad was much larger than today, and that other large lakes existed in now-dry Saharan basins. Many vivid rock paintings in the central Sahara show savanna animals and people herding livestock more than 3,000 years ago. Since then, the climate has become progressively drier, the desert margin has moved south, and most of the lakes and wildlife have disappeared. Other climate changes in the Sahel have occurred over the past 500 years or so. These changes, especially in the amount of rainfall, have tended to be cyclical: a decade or two of poor rains, followed by moister conditions, followed in turn by drought. Evidence of these fluctuations comes from written and oral accounts of droughts and famines, archaeological evidence, and the analysis of fossils. In addition, meteorological records show cycles of increasing and decreasing rainfall just since the early 20th century. For example, the Sahelian drought was preceded by two decades of far-above average rainfall, and was followed by several years of average rainfall. Records also suggest an overall downward trend in average rainfall since 1900. Scientists have suggested that the clearing of vegetation and global warming may account for this apparent trend. They argue that the removal of vegetation causes higher surface temperatures, increased evaporation, and reduced rainfall. D Vegetation African vegetation zones are closely linked to climatic zones, with the same zones occurring both north and south of the equator in broadly similar patterns. As with climatic zones, differences in the amount and seasonal distribution of precipitation constitute the most important influence on the development of vegetation. Moving across the continent into drier and drier climates, the typical sequence of vegetation is from tropical moist forest to moist savanna, dry savanna, semidesert, and finally desert. D1 Tropical Moist Forest Tropical moist forest occurs in humid tropical areas, usually with 1,500 mm (60 in) or more of precipitation and a dry season (or seasons) of three to four months or less. West of the highlands of East Africa and centered in the basin of the Congo River is a great tropical moist forest extending some 600 km (400 mi) north of the equator and a similar distance south of the equator. On the west, the forest extends to the Atlantic coast in the Congo, Gabon, and Cameroon, and stretches in an interrupted belt along the West African coast to Sierra Leone. Tropical moist forest also occurs along the eastern side of Madagascar. Scientists recognize two major subtypes of tropical forest: tropical rain forest and tropical wetland forest. Tropical rain forests are characterized by a dense mass of evergreens, oil palms, and numerous species of tropical hardwood trees divided vertically into strata, or layers. The upper canopy of treetops forms a dense cover over the middle layer of treetops and the surface layer of shrubs, ferns, and mosses below. Rising above the canopy are scattered tall trees, known as emergents. In dense forest environments, the shrub layer tends to be quite sparse, except along streams, because the canopy limits the amount of light that penetrates to the forest floor. Tropical rain forests are extremely diverse in species; pure stands of a single tree species are rare. Tropical wetland forests include both freshwater and saltwater subtypes. Freshwater swamp forests cover large parts of the Middle Congo River Basin. Saltwater swamp forests occur in many areas between Senegal and Angola on the Atlantic coast and between South Africa and the Red Sea on the coast of East Africa. Mangrove species, with their characteristic tall, arched roots, are highly adapted to the fluctuating water levels and brackish water found in estuaries and other tidal environments. Mangrove forests are tangles of roots, tree trunks, and branches reaching 8 to 23 m (25 to 75 ft) high. Significant areas of mangrove forest have been lost in order to clear land for rice cultivation, particularly in West Africa. D2 Tropical Savanna Surrounding the central tropical forest zone on the north, east, and south is a zone of tropical savanna vegetation that covers up to 65 percent of the continent. The savannas have somewhat drier climates compared to the tropical forests. They typically receive 500 to 1,500 mm (20 to 60 in) of precipitation per year, and have a pronounced dry season, usually from three to eight months long. Moving poleward, savanna vegetation transitions from moist woodland savanna to dry woodland savanna. Moist woodland savanna occurs close to the tropical forest, where climates are wetter and the dry season is only three to four months. The characteristic vegetation is a mixture of tall grasses and closely spaced trees. Vegetation is especially dense along river courses, with trees lining the banks in gallery forests. Shrubs and grasses are also dense in areas where intensive cultivation and past use of fire to clear vegetation have led to the degradation of once forested lands. Shea trees occur widely; their tough outer bark is fire-resistant and their kernels are a source of oil for cooking and other uses. Dry woodland savanna, frequently called Sudan savanna, occurs where there is less precipitation and a prolonged dry season of about five to eight months. As precipitation declines, grass becomes shorter and sparser, and the tree cover less dense. The baobab is the largest tree; it is widely distributed and is valued for its inner bark (from which rope may be made) and its edible leaves and fruits. Other common trees are the silk cotton and locust bean, and various species from the acacia and fig (ficus) families. D3 Semidesert and Desert In the semidesert, or steppe, zone that lies between the dry woodland savanna and the desert, annual precipitation is between 250 and 500 mm (10 to 20 in). Rainfall is limited, localized, irregular, and often violent. Semidesert vegetation fringes the Sahara on the south (where this zone is referred to as the Sahel savanna), north (especially the southern slopes of the Atlas Mountains) and east (on the Red Sea coast). It also occurs in Somalia and northeastern Kenya, in the Karoo plateau regions of South Africa, and on the inland margin of the Namib Desert of Namibia and Angola. In addition, the Kalahari Desert is actually a semidesert region, despite its name. In many of these areas, vegetation occurs in strips running along contours in the land, where there has been an accumulation of moisture and soil. These vegetated strips are usually separated by wide barren areas. With increasing aridity, the drought-resistant properties of plants become crucial for their survival. Trees must be adapted to the prolonged dry period, with deep root systems, thick bark, small leaves that may be shed, and thorns to discourage animals from feeding on them. With still greater aridity, only grasses and desert shrubs survive. Semidesert vegetation is vulnerable to damage from fire, clearing for cultivation, and overgrazing. Desert vegetation occurs in the Sahara and Namib deserts, areas with less than 250 mm (10 in) of rainfall per year. Desert plant life must adapt to the harsh conditions of sparse, infrequent rainfall and extreme temperatures. The seeds of some species lay dormant for prolonged periods, until there is moisture to support their growth. Large areas of desert are essentially bare; plant growth is concentrated in channels and depressions where water accumulates when it rains. Environmental conditions and species found in the Sahara differ from those in the Namib Desert of southern Africa. In the latter, cold ocean currents facilitate the accumulation of dew on the desert, which helps to support plant life. Succulents (plants that retain water) and annual plant species typically account for most species found in desert ecosystems, with different plant species characteristic of different desert types (sand, gravel, or rock). The arid environments of southern Africa have a remarkable number of different species. Namaqualand, a region in northwestern South Africa, is renowned for the profusion of brilliantly colored wildflowers that appear after spring rains. D4 Mediterranean Shrub Lands The shrub lands of the northwestern and southern tips of the continent have Mediterranean climates, with warm, dry summers and mild, rainy winters. Unlike most of Africa, this climate supports distinctive types of temperate plant species. Mediterranean vegetation is xerophytic (drought-resistant), an adaptation to the limited rainfall and prolonged dry season that occurs during the summer months. The tree species typical of the North African shrub lands resemble those found elsewhere around the Mediterranean Sea, including wild olive, cork, juniper, and oak. Large areas of North African shrub lands have been damaged or destroyed through thousands of years of herding and agriculture. Environmental impact was pronounced during the 20th century, when rapid population increase and commercialization encouraged the expansion of agriculture into marginal, environmentally sensitive lands. The Cape region of South Africa has a distinctive ecosystem known as fynbos, in which fine-leaved evergreen shrubs predominate. It is one of the richest plant life biomes in the world, with some 8,500 species of plants in a relatively small area of about 90,000 sq km (about 30,000 sq mi). Over two-thirds of the species are endemic, meaning that they are found nowhere else in the world. Distinct subtypes of fynbos occur along the coast and in the adjacent mountains. In several other parts of southern Africa infertile soils, fire, and a lengthy dry season constrain vegetation growth. D5 Highland Vegetation In Africa's discontinuous areas of high mountains and uplands, altitude plays a significant role in determining climate and vegetation. On high peaks such as Kilimanjaro, vegetation changes as altitude increases: savanna vegetation near the base, then, in turn, zones of montane forest, bamboo, hagenia, heather, and high-altitude alpine moorland, with rock and ice at the summits of the very highest peaks. The montane forests include hardwood trees and many unusual plant species, including giant heather, giant groundsel, and giant lobelia. Growing conditions are also unusual due to the high temperatures during the day and very low temperatures that prevail at night. Some 4,000 species are associated with the African alpine zone, and three-quarters of these species are found only in Africa's montane forests. Other comparatively high-altitude regions have distinctive plant life. The highlands of Ethiopia contain scattered patches of montane forest with many species that resemble temperate deciduous trees. At higher elevations, tree cover largely disappears--except in deep valleys--and grasses, sedges, and heathers prevail. In the highlands of South Africa above 1,100 m (3,500 ft), vast expanses of temperate grasslands occur. In the highest parts of the Ahaggar and Tibesti mountains of the Sahara, some plant species resemble those of the Mediterranean region, isolated there when the region's climate became drier thousands of years ago. E Soils Like vegetation zones, soil regions in Africa are closely linked to climatic zones. Rainfall and temperature determine the growth of vegetation, which inhibits soil erosion and enriches soil with nutrients from decaying organic material, called humus. The luxuriant vegetation of tropical forest environments produces large quantities of humus, which is concentrated on the forest floor. In savanna grasslands, humus extends to a greater depth in the soil. The sparse vegetation of semidesert and desert regions gives rise to soils with little organic content. Rainfall and temperature also determine the intensity of chemical weathering, physical weathering, and leaching--all of which affect the development of soil types. Soil development is highly influenced by the soil's parent material--the rock from which it is derived--and by topographic relief. Much of Africa's soil is derived from ancient, quartz-rich rocks that produce gen...