Structure and relief of Africa
A vast continental shield of Precambrian rocks, related in age and history to South America’s Brazilian Highlands, extends south of the Atlas Mountains to the Cape of Good Hope. On the east, the shield encompasses two landmasses—the Arabian Peninsula and Madagascar—that were split off from Africa during the Tertiary period. Among these ancient rocks some of the earliest traces of life on Earth—fossil microorganisms 3.2 billion years old—have been found. 
Geologically, the Atlas Mountains of North Africa are part of Europe, having been raised by the same forces that created the Alpine mountain ranges of southern and central Europe. The tectonic forces that split Africa and South America apart during the breakup of the supercontinent Gondwanaland, more than 150 million years ago (see Jurassic Period), have continued into more recent times, creating East Africa’s Great Rift Valley during the Tertiary Period and triggering eruptions there of the volcanic Mount Kenya and Kilimanjaro.Geography of Africa
2008/9 Schools Wikipedia Selection. Related subjects: African Geography
Africa is a continent comprising 61 political territories (including 53countries), representing the largest of the great southward projections from the main mass of Earth's surface. It includes, within its remarkably regular outline, an area of 30,368,609 km² (11,725,385 mi²), including adjacent islands.
Separated from Europe by the Mediterranean Sea and from much of Asia by the Red Sea, Africa is joined to Asia at its northeast extremity by the Isthmus of Suez (which is transected by the Suez Canal), 130 km (80 miles) wide. For geopolitical purposes, the Sinai Peninsula of Egypt – east of the Suez Canal – is often considered part of Africa. From the most northerly point, Ras ben Sakka in Tunisia, in 37°21′ N, to the most southerly point, Cape Agulhas in South Africa, 34°51′15″ S, is a distance approximately of 8,000 km (5,000 miles); from Cape Verde, 17°33′22″ W, the westernmost point, to Ras Hafun in Somalia, 51°27′52″ E, the most easterly projection, is a distance (also approximately) of 7,400 km (4,600 miles). The length of coast-line is 26,000 km (16,100 miles) and the absence of deep indentations of the shore is shown by the fact that Europe, which covers only 10,400,000 km² (4,010,000 square miles), has a coastline of 32,000 km (19,800 miles).
The main structural lines of the continent show both the east-to-west direction characteristic, at least in the eastern hemisphere, of the more northern parts of the world, and the north-to-south direction seen in the southern peninsulas. Africa is thus composed of two segments at right angles, the northern running from east to west, the southern from north to south, the subordinate lines corresponding in the main to these two directions.
Main Features
The average elevation of the continent approximates closely to 600 m (2,000 ft) above sea level, roughly near to the mean elevation of both North and South America, but considerably less than that of Asia, 950 m (3,117 ft). In contrast with other continents, it is marked by the comparatively small area of either very high or very low ground, lands under 180 m (600 ft) occupying an unusually small part of the surface; while not only are the highest elevations inferior to those of Asia or South America, but the area of land over 3,000 m (10,000 ft) is also quite insignificant, being represented almost entirely by individual peaks and mountain ranges. Moderately elevated tablelands are thus the characteristic feature of the continent, though the surface of these is broken by higher peaks and ridges. (So prevalent are these isolated peaks and ridges that a specialised term [Inselberg-landschaft] has been adopted in Germany to describe this kind of country, thought to be in great part the result of wind action.)
As a general rule, the higher tablelands lie to the east and south, while a progressive diminution in altitude towards the west and north is observable. Apart from the lowlands and the Atlas mountain range, the continent may be divided into two regions of higher and lower plateaus, the dividing line (somewhat concave to the north-west) running from the middle of the Red Sea to about 6 deg. S. on the west coast.
Africa can be divided into a number of geographic zones:
- The coastal plains - often fringed seawards by mangrove swamps - never stretching far from the coast, apart from the lower courses of streams. Recent alluvial flats are found chiefly in the delta of the more important rivers. Elsewhere, the coastal lowlands merely form the lowest steps of the system of terraces that constitutes the ascent to the inner plateaus.
- The Atlas range — orographically distinct from the rest of the continent, being unconnected with and separated from the south by a depressed and desert area (the Sahara).
Plateau region
The high southern and eastern plateaus, rarely falling below 600 m (2000 ft), and having a mean elevation of about 1000 m (3500 ft). The South African plateau as far as about 12° S, bounded east, west and south by bands of high ground which fall steeply to the coasts. On this account South Africa has a general resemblance to an inverted saucer. Due south the plateau rim is formed by three parallel steps with level ground between them. The largest of these level areas, the Great Karoo, is a dry, barren region, and a large tract of the plateau proper is of a still more arid character and is known as the Kalahari Desert.
The South African plateau is connected towards the north-east with the East African plateau, with probably a slightly greater average elevation, and marked by some distinct features. It is formed by a widening out of the eastern axis of high ground, which becomes subdivided into a number of zones running north and south and consisting in turn of ranges, tablelands and depressions. The most striking feature is the existence of two great lines of depression, due largely to the subsidence of whole segments of the earth's crust, the lowest parts of which are occupied by vast lakes. Towards the south the two lines converge and give place to one great valley (occupied by Lake Nyasa), the southern part of which is less distinctly due to rifting and subsidence than the rest of the system.
Farther north the western depression, known as the Great Rift Valley is occupied for more than half its length by water, forming the Great Lakes of Tanganyika, Kivu, Lake Edward and Lake Albert, the first-named over 400 miles (600 km) long and the longest freshwater lake in the world. Associated with these great valleys are a number of volcanic peaks, the greatest of which occur on a meridional line east of the eastern trough. The eastern depression, known as the East African trough or rift-valley, contains much smaller lakes, many of them brackish and without outlet, the only one comparable to those of the western trough being Lake Turkana or Basso Norok.
At no great distance east of this rift-valley are Mount Kilimanjaro - with its two peaks Kibo and Mawenzi, the latter being 5889 m (19,321 ft), and the culminating point of the whole continent - and Mount Kenya, which is 5184 m (17,007 ft). Hardly less important is the Ruwenzori Range, over 5060 m (16,600 ft), which lies east of the western trough. Other volcanic peaks rise from the floor of the valleys, some of the Kirunga (Mfumbiro) group, north of Lake Kivu, being still partially active.
The third division of the higher region of Africa is formed by the Ethiopian Highlands, a rugged mass of mountains forming the largest continuous area of its altitude in the whole continent, little of its surface falling below 1500 m (5000 ft), while the summits reach heights of 4600 m to 4900 m (15,000 to 16,000 ft). This block of country lies just west of the line of the great East African Trough, the northern continuation of which passes along its eastern escarpment as it runs up to join the Red Sea. There is, however, in the centre a circular basin occupied by Lake Tsana.
Both in the east and west of the continent the bordering highlands are continued as strips of plateau parallel to the coast, the Ethiopian mountains being continued northwards along the Red Sea coast by a series of ridges reaching in places a height of 2000 m (7000 ft). In the west the zone of high land is broader but somewhat lower. The most mountainous districts lie inland from the head of the Gulf of Guinea (Adamawa, etc.), where heights of 1800 m to 2400 m (6000 to 8000 ft) are reached. Exactly at the head of the gulf the great peak of the Cameroon, on a line of volcanic action continued by the islands to the south-west, has a height of 4075 m (13,370 ft), while Clarence Peak, in Fernando Po, the first of the line of islands, rises to over 2700 m (9000 ft). Towards the extreme west the Futa Jallon highlands form an important diverging point of rivers, but beyond this, as far as the Atlas chain, the elevated rim of the continent is almost wanting.
Plains
The area between the east and west coast highlands, which north of 17° N is mainly desert, is divided into separate basins by other bands of high ground, one of which runs nearly centrally through North Africa in a line corresponding roughly with the curved axis of the continent as a whole. The best marked of the basins so formed (the Congo basin) occupies a circular area bisected by the equator, once probably the site of an inland sea.
Running along the south of desert is the plains region known as the Sahel.
The arid region, the Sahara — the largest desert in the world, covering 9,000,000 km² (3,500,000 square miles) — extends from the Atlantic to the Red Sea. Though generally of slight elevation it contains mountain ranges with peaks rising to 2400 m (8000 ft) Bordered N.W. by the Atlas range, to the northeast a rocky plateau separates it from the Mediterranean; this plateau gives place at the extreme east to the delta of the Nile. That river (see below) pierces the desert without modifying its character. The Atlas range, the north-westerly part of the continent, between its seaward and landward heights encloses elevated steppes in places 160 km (100 miles) broad. From the inner slopes of the plateau numerous wadis take a direction towards the Sahara. The greater part of that now desert region is, indeed, furrowed by old water-channels.
The following table gives the approximate altitudes of the chief mountains and lakes of the continent:
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Hydrology
From the outer margin of the African plateaus, a large number of streams run to the sea with comparatively short courses, while the larger rivers flow for long distances on the interior highlands, before breaking through the outer ranges. The main drainage of the continent is to the north and west, or towards the basin of the Atlantic Ocean.
The high lake plateau of East Africa contains the headwaters of both the Nile and the Congo: the longest and second longest river in Africa respectively.
The upper Nile receives its chief supplies from the mountainous region adjoining the Central African trough in the neighbourhood of the equator. From there, streams pour eastward into Lake Victoria, the largest African lake (covering over 26,000 square m.), and to the west and north into Lake Edward and Lake Albert. To the latter of these, the effluents of the other two lakes add their waters. Issuing from there, the Nile flows northward, and between the latitudes of 7 and 10 degrees N. it traverses a vast marshy level, where its course is liable to being blocked by floating vegetation. After receiving the Bahr-el-Ghazal from the west and the Sobat, Blue Nile andAtbara from the Ethiopian highlands (the chief gathering ground of the flood-water), it separates the great desert with its fertile watershed, and enters the Mediterranean at a vast delta.
The most remote head-stream of the Congo is the Chambezi, which flows southwest into the marshy Lake Bangweulu. From this lake issues the Congo, known in its upper course by various names. Flowing first south, it afterwards turns north through Lake Mweru and descends to the forest-clad basin of west equatorial Africa. Traversing this in a majestic northward curve, and receiving vast supplies of water from many great tributaries, it finally turns southwest and cuts a way to the Atlantic Ocean through the western highlands.
North of the Congo basin, and separated from it by a broad undulation of the surface, is the basin of Lake Chad - a flat-shored, shallow lake filled principally by the Chari coming from the southeast.
West of this is the basin of the Niger, the third major river of Africa. With its principal source in the far west, it reverses the direction of flow exhibited by the Nile and Congo, and ultimately flows into the Atlantic — a fact that eluded European geographers for many centuries. An important branch, however - the Benue - flows from the southeast.
These four river-basins occupy the greater part of the lower plateaus of North and West Africa — the remainder consisting of arid regions watered only by intermittent streams that do not reach the sea.
Of the remaining rivers of the Atlantic basin, the Orange, in the extreme south, brings the drainage from theDrakensberg on the opposite side of the continent, while the Kunene, Kwanza, Ogowe and Sanaga drain the west coastal highlands of the southern limb; the Volta, Komoe, Bandama, Gambia and Senegal the highlands of the western limb. North of the Senegal, for over 1000 miles (1600 km) of coast, the arid region reaches to the Atlantic. Farther north are the streams, with comparatively short courses, reaching the Atlantic and Mediterranean from the Atlas mountains.
Of the rivers flowing to the Indian Ocean, the only one draining any large part of the interior plateaus is theZambezi, whose western branches rise in the western coastal highlands. The main stream has its rise in 11°21′3″ S 24°22′ E, at an elevation of 5000 ft. It flows to the west and south for a considerable distance before turning eastward. All the largest tributaries, including the Shire, the outflow of Lake Nyasa, flow down the southern slopes of the band of high ground stretching across the continent from 10 deg. to 12 deg. S. In the southwest, the Zambezi system interlaces with that of the Taukhe (or Tioghe), from which it at times receives surplus water. The rest of the water of the Taukhe, known in its middle course as the Okavango, is lost in a system of swamps and saltpans that was formerly centred in Lake Ngami, now dried up.
Farther south, the Limpopo drains a portion of the interior plateau, but breaks through the bounding highlands on the side of the continent nearest its source. The Rovuma, Rufiji, Tana, Jubba and Webi Shebeli principally drain the outer slopes of the East African highlands, the last of these losing itself in the sands in proximity to the sea. Another large stream, the Hawash, rising in the Ethiopian mountains, is lost in a saline depression near the Gulf of Aden.
Lastly, between the basins of the Atlantic and Indian Oceans, there is an area of inland drainage along the centre of the East African plateau, directed chiefly into the lakes in the great rift valley. The largest river is the Omo, which, fed by the rains of the Ethiopian highlands, carries down a large body of water into Lake Rudolf. The rivers of Africa are generally obstructed either by bars at their mouths, or by cataracts at no great distance upstream. But when these obstacles have been overcome, the rivers and lakes afford a vast network of navigable waters.
The calculation of the areas of African drainage systems, made by Dr A. Bludau (Petermanns Mitteilungen, 43, 1897, pp. 184-186) yields the following general results:
| mi² | Mm² | |
|---|---|---|
| Basin of the Atlantic | 4,070,000 | 10.541 |
| Basin of the Mediterranean | 1,680,000 | 4.351 |
| Basin of the Indian Ocean | 2,086,000 | 5.403 |
| Inland drainage area | 3,452,000 | 8.941 |
The areas of individual river basins are:
| mi² | Mm² | |
|---|---|---|
| Congo, length over 3000 mi (4800 km) | 1,425,000 | 3.691 |
| Nile, length fully 4000 mi (6500 km) | 1,082,000 | 2.802 |
| Niger, length about 2600 mi (4200 km) | 808,000 | 2.093 |
| Zambezi, length about 2000 mi (3200 km) | 513,500 | 1.330 |
| Lake Chad | 394,000 | 1.020 |
| Orange, length about 1300 mi (2100 km) | 370,505 | 0.9596 |
| Orange (actual drainage area) | 172,500 | 0.447 |
The area of the Congo basin is greater than that of any other river except the Amazon, while the African inland drainage area is greater than that of any continent but Asia, where the corresponding area is 4,000,000 square miles (10 Mm²).
The principal African lakes have been mentioned in the description of the East African plateau, but some of the phenomena connected with them may be spoken of more particularly here. As a rule, the lakes found within the great rift-valleys have steep sides and are very deep. This is the case with the two largest of the type, Tanganyika and Nyasa, the latter with depths of 430 fathoms (790 m).
Others, however, are shallow, and hardly reach the steep sides of the valleys in the dry season. Such are Lake Rukwa, in a subsidiary depression north of Nyasa, and Eiassi and Manyara in the system of the eastern rift-valley. Lakes of the broad type are of moderate depth, the deepest sounding in Lake Victoria being under 50 fathoms (90 m).
Besides the East African lakes, the principal are: - Lake Chad, in the northern inland watershed; Bangweulu andMweru, traversed by the head-stream of the Congo; and Lake Mai-Ndombe and Ntomba (Mantumba), within the great bend of that river. All, except possibly Mweru, are more or less shallow, and Lake Chad appears to be drying up.
Divergent opinions have been held as to the mode of origin of the East African lakes, especially Tanganyika, which some geologists have considered to represent an old arm of the sea, dating from a time when the whole central Congo basin was under water; others holding that the lake water has accumulated in a depression caused by subsidence. The former view is based on the existence in the lake of organisms of a decidedly marine type. They include jellyfish, molluscs, prawns, crabs, etc.
Islands
With exception - Madagascar - the African islands are small. Madagascar, with an area of 229,820 square miles (595,230 km²), is, after Greenland, New Guinea and Borneo, the fourth largest island on the Earth. It lies off the S.E. coast of the continent, from which it is separated by the deep Mozambique channel, 250 miles (400 km) wide at its narrowest point. Madagascar in its general structure, as in flora and fauna, forms a connecting link between Africa and southern Asia. East of Madagascar are the small islands of Mauritius and Réunion. Socotra lies E.N.E. of Cape Guardafui. Off the north-west coast are the Canary and Cape Verde archipelagoes. which, like some small islands in the Gulf of Guinea, are of volcanic origin.
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