Africa

I

Introduction

Africa, second-largest of the Earth's seven continents, with adjacent islands, covering about 30,330,000 sq km (11,699,000 sq mi), or about 22 per cent of the world's total land area. At the end of the 20th century more than 13 per cent of the world's population lived in Africa.

Straddling the equator, Africa stretches 8,050 km (4,970 mi) from its northernmost point, Cape Blanc (Ra’s al Abyad;) in Tunisia, to its southernmost tip, Cape Agulhas in South Africa. The maximum width of the continent, measured from the tip of Cape Vert in Senegal, in the west, to Cape Xaafuun (Ras Hafun) in Somalia, in the east, is about 7,560 km (4,700 mi). The highest point on the continent is the perpetually snowcapped Mount Kilimanjaro (5,892 m/19,330 ft) in Tanzania; the lowest is ‘Asal Lake (153 m/502 ft below sea level) in Djibouti. Africa has a regular coastline characterized by few indentations. Its total length is about 30,490 km (18,950 mi), which in proportion to its area is less than that of any other continent.

The main islands associated with Africa, which have a combined area of some 621,600 sq km (240,000 sq mi), are Madagascar, Zanzibar, Pemba, Mauritius, Réunion, the Seychelles, and the Comoro Islands in the Indian Ocean; São Tomé, Príncipe, and Bioko in the Gulf of Guinea; St Helena, Ascension, and the Bijagós Archipelago in the Atlantic; and the Cape Verde Islands, the Canary Islands, and the Madeira Islands in the North Atlantic. Although considered geographically part of Africa, St Helena, Ascension, the Bijagós Archipelago, the Canary Islands, and the Madeira Islands have few, if any, economic, political, or cultural links with the continent. Their ties are rather with western Europe: St Helena and Ascension are dependencies of the United Kingdom; the Canary and Madeira islands are an integral part of metropolitan Spain and Portugal respectively.

II

The Natural Environment

The African continent comprises a vast, rolling plateau; just 10 per cent of its land area lies at less than 500 ft above sea level, compared with 54 per cent of Europe and 25 per cent of North America. Only in the extreme south and north have great folded mountain ranges been built up. Elsewhere in the continent differences in elevation have been caused either by the faulting which produced the Rift Valley, or by the creation of the enormous river basins—notably those of the Congo, the Niger, the Nile, the Volta, and the Zambezi—which are a more prominent feature of the geography of Africa than that of any other continent. At its margins the plateau gives way, via steep escarpments, to the narrow coastal plain that surrounds the continent. All the great rivers of Africa, except the Niger-Benue and Zambezi-Shire systems, plunge in falls or rapids over the escarpments, making effective navigation inland from the sea impossible.

A

Geological History

The African continental plateau is a vast shield of ancient, hard rock, dating from the Precambrian and related in age and history to South America's Brazilian Highlands. The shield extends south from the Atlas Mountains to the Cape of Good Hope. To the east, it includes the Arabian Peninsula and Madagascar, which were split off from Africa during the Tertiary Period (see Plate Tectonics). Within these ancient rocks some of the earliest traces of life on Earth—fossil micro-organisms 3.2 billion years old—have been found. Geologically, the folded Atlas Mountains of north-west Africa are part of Europe, having been created by the same forces that produced 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, over 150 million years ago (see Jurassic Period), have continued into more recent times, creating the Rift Valley during the Tertiary Period and triggering eruptions of the East African volcanic cones, Mount Kenya and Mount Kilimanjaro.

B

Physiographic Regions

Africa may be divided into three major regions: the Northern Plateau, the Central and Southern Plateau, and the Eastern Highlands. In general, elevations increase across the continent from north-west to south-east, the average being about 560 m (1,900 ft). The coastal plains, with the exception of the Mediterranean and the Guinea coasts, are generally narrow.

The outstanding feature of the Northern Plateau is the Sahara, the great desert that occupies more than one-quarter of Africa. At the fringes of the Northern Plateau are several mountainous regions. To the north-west lie the Atlas Mountains, a chain of rugged peaks linked by high plateaux, extending from Morocco into Tunisia. Other prominent uplands are the Fouta Djallon, in the south-west, and the Adamawa Plateau and the Cameroon mountain range, in the south. The Lake Chad basin is situated in the approximate centre of the Northern Plateau.

The Central and Southern Plateau is considerably higher than the Northern Plateau, averaging more than 900 m (3,000 ft) in height. It includes west-central and southern Africa, and contains several major depressions, notably the Congo River basin and the Kalahari. South of this plateau is the folded chain of the Drakensberg of South Africa, which runs some 1,100 km (700 mi) along the south-eastern coast of the continent. In the extreme south is the Karoo, an arid plateau covering about 259,000 sq km (100,000 sq mi).

The Eastern Highlands, the highest part of the continent, lie near the eastern coast, extending from the Red Sea south to the Zambezi along the fault line of the Rift Valley. The region has an average elevation of more than 1,500 m (5,000 ft), and in the Ethiopian Plateau it rises in stages to about 3,000 m (10,000 ft). Ras Dashen (4,620 m/15,157 ft) in northern Ethiopia is the highest point of the plateau. South of the Ethiopian Plateau are a number of towering volcanic peaks, including Mount Kilimanjaro, Mount Kenya, and Mount Elgon. The most distinctive feature of the Eastern Highlands is the Rift Valley, the vast geologic fault system that begins in Anatolia, in eastern Turkey, stretches through the Jordan Valley and the Dead Sea, and then follows down the length of the Red Sea to Lake Turkana (formerly Lake Rudolph). At the southern end of Lake Turkana, the rift divides around Lake Victoria, but joins again at the head of Lake Malawi (Lake Nyasa), from where it runs down the Shire and Zambezi rivers, and finally out to sea. Altogether the Rift Valley extends around almost one-fifth of the Earth, and contains some of its deepest lakes. West of the Rift Valley is the Ruwenzori Range, which rises up to 5,109 m (16,762 ft) above sea level. The topography of the island of Madagascar features a rugged central highland extending in a generally north-south direction near the eastern coast.

Except for a few incursions from the sea, Africa has been a land area since Precambrian times. Its soils have therefore developed locally, chiefly by weathering. A few areas have alluvial soils laid down by rivers or ocean currents. African soils, for the most part, have irregular drainage and no definite water tables. Being typical tropical soils, most are relatively infertile, lacking humus and subject to mineral leaching from heavy rainfall and high temperatures. Desert soils (aridisols and entisols), which have the least organic content, cover large areas. The most fertile soils include the mollisols, also known as chernozems and black soils, of eastern Africa, and the alfisols, or podzolic soils, of parts of western and southern Africa.