(From Grolier Multimedia Encyclopedia)

Sahara

The Sahara Desert in northern Africa is the largest desert in the world. It spans the continent from the Atlantic Oceanto the Red Sea and extends northward from the Niger River and Lake Chad to the Atlas Mountains and the Mediterranean Sea. The name Sahara is from the Arabic word for "desert" or "steppe." The grasslands of West Africa, which form the southern boundary, are called the Sahel (Arabic for "coast" of the desert). The Sahara covers about 9,000,000 km2 (3,475,000 mi2), although climatic changes cause its borders to expand and contract.

Climatic history and fossilized remains reveal that the Sahara has experienced successive wet and dry eras. The latest arid period began about 3000 B.C., a result of the southward shifting of the trade-wind belt. Natural regeneration is thwarted by overgrazing and wood gathering along the desert edges.

Powerful states controlled the Sahara prior to the late 19th century, when the European powers began to extend their control and spread inland from the Mediterranean. Today ten independent states share the Sahara. Morocco, Algeria, Tunisia, Mali, Niger, Chad, and Sudan have large desert regions, and most of Libya, Egypt, and Mauritania are located in the Sahara.

Topography and Geology

The central Saharan uplands are a series of high plains, plateaus, and mountains that extend in a vast semicircle from southern Algeria to northeastern Mali, northern Chad, and western Sudan. The uplands average about 180 to 365 m (600 to 1,200 ft) in elevation. Two mountain chains are located in the central uplands: the Ahaggar Range in Algeria, which rises to 3,003 m (9,852 ft); and the Tibesti Massif in Chad, where the Sahara's highest point, Emi Koussi, reaches 3,415 m (11,204 ft). The Aïr Massif is located to the south in Niger.

The western Sahara is a vast, monotonous plain and plateau region that rises gradually from the Atlantic coast. Sandstone ridges and extensive basins break the monotony of the plains. The sand deserts of El Djouf, the Erg Iguidi, and the Erg Chech are prominent features. Elsewhere, large areas are covered by loose sands and gravel. In the northern Sahara, valleys are cut into the plains that extend southward from the Atlas Mountains. The great sand deserts of the Great Erg in Algeria and the Fezzan basin in southern Libya are prominent landscape features. The Libyan Desert dominates the northern part of the eastern Sahara. East of the Nile River are the Arabian Desert and the Nubian Desert.

Rocks of the Precambrian Era (prior to 543 million years ago), once folded and then denuded, compose the Saharan platform. Much of the Sahara was later submerged, and vast areas of sandstone and limestone were deposited. Volcanic topography dating from the Tertiary and Quaternary periods (beginning 65 million years ago) is found in the central Saharan uplands and its outliers. During the Quaternary Period (beginning 2.5 million years ago), when much of the Sahara received more rainfall, plateaus and highlands were deeply eroded, and sediments were deposited in basins. The soils of the Sahara are thin but fertile; only 20% of the soil cover is sand.

Climate

Winter high-pressure systems over the Sahara bring annual precipitation of about 100 to 200 mm (4 to 8 in) to the northern Sahara. In summer, wet monsoon winds from the Gulf of Guinea penetrate inland to the Sahel. Summer rainfall averages about 250 to 500 mm (10 to 20 in) annually, compared with less than 127 mm (5 in) in the winter. Most of the Sahara, however, receives less than 127 mm of rainfall annually, and large areas receive no rain for years at a time.

The Sahara is also very hot. The mean average annual temperature is 27° C (80° F). Because of high temperatures and clear skies, evaporation rates are high, creating relative humidities as low as 2.5%, the lowest in the world. Parts of the Sahara experience 50 to 75 days per year of wind and blowing sand that desiccate plant and animal life. Although the climate has remained relatively uniform, since c.3000 B.C. extended periods of drought have been common. The Sahel drought of 1968–74 was followed by another one in the 1980s.

Drainage, Vegetation, and Animal Life

The Sahara has an extensive network of dry streambeds, or wadis, that were formed during earlier wetter periods. Numerous intermittent streams now occupy some of the wadis following precipitation; these streams drain from the Atlas Mountains and central Saharan uplands into surrounding basins, where occasional salt marshes, or sebkhas, are found. Underground water resources, or oases, sufficient to support irrigated agriculture are found in many wadis and depressions. The Aswan High Dam, which controls the flow of the Nile River, has reduced the size of the adjacent desert through irrigation projects and has increased Egypt's cultivated area by almost 35%.

Few areas of the desert are completely void of vegetation. A sparse cover of xerophytic shrubs, those adapted to hot, dry climates, extends the northern edge of the desert; coarse grasses grow in depressions; and palm trees grow in valleys and oases. Thorn woodlands and wooded grasslands are found in the Sahel. Some animal life exists even in the desert's interior: insects, small rodents, reptiles, and gazelles on the plateaus. The Djoudj Sanctuary at the southern edge of the Sahara in northeastern Senegal is a major refuge for migrating birds.

People and Economy

The majority of the estimated 2 million people who live in the Sahara live along the margins of the desert, at the oases, and in the less arid, more isolated highlands. The overall population density is only about 0.2 persons per km2 (0.6 per mi2). Most inhabitants are Arabic-speaking Muslims. The people of the northern and western desert are of mixed Arab and Berber stock, with the Berber language surviving in pockets. The Tuareg of the Ahaggar, the Tibbu of the Tibesti, and the Nubians of western Sudan maintain their distinctive identities.

The principal socioeconomic distinction of the inhabitants is between the sedentary population and the pastoral nomads whose flocks are moved seasonally among traditional pastures. The sedentary population is concentrated in the oases, where dates are the principal crop.

The Sahara is rich in mineral resources. It has substantial proved crude petroleum reserves. Libya and Algeria are the largest oil producers, and Algeria is also a major producer and exporter of natural gas. Algeria, Mauritania, and Libya possess Saharan iron-ore deposits; Algeria has manganese; and Mauritania has rich deposits of copper. Morocco is a leading exporter of phosphate. Numerous metals are found in the central Saharan uplands, and uranium is mined in northern Niger.

Transportation

The traditional camel caravans have today been replaced by truck convoys over the same routes. The Saharan road system is steadily expanding and includes plans to construct a trans-Saharan highway linking the port of Tangier, Morocco, to Lagos, Nigeria, although only two main routes cross the desert from central Algeria to Nigeria and to southern Mali. Rail service is poor. Many international air routes cross the Sahara, however. The best road and rail transportation is associated with mineral exploitation.

History

Archaeological evidence exists of extensive settlement in the Sahara during the Paleolithic and Neolithic periods. Radar studies made by the space shuttle Columbia revealed buried sites of ancient settlements and subterranean traces of long-vanished rivers. Prehistoric stone artifacts and rock drawings are found throughout the central Sahara uplands, the most spectacular in the Tassili n'Ajjer, in Algeria. The Romans established colonies in Libya and Tunisia along the Mediterranean coast and reached their period of greatest expansion during the 2d century A.D. One of the earliest known Saharan states was the kingdom of the Garamantes, which peaked in the early centuries of the first millennium A.D. The origin of the Garamantes is unknown, but they are believed to have introduced writing, the horse, the camel, and wheeled transportation into what is now western Libya, where they practiced sophisticated irrigated agriculture and traded with Roman settlements along the Mediterranean coast, from which they borrowed the classical architecture of their desert towns. By A.D. 750, Arabic-speaking people spread Islam into the northern Sahara, and Berber kingdoms ruled the western Sahara from Senegal to Morocco.

By the end of the 12th century A.D., Islam had spread throughout most of the Sahara and along the trade routes to the Sahel. Subsequently, the rich and powerful medieval kingdoms of Kanem-Bornu, Songhai, and Ghana, and the empire of Mali controlled the central and southern Sahara. Although European explorers did not penetrate the Sahara until the end of the 18th century, the French were able to extend their control in Algeria to the edge of the desert by 1850; after that time the Sahara was divided among the colonies and protectorates of France, Great Britain, Italy, and Spain. The period of European control finally ended in 1976, when the Spanish relinquished their claim to Western Sahara.

Gary L. Fowler


Bibliography
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Crewe, Q., In Search of the Sahara (1984).
Gautier, Émile F., Sahara, trans. by Dorothy Ford Mayhew (1935; repr. 1970).
Langewiesche, W., Sahara Unveiled (1996).
Moorhouse, G., The Fearful Void (1974; repr. 1989).
Webb, J. L., Desert Frontier (1994).