Largest desert in the world, occupying around 9,065,000 sq km/3,500,000 sq mi of north Africa from the Atlantic to the Red Sea, with an interruption in the fertile Nile valley, covering: most of Egypt; part of west Sudan; large parts of Mauritania, Mali, Niger, and Chad; and southern parts of Morocco, Algeria, Tunisia, and Libya. Small areas in Algeria and Tunisia are below sea level, but it is mainly a plateau with a central mountain system, including the Ahaggar Mountains in Algeria, the Aïr Massif in Niger, and the Tibesti Massif in Chad, of which the highest peak is Emi Koussi, 3,415 m/11,208 ft.
Oases, often where water flowing underground from the rainy Atlas Mountains reaches the surface, punctuate the caravan routes, now modern roads. Resources include oil and gas in the north. Satellite observations have established a pattern below the surface of dried-up rivers that existed 2 million years ago. Cave paintings confirm that 4,000 years ago running rivers and animal life existed. Satellite photos taken during the 1980s have revealed that the Sahara expands and contracts from one year to another depending on rainfall; there is no continuous expansion, as had been believed.
Mountains and highland The Sahara is mostly an elevated plateau with a mean altitude of 600 m/1,968 ft, the surface of which is diversified by high tablelands and mountains (some 43% of the area is mountains). The summits of the central Ahaggar Mountains and of the Tibesti Mountains, farther to the east, rise as high as 2,600 m/8,530 ft and are snow-capped for several months of the year. To the northeast and northwest of the Ahaggar peaks extends the ridge of the Muidir plateau (320 km/200 mi), and that of the Tasili of the Asjer (480 km/300 mi), which has a mean altitude of 1,500 m/4,920 ft. South of the Ahaggar are the mountains of Air (2,000 m/6,562 ft). The Igidi, which with the West and East Erg stretches from Cape Blanco to the south of Tunisia (2,000 km/1,243 mi), is a vast belt of sand-hills. These account for only one-ninth of the area of the Sahara.
Surface covering Rocky wastes, with the bare exposure of fissured rocks as dominant features of the scene, form the hammada type of the Sahara. Numerous shrubs of different kinds find shelter in the clefts, and here, too, may be found rough camel pasture. Much of the central plateaus of Ahaggar and Tibesti is of this character. The Hammada al-Homra and the hammada of Murzuk are vast, undulating granite-strewn surfaces; and there are also vast tracts of stones and water-worn pebbles, called serir. Some 28% of the area is covered in soft sand, sometimes blown by the wind into high dunes, which may reach a height of over 100 m/328 ft, continually shifting and changing contour. The most extended area of these sandy wastes, known as erg, is the Libyan Desert, which stretches southwards for 1,795 km/1,115 mi from the Siwah group of oases.
Water and oases Very little rain falls on the Sahara. However there are subterranean sources of water. The wells of the oases, for example those of the Tuat group in the north-central Sahara, make cultivation on a limited scale possible (tropical fruit trees, palms, cereals). This combined with the nodal position of the oases on lines of trans-Saharan communication, gives them considerable economic and commercial significance. Ancient watercourses prove that rivers formerly flowed through the desert, and the presence of salt and marine shells indicates that parts of it were once under the sea.
Desertification It has long been suggested that the margins of the Sahara are becoming increasingly arid. The major evidence to support these claims are the presence of deserted (or semi-deserted) cities, abandoned caravan routes, and the fact that many settlements have been abandoned in the past 50 years. The idea of desert margins advancing was put forward in the 1970s but has subsequently been largely disproved. It has been shown that while human destruction of vegetation through fuelwood collection and cattle grazing may cause major soil degradation, fluctuations of the margins of the desert are mainly due to vegetation responding to seasonal variations in rainfall.
Climate The interior areas have the highest temperatures of Africa, as afternoon temperatures may exceed 44°C in summer, but in winter the mean in the interior is below 15°C and frosts are not unusual. Rainfall levels vary across the Sahara, with the very arid centre receiving less than 50 mm/2 in per year.
Resources The Sahara contains rich mineral resources, of which oil and natural gas are the most important. Hasei R'Mel, Edjeleh, and the Allizi Basin are the main centres of production in Algeria, and the Sarir field is the main source of oil in the Libyan Desert. Other mineral resources in the Sahara found in substantial quantities include iron ore in Algeria and Mauritania, and phosphates in Morocco and Western Sahara.
Communications Traders used to cross the desert by recognized routes linking oases. Major trans-Saharan routes are now at least partially tar-surfaced, with the most commonly used routes running from Beni Abbès (Algeria) to Niamey (Niger) via Bourem (Mali), and from El Goléa (Algeria) to Kano (Nigeria) via the Ahaggar plateau. Bus services run along these routes, with the majority of freight now carried by lorry as opposed to camel. It is also possible to skirt the edge of the desert via Morocco and Mauritania, a route of increasing importance since the sporadic clashes between Tuareg nomads and the Mali and Niger governments in the 1990s.
Exploration British explorers include Major Laing who crossed to Timbuktu in 1826, John Davidson in 1836, and John Richardson in 1845. B Khun de Prohok, between 1920 and 1930, was the first person to undertake motor expeditions.
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