Sahara Desert is the world's second largest desert, after Antarctica. At over 9,400,000 square kilometres (3,630,000 sq mi), it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean. To the south, it is delimited by the Sahel, a belt of semi-arid tropical savanna that composes the northern region of central and western Sub-Saharan Africa.
The Sahara's boundaries are the Atlantic Ocean on the west, the Atlas Mountains and the Mediterranean Sea on the north, the Red Sea and Egypt on the east, and the Sudan and the valley of the Niger River on the south. The Sahara is divided into western Sahara, the central Ahaggar Mountains, the Tibesti Mountains, the Air Mountains (a region of desert mountains and high plateaus), Ténéré desert and the Libyan desert (the most arid region). The highest peak in the Sahara is Emi Koussi (3,415 m/11,204 ft) in the Tibesti Mountains in northern Chad.
The flora of the central Sahara Desert is quite depauperate, and is estimated to include only 500 species. This is extremely low considering the huge extent of the area. It mainly consists of xerophytes and ephemeral plants (called also locally Acheb), with halophytes in moister areas. The flora has one near endemic family, a number of isolated monotypic genera of both wide and narrow distribution, and perhaps as many as 162 endemic species. The monotypic genera suggest a Tertiary origin with probable extinction of linking forms. Vegetation is very contracted along the wadis and the dayas with Acacia sp, Tamarix sp., and Calotropis procera. Where there is sufficient groundwater, hammadas are covered by Anrthirrnum ramosissimuma and Ononis angustissima.
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