Desierto de Namib, África

Namib, Namibe portugués, un desierto costero fresco que se extiende por 1.200 millas (1.900 km) a lo largo de la costa atlántica de África desde Namibe (anteriormente Moçâmedes) en Angola hacia el sur a través de Namibia hasta el río Olifants en la provincia de Western Cape de Sudáfrica. Alcanza el interior de 80 a 100 millas (130 a 160 km) al pie de la Gran Escarpa. La porción sur se fusiona con el Kalahari en la meseta sobre la escarpa. Su nombre se deriva del idioma Nama, lo que implica "un área donde no hay nada".

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El Namib es árido y está casi totalmente deshabitado, a excepción de un pequeño número de pueblos dispersos. Es importante debido a las rutas comerciales que lo cruzan, sus depósitos minerales, las pesquerías del mar fronterizo y su creciente utilización para fines recreativos.

Características físicas

Namib se divide en tres franjas sucesivas de tendencia norte-sur: la región costera muy estrecha a lo largo del Atlántico, fuertemente sujeta a influencias marinas; el Namib exterior, que ocupa el resto de la mitad occidental del desierto; y el Namib Interior, que constituye la porción oriental. Los límites entre ellos consisten en amplias zonas de transición.

Fisiografía

El desierto consiste básicamente en una plataforma relativamente lisa de roca madre truncada de varios tipos y edades. Predominan el esquisto de mica y otras metamórficas y granitos e intrusiones similares. La plataforma se eleva gradualmente desde la costa hasta unos 3.000 pies (900 metros) al pie de la Gran Escarpa. Montañas aisladas dispersas se elevan abruptamente y abruptamente sobre la plataforma, y ​​en la mitad norte varias corrientes han tallado profundas gargantas con paredes empinadas.

In much of the southern half of the desert the platform is surmounted by a vast expanse of sand—yellow-gray near the coast and brick red inland—which is derived from the Orange River and from other rivers that flow westward from the escarpment but never reach the sea. The dunes run in lines from north-northwest to south-southeast, individual dunes having lengths of 10 to 20 miles (16 to 32 km) and reaching heights of 200 to 800 feet (60 to 240 metres). The troughs between these lines of dunes are interrupted by smaller transverse dunes. The extreme southern coastal area consists of wind-scoured bedrock and a few rapidly moving crescent-shaped barchans (i.e., dunes convex to the wind). The northern third—the Kaokoveld region—consists of gravel plains and rock platforms occurring between scattered rugged mountains, interspersed with a few large dune fields.

Drainage

Being an almost rainless area, the Namib has a poorly developed and fragmentary drainage pattern. Water from the interior plateau flows through or into the desert. In the northern half the larger streams reach the sea, but between the Kuiseb and the Orange rivers every stream terminates in a vlei (salt pan or mud flat) against or among the dunes.

A portion of the water of major streams seeps through the sands of the streambeds. The underflow of the Kuiseb River has been tapped 25 miles (40 km) inland to provide water supplies for the towns of Walvis Bay and Swakopmund. A pipeline 80 miles (130 km) long supplies the town of Lüderitz with water from the seepage of the Koichab, a stream that terminates in the dunes. Only the Cunene (Kunene) and Orange rivers flow permanently on the surface. Other streams have surface flow only after heavy rainfall in the interior plateaus; they normally flow for no more than a few days in several years.

Soils

Large areas of the Namib are completely soilless, with bedrock at the surface. Other areas are covered with shifting sand. Soils that do occur are often highly saline, impregnated with gypsum, or cemented firmly by calcium carbonate, the latter forming a calcrete layer just below the surface. Arable soils in the Namib are limited to floodplains and the terraces of major rivers and are subject to occasional inundation.

Climate

The coastal area is almost totally rainless, yet its air is almost always at or near the saturation point. The cold Benguela Current flows northward along the coast, chilling the air above it and thus producing fog. This cool air moves inland as a southwest sea breeze, creating a temperature inversion about 1,000 feet (300 metres) thick, with fog below and hot, dry air above.

At the coast there is little difference in temperature between day and night or between winter and summer. Temperatures are usually between 50 and 60 °F (10 and 16 °C). Along the inland margins, summer temperatures normally reach the upper 80s F (low 30s C). Only in areas sheltered from the cooling sea breeze (lee sides of mountains and bottoms of canyons) do temperatures frequently approach those expected in low-latitude deserts—i.e., in excess of 100 °F (38 °C). Freezing temperatures occur occasionally along the inner edge of the desert. A few days each year, usually in fall or spring, berg (mountain) winds blowing from the east bring high temperatures (above 100 °F), together with dry air and clouds of dust, across the desert to the coast itself. The rare rains occur usually as short-lived torrential thunderstorms.

Average annual precipitation is generally about 0.5 inch (13 mm) at the coast, increasing inland until it reaches 2 inches (51 mm) at the foot of the escarpment. In some years, however, no rain falls at all. Dew, on the other hand, is heavy and for some types of vegetation is more important than the rainfall. In the extreme south, some winter precipitation occurs from frontal storms passing farther south over the Cape region; on rare occasions, snow may fall on the higher southern mountains.

Plant life

Six vegetational regions are found in the Namib: (1) the coastal region, with highly succulent vegetation, which uses moisture derived from the fog, (2) the almost completely barren Outer Namib, (3) the steppes of the Inner Namib, which in many years are barren but which in wet years are covered with short grasses, both annual and perennial, (4) the dunes of the Inner Namib, which produce a surprisingly rich flora of bushes and tall grasses, (5) the larger river channels, along which large trees, particularly acacias, grow, and (6) the southern winter rainfall area, where a succulent bush growth occurs. A curious Namib plant is the tumboa, or welwitschia (Welwitschia mirabilis), whose two gigantic leaves sprawl over the surface of the ground from the crest of its huge root crown (see Welwitschiaceae).

Animal life

The plains and the dunes of the Inner Namib support large numbers of several varieties of antelope, especially gemsbok (oryx) and springbok, as well as ostriches and some zebras. Elephants, rhinoceroses, lions, hyenas, and jackals are found in the northern Namib, especially along the rivers that flow from the interior highlands to the Atlantic. The dunes of the Outer Namib provide habitats for various types of insects and reptiles, especially beetles, geckos, and snakes, but virtually no mammals. The shore area is densely populated by marine birds—notably flamingos, pelicans, and, in the southern part, penguins—as well as a few jackals, some rodents, and a few colonies of seals. Large quantities of guano are scraped annually from the rocks of several offshore islands.