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 الدرع الأفريقي African Shield

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مُساهمةموضوع: الدرع الأفريقي African Shield   الثلاثاء أبريل 13, 2010 9:27 am


African Shield, or Afro-Arabian Shield, or Arabian-Nubian Massif, or
Arabian-Nubian Shield, or Arabian Shield, or Ethiopian Shield (geology)



In antiquity the Greeks are said to have called the continent Libya and the Romans to have called it Africa, perhaps from the Latin aprica (“sunny”) or the Greek aphrike (“without cold”). The name Africa, however, was chiefly applied to the northern coast of the continent, which was, in effect, regarded as a southern extension of Europe. The Romans, who for a time ruled the North African coast, are also said to have called the area south of their settlements Afriga, or the Land of the Afrigs—the name of a Berber community south of Carthage.

The second largest continent, after Asia, covering about one-fifth of the total land surface of the Earth. The continent is bounded on the west by the Atlantic Ocean, on the north by the Mediterranean Sea, on the east by the Red Sea and the Indian Ocean, and on the south by the mingling waters of the Atlantic and Indian oceans.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

Africa's total land area is approximately 11,724,000 square miles (30,365,000 square km), and the continent measures about 5,000 miles (8,000 km) from north to south and about 4,600 miles (7,400 km) from east to west. Its northern extremity is Al-Ghiran Point, near Al-Abyad Point (Cape Blanc), Tunisia; its southern extremity is Cape Agulhas, South Africa; its farthest point east is Xaafuun (Hafun) Point, near Cape Gwardafuy (Guardafui), Somalia; and its western extremity is Almadi Point (Pointe des Almadies), on Cape Verde (Cap Vert), Senegal. In the northeast, Africa was joined to Asia by the Sinai Peninsula until the construction of the Suez Canal. Paradoxically, the coastline of Africa—18,950 miles (30,500 km) in length—is shorter than that of Europe, because there are few inlets and few large bays or gulfs. The African Shield, sometimes called the Ethiopian Shield, extends eastward to include western Saudi Arabia and the eastern half of Madagascar.

Off the coasts of Africa a number of islands are associated with the continent. Of these Madagascar, one of the largest islands in the world, is the most significant. Other smaller islands include the Seychelles, Socotra, and other islands to the east; the Comoros, Mauritius, Réunion, and other islands to the southeast; Ascension, St. Helena, and Tristan da Cunha to the southwest; Cape Verde, the Bijagós Islands, Bioko, and São Tomé and Príncipe to the west; and the Azores and the Madeira and Canary islands to the northwest.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

The continent is cut almost equally in two by the Equator, so that most of Africa lies within the tropical region, bounded on the north by the Tropic of Cancer and on the south by the Tropic of Capricorn. Because of the bulge formed by western Africa, the greater part of Africa's territory lies north of the Equator. Africa is crossed from north to south by the prime meridian (0° longitude), which passes a short distance to the east of Accra, Ghana.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

The whole of Africa can be considered as a vast plateau rising steeply from narrow coastal strips and consisting of ancient crystalline rocks. The plateau's surface is higher in the southeast and tilts downward toward the northeast. In general the plateau may be divided into a southeastern portion and a northwestern portion. The northwestern part, which includes the Sahara (desert) and that part of North Africa known as the Maghrib, has two mountainous regions—the Atlas Mountains in northwestern Africa, which are believed to be part of a system that extends into southern Europe, and the Ahaggar (Hoggar) Mountains in the Sahara. The southeastern part of the plateau includes the Ethiopian Plateau, the East African Plateau, and—in eastern South Africa, where the plateau edge falls downward in a scarp—the Drakensberg range.

One of the most remarkable features in the geologic structure of Africa is the East African Rift System, which lies between 30° and 40° E. The rift itself begins northeast of the continent's limits and extends southward from the Ethiopian Red Sea coast to the Zambezi River basin.

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]
[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]


Africa contains an enormous wealth of mineral resources, including some of the world's largest reserves of fossil fuels, metallic ores, and gems and precious metals. This richness is matched by a great diversity of biological resources that includes the intensely lush equatorial rainforests of Central Africa and the world-famous populations of wildlife of the eastern and southern portions of the continent. Although agriculture (primarily subsistence) still dominates the economies of most African countries, the exploitation of these resources has become the most significant economic activity in Africa in the 20th century.

Climatic and other factors have exerted considerable influence on the patterns of human settlement in Africa. While some areas appear to have been inhabited more or less continuously since the dawn of humanity, enormous regions—notably the desert areas of northern and southwestern Africa—have been largely unoccupied for prolonged periods of time. Thus, although Africa is the second largest continent, it contains only about 10 percent of the world's population and can be said to be underpopulated. The greater part of the continent has long been inhabited by black peoples, but in historic times there also have occurred major immigrations from both Asia and Europe. Of all foreign settlements in Africa, that of the Arabs has made the greatest impact. The Islamic religion, which the Arabs carried with them, spread from North Africa into many areas south of the Sahara, so that many western African peoples are now largely Islamized.

A unique late Precambrian evolution is recorded in the so-called Arabian-Nubian Shield of northeastern Africa and Arabia. There, large volumes of volcanic and granitoid rocks were generated in an island-arc, marginal-basin setting—an environment similar to that of the present southwestern Pacific Ocean. Rocks were accreted onto the ancient African continent, the margin of which was then near the present Nile River, by subduction processes identical to those observed today. (Subduction involves the descent of the edge of one lithospheric plate beneath that of another where two such plates collide.)

[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]

The interiors of the ancient cratons were not affected by the above tectonic events, and intracratonic sedimentary and volcanic sequences accumulated in large basins. The most important of these are the Transvaal basin on the Kaapvaal craton that contains economically important iron ore deposits; the Congo basin; and the West African basin, with its thick late-Proterozoic sediments including a prominent tillite horizon that marks a major glaciation event at the end of the Precambrian.

After the Precambrian, Africa's geologic history is characterized by the following events: the formation of fold belts in the Paleozoic Era (570 to 245 million years ago) in South Africa (the Cape fold belt), Morocco (the Anti-Atlas belt), and Mauritania (the Mauritanide belt) bordering the older cratons; voluminous basaltic volcanism some 230 to 200 million years ago in South Africa, Namibia, and East Africa, known as the Karoo System, that was probably related to the beginning of the breakup of the Gondwana supercontinent; the formation of a young mountain belt in northwestern Africa some 100 to 40 million years ago as a result of collision between the African and European plates, together with the closure of the ancestral Mediterranean Sea (the Tethys Sea); and the development of the East African Rift System during and after the Tertiary Period (i.e., the last 66.4 million years), leading to the opening of the Red Sea, the northeast drift of the Arabian Plate, and the fracturing of the ancient crust of Africa along several long rift valleys, accompanied by extensive volcanism.






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