AFRICAGEOLOGIC HISTORY

General considerations The African continent essentially consists of five ancient Precambrian cratons (Kaapvaal, Zimbabwe, Tanzania, Congo, and West African) that were formed between about 3.6 and 2.0 billion years ago and that basically have been tectonically stable since that time; these cratons are bounded by younger fold belts formed between 2.0 billion and 300 million years ago. All these rocks have been extensively folded and metamorphosed (that is, they have been modified in composition and structure by heat and pressure). Precambrian rock outcrops appear on some 57 percent of the continent's surface, while the rest of the surface consists of largely undeformed younger sediments and volcanic rocks. The oldest rocks are of Archean age (more than 3.6 to 2.5 billion years old) and are found in the so-called granite-gneiss-greenstone terrains of the Kaapvaal, Zimbabwe, and Congo cratons. They consist of gray, banded gneisses, various granitoids, and rather well-preserved volcanic rocks that show evidence of submarine extrusion (i.e., emission of rock material in molten form) and formation under high temperatures. The rock type komatiite is particularly diagnostic of these volcanic sequences and is almost exclusively restricted to the Archean Eon. The cratons were tectonically stabilized by voluminous granite intrusions toward the end of the Archean and were then covered by clastic sediments, some of which contain economically important gold and uranium deposits (e.g., the Witwatersrand System in South Africa). The Proterozoic Eon (2.5 billion to 570 million years ago) is characterized by the formation of several mobile belts, which are long, narrow zones of strongly deformed and metamorphosed rocks that occur between the cratons and probably resulted from the collision between the cratons due to plate tectonic processes. The oldest mobile belts are found in Archean rocks, such as the Limpopo belt separating the Kaapvaal from the Zimbabwe craton. Younger belts were formed during a continentwide thermotectonic event known as the Eburnian (2.2 to 1.8 billion years ago), which gave rise to the Birimian assemblage in western Africa, the Ubendian assemblage in east-central Africa, and large volumes of rocks in Angola. Still younger belts of the Kibaran thermotectonic event (1.2 billion to 950 million years ago) are found in eastern and southern Africa. The end of the Precambrian was marked by a major event of mobile-belt formation known as the Pan-African episode (c. 950 to 550 million years ago), which generated long fold belts, such as the Mozambique belt along the east coast of Africa, the Damara and Katanga belts extending from Namibia into Congo (Kinshasa) and Zambia, the West Congo belt between Angola and Gabon, the Dahomey-Ahaggar belt between Ghana and Algeria, and the Mauritanide belt from Senegal to Morocco. 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.) 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. Rock types and structural evolution The Precambrian The oldest rocks consist of gneisses, granites, metasediments, and metavolcanic rocks 3.6 to 2.5 billion years old; all are variably deformed and metamorphosed to some degree. The best-preserved assemblages occur in the Kaapvaal and Zimbabwe cratons and contain large deposits of gold and sulfide minerals. The volcanic suites are dominated by basaltic and komatiitic lavas, often interlayered with metasediments and generally referred to as greenstone belts. These are often found together with layered gneisses, or they are intruded by granitoid plutons. Several generations of greenstones have been recognized. The oldest formed about 3.4 billion years ago, the second some 3.0 to 2.9 billion years ago, and the third some 2.7 to 2.6 billion years ago. Some of the oldest traces of life are preserved as unicellular algae in Precambrian cherts of the Barberton greenstone belt in the Transvaal region of South Africa. The end of the Archean is marked by voluminous granite intrusions, after which Africa's cratons became tectonically stable. One of the most spectacular features marking the end of the Archean is the intrusion of the Great Dyke in Zimbabwe, a large, layered body of mafic-ultramafic rocks with substantial deposits of chromium, asbestos, and nickel. It is still not clear whether Archean evolution was characterized by the same plate tectonic processes that are seen today, and there are suggestions that the greenstone belts are remnants of ancient oceanic crust. Cratonic (essentially undeformed) sediments appear in the stratigraphic record for the first time in the late Archean and are best developed in the Kaapvaal craton of southern Africa. The early Proterozoic (2.5 to 1.6 billion years ago) is characterized by cratonic clastic sediments on the stable cratons-the best examples are the Witwatersrand-Ventersdorp-Transvaal basin of southern Africa and the Francevillian basin in Gabon-and by metavolcanic-metasedimentary rocks and granitoids in noncratonic areas such as the extensive Birimian terrain of western Africa extending from Senegal to Ghana. Of particular interest are extensive stromatolite-bearing limestones and economically important iron formations in the Transvaal sequence of South Africa that provide evidence for an oxygen-rich atmosphere by about 2.2 billion years ago. About 2 billion years ago the Bushveld Complex-which is one of the largest differentiated igneous bodies on Earth, containing major deposits of platinum, chromium, and vanadium-was emplaced in the northern Kaapvaal craton. The middle part of the early Proterozoic was dominated by powerful orogenic (mountain-building) processes that gave rise to fold belts in which sedimentary and volcanic rocks originally deposited in deep basins along the continental margins were severely deformed, metamorphosed, intruded by granitoid plutons, and finally uplifted into mountain ranges, probably as a result of continental collision. This Eburnian event was particularly active in western Africa, where it deformed the Birimian assemblages; but it was also active in eastern Africa, where it generated the Ubendian belt in southern Tanzania, and in southwestern Africa, where it formed major rock units in Angola and northern Namibia. By the end of the early Proterozoic, the Archean crustal blocks had grown into cratons of considerable size. The record of the middle Proterozoic (1.6 billion to 900 million years ago) shows deposition of continental sediments and volcanic rocks on the cratons and adjacent to the earlier fold belts (molasse deposits). Undeformed or only mildly folded successions are found in southern Africa (Waterberg and Matsap sequences), in northern Zambia, and in Congo (Kinshasa). Elsewhere, sedimentary and volcanic sequences were deposited in elongate basins that were later subjected to intense deformation and metamorphism during the Kibaran event. This important thermotectonic episode gave rise to the Kibaran-Burundian fold belt in east-central Africa, the Ruwenzori belt in Uganda, and the Namaqua-Natal belt in South Africa and Namibia. The late Proterozoic (900 to 570 million years ago) is again characterized by platform deposits in stable areas, such as the West African craton (Taoudeni and Tindouf basins), the Congo craton, the Kalahari craton (Nama basin of Namibia), and the Tanzania craton (Bukoban beds). Tectonic and magmatic activity was concentrated in mobile belts surrounding these stable areas and took place throughout the late Proterozoic, during the so-called Pan-African thermotectonic event. Long, linear belts-such as the Damara-Katanga of central and southwestern Africa, the Mozambique belt of eastern Africa, and the Dahomey-Ahaggar belt of western Africa-formed during this time, and some of these belts contain diagnostic rock assemblages that indicate that they resulted from continental collisions. Many late Precambrian sequences of Africa contain one or two beds of tillites (sedimentary rocks that are composed of lithified clay and rock sediments produced by the action of ice), which are thought to have resulted from an extensive glaciation that covered much of Africa at this time. In the Arabian (Eastern) Desert of Egypt and in the Red Sea Hills of The Sudan, a predominance of volcanic rocks and granitoids, together with frequent remnants of ancient oceanic crust, document an evolution similar to what is now occurring in the island-arc systems of the southwestern Pacific. These rocks clearly demonstrate that plate tectonic processes operated in the late Precambrian. The economy With the exception of South Africa and the countries of North Africa, all of which have diversified production systems, the economy of most of Africa can be characterized as underdeveloped. Africa as a whole has abundant natural resources, but much of its economy has remained predominantly agricultural, and subsistence farming still engages more than 60 percent of the population. Until the beginning of the 20th century this system of farming relied on simple tools and techniques, as well as on traditional organization of the family or community for its labour. Because of poor transport and communications, production was largely for domestic use. There was little long-distance trade, and wage labour was virtually unknown. The small size and vast heterogeneity of polities at that time also made exchanges very limited. There were, however, notable exceptions, especially in West Africa, where for many centuries societies had engaged in long-distance trade and had elaborate exchange and craft facilities, communications, and a political infrastructure to maintain their trade routes. Africa has experienced considerable economic development during the 20th century; and, while this has provided many benefits, it also has given rise to a number of serious problems. The first significant changes occurred under colonial rule in the first half of the century: wage labour was introduced, transportation and communications were improved, and resources were widely developed in the colonial territories. The legacy of this, however, has been that the export of two or three major agricultural products or minerals-such as peanuts, petroleum, or copper-has come to provide most of the foreign-exchange earnings for nearly all African nations. Fluctuations in the prices of these commodities have made the economies of these countries vulnerable and fragile. The situation has been exacerbated in countries in the marginal dryland zones, where the increasing frequency of drought conditions have undermined agricultural productivity. The second major change was the vigorous promotion of industrial development, often with foreign assistance, that took place in the two decades (1960-80) following the political independence of most African countries. The political fragmentation of the continent, however, also became a major constraint to industrial growth, because it created numerous small markets. Consequently, most African countries have become saddled with excess industrial capacity, coupled with enormous foreign debts that were incurred in large part to build this capacity. In nearly all African countries a poor economic situation has been aggravated by rapid population growth, which has kept per capita gross domestic product low or in some cases has caused it to decline. Thus, any hope for improving economic conditions in most of Africa rests on two factors: population control within individual countries to give their economies the chance to grow; and the organization of groups of states into regional economic blocs in order to create internal markets large enough to sustain growth. Resources Mineral resources Africa's known mineral wealth places it among the world's richest continents. Its very large share of the world's mineral resources includes coal, petroleum, natural gas, uranium, radium, low-cost thorium, iron ores, chromium, cobalt, copper, lead, zinc, tin, bauxite, titanium, antimony, gold, platinum, tantalum, germanium, lithium, phosphates, and diamonds. Major deposits of coal are confined to four groups of coal basins-in southern Africa, North Africa, Congo (Kinshasa), and Nigeria. Proven petroleum reserves in North Africa occur in Libya, Algeria, Egypt, and Tunisia. Exploration has been concentrated north of the Ar-Ahaggar massifs; there may also be major Saharan reserves to the south. The other major oil reserves are in the West African coastal basin-mainly in Nigeria and also in Cameroon, Gabon, and Congo (Brazzaville)-and in Angola. Africa's natural gas reserves are concentrated in basins of North and coastal central Africa. Southern Africa is said to be one of the world's seven major uranium provinces. In South Africa the unusual degree of knowledge of reserves derives from the joint occurrence of uranium with gold, a condition that also decreases the cost of production. Other countries with significant uranium deposits are Niger, Gabon, Congo (Kinshasa), and Namibia.