Africa (Encyclopedia of Environmental Issues, Revised Edition)
The image of the African continent containing primarily dense tropical jungles holds true for limited regions only—those near the equator and subject to heavy seasonal rainfall. In fact, Africa has several different environmental regions. These correspond mainly, but not only, to relative latitudinal position north or south of the equator. Geographers generally separate the continent into five zones: North, South, East, West, and Central. The environmental conditions that are found in the zones vary widely.
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General Environmental Characteristics (Encyclopedia of Environmental Issues, Revised Edition)
In comparison to other continents, Africa possesses relatively few major mountain ranges. The most important tectonic upliftings are in South Africa—notably the Drakensberg range, which is 1,000 kilometers (620 miles) long, with elevations reaching in places nearly 3,500 meters (11,500 feet)—and in the northwestern and northeastern areas of the continent (the Aurès and Atlas mountains in Tunisia and Morocco, and the Simyen and Bale mountains in Ethiopia). The geology of the Ethiopian ranges, which boast peaks such as Ras Dashen in the Simyens, more than 4,500 meters (14,800 feet) high, and many volcanic peaks, is closely connected with the tectonic phenomena that created the Great Rift Valley running north to south from the Red Sea to Malawi in Southern Africa.
Whereas these long ranges contribute to extensive regional environmental patterns—combining patterns of flora and fauna that determine the livelihoods of entire sections of Africa—other quite different mountains (mostly volcanic “monoliths”) lend themselves to more localized environmental, and therefore human cultural, settings. Prototypical examples would be Mount Kenya and Mount Kilimanjaro in Tanzania (the latter being the highest mountain in Africa—reaching almost 6,000 meters, or 19,700 feet) and Mount Cameroon near the Gulf of Guinea coast in West Africa. Mount Kenya became the site of one of Africa’s earliest national...
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Environmental Issues (Encyclopedia of Environmental Issues, Revised Edition)
Among the environmental issues affecting Africa are concerns about declining numbers and even extinctions of indigenous plant and animal species. The list of menaced African species ranges from the cichlid tropical fish populations in East Africa’s freshwater lakes through the African elephant.
Various factors have caused a sharp decline in what was once a thriving habitat for more than five hundred species of cichlids in Lake Victoria. It is estimated that almost half of the cichlids have disappeared as the result of the decision to introduce Nile perch into the lake waters to increase local fishing harvests for food and commercial sale. The nonnative perch proved to be predatory and fed off the local cichlid populations.
There are many other examples of rising concerns among environmentalists who study declining numbers of animal species that were once very numerous in different areas of Africa. Those that have drawn the greatest attention are the large mammals, including mountain gorillas, elephants, and members of the “great cat” family (lions, leopards, and cheetahs). Questionable human actions—including poaching for food or trophies—are generally behind the threats to these species.
By the end of the twentieth century, organized conservation efforts were on the rise in Africa. The obvious importance of saving large game species led, for example, to “safari tourism” in protected parks....
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Further Reading (Encyclopedia of Environmental Issues, Revised Edition)
Doyle, Shane. Crisis and Decline in Bunyoro. London: British Institute in Eastern Africa, 2006.
Goldblatt, Peter, ed. Biological Relationships Between Africa and South America. New Haven, Conn.: Yale University Press, 1993.
Maddox, Gregory H. Sub-Saharan Africa: An Environmental History. Santa Barbara, Calif.: ABC-CLIO, 2006.
Shorrocks, Bryan. The Biology of African Savannahs. New York: Oxford University Press, 2007.
United Nations Environment Programme. Africa: Environment Outlook. Nairobi: Author, 2006.
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Africa (Encyclopedia of Science)
Africa is the world's second largest continent, encompassing an area of 11,677,240 square miles (30,244,051 square kilometers), including offshore islands. Recognized as the birthplace of the human race and of many other animal and plant species, it also possesses the world's richest and most concentrated deposits of minerals such as gold, diamonds, uranium, chromium, cobalt, and platinum.
Origin of Africa
Geologically, Africa is 3.8 billion years old (Earth is 4.6 billion years old). Present-day Africa, occupying one-fifth of Earth's land surface, is the central remnant of the ancient southern supercontinent called Gondwanaland, a landmass once made up of South America, Australia, Antarctica, India, and Africa. This massive supercontinent broke apart between 195 million and 135 million years ago, split by the same geological forcesontinental drifting, earthquakes, volcanoshat continue to transform Earth's crust today.
Africa has fewer high peaks than any other continent and few extensive mountain ranges. The major ranges are the Atlas Mountains along the northwest coast and the Drakensberg Mountains in South Africa. The highest point on the continent is Kibo (19,340 feet/5,895 meters), a peak of Mount Kilimanjaro in northeast Tanzania. Despite its location near the equator, the peak is...
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Africa (World of Earth Science)
From the perspective of geologists and paleontologists, Africa takes center stage in the physical history and development of life on Earth. Africa is the world's second largest continent. Africa possesses the world's richest and most concentrated deposits of minerals such as gold, diamonds, uranium, chromium, cobalt, and platinum. It is also the cradle of human evolution and the birthplace of many animal and plant species, and has the earliest evidence of reptiles, dinosaurs, and mammals.
Present-day Africa, occupying one-fifth of Earth's land surface, is the central remnant of the ancient southern super-continent called Gondwanaland, a landmass once made up of South America, Australia, Antarctica, India, and Africa. This massive supercontinent broke apart between 195 million and 135 million years ago, cleaved by the same geological forces that continue to transform Earth's crust today.
Plate tectonics are responsible for the rise of mountain ranges, the gradual drift of continents, earthquakes, and volcanic eruptions. The fracturing of Gondwanaland took place during the Jurassic Period, the middle segment of the Mesozoic Era when dinosaurs flourished on Earth. It was during the Jurassic that flowers made their first appearance, and dinosaurs like the carnivorous Allosaurus and plant eating Stegasaurus lived.
Geologically, Africa is 3.8 billion years old, which means that in its present form or joined with other continents as it was in the past, Africa has existed for four-fifths of Earth's 4.6 billion years. Africa's age and geological continuity are unique among continents. Structurally, Africa is composed of five cratons (structurally stable, undeformed regions of Earth's crust). These cratons, in south, central, and west Africa are mostly igneous granite, gneiss, and basalt, and formed separately between 3.6 and 2 billion years ago, during the Precambrian Era.
The Precambrian, an era which comprises more than 85% of the planet's history, was when life first evolved and the earth's atmosphere and continents developed. Geochemical analysis of undisturbed African rocks dating back 2 billion years has enabled paleoclimatologists to determine that Earth's atmosphere contained much higher levels of oxygen than today.
Africa, like other continents, "floats" on a plastic layer of Earth's upper mantle called the asthenosphere. The overlying rigid crust or lithosphere can be as thick as 150 mi (240 km) or under 10 mi (16 km), depending on location. The continent of Africa sits on the African plate, a section of the earth's crust bounded by mid-oceanic ridges in the Atlantic and Indian Oceans. The entire plate is creeping slowly toward the northwest at a rate of about 0.75 in (2 cm) per year.
The African plate is also spreading or moving outward in all directions, and therefore Africa is growing in size. Geologists state that sometime in the next 50 million years, East Africa will split off from the rest of the continent along the East African rift which stretches 4,000 mi (6,400 km) from the Red Sea in the north to Mozambique in the south.
Considering its vast size, Africa has few extensive mountain ranges and fewer high peaks than any other continent. The major ranges are the Atlas Mountains along the northwest coast and the Cape ranges in South Africa. Lowland plains are also less common than on other continents.
Geologists characterize Africa's topography as an assemblage of swells and basins. Swells are rock strata warped upward by heat and pressure, while basins are masses of lower lying crustal surfaces between swells. The swells are highest in East and central West Africa where they are capped by volcanic flows originating from the seismically active East African rift system. The continent can be visualized as an uneven tilted plateau, one that slants down toward the north and east from higher elevations in the west and south.
During much of the Cretaceous Period, from 130 million to 65 million years ago, when dinosaurs like tyrannosaurus, brontosaurus, and triceratops walked the earth, Africa's coastal areas and most of the Sahara Desert were submerged underwater. Global warming during the Cretaceous Period melted polar ice and caused ocean levels to rise. Oceanic organic sediments from this period were transformed into the petroleum and natural gas deposits now exploited by Libya, Algeria, Nigeria, and Gabon. Today, oil and natural gas drilling is conducted both on land and offshore on the continental shelf.
The continent's considerable geological age has allowed more than enough time for widespread and repeated erosion, yielding soils leached of organic nutrients but rich in iron and aluminum oxides. Such soils are high in mineral deposits such as bauxite (aluminum ore), manganese, iron, and gold, but they are very poor for agriculture. Nutrient-poor soil, along with deforestation and desertification (expansion of deserts) are just some of the daunting challenges facing African agriculture in modern times.
The most distinctive and dramatic geological feature in Africa is undoubtedly the East African rift system. The rift opened up in the Tertiary Period, approximately 65 million years ago, shortly after the dinosaurs became extinct. The same tectonic forces that formed the rift valley and which threaten to eventually split East Africa from the rest of the continent have caused the northeast drifting of the Arabian plate, the opening of the Red Sea to the Indian Ocean, and the volcanic uplifting of Africa's highest peaks including its highest, Kilimanjaro in Tanzania. Mount Kibo, the higher of Kilimanjaro's two peaks, soars 19,320 ft (5,796 m) and is permanently snowcapped despite its location near the equator.
Both Kilimanjaro and Africa's second highest peak, Mount Kenya (17,058 ft; 5,117 m) sitting astride the equator, are actually composite volcanos, part of the vast volcanic field associated with the East African rift valley. The rift valley is also punctuated by a string of lakes, the deepest being Lake Tanganyika with a maximum depth of 4,708 ft (1,412 m). Only Lake Baikal in Eastern Russia is deeper at 5,712 ft (1,714 m).
Seismically, the rift valley is very much alive. Lava flows and volcanic eruptions occur about once a decade in the Virunga Mountains north of Lake Kivu along the western stretch of the rift valley. One volcano in the Virunga area in eastern Zaire which borders Rwanda and Uganda actually dammed a portion of the valley formerly drained by a tributary of the Nile River, forming Lake Kivu as a result.
On its northern reach, the 4,000-mi (6,400-km) long rift valley separates Africa from Asia. The rift's eastern arm can be traced from the Gulf of Aqaba separating Arabia from the Sinai Peninsula, down along the Red Sea, which divides Africa from Arabia. The East African rift's grabens (basins of crust bounded by fault lines) stretch through the extensive highlands of central Ethiopia which range up to 15,000 ft (4,500 m) and then along the Awash River. Proceeding south, the rift valley is dotted by a series of small lakes from Lake Azai to Lake Abaya and then into Kenya by way of Lake Turkana.
Slicing through Kenya, the rift's grabens are studded by another series of small lakes from Lake Baringo to Lake Magadi. The valley's trough or basin is disguised by layers of volcanic ash and other sediments as it threads through Tanzania via Lake Natron. However, the rift can be clearly discerned again in the elongated shape of Lake Malawi and the Shire River Valley, where it finally terminates along the lower Zambezi River and the Indian Ocean near Beira in Mozambique.
The rift valley also has a western arm which begins north of Lake Mobutu along the Zaire-Uganda border and continues to Lake Edward. It then curves south along Zaire's eastern borders forming that country's boundaries with Burundi as it passes through Lake Kivu and Tanzania by way of Lake Tanganyika.
The rift's western arm then extends toward Lake Nysasa (Lake Malawi). Shallow but vast, Lake Victoria sits in a trough between the rift's two arms. Although the surface altitude of the rift valley lakes like Nyasa and Tanganyika are hundreds of feet above sea level, their floors are hundreds of feet below due to their great depths.
The eastern arm of the rift valley is much more active than the western branch, volcanically and seismically. There are more volcanic eruptions in the crust of the eastern arm with intrusions of magma (subterranean molten rock) in the middle and lower crustal depths. Geologists consider the geological forces driving the eastern arm to be those associated with the origin of the entire rift valley and deem the eastern arm to be the older of the two.
It was in the great African rift valley that hominids, or human ancestors, arose. Hominid fossils of the genus Australopithicus dating 3 million years ago have been unearthed in Ethiopia and Tanzania. And the remains of a more direct ancestor of man, Homo erectus, who was using fire 500,000 years ago, have been found in Olduvai Gorge in Tanzania as well as in Morocco, Algeria, and Chad.
Paleontologists, who study fossil remains, employ radioisotope dating techniques to determine the age of hominid and other species' fossil remains. This technique measures the decay of short-lived radioactive isotopes like carbon and argon to determine a fossil's age. This is based on the radioscope's atomic half-life, or the time required for half of a sample of a radioisotope to undergo radioactive decay. Dating is typically done on volcanic ash layers and charred wood associated with hominid fossils rather than the fossils themselves, which usually do not contain significant amounts of radioactive isotopes.
Present-day volcanic activity in Africa is centered in and around the East African rift valley. Volcanoes are found in Tanzania at Oldoinyo Lengai and in the Virunga range on the Zaire-Uganda border at Nyamlagira and Nyiragongo. There is also volcanism in West Africa. Mount Cameroon (13,350 ft; 4,005 m) along with smaller volcanos in its vicinity, stand on the bend of Africa's West Coast in the Gulf of Guinea, and are the exception. They are the only active volcanoes on the African mainland not in the rift valley.
However, extinct volcanoes and evidence of their activity are widespread on the continent. The Ahaggar Mountains in the central Sahara contain more than 300 volcanic necks that rise above their surroundings in vertical columns of 1,000 ft or more. Also, in the central Sahara, several hundred miles to the east in the Tibesti Mountains, there exist huge volcanic craters or calderas. The Trou au Natron is 5 mi (8 km) wide and over 3,000 ft (900 m) deep. In the rift valley, the Ngorongoro Crater in Tanzania, surrounded by teeming wildlife and spectacular scenery, is a popular tourist attraction. Volcanism formed the diamonds found in South Africa and Zaire. The Kimberly diamond mine in South Africa is actually an ancient volcanic neck.
The only folded mountains in Africa are found at the northern and southern reaches of the continent. Folded mountains result from the deformation and uplift of the earth's crust, followed by deep erosion. Over millions of years this process built ranges like the Atlas Mountains, which stretch from Morocco to Algeria and Tunisia.
Geologically, the Atlas Mountains are the southern tangent of the European Alps, geographically separated by the Strait of Gibraltar in the west and the Strait of Sicily in the east. The Atlas are strung across northwest Africa in three parallel arrays; the coastal, central, and Saharan ranges. By trapping moisture, the Atlas Mountains carve out an oasis along a strip of northwest Africa compared with the dry and inhospitable Sahara Desert just to the south.
The Atlas Mountains are relatively complex folded mountains featuring horizontal thrust faults and ancient crystalline cores. On the other hand, the Cape ranges are older, simpler structures, analogous in age and erosion to the Appalachian Mountains of the eastern United States. The Cape ranges rise in a series of steps from the ocean to the interior, flattening out in plateaus and rising again to the next ripple of mountains.
For a continent of its size, Africa has very few islands lying off its coast. The major Mediterranean islands of Corsica, Sardinia, Sicily, Crete, and Cyprus owe their origins to the events that formed Europe's Alps, and are a part of the Eurasian plate, not Africa. Islands lying off Africa's Atlantic Coast like the Canaries, Azores, and even the Cape Verde Islands near North Africa are considered Atlantic structures. Two islands in the middle of the South Atlantic, Ascension and St. Helena, also belong to the Atlantic. Islands belonging to Equatorial Guinea as well as the island country of Sao Tome and Principe at the sharp bend of Africa off of Cameroon and Gabon are related to volcanic peaks of the Cameroon Mountains, the principal one being Mount Cameroon.
Madagascar, the world's fourth largest island after Greenland, New Guinea, and Borneo, is a geological part of ancient Gondwanaland. The island's eastern two-thirds are composed of crystalline igneous rocks, while the western third is largely sedimentary. Although volcanism is now quiescent on the island, vast lava flows indicate widespread past volcanic activity. Madagascar's unique plant and animal species testify to the island's long separation from the mainland.
Marine fossils, notably tribolites dating from the Cambrian Period (50570 million years ago; the first period of the Paleozoic Era) have been found in southern Morocco and Mauritania. Rocks from the succeeding period, the Ordovician (50025 million years ago) consist of sandstones with a variety of fossilized marine organisms; these rocks occur throughout northern and western Africa, including the Sahara.
The Ordovician Period was characterized by the development of brachiopods (shellfish similar to clams), corals, starfish, and some organisms that have no modern counterparts, called sea scorpions, conodonts, and graptolites. At the same time, the African crust was extensively deformed. The continental table of the central and western Sahara was lifted up almost a mile (1.6 km). The uplifting alternated with crustal subsidings, forming valleys that were periodically flooded.
During the Ordovician Period, Africa, then part of Gondwanaland, was situated in the southern hemisphere on or near the South Pole. It was toward the end of this period that huge glaciers formed across the present-day Sahara and the valleys were filled by sandstone and glacial deposits. Although Africa today sits astride the tropics, it was once the theater of the Earth's most spectacular glacial activity. In the next period, the Silurian (42595 million years ago), further marine sediments were deposited.
The Silurian was followed by the Devonian, Mississippian, and Pennsylvanian Periods (40886 million years ago), the time interval when insects, reptiles, amphibians, and forests first appeared. A continental collision between Africa (Gondwanaland) and the North American plate formed a super-supercontinent (Pangaea) and raised the ancient Mauritanide mountain chain that once stretched from Morocco to Senegal. During the late Pennsylvanian Period, layer upon layer of fossilized plants were deposited, forming seams of coal in Morocco and Algeria.
When Pangaea and later Gondwanaland split apart in the Cretaceous Period (1446 million years ago), a shallow sea covered much of the northern Sahara and Egypt as far south as the Sudan. Arabia, subjected to many of the same geological and climatic influences as northern Africa, was thrust northward by tectonic movements at the end of the Oligocene and beginning of the Miocene Epochs (around 30 million years ago). During the Oligocene and Miocene (55 million years ago; segments of the modern Cenozoic Era) bears, monkeys, deer, pigs, dolphins, and early apes first appeared.
Arabia at this time nearly broke away from Africa. The Mediterranean swept into the resulting rift, forming a gulf that was plugged by an isthmus at present-day Aden on the Arabian Peninsula and Djibouti near Ethiopia. This gulf had the exact opposite configuration of today's Red Sea, which is filled by waters of the Indian Ocean.
As the Miocene Epoch ended about five million years ago, the isthmus of Suez was formed and the gulf (today's Red Sea) became a saline (salty) lake. During the Pliocene (1.6 million years ago) the Djibouti-Aden isthmus subsided, permitting the Indian Ocean to flow into the rift that is now the Red Sea.
In the Pleistocene Epoch (11,000.6 million years ago), the Sahara was subjected to humid and then to dry and arid phases, spreading the Sahara desert into adjacent forests and green areas. About 5,000,000 years ago in the post glacial period of the modern epoch, the Holocene, a further succession of dry and humid stages, further promoted desertification in the Sahara as well as the Kalahari in southern Africa.
Earth scientists state the expansion of the Sahara is still very much in evidence today, causing the desertification of farm and grazing land and presenting the omnipresent specter of famine in the Sahel (Saharan) region.
Africa has the world's richest concentration of minerals and gems. In South Africa, the Bushveld Complex, one of the largest masses of igneous rock on Earth, contains major deposits of strategic metals such as platinum, chromium, and vanadiumetals that are indispensable in tool making and high tech industrial processes. The Bushveld complex is about 2 billion years old.
Another spectacular intrusion of magmatic rocks composed of olivine, augite, and hypersthene occurred in the Archean Eon over 2.5 billion years ago in Zimbabwe. Called the Great Dyke, it contains substantial deposits of chromium, asbestos, and nickel. Almost all of the world's chromium reserves are found in Africa. Chromium is used to harden alloys, to produce stainless steels, as an industrial catalyst, and to provide corrosion resistance.
Unique eruptions that occurred during the Cretaceous in southern and central Africa formed kimberlite pipesertical, near-cylindrical rock bodies caused by deep melting in the upper mantle. Kimberlite pipes are the main source of gem and industrial diamonds in Africa. Africa contains 40% of the world's diamond reserves, which occur in South Africa, Botswana, Namibia, Angola, and Zaire.
In South Africa, uranium is found side-by-side with gold, thus decreasing costs of production. Uranium deposits are also found in Niger, Gabon, Zaire, and Namibia. South Africa alone contains half the world's gold reserves. Mineral deposits of gold also occur in Zimbabwe, Zaire, and Ghana. Alluvial gold (eroded from soils and rock strata by rivers) can be found in Burundi, Côte d'Ivoire, and Gabon.
As for other minerals, half of the world's cobalt is in Zaire and a continuation into Zimbabwe of Zairian cobalt-bearing geological formations gives the former country sizable reserves of cobalt as well. One quarter of the world's aluminum ore is found in a coastal belt of West Africa stretching 1,200 mi (1,920 km) from Guinea to Togo, with the largest reserves in Guinea.
Major coal deposits exist in southern Africa, North Africa, Zaire, and Nigeria. North Africa is awash in petroleum reserves, particularly in Libya, Algeria, Egypt, and Tunisia. Nigeria is the biggest petroleum producer in West Africa, but Cameroon, Gabon, and the Congo also contain oil reserves. There are also petroleum reserves in southern Africa, chiefly in Angola.
Most of Africa's iron reserves are in western Africa, with the most significant deposits in and around Liberia, Guinea, Gabon, Nigeria, and Mauritania. In West Africa as well as in South Africa where iron deposits are also found, the ore is bound up in Precambrian rock strata.
Africa, like other continents, has been subjected to gyrating swings in climate during the Quartenary Period of the last 2 million years. These climatic changes have had dramatic affects on landforms and vegetation. Some of these cyclical changes may have been driven by cosmic or astronomical phenomena including asteroid and comet collisions.
But the impact of humankind upon the African environment has been radical and undeniable. Beginning 2,000 years ago and accelerating to the present day, African woodland belts have been deforested. Such environmental degradation has been exacerbated by overgrazing, agricultural abuse, and man-made changes, including possible global warming partially caused by the buildup of man-made carbon dioxide, chlorofluorocarbons (CFCs), and other greenhouse gases.
Deforestation, desertification, and soil erosion pose threats to Africa's man-made lakes and thereby Africa's hydroelectric capacity. Africa's multiplying and undernourished populations exert ever greater demands on irrigated agriculture, but the continent's water resources are increasingly taxed beyond their limits. To stabilize Africa's ecology and safeguard its resources and mineral wealth, many earth scientists argue that greater use must be made of sustainable agricultural and pastoral practices. Progress in environmental and resource management, as well as population control is also vital.
See also Earth (planet)