Friday, July 20, 2007
Africa — Earth Science paradise
Africa — Earth Science paradise - Geo-Currents a Look at Recent Geological News
Lewis D. Ashwal
Rocks & Minerals, Jan-Feb, 2002 by Lewis D. Ashwal
Ex Africa semper aliquid novi. (There is always something new out of Africa.) --Pliny the Elder (AD 23-79)
Geologists and other earth scientists who live and work in Africa often rationalize their current place of residence in terms of the spectacularity of African geology. Some of these features are discussed here; their locations are shown in the accompanying digital elevation image of Africa (fig. 1). Another aspect that stimulates many relates to the large tracts of Africa that represent unstudied or poorly known land. Here, first-order scientific discoveries can still be made by those who are prepared to deal with the real (or perceived) impediments to geoscience work in the Third World. In many cases, this "risk" turns out to be imaginary, and scientifically minded adventurers who come here find themselves returning repeatedly. It is in Africa where critical data exist that spawned the theories of continental drift (Du Toit 1937), the origins of life (Schopf 1975), and the emergence of humans (Berger and Hilton-Barber 2000) and their subsequent migrations out of Africa (Stringer and McKie 1996). More recently, the superb rock exposures in Namibia and elsewhere have led to the idea of the "snowball earth" (Hoffman et al. 1998).
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Africa is the second-largest continent, occupying more than 20 percent of Earth's land surface. There are extreme variations in topography and climate, from the Sahara Desert (the world's largest, at more than 9 million square kilometers), to the equatorial tropical rain forests, to the snow-capped volcanoes of the East African Rift (fig. 2). Many visitors to Johannesburg are surprised to find themselves at an elevation of over 1,700 meters, which is higher than that of Denver. Topographically, the African continent appears to be tilted toward the northwest (fig. 1); the reasons for the unusually high elevations of southern Africa are not well understood and are currently being discussed in terms of deep-mantle processes (Gurnis 2001).
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Africa's deserts (fig. 2) are spectacular in their own right but have recently yielded some surprising discoveries, including meteorites. The Sahara  (numbers in brackets are keyed to locations on figs. 1 and 2) is now recognized as the world's second-largest meteorite repository (after Antarctica); it has yielded thousands of new specimens, including several stones from the moon and Mars. In the Namib Desert, an iron meteorite said to be the world's largest (~60 tons) can be visited in situ at Hoba , and specimens from the huge (30,000 square kilometers) Gibeon strewn field southeast of Windhoek  continue to be widely distributed. The inventory of Namibian meteorites has more than doubled in the past twenty-five years and include the recovery of some very large and extraordinary specimens (Ashwal 2001).
The Nile Delta , at the terminus of the world's longest river, is familiar to most people, but few are aware of the Okavango Delta (or, more precisely, alluvial fan) (fig. 3) of Botswana . Here, a wetland basin with perennial swamps is situated in the middle of the Kalahari Desert (McCarthy 1993). This geological peculiarity owes its existence to a downthrown graben, an extension of the East African Rift System, which has trapped the Okavango River. Game viewing here is as good as or better than in the Serengeti of Kenya and Tanzania.
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The East African Rift System (Morley 1999), the world's largest (>3,700 kilometers long) and perhaps best example of continental-rifting processes, has created some of the most spectacular horst and graben landscapes on Earth; fault scarps with >4,000 meters of relief are not uncommon (e.g., in the Ruwenzori Mountains of Uganda) . Near-continuous volcanism has been occurring for at least the past 30 million years and continues to produce a wide variety of alkaline lavas and other volcanic products. Well-known composite volcanoes include the snow-capped Mount Kenya (5,199 meters), right on the equator , and Kilimanjaro (Africa's highest point, at 5,895 meters) , to the south in Tanzania. Of particular interest is the remarkable Oldoinyo Lengai volcano (fig. 4) of northern Tanzania , in which carbonatitic lavas composed almost entirely of sodium carbonate can frequently be seen erupting (Bell and Keller 1995).
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In the Late Carboniferous (~330 Ma [million years ago]), when Africa was part of the Gondwana supercontinent, its southern tip was near the South Pole. As Gondwana drifted northward and began to fragment into the present continental configurations, a superb record of tectonically linked climate change was being formed in sedimentary rocks of the Karoo Basin (Smith, Ericksson, and Botha 1993) in southern Africa . Starting with the glacial conditions that formed tillites of the well-known Dwyka Formation, through the warm, wet swamps that generated enormous coal deposits of Permo-Carboniferous age, to the arid desert environments of the Triassic, the Karoo is one of the world's best-preserved foreland basins. These rocks have yielded some astonishing fossils, including huge dinosaurs and their mammal-like reptile precursors. Karoo sedimentation culminated in the mid-Jurassic with a vast outpouring of basalt and emplacement of related dolerite sills and dikes, with a collective volume of at least 1.5-2 million cubic kilometers (Cox 1988). This classic flood basalt province seems to have formed almost instantaneously at 183 Ma. Concurrently, and up until the early Eocene (~53 Ma), southern Africa was punctured by thousands of kimberlite pipes (Gurney 1990). The diamonds recovered directly from the pipes, as well as those transported fluvially into offshore regions, have sustained the economies of South Africa, Botswana, Namibia, and Angola.
Africa contains the largest area of Precambrian rocks of any continent. It is here that the concept of ancient, stable cratons, surrounded by younger mobile belts formed during collisional events, was fruitfully applied on a continental scale. Surprisingly, although Africa's Archean cratons have been studied for many decades, and have been shown to be repositories of huge ore deposits, the state of their first-order characterization--for example, by precise geochronology--lags far behind that of their cratonic counterparts in Canada and Australia. This is especially true not only for the huge West African, Congo, and Tanzania Cratons, but also for the better-studied Kaapvaal and Zimbabwe Cratons (collectively referred to as the Kalahari Craton) to the south. In partial response to this, the Kaapvaal Craton Project was recently initiated (James et al. 2001), in which state-of-the-art geophysics, coupled with studies of surface geology as well as deep lithospheric samples provided by xenoliths in kimberlites, are addressing questions about the deep structure, composition, and stability of cratonic roots. This has obvious relevance to the origin of, and exploration for, diamonds.
The oldest rock yet discovered in Africa (3,644 [+ or -] 4 Ma; Compston and Kroner 1988) occurs in Swaziland, as part of the Ancient Gneiss Complex . Nearby, the Barberton Greenstone Belt  contains some of the world's best-preserved Archean volcanic and sedimentary rocks (~3.45 Ga [billion years old]), including the famous komatiites of Spinifex Stream (Dann 2000). Some argue that the Belingwe Greenstone Belt (fig. 5) of Zimbabwe  displays an even better record of early Earth processes, including spectacular unconformities, basaltic pillow lavas that look like they formed yesterday, and glasses of komatiitc composition. Some of Earth's earliest bacterial life forms are preserved in Barberton cherts. African greenstone belts (De Wit and Ashwal 1997) are peppered with mines exploiting the rich deposits of gold and base metals associated with the mafic lavas and intrusives. Archean greenstone belts, in various states of preservation and exposure level, can also be found in Tanzania, Kenya, Uganda, Sierra Leone, Liberia, Ivory Coast, and Mauritania.
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Mobile belts of various ages seem to have stitched Africa's Archean cratons together by Himalayan-style continental collisions. With an age of about 2.7 Ga, the Limpopo Belt  (Van Reenen et al. 1992), located astride the boundary between Zimbabwe and South Africa, is arguably one of the world's oldest and best-exposed examples of such a collisional zone. Here, the deep crustal roots of the collision between the Kaapvaal and Zimbabwe Cratons are exposed at the surface (fig. 6). The Mozambique Belt, or East African Orogen (Stern 1994), of Late Proterozoic (or "Pan African") age, consists of variably deformed rocks that occur in a huge tract between southern Mozambique, through Malawi, Tanzania, Kenya, Ethiopia, and beyond; obvious correlatives have been found in Madagascar, India, Australia, and Antarctica. These rocks are continuing to provide clues to the collisional events involved in the construction of Gondwana. Many other mobile belts of various ages crisscross the African cratonic nuclei--these are all ripe for study.
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Some of the world's classic and largest igneous complexes occur in Africa. The latest Archean (2.57 Ga) Great Dyke of Zimbabwe  is 480 kilometers long, averages 8 kilometers wide, and consists of layered ultramafic rocks in four coalescent lopolithic subcomplexes (Wilson 1996). The 2.06-Ga Bushveld Complex of South Africa  is the world's largest layered mafic intrusion. It is exposed over 60,000 square kilometers and consists of layered cumulate rocks to 10 kilometers thick (Eales and Cawthorn 1996). Bushveld and Great Dyke represent the world's largest repositories of magmatic ore deposits, including platinum group elements, chromium, and vanadium. The ~1.4-Ga Kunene Complex (fig. 7) is the world's largest massif-type anorthosite complex (Ashwal and Twist 1994); it is exposed over ~15,000 square kilometers in southem Angola and northern Namibia . Other African superlatives include the world's largest (originally ~300 kilometers in diameter, now eroded to a diameter of ~70 kilometers) and oldest (2.02 Ga) meteorite impact crater at Vredefort  (Reimold and Gibson 1996) and the world's largest and most productive deposits of gold (the Witwatersrand Basin ) and manganese (the Kalahari manganese field ), all in South Africa (Wilson and Anhaeusser 1998).
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As if this spectacular geology were not enough, some of the world's most renowned mineral and fossil specimens can be found in southern Africa (Cairncross and Dixon 1995; MacRae 1999). The Tsumeb mine in Namibia (Hoal 1992; see Cook, Nicolson, and Bruce, this issue) is arguably the most famous mineral locality on earth, and the Kalahari manganese field (see Gutzmer and Cairncross, next issue) does not lag far behind. The Triassic mammal-like reptile fossils (see Rubidge and Hancox, this issue) provide critical evidence in the evolutionary succession between reptiles and primitive mammals. The hominid finds at Olduvai Gorge in Kenya, the Sterkfontein caves near Johannesburg, and the Taung child near Kimberley are now legendary. The African continent is clearly a place where those with a passionate interest in Earth, at a level ranging from the professional to the casual, can feel comfortable. The prospects for making new, important discoveries are high. Your visits and ideas are welcomed.
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Lewis D. Ashwal Department of Geology School of Geosciences University of the Witwatersrand Private Bag 3 Wits, 2050 Johannesburg, South Africa LDA@cosmos.wits.ac.za
Lewis D. Ashwal is a professor in the Department of Geology at the University of the Witwatersrand.
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