Chapter 4
Because much of Africa’s high interior constitutes an eroding surface, the bedrock geology must be taken into account in interpreting its ecology. This brings into play distinctions between granitic (‘felsic’) and volcanic (‘mafic’) formations on soil features. Sedimentary deposits are limited in their extent mostly to South Africa and the extension of Kalahari sands in the west. The Congo Basin has accumulated fluvial (river-washed) sediments, but is peripheral to the story.
The important geochemical feature of the bedrock is its silica content, contributing to the granular texture of the soils formed (Box 4.1). Silica-rich granite and sandstone yields sandy soils of low intrinsic fertility. Volcanically derived basalts as well as mudstones produce finely textured soils that are inherently richer in mineral nutrients. Furthermore, the clay content retains the mineral nutrients supporting plant growth against the forces of leaching. The consequent soil fertility underlies a functional subdivision within Africa’s savannas between places with lots of herbivores, and regions where herbivores are locally concentrated in current or former wetlands. Geology can become somewhat complex. In relation to ecology, the summary outline presented in Box 4.1 is adequate. Silica-rich rocks are typically pale pink or brown in colour while volcanic rocks are generally dark brown or reddish (Figure 4.1).
Box 4.1Rock Types
The products of bedrock weathering forming soils are dependent on how the rock was formed as well as its geochemical composition. Igneous rocks originate from the cooling of molten magma, either deep beneath the land surface or following surface eruptions. The mineral content and rate of cooling of the molten material influence the grain or crystal size. Felsic rocks have high contents of quartz (silica oxide) and feldspar (alumino-silicates) and are thus coarsely crystalline, especially if formed deep underground (‘plutonic’). They include forms of granite, typically light grey or pinkish in colour because of the low iron content (Table 4B.1). Gneiss is a metamorphosed (melted and recrystallised) form of granite or other rocks, sometimes showing banding because specific minerals have separated. Felsic rocks are labelled ‘acidic’, because the soils they form typically show a low pH.
Table 4B.1Classification of rock types based on their texture or grain size and mineral composition, particularly of silica relative to iron and magnesium minerals
Mafic rocks have high contents of magnesium and iron silicates (pyroxene) and are labelled ‘basic’. Mafic rocks are intrinsically dark grey in colour due to their high iron oxide content, but weather to reddish brown on their surfaces. Basalt is formed when mafic lava is extruded on the land surface and cools rapidly, producing fine-grained crystals. Dolerite (or diabase) is similar in mineral composition to basalt, but is formed intrusively in fissures or cracks beneath the land surface, generating somewhat coarser crystals. Gabbro, produced by slow cooling of mafic lava deep underground, is still more coarse-textured. Volcanic substrates containing greater amounts of silica give rise to rhyolite or granodiorite. Phonolite, also of volcanic origin, is intermediate in its chemical composition between felsic and mafic. Very ancient (‘Archean’) volcanic material that has subsequently been metamorphosed yields greenstone. This is especially rich in magnesium and hence is labelled ‘ultramafic’. Banded ironstone is derived from alternating precipitates of iron oxide and silica formed in ancient oceans, before there was much oxygen in the atmosphere. Granite can vary quite widely in its mineral composition, affecting the texture of the soils formed.
Sedimentary rocks are generated where erosion deposits have been compressed under pressure. The texture of the sedimentary source distinguishes pebbly conglomerates, coarse sandstones, finer-grained siltstones and mudstones or shales. Limestone and dolomite (also called dolostone) are derived from marine sediments that are rich in calcium from shell fragments. Sedimentary deposits may become metamorphosed by melting under extreme pressure followed by recrystallisation. Sandstone is thereby transformed into quartzite. Granites, gneisses and allied igneous rocks constitute the basement shield upon which more recent sedimentary rocks and volcanic material have been deposited.
Figure 4.1
Basic contrasts in geological substrates. (A) Felsic granitic–gneiss pinky-beige in colour; (B) mafic dolerite dark brown in colour.
(both photos from South African Lowveld region near Kruger NP)
I became aware of geological influences on ecology during my doctoral research within the Hluhluwe-iMfolozi Park located in northern KwaZulu-Natal of South Africa. The escarpment foothills in this region are mostly underlain by shale and sandstone layers derived from sediments deposited while Africa was part of Gondwana, representing the Karoo Supergroup. Locally, dolerite sills intrude, derived from the feeder pipes conveying molten basalt to the surface via the fissure eruptions that initiated the break up of Gondwana. The underlying geology affected not only the kinds of trees and grasses that grew, but also where white rhinos were most likely to be found.
The Basement Shield
Granites, gneisses and allied igneous or metamorphosed rocks form the basement shield upon which more recent sedimentary rocks and volcanic material have been deposited. These plutonic rocks formed originally deep beneath the Earth’s surface have become exposed by erosion over many millions of years, initiated by the break up of Gondwana and renewed by subsequent uplift during the Pliocene and Pleistocene.1 The rocks themselves were formed between 3.6 and 2 billion years ago, initially taking the form of Archean greenstones and banded ironstones formed before there was much oxygen in the Earth’s atmosphere. Basement granitic rocks constitute the prevalent geological substrate from northern parts of South Africa through Zimbabwe, Zambia and southern Tanzania into sections of Kenya and Uganda, as well as through much of western Africa (Figure 4.2). Archean greenstone and ironstone outcrop in the Witwatersrand ridges, Barberton mountain-land adjoining Swaziland, Great Dyke running through Zimbabwe and near Lake Victoria in eastern Africa. The Bushveld Igneous Complex, formed about 2 billion years ago, fills an extensive basin situated north of the Witwatersrand. It is mined for platinum and chrome. It is rimmed in the south by quartzite forming the Magaliesberg range of hills, which separates Highveld grassland from the bushveld region to the north. Dipping deep beneath the Witwatersrand watershed are the conglomerate layers, formed in ancient stream channels 2.8 billion years ago, which have yielded most of South Africa’s gold.
Figure 4.2
Surface geology map of Africa. Notable features are (i) widespread presence of basement (‘acid’ or ‘metamorphic’) igneous rocks, (ii) local prevalence of volcanic intrusions in the rift valley region extending from Ethiopia into northern Tanzania, and again in pockets in southern Africa, (iii) geological complexity within eastern Africa, (iv) sedimentary deposits forming the Karoo Supergroup surrounding the volcanic Drakensberg basalt in South Africa, and (e) deposits of wind-blown Kalahari sand extending over a vast area in the west.
(from Jones et al. (2013) Soil Atlas of Africa)
Sedimentary Formations
Most of South Africa is underlain by rocks formed from sediments laid down in the interior basin north of the Cape Fold Mountains, during the time around 250 Ma when Africa was situated centrally within Gondwana (Figure 4.2). The various layers form the Karoo Supergroup. Tillite of glacial origin lies at the base, testifying to the former polar location of the subcontinent. Above it are the layers deposited during the Permian period when the mammal-like reptiles (or therapsids) were the predominant big animals. The coal beds it contains, derived from swampy vegetation, are mined. The sequence upwards from mudstones through shales and sandstones reflects the progressive dryness that prevailed through the Triassic into the early Jurassic. The sequence was terminated by the eruption of the Drakensberg lavas. However, around Johannesburg more ancient formations have been exposed as a result of the continental uplift. They include dolomites deposited on a sea floor 2.5 billion years ago, which generated the caves or sinkholes accumulating fossils in the ‘Cradle of Humankind’ proclaimed as a World Heritage Site.
Permian sediments allied with the Karoo Supergroup are also found in western Zimbabwe where coal is mined. They are present locally in Zambia’s Luangwa Valley and from north-eastern Malawi to the Rukwa Basin in southern Tanzania. In north-west Zambia and adjoining parts of Congo DRC, copper is mined where ancient sandstone, shale and limestone formations outcrop. Sediments of Permian age extend again from the coastal region of Kenya near Mombasa into adjoining Ethiopia. Sedimentary deposits of Jurassic age containing dinosaur fossils occur in the Tendaguru formation in southern Tanzania and in the Zambezi and Limpopo valleys.
Unconsolidated aeolian (wind-blown) or colluvial (water-washed) sands that were eroded from the high-lying interior accumulated in the Kalahari Basin in the west. They cover a vast area extending from northern South Africa through Angola, western Zimbabwe and Zambia as far north as the Bateke Plateau in Congo-Brazzavillle and adjoining Gabon (Figure 4.2). Fluvial (water-born) sediments fill the Congo Basin. The coastal plain extending from southern Mozambique into northern KwaZulu-Natal in South Africa has been formed by sand of Cretaceous or later origin deposited by the Limpopo and Zambezi rivers. Sediments of Miocene or later age, containing fossils covering the crucial periods in human and mammalian evolution, are found mostly within or adjoining the East African Rift System. The fossil-bearing sediments of Olduvai Gorge and Laetoli in Tanzania to the south of the Serengeti plains were laid down in an ancient lake basin to the west of the highlands bordering the Eastern Rift.2
Volcanic Intrusions
Volcanic deposits associated with rift valley formation are prominent within an area extending from Ethiopia as far south as Tanzania to the east of Lake Victoria (Figure 4.2). In Ethiopia, volcanic eruptions commenced around 30 Ma during the Oligocene, building up the broad basalt platform underlying the Simien Mountains.3,4 The thickness of the volcanic deposit there originally approached 3000 m, but has been reduced by erosion.5 To the south-east, the Bale Mountains were formed by lava flows over a base of sedimentary sandstone and limestone. The Afar Depression to the east is covered by volcanic lavas of Quaternary age up to 1500 m thick.6 Lava flows had spread into the Samburu region of central Kenya by 15 Ma in the early Miocene. The Aberdare Range adjoining the Eastern Rift in central Kenya was formed by basaltic lava erupted during the late Miocene, followed by further eruptions in the Pliocene and late Pleistocene.2 Further south in Kenya, phonolite lavas dated to ~13 Ma occur around Nairobi and border the Yatta Plateau in Tsavo East NP. Basaltic eruptions that took place merely 150 years ago feature in the Chyulu Hills and adjoining regions of Tsavo West NP (Figure 4.3D). Mount Longonot, situated within Kenya’s rift valley, spewed volcanic ash only 100 years ago. In northern Tanzania, volcanic deposits span an east–west distance exceeding 200 km near the southern extremity of the Eastern Rift, from mounts Kilimanjaro and Meru in the east to west of the Ngorongoro highlands. Mount Oldoinyo Lengai still periodically spews volcanic ash towards the nearby Serengeti Plains. Lavas associated with both rift valley arms are notable chemically for their high sodium and carbonatite contents plus unusually low silica, making them exceptionally alkaline.2
Along the Western Rift, Mount Nyamuragira has erupted 40 times since 1885, while Mount Nyiragongo erupted 34 times during the same period. Remnant cones exemplify recent volcanic activity on the Ugandan side of the Congo border within the Queen Elizabeth (formerly Rwenzori) NP. Rwanda is mostly covered by volcanic material emanating from Western Rift volcanoes. In western Africa, a region of ongoing volcanic activity extends from the Cameroon highlands to islands in the Gulf of Guinea. Northern Nigeria has remnants of ancient lava flows intruding through basement granite on the Jos and Bui plateaus. In Guinea, dolerite and gabbro sills penetrate the predominant sandstone on the Fouta-Djallon highlands.
In South Africa, basalt derived from fissure eruptions initiated 183 Ma covers quite a small area, because most has been eroded away (Figure 4.2). Nevertheless, the massive outpouring of lava left a spectacular feature along the border between South Africa and Lesotho. Cliffs forming the Drakensberg/Maloti escarpment rise almost 1000 m vertically (Figure 4.3A). Further remnants of this volcanic mantle persist in the Lebombo range of hills running along South Africa’s border with Mozambique, in the Springbok flats north of Pretoria, and from south of Victoria Falls in Zimbabwe into western Zambia. In Namibia, the Etendeka lavas that erupted preceding the split with South America occur in the north-west. Basalt lies beneath the Kalahari sand cover through much of Botswana and adjoining parts of northern Namibia, Angola and Zambia.7 Widely prevalent, but not shown on the continental map, are numerous dolerite dykes and sills, remnants of feeder pipes supplying the surface-erupted lavas. They are especially prominent in the Karoo region of South Africa (Figure 4.3C). Intrusions of gabbro, much earlier than the basalt in their origin, occur in the eastern highlands of Zimbabwe and in a strip of the Lowveld extending into the Kruger NP in South Africa. Even more ancient Ventersdorp lavas dating back more than 2.7 billion years outcrop in the Witwatersrand region.
Figure 4.3
Volcanic overlays. (A) Basalt ramparts of the Drakensberg escarpment in South Africa; (B) basalt traps forming the Simien Mountains, Ethiopia (photo: Craig R. Sholley); (C) outcropping columnar dolerite in Karoo region, South Africa (photo: Trishya Owen-Smith); (D) recent basalt lava flow in Tsavo West NP, Kenya, with Mount Kilimanjaro vaguely discernible behind.
The influence of the rift valley volcanoes extends well beyond the surface area that they cover. Those in the Ngorongoro highlands contributed to the formation of the Serengeti plains, and their influence on soils can be detected over 100 km to the west. In southern Africa, most of the basalt may be gone, but its legacy in the form of dolerite dykes remains widespread and contributes to local heterogeneity in soils and vegetation. Volcanic material is absent from Malawi through southern Tanzania in south-central Africa.
Overview
Basement granitic substrates form the bedrock over much of Africa. Sedimentary deposits of Kalahari sand occur extensively through the south-west, while much of southern Africa is underlain by sedimentary layers formed in the Karoo basin. Volcanic intrusions are most widespread in the north-east, from Ethiopia southwards in association with rift valley faults. South Africa retains a more ancient legacy of volcanism in the form of the dolerite intrusions representing feeder pipes to the flood basalts associated with the breakup of Gondwana. Volcanic eruptions continue along the Western Rift and occurred quite recently elsewhere in eastern Africa.
Compared with the Pacific Rim and southern Europe, the extent of the volcanic influence in Africa is quite trivial. Nevertheless, it is more widespread than in the other two southern continents. Basalt contemporaneous with the Etendeka lavas in Namibia covers parts of the Parana plateau of Brazil and adjoining Uruguay, but most of the Brazilian plateau is underlain by sandstone. To the south, the land cover in the Argentinian pampas is mainly windblown silt (or loess) originating from the Andean volcanics. Parts of Patagonia are covered by volcanic lavas, tufts and ashes of late Cretaceous age. Lavas produced by volcanoes that erupted during the Plio–Pleistocene cover sections of the high interior plateau of central Mexico. An active mantle plume underlies the high plateau of Yellowstone NP with its geysers and hot springs. In Australia, basalt eruptions dated to 60 Ma underlie parts of the eastern tablelands. The Deccan traps (‘stairs’) in west-central India were formed by multiple layers of flood basalts more than 2000 m deep covering a vast area. They date back to the end of the Cretaceous 66 Ma, intriguingly synchronous with the terminal extinctions of the dinosaurs. Much of central Eurasia and North America is covered by windblown deposits of loess generated by continental glaciation during the Pleistocene, highly fertile but divorced from the underlying geology.
Although volcanic intrusions are limited in their extent in Africa, their consequences for large herbivores and hence human evolution were profound through their effects on soil fertility, as will be described in the next chapter.
SUGGESTED FURTHER READING
McCarthy, T; Rubidge, BS. (2005) The Story of Earth and Life. A Southern African Perspective on a 4.6-Billion-Year Journey. Struik, Cape Town.
REFERENCES
1.Furon, R. (1963) Geology of Africa. Oliver and Boyd, London.
2.Scoon, RN, et al. (2018) Geology of National Parks of Central/Southern Kenya and Northern Tanzania. Springer, Cham.
3.Macgregor, D. (2015) History of the development of the East African Rift System: a series of interpreted maps through time. Journal of African Earth Sciences 101:232–252.
4.Partridge, TC. (2010) Tectonics and geomorphology of Africa during the Phanerozoic. In Werdelin, L; Sanders, WJ (eds) Cenozoic Mammals of Africa. University of California Press, Berkeley, pp. 3–17.
5.Asrat, A. (2016) The Ethiopian highlands. In Viljoen, R (ed.) Africa’s Top Geological Sites. Struik, Cape Town, pp. 197–205.
6.Asrat, A. (2016) The Danakil depression of Ethiopia. In Viljoen, R (ed.) Africa’s Top Geological Sites. Struik, Cape Town, pp. 189–196.
7.Haddon, IG. (2004) The Sub-Kalahari Geology and Tectonic Evolution of The Kalahari Basin, Southern Africa. University of the Witwatersand, Johannesburg.