Africa emerged from the middle of the supercontinent called Gondwana, splitting from the land masses that became South America, Australia, India and Antarctica (Figure I.1). The rupture was initiated by massive outpourings of flood basalts, which commenced 183 million years ago (Ma) during the early Jurassic period in what is now southern Mozambique. The volcanic overlay spread inland from there at least as far as south-western Zambia. By 160 Ma, a widening trough separated Africa from eastern Antarctica and Madagascar, filled by the proto-Indian Ocean. In the west, the separation of South America from Africa began with lava eruptions in what is now Namibia, initiated around 123 Ma, and the South Atlantic Ocean began opening. Unencumbered by adjoining land masses, Africa drifted slowly northward, and rotated a little anticlockwise. The location of the equator shifted from the southern Sahara region towards its current middle position, with similar portions of the continent to its north and south. Once the continents eventually halted their drift, South America lay almost 3000 km from the nearest point of Africa, while Australia ended up almost 10,000 km distant on the other side of the Indian Ocean. The Tethys Sea separated Africa from Europe.
Stages of the breakup of the supercontinent called Gondwana. (a) 200 Myr, (b) 160 Myr, (c) 130 Myr, (d) 100 Myr.
(from Storey (1995) Nature 377: 301)
Following its parting from the other southern continents, Africa’s high-lying land surface became progressively worn down by erosion, lowering the hilltops and filling in the valleys. By 66 Ma, when the Cretaceous period ended with the demise of the dinosaurs, a gently undulating plain had been formed over most of the continent. This is known as the African erosion surface. The only mountain ranges lay in the far south and far north. The Cape Fold Mountains were formed during the Permian ~250 Ma, when Africa’s land mass pressed against Antarctica, while the Atlas Range was formed much later where Africa’s drift northward butted against Eurasia. Freed from the adjoining land masses, Africa’s coastal margins tilted upward. With the passage of time, the coastal escarpments became eroded back by as much as 200 km in the east and 50 km in the west. Material removed from the high country accumulated in the Kalahari, Congo and Chad basins and extended shorelines especially in the east and south. Through the interior, low hills emerged where more resistant rocks intruded. Africa’s surface probably resembled Australia, worn down and depleted of nutrients. Fossil deposits preserve remains of the mammal-like reptiles and early dinosaurs that thronged Gondwana, but record little of the later dinosaurs present in Africa during the late Jurassic and Cretaceous periods. The fragments of their bones were mostly swept from the eroding land surface.
This static situation ended around 45 Ma, when a plume from deep within the Earth’s mantle began pushing up in the north-east under present-day Ethiopia, initiating volcanic eruptions. The doming of the land surface caused faults to develop by 25 Ma, which opened the Red Sea and propagated southward through the continent. Massive outpourings of volcanic lava produced layers upon layers of basalt. The raised land surface deflected rain-bearing winds and altered river courses.
This tectonic activity not only reshaped the physical features of the continent; it nurtured the plants and animals that evolved on its surface, including the walking, meat-eating, cerebral and culturally sophisticated ape that became us. The chapters forming this first section of the book establish the interconnected consequences of the tectonic uplift for land forms, climates, rivers and soils. They present largely an east-side story, but expanded coast-to-coast in the south. West-central and western Africa remained mostly low-lying and rather similar to tropical regions of South America, Australia and Asia in their ecology, and thus feature less in the story.
Suggested Further Reading
McCarthy, T; Rubidge, B. (2005) The Story of Earth & Life. A Southern African Perspective on a 4.6-Billion-Year Journey. Struik Nature, Cape Town.
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.