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The Great Rift Valley
A continent tearing in two, and the trench it leaves behind
Africa is tearing in two, slowly, and this is the seam. The Great Rift Valley is the surface expression of that split: a system of faults and troughs running roughly 6,000 kilometers from the Jordan Valley and the Red Sea south through Ethiopia, Kenya and Tanzania to Mozambique, in places a broad floor walled by escarpments a thousand meters high. Where the crust thins, magma rises, which is why the rift is lined with volcanoes and hot springs. Where the floor drops, water gathers, which is why it holds a chain of lakes from the shallow soda pans of Kenya to the abyssal depths of Tanganyika. In perhaps ten million years, East Africa may be an island.
Geologists now speak of the East African Rift System, with an eastern branch through Kenya and a western branch curving around Lake Victoria, and there is genuine argument about how far north to extend it — the Dead Sea Transform is a different kind of fault, though it is often bundled in. Volcanoes rise along the flanks: Kilimanjaro at 5,895 meters, Mount Kenya, the Virunga chain, and the fiercely active Nyiragongo above Goma. Lake Tanganyika, in the western branch, is the second deepest lake on Earth, and Lake Malawi holds more fish species than any other lake. The Ethiopian Highlands, themselves split by the rift, form the largest continuous highland in Africa.
The rift is also the reason so much of the human story is legible in East Africa. Faulting exposes old sediments interleaved with volcanic ash that can be dated to within a few thousand years, so hominin fossils that would be buried beyond reach anywhere else lie near the surface at Olduvai, Hadar and Turkana — Lucy, Homo habilis, the Laetoli footprints. Today some of the continent's densest farming populations work the volcanic soils of the rift highlands, while the lakes support fisheries feeding millions. The heat beneath the rift also makes power: Kenya draws a large share of its electricity from geothermal fields at Olkaria, an elegant use for a continent's slow disintegration.