Look at a satellite image of East Africa and you will see, running south from Ethiopia through Kenya and Tanzania and into Mozambique, a long string of lakes, volcanoes, and deep valleys that does not look like the rest of the African continent. The string of features is the East African Rift — the southern half of a much longer rift system that runs all the way north through the Red Sea, the Dead Sea valley, and on into the Beqaa valley of Lebanon. This 6,000-kilometre crack in the Earth's surface is the Great Rift Valley, and it is one of the most geologically active places on the planet. The crack is the surface expression of a slow tectonic process in which the African continent is gradually splitting in two: the larger Nubian Plate on the west, the smaller Somali Plate on the east, separating at roughly 6-7 millimetres per year. Given enough time — perhaps 10 to 20 million years — the East African Rift will widen into a new ocean basin and the Horn of Africa will become a continent of its own. The process is far enough along to be measured today and far enough off that no living person will see its completion.
The Length
The full Great Rift Valley is sometimes drawn from the Beqaa Valley in Lebanon, through the Jordan Valley, the Dead Sea (the lowest point on land at -430 metres), the Gulf of Aqaba, the Red Sea, the Afar Triangle in Ethiopia, and southward through the East African Rift to the Limpopo River valley on the Mozambique border. The total length is around 6,000 kilometres. The Red Sea section is geologically the most advanced, with the rift already wide enough to be filled with ocean water and the seafloor producing new oceanic crust at a slow but measurable rate. The East African portion is the active continental rift section, where the original continental crust is still in the process of being pulled apart but has not yet been thin enough to allow seawater to flood in.
The Eastern and Western Branches
The East African Rift is not a single linear feature but a system of two roughly parallel branches that wrap around the high plateau of central Africa. The Eastern Rift (sometimes called the Gregory Rift after the British geologist who first mapped it) runs through Ethiopia, Kenya, and Tanzania, containing Lake Turkana, Lake Naivasha, Lake Magadi, and (further south) Lake Eyasi. The Western Rift runs through Uganda, Rwanda, Burundi, Tanzania, Zambia, Malawi, and Mozambique, containing some of the largest and deepest lakes in the world — Lake Albert, Lake Edward, Lake Kivu, Lake Tanganyika, and Lake Malawi. The two branches converge near the Tanzanian border and connect with the Malawi Rift in the south. The high plateau between the two branches contains lakes that are not in the rift system, including Lake Victoria — which despite its central African location is not a rift lake but a depression created by uplift of the surrounding terrain.
The Volcanoes
The rifting process produces extensive volcanism. The Ethiopian highlands include several active and dormant volcanoes (Erta Ale, with its persistent lava lake, is one of the most visited; Dallol, with its surface hot springs and salt formations, is among the world's strangest landscapes). The Kenyan rift contains the still-active Suswa and Longonot volcanoes plus dozens of smaller cones, and the heavily eroded volcanic mountain Mount Kenya. In northern Tanzania, the rift cluster includes Ol Doinyo Lengai (the only volcano in the world that erupts natrocarbonatite lava, which is black when fresh and turns white within hours of cooling) and Mount Kilimanjaro (Africa's highest mountain, technically just outside the rift but produced by the same processes). The Western Rift includes the Virunga volcanoes between Rwanda, Uganda, and the Democratic Republic of Congo, several of which are still active and have been the source of major recent eruptions including the 2002 destruction of Goma by Nyiragongo.
The Lakes
The deep, elongated lakes of the rift system are some of the most biologically and physically remarkable bodies of water on Earth. Lake Tanganyika, in the Western Rift, is the world's second-deepest lake (1,470 metres maximum depth), the world's second-largest by volume, and the longest freshwater lake on Earth (660 kilometres). Lake Malawi (Lake Nyasa) is similarly long and deep. Lake Kivu, smaller but deeper than its surface area would suggest, has a unique feature — its deeper layers contain enormous volumes of dissolved methane and carbon dioxide produced by underwater volcanic activity, which could in principle erupt catastrophically if the lake's thermal stratification ever becomes unstable. The Kivu basin's methane is being slowly extracted commercially for power generation, partly to reduce the long-term explosion risk. Lake Turkana in northern Kenya, also called the Jade Sea for its colour, is alkaline rather than fresh and is the largest desert lake in the world. Each of the major rift lakes has produced extraordinary biological radiation events; Lake Malawi alone is thought to contain over 1,000 endemic cichlid fish species.
Human Evolution
The Great Rift Valley is the area where most of the major fossil discoveries of human evolution have been made. The hominid family — including modern humans and our extinct relatives — emerged in this region, and the rift's combination of active sediment deposition (in lake basins and volcanic ash falls) and active erosion (in the river systems cutting through those sediments) has produced ideal conditions both for fossils to form and for them to be exposed for discovery several million years later. The Olduvai Gorge in northern Tanzania, in the Eastern Rift, has been a particularly productive site since the 1930s and is associated with the Leakey family's discoveries of Homo habilis, Paranthropus boisei, and various early Homo erectus specimens. The Afar region of Ethiopia, in the northern rift, has produced "Lucy" (Australopithecus afarensis, 3.2 million years old) and many other key specimens. The Lake Turkana basin in northern Kenya has produced specimens of Homo ergaster and Kenyanthropus platyops. Most of what is known about human evolutionary biology comes, directly or indirectly, from rift valley fossils.
The Afar Depression
The northernmost section of the East African Rift, the Afar Depression in northeast Ethiopia, is one of the few places on Earth where active continental rifting can be observed at the surface. The depression sits 100 metres below sea level in places (which is why the Danakil region has its famous salt deposits) and is geologically the site where three plates meet — the African continent, the Arabian Peninsula, and the Indian Ocean basin. The active rifting in this region has produced visible new fissures during eruption events; in 2005, a single rifting event in the Dabbahu volcanic complex opened a series of new cracks across the landscape that totalled about 60 kilometres of new fault length in a few days. The region is geologically extremely active. It is also extreme in every other way: temperatures regularly exceed 50°C, rainfall is essentially zero, and the Danakil salt mines have been producing for over a thousand years from one of the harshest workable environments on Earth.
The Geological Future
The East African Rift is producing the early stages of what will eventually be a new ocean basin. The current spreading rate is approximately 6-7 millimetres per year — slow by tectonic standards but consistent and continuous over the past several million years. At that rate, the continental crust under the rift will become thin enough for seawater to flood in through the existing connection to the Red Sea within approximately 5-10 million years. The Horn of Africa — Ethiopia, Somalia, Eritrea, Djibouti, parts of northern Kenya — will then separate from the rest of the continent as a new minor continent. The Western Rift, with its different geometry, may produce a separate spreading axis or may stall as a "failed rift" similar to the Mississippi embayment in North America. The geological future of the region is the only place on Earth where the early stages of a new ocean basin formation can be observed on land.
Modern Settlement
The rift valley contains some of the most agriculturally productive and densely populated land in Africa. The Kenyan section of the Eastern Rift includes the Mau Escarpment and the surrounding highlands, which were the original "White Highlands" of British colonial Kenya and are still among the country's most productive agricultural areas. The Ethiopian highlands east and west of the rift are among the most densely settled rural areas on the continent. The Western Rift valleys around Lakes Edward and Albert, in Uganda and the eastern DR Congo, are also densely settled, despite the area being one of the most politically unstable in Africa. The combination of fertile volcanic soils, regular rainfall (from the rift's elevation), and abundant water (from the lakes) makes the rift system one of the most productive zones on the continent, even as the underlying geological processes that create those conditions occasionally produce eruptions, earthquakes, and landslides.