Great Rift Valley

Earth's outer shell is made of giant moving pieces.

Here, the pieces are slowly moving apart. The land between them sinks down, making a deep valley. Hot melted rock comes up and makes volcanoes.

A rift valley forms where the land is being pulled apart. Britannica explains that a long strip of Earth's crust sinks down between two sets of cracks called faults, leaving steep walls on each side. The East African rift has been forming for about 30 million years, as Africa and Arabia split apart.

The US Geological Survey (USGS) explains that when the crust stretches too far, cracks open and melted rock rises through them, sometimes erupting as volcanoes. That is how giant volcanoes such as Kilimanjaro and Mount Kenya formed near the rift. The USGS says that if the spreading continues, the Horn of Africa could one day split off as a big island, with a new ocean in between.

Britannica defines a rift valley as an elongated trough formed by the subsidence of a crustal segment between dip-slip (normal) faults; such valleys occur in divergence zones where lithospheric plates separate, often with volcanic cones or deep lakes on their floors, and those of the East African Rift System are the most extensive continental examples. The system has formed over some 30 million years as Africa and Arabia separated, with extensive volcanism producing massifs such as Kilimanjaro and Mount Kenya.

The USGS notes that spreading has already separated Arabia from Africa to form the Red Sea, that the African and Arabian plates meet in a triple junction where the Red Sea meets the Gulf of Aden, and that a new spreading center may be developing along the East African Rift Zone: as continental crust is stretched, tension cracks form, magma rises and the rift widens. If spreading continues, the plates meeting at the edge of Africa may separate completely, letting the Indian Ocean flood in and turning the Horn of Africa into a large island.

Scientists dig for very old bones in the rift.

They have found bones of early humans and our ancient relatives. These finds help us learn where people came from.

The rocks of the Great Rift Valley hold some of the most important fossils for understanding the human story. Around Lake Turkana, at a place called Koobi Fora, scientists have been finding fossils since 1969. UNESCO says they have found remains of five kinds of humans and human relatives there, including Homo habilis, which lived about 2 million years ago, and Homo erectus.

The fossils also show that the climate changed: petrified forests show the area was once moist woodland and grassland before it became the hot desert it is today.

UNESCO's Lake Turkana listing records some one hundred archaeological and paleontological sites, petrified forests and fossil-rich sediments. At Koobi Fora, north of Allia Bay, finds since 1969 include Paranthropus boisei and Homo habilis, evidence of a relatively intelligent hominin about two million years ago; remains of five species, Australopithecus anamensis, Homo habilis/rudolfensis, Paranthropus boisei, Homo erectus and Homo sapiens, come from one locality. UNESCO credits the Koobi Fora deposits with contributing more to understanding paleo-environments than any other site on the continent.

The petrified forests record a climatic shift from moist forest and grassland to today's hot desert. UNESCO lists more than 350 bird species and mammals such as zebra, gazelle, lion and cheetah in the Turkana parks today.

The Rift Valley has steep cliffs and hot, dry places.

Stay back from cliff edges. Drink water and wear a hat. Watch wild animals from a car or with a guide.

Many visitors enjoy the rift from viewpoints on its edge, where the land drops steeply to the valley floor, so stay behind barriers and well back from cliff edges. Down on the floor, places such as Lake Turkana are hot and dry, so carry plenty of water and protect yourself from the sun.

The lakes are home to large animals such as crocodiles, hippos and rhinos, so watch wildlife from a vehicle or with a trained guide, and follow park rules. Near hot springs and geysers, stay on the marked paths.

The rift's escarpments drop hundreds to thousands of meters, more than 2,700 m at the Gikuyu and Mau escarpments, so viewpoints call for caution at edges. Its floor includes hot, arid country around Lake Turkana, which UNESCO describes as an arid landscape often devoid of life, and geothermal features such as the hot springs and geysers of the Kenya lakes, where visitors should stay on marked paths.

Large animals, including Nile crocodiles, hippopotamuses, rhinos and big cats, live in the protected areas, so visits should follow park rules, and use vehicles or trained guides. Sites such as Koobi Fora are scientific heritage: look, photograph and leave fossils where they lie.