The East Arican Rift System is a massive rift zone spanning east Africa from the Afar Depression, by the Red Sea, south and then into the ocean near Mozambique. The massive rift valley is produced by a geological process in which the African Plate is gradually coming apart in this region. Ultimately two plates will become separate entities here: the Nubian Plate to the west, and the Somali Plate to the east.
In geological terms, a rift occurs at a place where the Earth's crust is being pulled apart. Characteristic consequences are a massive rift valley between the two plates, as well as an unusual level of volcanic activity. Both these features are true of the East African Rift System. The system includes both historically active but now dormant volcanoes. Mt. Kilimanjaro, the Ol Doinyo Lengai volcano (still active), Tanzania's Crater Highlands, Mt. Kenya, the Menengai Crater are all major landmarks lying along the rift. The Menegai Crater of Kenya, despite its name, is actually a vast shield volcano, the slopes of which are now farmland.
The most striking feature of the rift, however, is its rift valley, the Great Rift Valley. The Great Rift Valley is thousands of miles long, stretching from the Middle East to Mozambique, although geologists now recognize that only part of it (that lying south of the Afar Depression) belongs to the East African Rift System. (The northern part, in the Middle East, is part of a separate rift system called the Dead Sea Transform).
As the East African Rift slowly reworks the surface of the Earth, it has produced one further consequence. This one which is archaeological rather than geological, but is perhaps the most important reason that many people recognize the name of the rift. The deep valley and gorges produced by geological activity here have exposed to the surface layers which lay buried for hundreds of thousands and even millions of years. This makes the land ideal for fossil hunters - and, because it's eastern Africa, particularly ideal for those searching for evidence of theoretical human ancestors. Lucy, the first skeleton of the Australopithecus afarensis species, was found to the north, in the Afar Depression. Part of the rift system includes the Olduvai Gorge, where decades of archaeological research has unearthed evidence of early tool-using cultures. The name of this gorge was given to the stone tools used by some cultures in the Lower Paleolithic, the so-called "Oldowan" tool set.