The creation of the East African Rift Valley

As tectonic plates separated, upwelling magma gradually caused the formation of the Ethiopian and the Kenyan domes, each reaching about nine thousand feet above sea level. During this time, the continent was covered by dense rainforest, home to an increasing number of ape species. As the domes grew, rainfall was disrupted, and the eastern forests began to fragment, producing open country and a variety of habitats. Tension grew in the tectonic plates, and massive faulting occurred, resulting in a rift several thousand feet deep which runs three thousand miles from the Red Sea to Mozambique. This resulted in yet more microenvironments and ecological barriers. The whole effect was to act as an engine of evolution. (Leakey, 1995)