The Great Rift Valley and East African Highlands

The Great Rift Valley is the surface expression of the East African Rift System, a divergent plate boundary roughly 6,000 km long that runs from the Afar Triple Junction in Ethiopia and Djibouti south through Kenya, Tanzania, and Mozambique. There the Somali plate is pulling away from the larger Nubian (African) plate at a rate of only a few millimeters per year, a process that began in the Miocene epoch some 25 to 30 million years ago and that, if it continues uninterrupted over many millions of years, is expected eventually to open a new ocean basin and separate a Somali sub-continent from the rest of Africa. The rift bifurcates into two principal arms: the Eastern, or Gregory, Rift, which passes through Ethiopia and Kenya and is lined with active and dormant volcanoes, and the Western, or Albertine, Rift, which arcs through Uganda, Rwanda, Burundi, and the eastern Democratic Republic of the Congo and holds the deepest of the rift lakes. Lake Victoria, Africa's largest lake by surface area, sits not within either rift trough but in the shallow sag of land between the two arms, which is why, unlike the narrow, elongated rift lakes, it is broad and comparatively shallow.