Rift Valley (Great Rift Valley) Steep-sided, flat-floored valley in sw Asia and e Africa. It runs from n Syria, through the Jordan Valley and the Dead Sea, and then continues as the trough of the Red Sea through e Africa to the lower valley of the Zambezi River in Mozambique. Dotted along its course are a number of significant lakes, including Tanganyika and Turkana. Length: c.6400km (4000mi).

rift valley Depression formed by the subsidence of land between two parallel faults. Rift valleys are believed to be formed by thermal currents within the Earth's mantle that break up the crust into large slabs or blocks of rock, which then become fractured.