rift valley An elongate trough, of regional extent, bounded by two or more faults. Many rifts on land are associated with alkaline volcanicity and, because their margins are uplifted, many are starved of clastic sediments and so contain lakes; the E. African rift system is an outstanding example. Some rifts are thought to be at the embryonic stage of ocean development of the Wilson cycle, whilst others may become ‘failed rifts’ (or ‘failed arms’) and fill with sediment to become aulacogens. The rift valley developed along the axis of slow-spreading oceanic ridges is known as the median valley (or axial rift or axial trough) and is associated with the production of basaltic magmas. Tibetan rifts form at the end of the Wilson cycle as a result of the northward indentation of India into Asia and the spreading of the thickening Tibetan crust. Graben (the German word for ‘ditch’) can be used synonymously for ‘rift valley’ and also for an infilled, fault-bounded trough of any size, with or without topographic expression.

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 An elongate trough of regional extent, bounded by two or more faults. Many rifts on land are associated with volcanic activity and many contain lakes. The East African rift system is an outstanding example. Some rifts are thought to be at the embryonic stage of ocean development of the Wilson cycle; others may become ‘failed rifts’ (or ‘failed arms’) and fill with sediment to become aulacogens. The rift valley developed along the axis of a slow-spreading oceanic ridge is known as the median valley (or axial rift or axial trough). ‘Graben’ (the German word for ‘ditch’) can be used synonymously for ‘rift valley’ and also for an infilled, fault-bounded trough of any size.

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.

rift val·ley • n. a large elongated depression with steep walls formed by the downward displacement of a block of the earth's surface between nearly parallel faults or fault systems.