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.