RIFT VALLEY

any elongated trough formed by the subsidence of a segment of the Earth's crust between dip-slip, or normal, faults. Such a fault is a fracture in the terrestrial surface in which the rock material on the upper side of the fault plane has been displaced downward relative to the rock below the fault. A rift valley constitutes a type of tectonic valley and, as such, differs from river and glacial valleys, which are produced by erosional forces. Rift valleys are usually narrow and long, some measuring hundreds of kilometres in length. Their floors are relatively flat, owing in large part to volcanic deposition and marine or lacustrine sedimentation. The sides of rift valleys drop steeply away in the form of steps and terraces. At their margins, the walls of the valleys may rise hundreds of metres. Rift valleys are found both on the continents and on the floor of ocean basins. In terms of the theory of plate tectonics (q.v.), they occur in divergence zones, belts where two of the various lithospheric plates that make up the Earth's surface are separating. Numerous submarine rift valleys have been discovered along the crests of the large ridges that run throughout the Earth's oceans. These ridges are centres of seafloor spreading: areas where magma from the mantle is welling up, cooling to form new oceanic crust, and is moving away from the crests in either direction. The distribution of rift valleys on the continents is irregular and relatively sparse, but they seem to occur at sites of incipient plate spreading. Many have volcanic cones on their floors or contain deep lakes. The most extensive of the continental rift valleys are those of the East African Rift System, which extend northward to the Red Sea and eastward into the Indian Ocean. Other notable examples include the Baikal Rift Valley (Russia) and the Rhine Rift Valley (Germany).