rift valley, elongated depression, trough, or graben in the earth's crust, bounded on both sides by normal faults and occurring on the continents or under the oceans. The central flat block forming the trough slips downward relative to the crustal blocks on either side. The appearance is that of a fallen keystone in a broken arch. Rift valleys form by tensional forces, typically those associated with the initiation of plate separation (see plate tectonics). The development of a rift valley in a continent is believed to be a precursor to the breakup of the continent and the development of a new ocean basin by seafloor spreading. Rift valleys, such as the Red Sea and the African rift valleys, are commonly the sites of volcanism and the locus of much earthquake activity.