<P> A rift valley is a linear - shaped lowland between several highlands or mountain ranges created by the action of a geologic rift or fault . A rift valley is formed on a divergent plate boundary, a crustal extension, a spreading apart of the surface, which is subsequently further deepened by the forces of erosion . When the tensional forces were strong enough to cause the plate to split apart, a center block dropped between the two blocks at its flanks, forming a graben . The drop of the center creates the nearly parallel steeply dipping walls of a rift valley when it is new . That feature is the beginning of the rift valley, but as the process continues, the valley widens, until it becomes a large basin that fills with sediment from the rift walls and the surrounding area . One of the best known examples of this process is the East African Rift . On Earth, rifts can occur at all elevations, from the sea floor to plateaus and mountain ranges in continental crust or in oceanic crust . They are often associated with a number of adjoining subsidiary or co-extensive valleys, which are typically considered part of the principal rift valley geologically . </P> <P> The most extensive rift valley is located along the crest of the mid-ocean ridge system and is the result of sea floor spreading . Examples of this type of rift include the Mid-Atlantic Ridge and the East Pacific Rise . </P>

A rift valley is evidence of which kind of plate boundary