The East African Rift System has many of the geophysical characteristics of a mid-ocean ridge and is laterally continuous with the Red Sea and Gulf of Aden spreading centres. Although all three were initiated simultaneously on continental crust in pre-Miocene times, that in East Africa did not evolve oceanic crust.
Seismic, thermal and geochemical evidence suggest that the lithosphere under much of Africa is about 300 km thick, whereas under the rift it is today 40–50 km. Thin lithosphere can result either from mechanical necking or from the upward migration of the isotherm defining the lithosphere/asthenosphere boundary. Both processes are important in the evolution of a continental rift into a spreading ocean, but the limited extension in East Africa indicates that here the latter must dominate. It is, however, difficult to transfer sufficient heat into the lithosphere from below in the time available; conductive processes are too slow, and transfer by magma, although fast enough, requires more mass addition to the lithosphere than is compatible with the limited surface extension. Hot volatiles might transfer heat to the lithosphere from below and depress melting temperatures without adding significantly to its mass.
Rifts have been attributed to convective mantle plumes or to membrane stresses in the lithosphere. If the lithosphere is thick, mantle plumes are unlikely to generate rifts in the time available.
