The East African Rift System

Abstract

The East African Rift System (EARS) is driven by extension and thinning of the African Plate. Rifting commenced during the Oligocene (at approximately 30 Ma) and has persisted intermittently through the Miocene, Pliocene and Pleistocene into the Holocene. Rifts are invariably associated with intense volcanism and East Africa includes one of Earth’s largest volcanic provinces. The occurrence of active cones is consistent with a long-lived mantle plume that may have rejuvenated the older Gondwana-age plume. Major seismic activity in Africa is uncommon but moderate-sized earthquakes can be triggered by active faults. Three discrete branches of the EARS are recognised: Ethiopian, Albertine and Gregory. The Ethiopian Rift is part of an active triple junction that includes the Red Sea and Gulf of Aden. Rifting propagated southwards from Ethiopia, synchronously forming the western or Albertine Rift, the most extensive branch, and the smaller, eastern or Gregory Rift. Rifting involves three distinct stages; pre-rift, half-graben and full graben. The pre-rift stage is manifested by regional doming and includes an early phase of intense volcanism. The half-graben stage includes joining of isolated warps into linear features. The development of a rift valley, or full graben, i.e. a down-faulted block enclosed by uplifted rift platforms on either side, is one of the most intriguing of geological features. Rifting has had a pronounced effect on the geomorphology of East Africa. The drainage patterns, including major rivers such as the Nile, have been severely affected. A notable difference between the Albertine and Gregory Rifts is that whereas the former contains some of the world’s largest and deepest freshwater lakes, the latter is far more arid and is characterised by a chain of mostly small and shallow, alkaline lakes.