East African Rift System

The East African Rift System is a long, active continental rift in eastern Africa. In a commonly used definition it extends from the Afar region, near the Horn of Africa, south through Ethiopia and Kenya toward western Mozambique. The U.S. Geological Survey describes the African system as about 3,000 kilometres long from Afar to western Mozambique; the Geological Society of London gives about 3,500 kilometres for a route beginning near the Red Sea. Broader descriptions of the Afro-Arabian rift may include additional northern branches, so quoted lengths depend on the boundaries being used.

The main rift separates the Nubian part of the African plate from the Somali plate. In the north, these plates meet the Arabian plate around Afar, forming a three-plate junction. Extension is uneven along the system and did not begin simultaneously in every sector. Regional volcanism and faulting predate the development of the main rift, so geological accounts distinguish earlier activity from the later growth of the present rift branches.

A continental rift develops where tectonic forces stretch the crust. In East Africa, extension has fractured the brittle upper crust and produced normal faults: blocks of crust move down between higher fault-bounded shoulders, creating elongated valleys and steep escarpments. Continued stretching also thins the crust, while magma and volcanic rocks occur along parts of the system. Faulting, uplift, volcanism and erosion together shape the varied rift landscape.

Researchers study the East African Rift as an unusually extensive active continental rift. The USGS notes that its earthquakes and volcanic activity are associated with extension and active magmatism. The processes do not proceed at a uniform rate along the full system, and the rift's long-term evolution should not be confused with a prediction of imminent continental separation.

The system is commonly described as having two principal branches. The eastern branch runs through Ethiopia and Kenya and is particularly associated with volcanism. The western branch curves from the Uganda region toward Malawi and contains a chain of large lakes, including Tanganyika and Kivu. Both branches contain fault-bounded basins and volcanic features, but their geology is not identical: the western branch is marked by especially deep lakes and has comparatively less volcanic activity than the eastern branch.

The branches and their associated highlands, valleys and lakes are not simply a single continuous trench. They are a linked but structurally varied set of rift segments, with local differences in topography, sediment, faulting and volcanic history.

The rift offers geologists a broad natural setting in which to examine how continental crust responds to extension. Its active faults, earthquakes, volcanoes and sediment-filled basins preserve evidence of processes that, over geological time, can transform continental crust and contribute to the development of new ocean basins. The USGS describes the system as a rare continent-scale example through which scientists can study how continental rifts evolve toward oceanic spreading centres. This is a description of a geological process over immense timescales, not a forecast that a new ocean will form in the near future.