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KENYA: Africa splitting into two

Evidence for the existence of this hotter-than-normal mantle plume has been found in geophysical data and is often referred to as the “African Superswell”.

This superplume is not only a widely-accepted source of the pull-apart forces that are resulting in the formation of the rift valley but has also been used to explain the anomalously high topography of the Southern and Eastern African Plateaus.

Breaking up isn’t easy

Rifts exhibit a very distinctive topography, characterised by a series of fault-bounded depressions surrounded by higher terrain. In the East African system, a series of aligned rift valleys separated from each other by large bounding faults can be clearly seen from space.

Not all of these fractures formed at the same time, but followed a sequence starting in the Afar region in northern Ethiopia at around 30 million years ago and propagating southwards towards Zimbabwe at a mean rate of between 2.5-5 cm a year.

Although most of the time rifting is unnoticeable to us, the formation of new faults, fissures and cracks or renewed movement along old faults as the Nubian and Somali plates continue moving apart can result in earthquakes.

However, in East Africa most of this seismicity is spread over a wide zone across the rift valley and is of relatively small magnitude. Volcanism running alongside is a further surface manifestation of the ongoing process of continental break up and the proximity of the hot molten asthenosphere to the surface.

Diaz writes that the East African Rift is unique in that it allows us to observe different stages of rifting along its length. To the south, where the rift is young, extension rates are low and faulting occurs over a wide area. Volcanism and seismicity are limited.

Towards the Afar region, however, the entire rift valley floor is covered with volcanic rocks.

This suggests that, in this area, the lithosphere has thinned almost to the point of complete break up. When this happens, a new ocean will begin forming by the solidification of magma in the space created by the broken-up plates.

Eventually, over a period of tens of millions of years, seafloor spreading will progress along the entire length of the rift.

The ocean will flood in and, as a result, the African continent will become smaller and there will be a large island in the Indian Ocean composed of parts of Ethiopia and Somalia, including the Horn of Africa.

Dramatic events, such as sudden motorway-splitting faults or large catastrophic earthquakes may give continental rifting a sense of urgency but, most of the time, it goes about splitting Africa without anybody even noticing.

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