The East African Rift System and its Likely Origins

Written by Dr Carina Lemmer
Published: 25 May, 2020

The East African Rift System is said to be one of the geologic wonders of the world with a triple-junction of rifts in the north over Ethiopia and two branches of rifts that stretch southwards for thousands of kilometres along the east coast of Africa.

By all accounts the Ethiopian Rift is likely to have a hotspot that is currently active underneath it.  From a new interpretation of the South African experience it sounds all too familiar:  a deep rift (that could be the northern extreme of an ancient spreading ridge underneath Africa inferred to be along the eastern coast of Africa) with an active hotspot underneath it that is interpreted to be splitting a “Somalian Plate” off from the rest of Africa (the “Nubian Plate”).  These two plates are said to be moving away from each other.

From what can be deduced from aeromagnetic data and billions of years of Southern African history it is likely that deep down the African basement will remain in place after this hotspot, with deeper down the ancient spreading ridge underneath it in place.  However, the top crust carrying the Somalian Plate will likely move a certain distance and then stop.  It is also likely that the Somalian Plate will carry a “mold” of the rift or spreading ridge along its western edge with it.  If enough hot magma has been inserted underneath the top crust of the Nubian Plate, with new layers of lava on top, meaningful movement of a much smaller “platelet” will result over the Nubian Plate itself.

The series of rifts to the south includes a western branch which contains the East African Great Lakes and an eastern branch all the way down through Kenya, ending up in Mozambique.  From what aeromagnetic data over Southern Africa have shown, these rifts are situated today on platelets that formed at various stages over the single line that is the ancient spreading ridge.  After formation the platelets moved to the west or to the east for certain distances and then stopped.  Because they carried with them a mold of the spreading ridge, lakes could form in their low lying areas.  Due to their movements during various epochs they are no longer aligned in a single line that is the original spreading ridge.

Another manifestation of the ancient spreading ridge is the Great Dyke of Zimbabwe, which today exhibits a linear north-south geological feature roughly down the centre of Zimbabwe. It bears testimony to layered ultramafic intrusions into the spreading ridge, mainly due to a hotspot around about 2.5 billion years ago on its way south.  The lava of this hotspot was mineralized with chrome, platinum and nickel.  This feature is also out of alignment with the ancient spreading ridge, for two reasons.  The platelet that the hotspot created moved, but the hotspot also could no longer find access to the surface directly through the spreading ridge because of the build-up in the ridge by previous hotspots.  Its track nevertheless hugged the spreading ridge as best as possible. So these features along the eastern side of Africa bear testimony to the existence of an ancient spreading ridge buried deep down under Africa.  Over the billions of years of its existence hotspots seemed to seek it out for access to the surface for their magma.  From what could be ascertained over Southern Africa the movements further entailed coming southwards along the spreading ridge, making a loop over Southern Africa and returning northwards again, as close to the spreading ridge as possible.