A brief 19–17 Ma episode that marks the onset of magmatism has been dated from Rb-Sr isochron and 40Ar/39Ar geochronology of lavas from the Rungwe volcanic province. According to 40Ar/39Ar and preliminary K-Ar geochronology, volcanism and rifting in East Africa initiated in the Eocene in the Turkana depression between the Ethiopian and East African plateaus. A strong thermal impact from a mantle plume impinged on the lithosphere beneath the Ethiopian plateau caused numerous volcanic eruptions along the central segment of the Ethiopian rift about 30 Ma ago and sublithospheric thermal erosion on the periphery of the East African plateau, since 23 Ma in the Kenya rift and since 19 Ma in the Western rift.
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