The area of the triple-rift junction in W Kenya, between the E-W Kavirondo Rift, the N-S Northern Kenya Rift and the NW-SE Central Kenya Rift, has been the site of continuous volcanic and tectonic activity since the Middle Miocene. From about 20 to 7 Ma the area was dominated by the accumulation of a thick pile of nephelinitic and phonolitic volcanics in the Timboroa area. From 7 Ma to the present day, apart from local eruptions of basanite in the W, the volcanic and tectonic activity has been progressively concentrated towards the axial zone of the Kenya Rift with alkali basalt-trachyte sequences predominating. During this period the Kavirondo Rift was inactive. The stratigraphic sequence in the area has been dated by 19 new K-Ar age determinations. These enable the absolute and relative ages of the Tinderet, Timboroa, Kapkut, Londiani and Kilombe volcanoes to be established for the first time.
Topographical and structural considerations show that the Kavirondo Rift largely fades out 50 km W of the Kenya Rift and is barely perceptible at its edge. The W wall of the Kenya Rift adjacent to the Kavirondo Rift is a monocline and is very similar to the South Turkana region. Although the area is adjacent to major rift-bounding faults there is little faulting within it, the tectonic pattern being dominated by monoclines and volcano-tectonic structures.