Character of Quaternary volcanism in the Gregory Rift Valley

Abstract

The latest stages in the evolution of the rift in Kenya were marked by intense volcanic activity along the graben and massive eruptions across the plateau to the east. Within the rift, trachytic volcanoes south of the equator have calderas at least 10 km across. They erupted more abundant ash-fall and ash-flow deposits than the volcanoes in a northern group which are characterised by smaller calderas. Flood trachytes occupy much of the rift floor in central and southern Kenya, but fissure basalts become increasingly more prominent to the north where they are conspicuous in late eruptions across the caldera volcanoes. Variations in the nature of almost exclusively basaltic activity east of the rift produced impressive linear multicentre ranges in some areas, thin yet extensive flows in others, and one field dominated by maar formation. In northern Tanzania, strongly alkaline volcanism is superimposed on both the rift floor and plateau regimes mentioned above. Late caldera collapse on some trachytic shields was accompanied by build-up of large nephelinite cones and a complex volcano having a broad basaltic lava pile surmounted by phonolites and trachytes. Volcanic and tectonic processes combined to exert a profound influence on patterns of drainage and sedimentation throughout the region, thereby both creating and destroying environments favourable for occupation by early man.