The trachytic Korosi shield volcano lies at the north end of Lake Baringo.  Unlike other rift valley volcanoes, it does not contain a summit caldera.  Lava domes on the northern flanks have fed trachytic lava flows.  The major stage of Korosi volcanism spanned a period between 0.4 and 0.1 million years ago and included the eruption of voluminous fluid basaltic lava flows and pyroclastic cone formation along a NNE-trending fissure cutting across the axis of the shield volcano.  This was followed by the eruption of trachytic lava domes and pumice/scoria cones.  The youngest activity, consisting of fissure-fed basaltic lava flows on the lower northern flanks between Korosi and Paka, was probably coeval with the latest eruptions on Ol Kokwe to the south, which are only a few hundred to a few thousand years old (Dunkley et al. 1993).  Fumaroles and hot steaming ground occur around the summit cones and NW flanks over an area of 30 sq km.