The trachytic Korosi shield volcano lies at the N end of Lake Baringo. Unlike other rift valley volcanoes, it does not contain a summit caldera. Lava domes on the N flanks have fed trachytic lava flows. The major stage of 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 N flanks between Korosi and Paka, was probably coeval with the latest eruptions on Ol Kokwe to the S, 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 km3.