The present outcrop area of this volcano is 224 km2 but it may originally have been four times that. It has a caldera with well preserved western and northern walls (Dunkley et al., 1993). The northern and southern flanks are covered by younger basalts and the western margin obscured by Quaternary sediments. The eastern margin is defined and cut by faults marking the western side of the rift valley. The volcanic rocks are predominantly trachyte lavas but with intercalations of ignimbrite and air fall tuffs. They are cut by numerous trachyte and basalt dykes and stocks.