The Tarosero and Monduli volcanoes are built on a plateau of basalt and trachybasalt. The lower slopes are essentially basaltic with rare nephelinite, but on Tarosero the basalts are overlain by soda trachytes, in the summit area by sanidine-phyric soda trachytes while peralkaline rhyolites occur on the lower northwestern slopes (J.B. Dawson, pers. comm., 1990). On the northern slopes of this volcano there are two parasitic cones of trachytic agglomerate and tuff and a trachyte dome occurs within a small caldera. South of Tarosero is the Monduli Juu Caldera which may have been formed when the surrounding trachyte tuffs were erupted. Analyses of two rocks from Tarosero and three from Monduli, including trace element and Sr, Nd and Pb isotope data, are in Paslick et al. (1995).