The massive Menengai shield volcano occupies the floor of the East African Rift. Construction of a 30 km3 shield volcano beginning about 200,000 years ago was followed by the eruption of two voluminous ash-flow tuffs, each preceded by major pumice falls. The first took place about 29,000 years ago and produced a large caldera. The second, producing about 30 km3 of compositionally zoned peralkaline trachytic magma about 8000 years ago, was associated with formation of the present-day 8 x 12 km summit caldera. More than 70 post-caldera lava flows cover the caldera floor, the youngest of which may be only a few hundred years old. No historical eruptions are known. Fumarolic activity is restricted to the caldera.