The Olkaria volcanic complex, located immediately S of Lake Naivasha and W of Longonot volcano, comprises up to 80 individual alkali rhyolite centers. A thick, geochemically diverse basal complex is overlain by a dominantly silicic lava-dome and lava-flow complex constructed within a largely obscured 11 x 7.5-km-wide caldera that formed about 20,000 years ago following the eruption of welded alkali rhyolite ignimbrites. The youngest known eruption originated from the Ololbutot fissure on the SW side of the complex and produced a 5 km3 lava flow and a pumice flow that has a radiocarbon age of about 180 years. Fumarolic activity occurs at the Njorowa Gorge, and at the Orengingnai and Olkaria (Orgaria) pumice cones. Olkaria is the site of a large 50-80 km3 high-temperature geothermal field.