Central Island in Lake Turkana is approximately circular, rises to 170 m and covers 9 km2, but the bathymetry indicates that the edifice widens to a diameter of about 5 km at a depth of 45 m beneath the lake surface (Dunkley et al., 1993). The island is dominated by two large craters with crater lakes, the rest of the island comprising coalescing tuff cones and rings. It is built of stratified and palagonitised lapilli tuffs, ashes and breccias with minor lava flows and includes basanite, basalt, benmoreite and phono-tephrite (Dunkley et al., 1993). Bloomer et al. (1989) report the presence of olivine basalt, mugearite and trachyte flows and give analyses of seven rocks, including a range of trace elements. There is widespread geothermal activity on the island consisting of vigorous fumaroles and solfataras with sulphur chimneys common (Dunkley et al., 1993).