Silali

The 30-km-wide Silali trachytic shield volcano, the largest Quaternary volcano in the northern Gregory Rift, completely straddles the East African Rift. The 5 x 8 km summit caldera with 300 m deep walls is thought to have formed incrementally about 63,000 years ago, related to eruption of the Kantenmening basaltic and trachytic lava flows. A series of summit-area lava benches formed by eruptions from circumferential fissures is morphologically similar to Galapagos Islands volcanoes. The N, E, and S flanks are cut by a prominent broad rift zone 10 km wide and 30 km long, with numerous pyroclastic cones. Lava domes on the upper E flank formed during the late Pleistocene and early Holocene, until about 7,000 years ago. Youthful cones are common on the caldera floor and on the N and NE flanks; the youngest lava flows may be little more than a few hundred years old (Williams et al. 1984). Geothermal activity occurs within the caldera and on the upper E flanks.