Semisopochnoi, the largest subaerial volcano of the western Aleutians, is 20 km wide at sea level and contains a caldera 8 km wide that formed as a result of collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic pumice. The high point of the island is 1,221 meter Anvil Peak, a double-peaked cone that forms much of the island's northern part. The three-peaked, 774-meter high Mount Cerberus is a volcano within the caldera. Each of the peaks contains a summit crater; lava flows on the northern flank of Cerberus appear younger than those on the southern side. Other post-caldera volcanoes include the symmetrical 855 m high Sugarloaf Peak south-southeast of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the northeast part of the caldera. Most documented historical eruptions have originated from Cerberus, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone within the caldera could have been active during historical time.[3][4][5]
Semisophochoi's last known volcanic eruption was in April 1987. A historic eruption of Semisopochnoi was reported in 1873, and at least four others may have occurred in the previous hundred years, but documentation is scanty. These eruptions apparently emanated from the flanks of Mount Cerberus; the most recent flow appears to be less than a century old
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[link to www.avo.alaska.edu]