From Wood and Kienle (1990) 
: "Semisopochnoi is the largest young volcanic island in the western Aleutians and is composed of a variety of volcanic landforms. Basaltic pyroclastic material built a shield of ~20 km wide (at sea level) which culminated in a post-glacial pumice and ash eruption of dacite and andesite, producing an 8-km-wide caldera. Smaller composite cones are both pre- and post-glacial. Mount Cerberus is the most active of the three younger cones within the caldera. These young cones are dominantly two-pyroxene, high-alumina basalt, and andesite. One young composite cone (Sugarloaf) has olivine basalt. Dacite and andesite are found among the eruptive products of the pre caldera shield. Much of the island is covered by basaltic to andesitic ash derived from the younger cones. Semisopochnoi's tholeiitic differentiation trend (iron is enriched as silica increases) and relatively large volume are common in volcanoes near segment boundaries. Semisopochnoi is also on a small submarine ridge that extends northward as a part of the scorpion-tail-shaped Bower's Ridge; it is unclear if this setting influences its volcanism.
"An 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."