Volcanic heat keeps the crater on Thule Island ice free.
In 1962, steam was rising from the water in Thule's summit crater and ash was noted on the island's southwest flank. A caldera has been tenatively identified between the islands of Thule and Cook, thus these two islands may be pieces of a larger island that once existed there. Unlike most of the South Sandwich Islands, where most volcanoes are made of basalt, part of Cook Island is composed of andesite and dacite. Bellingshausen is a small youthful andesite cone with active fumaroles but no historic eruptions. Click here to view the sources used in the South Sandwich Islands section.