Umboi Island, 50 km in its longest dimension, is the largest of the volcanic islands off the N coast of New Guinea. Dominantly basaltic-andesitic, it is comprised of several coalescing stratovolcanoes cut by a 13 x 17 km caldera whose walls rise up to 1 km above its floor. The caldera is widely breached to the sea on the NE side and contains three youthful post-caldera cones with summit crater lakes, Talo, Soal, and Barik. Talo, displays several thermal areas and has satellitic cones on its flanks. No historical eruptions have been reported, but activity of the post-caldera cones is thought to have continued until the last few hundred years (Johnson et al., 1972).