The very small, 1 x 1.5 km Koniuji Island, between Atka and Kasatochi volcanoes, is the emergent top of a mostly submarine volcano. Located about 20 km N of the western side of Atka Island, the volcano is deeply dissected, with a steep, arcuate cliff on the west. New Ar-Ar dating indicates that the island emerged above sea level aout 15,000 years ago, and that several lava flows and domes were erupted during the Holocene. The summit lava dome is younger than about 3,000 years. Reports of historical eruptive activity may be erroneous and are considered to refer to eruptions of the relatively undissected Kasatochi, 25 km W.