The southern portion of Atka Island is older than the north, with some volcanic rock dating back 5 million years. The active northern part of the island once had one large cone, which was lost in a large eruption, and is now peppered with several smaller volcanos.
A volcanic complex can have several vents, and a widely varying composition of lava. Seismic activity within a complex can be difficult to pinpoint the source of the activity. Which vent is rumbling now? Some of those smaller vents have developed into stratovolcanoes.
Korovin Volcano has been very active in recent times, while Mount Kliuchef last erupted in 1812. The Atka Complex recently was elevated to a Level Yellow, due to seismic activity on the island. Interestingly, the swarm of activity is not near the known suspects, but several kilometers the the west and southwest, and approximately 10 miles from the community of Atka.