According to the history recorded by the Alaska Volcano Observatory: The first episode began in Pliocene or early Pleistocene time (the oldest known age of lavas is 0.93 Ma [Nye, 1990]) and produced extensive flows and subordinate pyroclastic deposits of basaltic and andesitic composition, which enlarged the island by several kilometers along the northwest coast. Radial dips of flows suggest that Makushin Volcano itself was the principal vent area. The older flows are extensively glaciated, which implies a minimum age of late Pleistocene. The summit of Makushin subsequently collapsed, forming a summit. Andesitic pyroclastic-flow and debris flow deposits occur in glaciated valleys on the north and south sides of the volcano indicating a Holocene age for the caldera-forming eruption.