Description

From Miller and others (1998) ^[1] : "Makushin volcano is a broad, truncated stratovolcano, 1800 m high and 16 km in basal diameter, which occupies most of the triangular northwest extension of Unalaska Island. A breached summit caldera, about 3 km across, contains a small cinder cone, eroded remnants of other cones, and several fumaroles. The volcano is capped by an icefield of about 40 square km; subsidiary glaciers descend the larger flanking valleys to elevations as low as 305 m.
"Makushin volcano was constructed during two periods of volcanism separated by an interval of pronounced erosion ^[2] . Bedrock is exposed as high as 975 m on the southeast flank of the volcano. The first episode began in Pliocene or early Pleistocene time (the oldest known age of lavas is 0.93 Ma [Nye, 1990, in Wood and Kienle, 1990 ^[3] ]) 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 caldera. 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 ^[4] . Reeder (1983) ^[5] reported a C14 age determination of 7950 +/- 90 ypb on organic soil directly beneath the pyroclastic flow deposits and Nye and others (1984) ^[6] report a limiting C14 age of 4280 +/- 280 ybp on organic material in a debris flow.
"Several monogenetic satellite vents composed of basaltic and andesitic lava flows, ash, and scoria cones occur within the summit caldera. These vents also form smaller cones on Makushin's flanks and surrounding area. Most of these vents have been slightly glaciated but blanket late Pleistocene topography indicating a latest Pleistocene or early Holocene age. Pakushin cone, a multiple-cratered composite cone, lies 8 km southwest of Makushin Volcano. Tabletop Mountain, the eroded remains of a pyroclastic cone encircled by flows originating from small flank vents, is 20 km northeast. Wide Bay cone, a small symmetric cone with an oval summit crater, occupies the northwest edge of Unalaska Bay, and Sugarloaf cone, built of steeply dipping, crudely bedded pyroclastics, is situated 14 km to the southwest. The Point Kadin vents, 10 small cones and explosion craters aligned along a rift zone trending N75W from the summit caldera, lie just south of an ash-flow deposit which fills a valley extending north from the volcano to the coast. The deposit is relatively undissected and may correlate with a blanket of airfall ash and cinders that covers part of the icefield on the volcano's northern flank. Based on geomorphic analysis, Arce (1983) ^[7] infers that the sequence of Holocene events to have been as follows: construction of Sugarloaf cone, activity at Tabletop Mountain, construction of Makushin cone, and lastly, construction of the Wide Bay cone and activity on the Pt. Kadin vents.
"Arce (1983) ^[7] concludes that at least 15 tephra layers of Holocene age are recognizable on northern Unalaska Island; however, he assigns some to satellite vents of Makushin Volcano and others to more distant sources, so the exact number of deposits attributable to Makushin Volcano is uncertain."