On 20 December, pilot T. Madsen (Aleutian Air) noticed anomalous amounts of steam rising from six large and closely spaced steam vents just E of the summit. The largest plume was 500-600 m high. No ash was observed in the white plumes. Air temperature at 2,400 m was -6.7°C, warm for that altitude. Steaming remained anomalously vigorous for the next two days before returning to a more normal level. Based on John Reeder's observations . . . since 1979, the summit steam activity is continuous and normally reaches heights of 100 m or slightly less.

Geological Summary. The ice-covered Makushin volcano on northern Unalaska Island is capped by a 2.5 km caldera. Its broad, dome-like structure contrasts with the steep-sided profiles of most other Aleutian stratovolcanoes. Much of the edifice was formed during the Pleistocene, but the caldera (which formed about 8,000 years ago), Sugarloaf cone on the ENE flank, and a cluster of about a dozen explosion pits and cinder cones at Point Kadin on the WNW flank, are of Holocene age. A broad band of NE-SW-trending vents cuts across the volcano. The composite Pakushin cone, with multiple summit craters, lies 8 km SW. Table Top (Pleistocene, 68 +/- 14 ka) and Wide Bay (Holocene) cinder cones are about 20 km ENE on the peninsula across the bay from the City of Unalaska. Frequent explosive eruptions have occurred during the past 4,000 years, sometimes accompanied by pyroclastic flows and surges. Geothermal areas are found in the summit caldera and on the SE and E flanks. Small-to-moderate explosive eruptions have been recorded since 1786.

Information Contacts: J. Reeder, Alaska Division of Geological and Geophysical Surveys (ADGGS).