During the first week of February, National Weather Service personnel in Cold Bay, 93 km ENE of Shishaldin, observed anomalous steaming. On 9 February a vigorous steam plume rose as high as 1,830 m above the vent and a long plume drifted downwind. Satellite imagery taken that day showed a thermal anomaly at the vent in addition to the steam plume. The steam activity decreased during the week, becoming only light puffs rising a few meters above the vent; however, the thermal anomaly at the vent persisted. A newly installed seismic net recorded slightly elevated seismicity beginning at the end of January.

The hazard status was raised to Yellow on 18 February due to the persistence of the thermal anomaly and the identification of low-level seismic tremor. Pilots and ground observers reported a large steam plume rising to 5,800 m on 18 February. No ash was detected on satellite imagery. Cloudy weather precluded ground observations for most of the following week.

Shishaldin volcano, located near the center of Unimak Island in the eastern Aleutian Islands, is a spectacular symmetrical cone with a basal diameter of approximately 16 km. A small summit crater typically emits a noticeable steam plume with occasional small amounts of ash. Shishaldin is one of the most active volcanoes in the Aleutian volcanic arc, situated near that part of the arc where the maximum rate of subduction occurs. It has erupted at least 27 times since 1775. Major explosive eruptions occurred in 1830 and 1932, and eight historical eruptions have produced lava flows. Steam and minor ash emission began in March 1986 and continued intermittently through mid-February, 1987. A poorly documented short-lived eruption of steam and ash, perhaps as high as 10 km, occurred in December 1995 (BGVN 21:01). Fresh ash was noted on the upper flanks and crater rim but no specific eruptive event was identified for the deposits.

Geological Summary. The symmetrical glacier-covered Shishaldin in the Aleutian Islands is the westernmost of three large stratovolcanoes in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." Constructed atop an older glacially dissected edifice, it is largely basaltic in composition. Remnants of an older edifice are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is covered by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. A steam plume often rises from the summit crater.

Information Contacts: Alaska Volcano Observatory, a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA (URL: http://www.avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA.