Snow contaminated by dark ash fell on the freight vessel United Spirit between 1200 and about midnight on 7 February, as it steamed from 48.8°N, 152.5°W to 49.2°N, 156.3°W, about 1,000 km SE of Westdahl. Satellite imagery (figure 3) shows that winds were driving the eruption cloud towards the vessel, about 24 hours before the ashfall began. Winds observed from the United Spirit during the snowfall were steadily from the NW.

Geological Summary. Westdahl is a broad glacier-covered volcano occupying the SW end of Unimak Island. Two peaks protrude from the summit plateau, and a new crater formed in 1978 cuts the summit icecap. The volcano has a somewhat of a shield-like morphology and forms one of the largest volcanoes of the Aleutian Islands. The sharp-topped, conical Pogromni stratovolcano, 6 km N, rises several hundred meters higher than Westdahl, but is moderately glacially dissected and presumably older. Many satellitic cones of postglacial age are located along a NW-SE line cutting across the summit of Westdahl. Some of the historical eruptions attributed to the eroded Pogromni may have originated instead from Westdahl (Miller et al. 1998). The first historical eruption occurred in 1795. An 8-km-long fissure extending east from the summit produced explosive eruptions and lava flows in 1991.

Information Contacts: JMA, Tokyo.