The current episode of eruptive activity, which began on 15 September (BGVN 21:08-21:10), persisted [through 3 January]. On 2 December, infrared video taken by the Alaska State Troopers confirmed that the E summit vent was more active than the W vent. The source of intense steaming low on the N flank, which had been intermittently visible to aerial and ground observers for several weeks, was not a new flank vent, but simply a site where lava was in contact with ice or meltwater. Meltwater channels extended down to the low pass between Pavlof and Pavlof Sister, then to the NW into the Cathedral River drainage. On the early morning of 4 December, seismic activity abruptly declined to about background, the lowest level after the onset of the eruption. The substantial decrease in seismicity implied that eruptive activity probably abated.

After a few days of quiescence, seismicity sharply increased on 10 December, accompanying intense long eruption pulses. Steam plumes reached an altitude of 8,500 m, and ash plumes rose to 7,700 m. On 11 December, pilots reported a steam plume at 8,700 m altitude and an ash cloud at 5,200 m. Satellite imagery indicated that a thick 20-km-wide plume extended as far as 160 km SE and a thinner, more diffuse part of the plume then turned E, extending 105 km. Seismicity declined on 12 December. However, lava fountaining from the summit vents and intermittent bursts of steam and ash to below 6,100 m continued; two lava flows were still active on the N flank. Seismicity decreased to about background levels on the evening of 13 December, but observers in Cold Bay, 60 km SW, reported lava fountaining prior to the seismic decrease. There was no steam or ash visible on the morning of 15 December.

Seismic activity began to build again around midnight on 25 December. In the early morning of 27 December seismicity quickly and steadily increased, reaching the highest level to date in this eruption episode. Early morning satellite images and pilot observations showed a summit hot spot, an active lava flow, and an ash plume extending tens of kilometers downwind (NW). That afternoon observers reported discontinuous bursts of ash and steam rising several hundred meters above the summit. Ground observers in Nelson Lagoon, 80 km NE, reported vigorous fire fountaining and a lava flow visible at night. On 28 December, visual reports in the afternoon from pilots and ground observers in Cold Bay indicated plumes reaching altitudes of 3,700-4,900 m and extending for up to 32 km WNW.

Seismicity started to decline in the afternoon, and the volcano was quiet during the night. On the morning of 29 December, pilots and ground observers in Cold Bay reported no eruptive activity. On 2 January, a pilot report indicated steam and ash drifting S from the summit. On the morning of 3 January, an observer in Cold Bay spotted a small burst of ash rising just above the summit. The eruptive pause continued during the week of 11-17 January with very low levels of seismicity.

Geological Summary. The most active volcano of the Aleutian arc, Pavlof is a Holocene stratovolcano that was constructed along a line of vents extending NE from the Emmons Lake caldera. Pavlof and Pavlof Sister to the NE form a dramatic pair of symmetrical, glacier-covered stratovolcanoes that overlook Pavlof and Volcano bays. Little Pavlof is a smaller cone on the SW flank of Pavlof volcano, near the rim of Emmons Lake caldera. Unlike Pavlof Sister, eruptions have frequently been reported from Pavlof, typically Strombolian to Vulcanian explosive eruptions from the summit vents and occasional lava flows. The active vents lie near the summit on the north and east sides. The largest recorded eruption took place in 1911, at the end of a 5-year-long eruptive episode, when a fissure opened on the N flank, ejecting large blocks and issuing lava flows.

Information Contacts: Alaska Volcano Observatory (AVO), 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.