Report on Pavlof (United States) — September 1981
Scientific Event Alert Network Bulletin, vol. 6, no. 9 (September 1981)
Managing Editor: Lindsay McClelland.
Pavlof (United States) Ash clouds; lava flow; seismicity
Please cite this report as:
Global Volcanism Program, 1981. Report on Pavlof (United States). In: McClelland, L (ed.), Scientific Event Alert Network Bulletin, 6:9. Smithsonian Institution. http://dx.doi.org/10.5479/si.GVP.SEAN198109-312030.
55.417°N, 161.894°W; summit elev. 2493 m
All times are local (unless otherwise noted)
NOAA weather satellite images revealed an eruption plume emerging from Pavlof at 1030 on 25 September. On the image at 1415, when weather clouds next permitted a clear view of the area, both Pavlof and Shishaldin (about 150 km to the SW) were emitting plumes. At 1545, data from infrared imagery indicated that the temperature at the top of Pavlof's cloud was -55°C, corresponding to an altitude of about 9 km. This cloud drifted nearly due E and was still visible at 1945 when imagery showed a new plume originating from Pavlof. By 2215, the new plume had reached 9-10.5 km altitude and feeding from Pavlof appeared to be continuing. By 0415 the next morning, the bulk of this plume had drifted SE and appeared to be largely disconnected from its source, although faint traces of plume may have extended back to Pavlof.
Fishermen in Pavlof Bay reported that activity continued through the night, dropping nearly 4 cm of ash on one boat. An ash sample from one of the boats was sent to the USGS in Anchorage. No certain activity could be distinguished on the satellite image returned at 0615, but there were unconfirmed reports of a renewed eruption by about 0700 and by 0930 the imagery again showed plumes from both Pavlof and Shishaldin. From infrared imagery, a temperature of -28°C was determined for the top of Pavlof's plume, indicating that its altitude was approximately 7.5 km. A Reeve Aleutian Airways pilot flying near Pavlof at 1000 observed a black eruption column and estimated the altitude of its top at roughly 6-7 km. He also reported incandescent material on the W flank. A faint plume extended ESE and was still connected to Pavlof on the satellite image at 1415. No eruption clouds have been observed on the imagery since then, and there have been no reports from pilots of renewed activity.
A visit to the volcano 2-3 October by Egill Hauksson and Lazlo Skinta revealed that lava had been extruded from a vent about 100 m below the summit and had flowed down the NNW flank to about the 600 m level. The lava covered an area of roughly 3 km2, and was 6-7 m thick at the thickest portion of the flow front, which was not advancing. A sample of the lava was sent to the Lamont-Doherty Geological Observatory. No ashfall thicknesses could be determined because of redistribution by very strong winds.
A Lamont-Doherty seismic monitoring station [8.5] km SE of the summit recorded occasional periods of harmonic tremor and an increase in the size of B-type events beginning about 2 weeks before the eruption. However, a few days before the eruption began both the number and size of events decreased; only five discrete shocks were recorded between 1500 on 22 September and 1500 on the 23rd, and only two during the next 24 hours, as compared to an average background level of 15-25/day. On 25 September, the day Pavlof's eruption was first observed on satellite imagery, the seismographs recorded a few more discrete events and intermittent, very low-amplitude harmonic tremor. Between 2000 on 25 September and 0300 on 26 September tremor amplitude increased gradually, and by about 0330 tremor was saturating the instruments. The strongest tremor was recorded between 0500 and 0900, then amplitudes began to decrease. However, tremor remained strong and continuous until 1220 on 27 September, when it declined to several-minute bursts, between which discrete events could be observed. About 100 discrete events and lower amplitude bursts of tremor were recorded during the 24-hour period ending at 1500 on 28 September. As of 5 October, B-type events and bursts of harmonic tremor were continuing.
Both the 1980 and the 1981 eruptions occurred from vents high on the N flank, but it was not certain whether these were the same vents.
Geologic Background. The most active volcano of the Aleutian arc, Pavlof is a 2519-m-high Holocene stratovolcano that was constructed along a line of vents extending NE from the Emmons Lake caldera. Pavlof and its twin volcano to the NE, 2142-m-high Pavlof Sister, form a dramatic pair of symmetrical, glacier-covered stratovolcanoes that tower above Pavlof and Volcano bays. A third cone, Little Pavlof, is a smaller volcano on the SW flank of Pavlof volcano, near the rim of Emmons Lake caldera. Unlike Pavlof Sister, Pavlof has been frequently active in historical time, typically producing 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 historical 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: T. Miller and J. Riehle, USGS, Anchorage; S. McNutt and E. Hauksson, LDGO; W. Younker, NOAA/NESS, Anchorage.