Report on Pavlof (United States) — July 1986
Scientific Event Alert Network Bulletin, vol. 11, no. 7 (July 1986)
Managing Editor: Lindsay McClelland.
Pavlof (United States) Continued Strombolian activity and vigorous seismicity
Please cite this report as:
Global Volcanism Program, 1986. Report on Pavlof (United States). In: McClelland, L (ed.), Scientific Event Alert Network Bulletin, 11:7. Smithsonian Institution. https://dx.doi.org/10.5479/si.GVP.SEAN198607-312030.
55.417°N, 161.894°W; summit elev. 2493 m
All times are local (unless otherwise noted)
Eruptive activity accompanied by strong seismicity continued through early August. Poor weather usually obscured the volcano, but seismicity remained high with only slight day-to-day variations. A thin ash plume was visible during the late evening of 18 July and reached ~5.5 km altitude the next day. Little activity was evident on 20 July, but several explosions were heard 23 July. No changes in seismicity could be correlated with these observations.
Seismicity increased substantially on 31 July at 1845, and an airline pilot reported explosions an hour later. Pulses of seismicity that varied in amplitude from 5-6 mm to about 40 mm saturated instruments for about 20 hours, and were recorded by stations as far as 60 km from the volcano. Seismic activity declined somewhat on 2 August, remaining vigorous but detected only on the instrument 7.5 km from the summit. As of 5 August, discrete high-amplitude volcanic events averaging 30-40 seconds long continued to be recorded.
When visited by geologists on 7 August, snowfields at 800 m altitude about 4 km N of the summit were covered by 2.5-7.5 cm of pea-sized to fist-sized tephra. At a location 10-12 km WNW of the summit, 15 cm of ash had fallen since the previous visit 2 years earlier. Some ash was clearly fresh, as it was visible on top of snowfields in nearby ravines, but geologists were unable to sample or measure the thickness of ash deposits on the snow. Several booming explosions were heard during ½ hour at this site, and several more explosions were heard later that day from S of the summit. Airline pilots continued to report emission of vapor and ash through early August (table 1).[Skip text table]
Date Time Activity Reported [Observers] 14 Jun 1710 New vent on Se summit ejected blocks and magma to 35 m, lava flow to ESE, ash plume towards SW. 15 Jun 1200- Incandescent blocks and lava fountains 30-60 m from new 2000 SE vent, lava flow on SE flank, ash and steam from four vents on SE summit, 3,300 m ash-and-steam plume 30 km to NNW. 16 Jun 1211 3,600-m ash plume 40 km to N. 17 Jun 1325 3,300-m steam and light ash plume 8 km to NNE. 18 Jun 1142- 3,000-4,600-m ash-and-steam plumes extending 80-120 km to 1619 the NE and NNE. 19 Jun 3,500-m steam plume with no detectable ash to NE. 21 Jun 1330 4,200-m steam plume with streaks of ash 40 km to SW. 22 Jun 1127 3,000-m steam plume with blue/brown haze 40 km to SW. 23 Jun 1200 3,000-m light ash plume drifting E. 24 Jun 1712 Wind-blown ash to 300 m above ground surface 9 km S of summit. 27 Jun 1045 3,300-m dark ash plume extending 40 km to SW. 08 Jul 1500 3,950-m steam-and-ash plume 40 km to WSW. 06-12 Jul Explosions were heard and felt at canoe Bay, 45 km ENE of Pavlof by U.S. Fish and Wildlife personnel working in the area. 13 Jul 1145 3,600-m ash plume 30 km to ENE. 13-19 Jul Almost continuous rumblings, 4-5 second intervals, earthquakes and ground vibrations strong enough to rattle windows, shake shelf items, and prevent sleep; some ashfall at USF&W camp, 45 km ENE. 16 Jul 1300 Extensive ash deposits in Cathedral Peaks region, 17.6 km W of summit, Pavlof and Pavlof's Sister covered with ash, SE vent ejected steam with dark vertical plume of ash at 3-5-minute intervals, N vent quiet. 17 Jul 1315 3,600-m ash plume to 30 km ENE. 18 Jul SE summit vent eruptions at several-minute intervals caused pulsating plume and incandescent lava flowing down SE flank that did not reach the ocean. 19 Jul 1320 3,300-m ash plume extending 24 km to ENE. 20 Jul Several vents observed at SE summit, none with active lava flows, one ejecting rocks and lava bombs tens of meters high, three separate SE-flank flows, one still steaming; none had entered the ocean. 22 Jul 1200 3,600-m dark ash plume extending to ENE. 29 Jul 1200 3,600-m steam-and-ash plume toward WSW. 20-30 Jul Very little rumbling and ground vibrations at USF&W camp. 31 Jul 1217 Small puffs of ash and steam to 150 m from SE vent. 1836 Active 600-m-long lava flow down E slope from SE vent located 120 m below summit. 01-03 Aug Explosions heard from 55 km NE of Pavlof, 3-5-minute intervals, heavy ash detected in streams and rivers in Aghileen Pinnacles and Cathedral Valley areas, 17.5 to W and NW of Pavlof. 02 Aug 1240 3,000-m steam-and-ash plume extending to NE. 05 Aug 4,500-m steam-and-ash pluem to E. 08 Aug 1300 Dark ash clouds from N summit vent to 30-50 m at several- minute intervals, SE near-summit vents emitting traces of steam. 13 Aug evening Summit lava fountaining. [ASM] 14 Aug midday Large white steam plume. [GW] 18 Aug 0850 Dark ash to 3,600 m, drifting 80 km SE. [PA] 1047 Steam-and-ash plume to 4,000 m, drifting 55 km ESE. [SP] 20 Aug 1556 Ash plume to 3,600 m, drifting SE. [NOAA] Sound heard just before emission of large ash puff. [JS] 21 Aug 0900 Plume alternating steam and ash at 3-minute intervals to 3,000 m; visible for 15 minutes, then obscured by weather. [TD & RS] 22 Aug 1152 Ash plume to 3,600 m, drifting at least 16 km ESE. [RA] 1241 Occasional steam plumes. [RA] 05 Sep 300-m dark nearly vertical column, drifting slightly NE. [SH] 10 Sep 1155 Ash and steam to 3,000 m, drifting NE for 8 km. [HB & AL] 12 Sep 1100 Minor steam emission from 100-m region near summit. [JC] 15 Sep 1430 White steam from old N vent did not rise above summit. [MB & CB]
The active spatter-fed flow on the E flank and an inactive flow probably associated with April explosions were composed of individual tephra fragments, and moved downslope as debris flows. Airline pilots reported that the flow front on the E flank remained at about 600 m altitude as of 31 July.
On 8 August personnel aboard a Reeve Aleutian Airways plane reported that they sighted what may be a large bulge on the W side of the volcano about 2/3 of the way upslope. It appeared to have a diameter of about 300 m.
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, USGS Anchorage; J. Taber, LDGO; J. Reeder, ADGGS.