Pavlof in eruption with a weak gas and ash plume trailing to the southeast.  Pavlof Sister at left.  Time of photo 6:32 pm local.

Pavlof in eruption with a weak gas and ash plume trailing to the southeast. Pavlof Sister at left. Time of photo 6:32 pm local.

Date: Aug 23rd, 2007
Volcano(es): Pavlof Sister Pavlof
Photographer: Mees, Burke

Pavlof 2007/8

From McGimsey and others, (2011): "Pavlof, one of the most frequently active volcanoes in the Aleutian arc, abruptly erupted on August 14, 2007, following an 11-year repose [see fig. 25 in original text]. The 31-day Strombolian eruption was preceded by less than 1 day of increased seismicity detected on the AVO seismic network, and produced a spatter-fed lava flow, minor ash clouds, and lahars that extended down the south flank into the sea. The following account is drawn in part from Waythomas and others (2008).
"The previous eruption of Pavlof Volcano was September 1996-January 1997 (Neal and others, 1997; McGimsey and Wallace, 1999). Minor activity (snowmelt, ash dustings, steam plumes, sulfur emissions) was noted in 1999 and 2001, and in 2005, steaming was observed at adjacent Hague volcano [see table 4b in original text]. Nothing unusual was observed during the summer of 2007 and the seismicity was at background levels through August 13. Abruptly on the morning of August 14, the 5-station seismic network on Pavlof began recording low-frequency earthquakes occurring at a rate of 2 to 7 events every 10 minutes, a pattern that had preceded eruptions in 1996, 1986, 1983, and 1981 (McNutt, 1987; McNutt, 1989; Roach and others, 2001). Although NWS observers in Cold Bay (37 mi southwest) with clear views of the volcano could see no anomalous steaming or other activity, and clear satellite views of the summit that morning also showed no signs of activity, the dramatic increase in seismicity prompted AVO to raise the Aviation Color Code/Volcano Alert Level to YELLOW/ADVISORY.
"During the night, an intense thermal anomaly (TA) was visible in satellite images (Advanced Very High Resolution Radiometer-AVHRR), and seismic activity continued to increase in both number and duration of events per hour, clear signs that the unrest was escalating. On the morning of August 15, based on observations of the TA and increasing seismicity, AVO elevated the Aviation Color Code/Volcano Alert Level to ORANGE/WATCH and announced that an eruption was expected. With the upgrade in color code, AVO began 24-hour surveillance of the volcano. Later in the day, AVO received eyewitness accounts from mariners of incandescent blocks rolling down the eastern-southeastern flank of the volcano during the previous night, beginning around midnight. Pilots reported a thin, low-level ash plume extending a few kilometers southwest from the summit. After receiving these reports, AVO established that the volcano was in eruption. Aerial photographs taken on August 15 show lava fountaining from a vent located about 200 m (650 ft) below the summit [see fig. 26 in original text].
"On August 16, strong seismic signals recorded at a single station (PVV), located 8.5 km (5.3 mi) southeast of the summit, heralded the passage of lahars down the south flank; more than 41 lahar events would be recorded by this station over the next 29 days. Satellite observations of a strong thermal anomaly (TA) [see fig. 27 in original text] and nighttime incandescence at the summit reported by local residents were indications of vigorous lava eruption at the summit vent [see fig. 28 in original text]. The seismic network recorded long periods of volcanic tremor with repetitive explosions that indicated nearly continuous Strombolian eruption. In addition to the generation of lahars, this activity produced low-level ash clouds (5-6 km ASL; 3.1-3.7 mi), and a spatter-fed lava flow that descended the southeastern flank. By August 18, AVO personnel in the field reported that vigorous eruption of lava at the summit continued. Using a Forward Looking Infrared (FLIR) camera, they determined that a 20- to 50-m-wide, 65- to 165 ft-wide) 600 °C (1,112 °F) lava flow extended 565 m (1,850 ft) from the vent down the southeast flank [see figs. 29 and 30 in original text]. Thermal data collected the next day indicated that the outer part of this flow was about 140 °C (284 °F) and had cooled considerably. The vent crater for the last eruption of Pavlof, in 1996, was located on the upper northwestern side of the summit. For this eruption, the active vent migrated to the upper southeastern side, about 200 m (650 ft) below the summit [see figs. 31-33 in original text].
"Seismicity at Pavlof was elevated and steady throughout the remainder of August and then began waxing and waning for the first week of September. A strong TA was present in satellite images, even through clouds, during this time. During the second week of September, the seismicity began showing signs of a steady decrease [see fig. 34 in original text], and by September 13, seismicity decreased to low levels and only a minor steam plume was visible above the volcano. A TA was last seen on September 15, and AVO declared that the activity had reached a lull by September 17. An AVO field crew with clear views reported that all eruptive activity had ceased during their visit on September 19, and the Aviation Color Code /Volcano Alert Level was downgraded to YELLOW/ADVISORY on September 20. The next 2 weeks of low seismicity and no further signs of activity or unrest prompted AVO to declare the eruption over (ending on September 13), and the Color Code/Volcano Alert Level was downgraded to GREEN/NORMAL on October 5.
"Ash, a blocky lava flow, and multiple lahars were generated by this eruption. Mixed ash and steam clouds produced during the most energetic eruptive period, mid-August to mid-September, reached altitudes of 5-6 km (about 20,000 ft) ASL. The plumes were diffuse, drifted primarily to the southeast over the North Pacific Ocean, and many could not be detected in satellite imagery. No ash reportedly fell on nearby communities and there were no significant impacts to aviation. AVO deployed a DRUM aerosol impactor (particle collector) in Sand Point, 90 km (56 mi) east of Pavlof, and collected fine ash (2.5-0.1 µm). Although no visible ash fallout was observed during aerosol sampling, these results demonstrate that volcanic ash was present in respirable size fractions downwind of the volcano even during periods of low ash emissions (Peter Rinkleff and Cathy Cahill, AVO/UAFGI, written commun., 2010).
"Analyzed samples from the lava flow are basaltic andesite in composition (53% SiO2), which is similar to the products of previous Pavlof eruptions (McNutt and others, 1991; Neal and McGimsey, 1997). Lahars were produced by interaction of hot blocks and spatter from the lava flow with snow and ice on the southeastern flank. The lahars inundated an area over 2 km2 (0.78 mi2) and formed a debris fan that extended 3.6 km (2.2 mi) from the base of the volcano into Pavlof Bay [see fig. 35 in original text]."

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