Ongoing ash explosions and lava flows until eruptive activity ended on 7 December 2022

Pavlof, one of the most consistently active volcanoes in the Aleutian Islands, has been erupting since mid-November 2021, with frequent weak explosions, periods of lava spatter, fountaining, small lava flows and emissions of steam and gas that sometimes contained ash (BGVN 47:08). This eruption continued through 7 December 2022, when both explosions and significant thermal activity had stopped (figure 24). The current report, which describes activity between August 2022-January 2023, is monitored remotely by the Alaska Volcano Observatory (AVO) using local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks, and the information below is based on AVO weekly reports and satellite data. During August to mid-December 2022, the Aviation Color Code (AVO) remained at Orange and the Volcano Alert Level (VAL) remained at Watch, the second highest level for both on a four-level scale, after which they were lowered. Weather clouds often prevented satellite and webcam views.

Figure 24. Graph of thermal anomalies at Pavlof during April 2022-March 2023, as recorded by the MIROVA system. Frequent mostly moderate power hotspots were identified during August through early December 2022. Courtesy of MIROVA.

During August, the minor eruption from the active vent on the E flank, just below the summit, continued, with occasional explosions that probably produced minor ash emissions (figure 24) that usually rose no higher than an altitude of 3 km and dissipated quickly. According to a pilot report, a small ash emission rose to an altitude of about 4 km on 24 August. A small pyroclastic debris flow was observed in satellite data on 26 August, extending about 590 m SE from the active vent. Seismic tremor persisted, and elevated surface temperatures were identified almost daily in satellite images. No active lava flows extending down the flank were detected.

Figure 25. A typical small ash plume is visible rising from Pavlof in this aerial photo at 1123 on 26 August. Photo by Pavel Izbekov; courtesy of AVO/University of Alaska Fairbanks, Geophysical Institute.

By 2 September a second small vent had re-opened ESE of the main vent. Elevated surface temperatures in the active vent area were identified in satellite images on most days, consistent with minor lava effusion. Strong incandescence at the vent and from an area within 200 m downslope was visible in webcam images starting on 14 September, indicating a short lava flow. Small explosions were detected during 18-21 September, and steam and gas plumes often containing minor amounts of ash were frequently visible. New lahar and minor ash deposits extending less than 900 m from the vent were visible during 11-13 September. Seismic tremor was nearly continuous in September.

Small explosions and persistent seismic tremor continued during October except for the first week, multiple explosions were recorded almost daily. Strongly elevated surface temperatures were often identified in satellite images, incandescence and glowing ejected rock fragments were frequently visible in nighttime webcam images, and diffuse steam plumes were sometimes visible in webcam views. Discolored snow near the vent was visible in webcam images on the morning of 7 October, suggesting low-level explosive activity; a small explosion was recorded at 1503 that same day. Webcam images on 20 October showed a new dark flow of lava and debris extending about a third of the way down the E flank. During 31 October-1 November, a diffuse steam-and-ash plume rose less than 3 km in altitude and deposited minor amounts of tephra on the E flank.

During November, multiple explosions were detected almost daily until mid-month, after which none were clearly recorded, but seismic tremor was nearly continuous. Webcam images taken on 12 and 15 November showed a lava flow and ash deposits on the upper flanks, and nighttime crater incandescence was sometimes visible. Elevated surface temperatures were observed in satellite data on most days, and occasional clear webcam views showed recent flow deposits on the volcano’s flank as well as weak steaming and light ash emissions and deposits, probably associated with lava spattering or fountaining. On 28 November, a flowage event likely resulting from the interaction of hot rocks with ice and snow on the flank was observed in seismic data and a series of webcam images, and satellite and webcam images showed that the deposit extended about 5 km down the SE flank, the largest flowage event during the current eruptive period. This event produced an ash and gas plume that reached an altitude as high as 6 km, before dissipating within tens of minutes.

Multiple small explosions were detected on 1-3 December that resulted in a narrow gully being eroded in the ice below the vent, extending about 2 km down the flank. Elevated surface temperatures and incandescent lava were not observed after 2 December. Activity significantly decreased after 3 December, with AVO reporting that the last explosion was detected on 7 December. Occasional small steam and gas plumes were visible in webcam images. Tremor had become more intermittent by mid-month, but overall seismicity remained above background levels through the end of December. On 17 December, due to the reduced activity, AVO decreased the ACC to Yellow and the VAL to Advisory, both the second lowest on a four-level scale. Weakly elevated temperatures were detected the last week of December, and a high-resolution satellite image on 27 December showed a small spot of strongly elevated temperatures within the recently active vent on the volcano’s upper E flank, but there was no sign of eruptive activity.

During the first two weeks of January 2023, seismicity remained above background levels and weakly elevated surface temperatures and minor steaming from the active vent were observed that AVO suggested might have been associated with the cooling of lava around the vent. However, no explosions or other eruptive activity was noted. By mid-January seismicity had decreased to background levels. On 19 January, AVO decreased the ACC and VAL to Green and Normal, respectively, stating that the eruption had ended. However, weakly elevated surface temperatures and minor steaming from the recently active vent continued to be observed intermittently in satellite and web camera imagery, consistent with cooling of previously erupted lava.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://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, 3354 College Rd., Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).

Information is preliminary and subject to change. All times are local (unless otherwise noted)

November 1973 (CSLP 144-73)

Harmonic tremor coincides with report of observed lava flow

Card 1744 (14 November 1973) Harmonic tremor coincides with report of observed lava flow

The following was received by cable from D.B. Stone. "Eruptive activity was observed about 10:00 p.m. Alaska Standard Time from a point 33 km NW of the summit of Pavlof. Reported as 'shooting flames' intermittently with a possible lava flow on the NW flank. A seismometer 7 km SE of the summit recorded increased microearthquake activity at about that time. Seismic records showed onset of harmonic tremor on November 13, 07:00 AST, coincident with reported eruption of lava."

Information Contacts: D.B. Stone and J. Kienle, Geophysical Institute, University of Alaska, Fairbanks AK; J.N. Davies, c/o White Alice Como Site, Cold Bay AK.

October 1975 (NSEB 01:01) Cite this Report

Columns of "smoke" to 2,400 m

For the past several weeks Pavlof has been producing columns 2,100-2,400 m high, carried NE up the Alaska Peninsula. Weather conditions commonly prevent observation of the volcano, but on 26 October it was active for 30 minutes of the hour that it was visible. On 31 October, for 1.5 hours, "lava" was reported streaming down the N side of the cone; this may have been a mud flow.

Information Contacts: P. Sventek, USAF, Cold Bay.

December 1975 (NSEB 01:03) Cite this Report

Ash eruptions, lava emission, and orange glow

10 December, 0100-0800: black ash and numerous sporadic orange mud/lava bursts. 18 December, 1300: white steam. 19 December, 1000: grey smoke. 23 December, 1000: white steam; fresh snow on N slope was darkened from the ashfall. 23 December, 1900: 10-second orange glow. 27 December, 1000: white steam; snow was white except for ash on N slope. 28 December, 0815: Tephra ejection like "a blowtorch" visible (in daylight) for 30 seconds; 1300: grey smoke. 30 December, 1755-1810: energetically emitted surges of glowing red-hot ejecta. In contrast to the sporadic 10 December activity, this emission was of a continuous pulsing nature and surges rose at least 150 m above the summit. This was the most energetic activity observed during the last several months. For the next five hours Pavlof was visible but quiet.

Information Contacts: P. Sventek, USAF, Cold Bay.

January 1976 (NSEB 01:04) Cite this Report

Intermittent steam and ash emissions

3-4 January: intermittent steam, grey smoke, and black ash. 5 January: black ash. 11-12 January: steam. At 2130 on 11 January a pilot reported the volcano erupting red-hot mud/lava up to 300 meters above the cone. 17 January: grey smoke. 22 January: steam. 24-25 January: grey smoke and steam. 29-30 January: constant steam. 31 January: very intermittent steam.

Information Contacts: P. Sventek, USAF, Cold Bay.

February 1976 (NSEB 01:05) Cite this Report

Ash eruptions and lava flows during 21-24 February

At approximately 1400 on 20 February, Pavlof was seen to eject ash clouds at about 3-minute intervals. The ash clouds moved S and dissipated into a layer near the 3,000 m level. After sunset, lava emissions became visible, but were not as energetic as those observed on 31 December.

At 2230 22 February, a USAF pilot flying at 12.5 km reported that lava was visible trailing as a rivulet down the entire NW flank. The volcano continuously emitted lava during the 45-minute pilot observation. On 23 February at 1530 a pilot reported that the activity was basically the same as on the 22nd. On 24 February at 1115 another pilot reported that the lava activity appeared to have ceased, but that strong winds were driving ash, smoke and steam down the S side of the volcano.

Information Contacts: P. Sventek, USAF, Cold Bay.

March 1976 (NSEB 01:06) Cite this Report

Activity remains low in March

Activity during March remained low. 3 March, 1200-1400: steaming continuously. 10 March, 1800: inactive; snow on the W flank was white. 12 March, 1400-1600: steaming quietly; 1730 snow umblemished on all sides of cone visible on fly-by. 19 March, 1600: steaming; snow umblemished on all sides of cone visible on fly-by. 22 March, 1200: steaming, W flank umblemished. 23 March, 1900: steaming weakly. 25 March, 1730: several radial ash sprays visible on N flank. 28 March, 1500-1700: inactive, with ash still visible on N flank.

Information Contacts: P. Sventek, USAF, Cold Bay.

April 1976 (NSEB 01:07) Cite this Report

Steam-and-ash emissions; ashfall seen on flanks

6 April, 1118: pilot reported ash extensively covering entire expanse of Pavlof's SE flank. Black smoke and intermittent steam were observed. 14 April, 0900-1900: steaming intermittently; 2 ash sprays visible on NW flank. 15 April, 0600-0700; 25 April 2300; and 27 April 1200-1900: steam emissions. 29 April, 1500: a pilot reported the volcano steaming. The entire E flank was reported to be ash-covered and very black. 30 April, 1200-1900: Although difficult to differentiate between ash and exposed rock (when viewed from Cold Bay, 60 km to the W) there appeared to be ash over the upper 100 m of the cone and along several radial areas along the near (SW) flank. Pavlof apparently produced extensive ash during the last few days of the month.

Information Contacts: P. Sventek, USAF, Cold Bay.

May 1976 (NSEB 01:08) Cite this Report

Ash emissions in mid-May

7 May, 1200: all flanks of volcano snow-covered. 14 May, 1500: dark ash visible on cone. 16 May, 2100: dark ash and steam emission visible. 20 May: upper 100 m of cone irregularly darkened, apparently from ash. Steam issuing from cone. At 2000, a thin layer of ash at the 3,000-m level was carried S for at least 25 km. At 2045, light grey material was ejected a few hundred meters. 21 May, 0900-0930: steaming. 31 May, 1200-1500: quiet. No longer possible to distinguish (at distance) between new ash darkening snow and dark bedrock exposed by melting snow.

Information Contacts: P. Sventek, USAF, Cold Bay.

June 1976 (NSEB 01:09) Cite this Report

Steam-and-ash emissions continue; ashfall on cone

2 June, 1450: pilot reported volcano steaming, extensive ash deposits visible on top and E flank of cone. 3 June, 1200-1800: volcano quiet, cone darkened by ash, or exposed rock due to melted snow. 4 June, 0900-2100: steamed continuously; occasionally ejected a light tan ash that settled into a thin layer at the 2500-m level and moved NE up the Alaska Peninsula. 5 June, 1500-2200: no activity noted. 6 June 1810-1830: no activity noted.

10 June, 2100: black smoke and ash emitted. Flanks extensively darkened by ash. 11 June, 1500: pilot reported volcano steaming. 12 June, 0900-2100: no activity apparent. 15 June, 0600-0700, 2200: steaming constantly. 19 June, 1630: steaming and emitting grey smoke. 26 June, 1400-2100: vertical steam column reached 300-500 m above cone. 27 June, 1200-1500: intermittent steam and ash emissions formed a thin layer stretching NE at the 2500-m level. 28 June, 1300-2200: ash layer extended NE at the 2,500-m level.

Information Contacts: P. Sventek, USAF, Cold Bay.

July 1976 (NSEB 01:10) Cite this Report

Ash on N slope; heavy steaming

4 July, 0600-2200: black ash noted on the N slope. 15 July, 2000-2300: heavy steam blown down the W slope. 18 July, 1600-2300: heavy steam emitted. 23 July, 2100-2200: no activity.

Information Contacts: R. Dean, USAF, Cold Bay.

August 1976 (NSEB 01:11) Cite this Report

Light steam plume seen on one day in August

31 July, 1730-2130: no activity. 3 August, 2200-2232: plume of light steam. 12 August, 1845-1900: no activity. 22 August, 1010-23 August, 1807: no activity.

Information Contacts: R. Dean, USAF, Cold Bay.

October 1976 (NSEB 01:13) Cite this Report

Ash covering upper slopes and the cone

10 September, 1520-1523: "smoke" emission with some intermixed ash. Ash deposits were noted on the SE slope. 22 September, 0745-1200: no activity. Lower slopes were snow-covered, the upper slopes were "dark," and the cone was black. 28 September, 0750-0843: a steady 150-m steam plume was observed. The cone was ash-covered, but snow blanketed the rest of the mountain.

Information Contacts: R. Dean, USAF, Cold Bay.

November 1976 (NSEB 01:14) Cite this Report

Continued steam-and-ash emissions

7 October, 1852: no activity. Ash covered the top 100 m of the cone. 9 October, 0930-1110: heavy steam emission was visible on a rare windless day. The cone was covered with ash. 28 October, 1430: a steady "smoke" plume and occasional small ash clouds issued from the vent.

6 November, 0330-0500: an eruption was reported by fishermen working in the Bering Sea off Cold Bay. 17 November, 1240: "smoking." 18 November, 1510, 1715: heavy "smoke" rose about 200 m above the vent. 19 November, 0915: steam and ash emission. Ashfall had darkened the cone. 1530: steam emission.

Information Contacts: R. Dean, USAF, Cold Bay.

January 1977 (NSEB 02:01) Cite this Report

Decreased seismicity; small steam emissions

Seismic activity at Pavlof, characterized by [50-1,000] events/day August-November 1976, had declined to 10-15 events/day during a 2-day period in mid-December. During periods of high seismic activity, individual events frequently blended together, producing an effect similar, but not identical, to harmonic tremor (one frequency dominant). Approximately one earthquake/week (probably tectonic in origin) is locatable.

Due to poor weather conditions, visual observations of Pavlof were impossible during January. When visited on 21 December, Pavlof was emitting very small amounts of steam.

Information Contacts: J. Davies, LDGO; R. Dean, USAF, Cold Bay.

March 1977 (NSEB 02:03) Cite this Report

Some steaming and ash near the summit

10 February, 1010-1640: no activity. Entire volcano covered with snow. 12 February, 1105-1355: steaming lightly. Part of cone covered with ash. 22 February, 0917-1625: no activity. Entire volcano covered with snow. 23 February, 0935-1710: no activity. 13 March, 1525: no activity. 16 March, 1530: overflight in private aircraft piloted by Capt. Young (USAF); steam was issuing from about 20 small vents in an ash-covered area approximately 50 m in diameter, l0 m below the base of the cone on the W slope. An older 30-m-diameter crater, partially filled with snow, was about 20 m below the base of the cone on the N slope.

Information Contacts: R. Dean, USAF, Cold Bay.

April 1977 (NSEB 02:04) Cite this Report

Ashfall darkens snow near the summit on 22 March

22 March, 1915: light "smoke" plume. Ash darkened the top 100 m of the cone; 2305: steaming.

Information Contacts: R. Dean, USAF, Cold Bay.

July 1980 (SEAN 05:07) Cite this Report

Steam and a little ash

Pavlof was emitting steam when viewed by J. Davies on 3 July. A few days later, according to second-hand reports, ash was present in the steam column.

Information Contacts: J. Davies and S. McNutt, LDGO.

November 1980 (SEAN 05:11) Cite this Report

Lava fountaining and ash emission; possible lava flow

An eruption from Pavlof 11-12 November ejected large lava fountains and ash clouds that reached 11 km altitude, and may have produced lava flows.

A seismic station [8.5 km SE] of Pavlof registered a 2.5-minute burst of low-amplitude harmonic tremor beginning 5 November at 1351. Emission of steam, ash, and some blocks from a vent high on the NE flank started 8 November at 1047 and lasted about 5 minutes, without accompanying seismicity. A second burst of low-amplitude tremor occurred between 0536 and 0541 on 9 November.

In contrast to the pattern observed before eruptions in 1973, 1974, 1975, and 1976, virtually no additional seismic activity was recorded until a group of 7 low-frequency volcanic earthquakes occurred at about 2300 on 10 November. After an explosion event appeared on seismic records at 0243 on 11 November, 10 more low-frequency volcanic earthquakes were recorded between 0300 and 0400. Continuous harmonic tremor, of fairly low amplitude, began at 0608, but amplitude intensified around 0900.

Reeve Aleutian Airways pilot Everett Skinner saw rocks up to 1 m in diameter rising 10-30 m at 1315 on the 11th. An observer in Cold Bay, 60 km to the W, noted an increase in activity about 1600. Skinner returned to the vicinity between 1630 and 1700, reporting lava fountaining from the summit, a black cloud hugging the upper N flank, and an eruption column reaching an estimated 6 km altitude. Between 1800 and 2000, various witnesses reported lava fountaining to a maximum height of 300 m, and incandescent material moving down the N flank. A satellite image returned at 1958 shows a nearly circular plume, 15 km in diameter, N of the volcano. Activity was visible through the night from Cold Bay and the Sand Point area (50-65 km to the ENE).

The next morning, at 0946, a satellite image revealed a plume 160 km long and almost as wide spreading N of Pavlof. Spectral analysis and weather balloon data indicated that the plume reached 8-9 km above sea level. Pilot reports on 12 November placed the top of the eruption cloud at 9 km at 1000, 6 km at 1100, and 11 km at 1400. The eruption clouds were described as varying from ash-rich to ash-poor. A helicopter crew from KENI television, Anchorage, videotaped pulses and bursts of lava fountaining, rising 150-300 m between 1600 and 1700. The fountains emerged from a pre-existing vent high on the NE flank, the only vent confirmed active during the eruption.

Very high-amplitude harmonic tremor accompanied the eruption, reaching its strongest levels between 2000 on 11 November and 0700 on 12 November. Tremor ceased at 1835 on the 12th, when many B-type earthquakes began to be recorded.

By the morning of 13 November, the eruption had ended. Several hundred B-type events per day were recorded 14-15 November. Renewed high-amplitude tremor began 15 November at 1306, lasting until 1711. B-type earthquakes continued 16-19 November, but fewer than 100/day were recorded.

Information Contacts: S. McNutt and J. Davies, LDGO; A. Till, USGS, Anchorage; J. Kienle, Univ. of Alaska; G. Roberts, Cold Bay Weather Station; Cmdr. J. Hair, Marine Environmental Branch, Juneau.

September 1981 (SEAN 06:09) Cite this Report

Ash clouds; lava flow; seismicity

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.

Information Contacts: T. Miller and J. Riehle, USGS, Anchorage; S. McNutt and E. Hauksson, LDGO; W. Younker, NOAA/NESS, Anchorage.

October 1983 (SEAN 08:10) Cite this Report

Large eruption column; tremor

Strong tremor started to appear on local seismic records on 14 November at about 1500 and by 1800 was saturating the instruments. The mayor of Sand Point, about 90 km E of Pavlof, saw glow over the volcano at 2330 that night. At 1220 the next day, an airline pilot reported an eruption column rising to about 5.5 km altitude through weather clouds that covered the summit and obscured the vent area. Twenty minutes later, the column had reached 7.5 km altitude. Tephra emission was continuing at 1300. The plume blew S and SE, and spread to about 50 km width, 50 km S of the volcano. Aircraft were diverted from the area. Tremor continued to saturate the seismic instruments through the afternoon.

Information Contacts: T. Miller, USGS, Anchorage; S. McNutt, LDGO.

November 1983 (SEAN 08:11) Cite this Report

Ash emission; vapor plume; volcanic tremor

On 19 November a small vapor cloud rose approximately 100 m above the vent. Bad weather prevented observations until 26 November when Pavlof was visible until mid-afternoon from Cold Bay. During the morning, a vapor plume containing a little ash rose to 4.5 km altitude. At intervals of approximately 30 minutes, puffs of dark ash were emitted. The intervals became shorter, and by 1500 ash emission was nearly continuous.

Through October and early November, a Lamont-Doherty seismic monitoring station near the volcano recorded background levels of 0-40 (usually 0-30) small low-frequency events per day. A 30-minute burst of volcanic tremor began at 2000 on 4 November, and a 6-minute burst at 1757 on 9 November. Between 1430 on 11 November and 1100 on 13 November, 15 explosions were recorded. Several 1-2 minute bursts of tremor occurred between 1700 and 1900, when continuous tremor started. Its amplitude gradually increased, and tremor began to saturate the seismograph at 1100 on 14 November. Tremor was strongest between midnight and 1200 on 15 November, and continued to saturate the seismograph until 2100 on 15 November when its amplitude began to decrease. Tremor remained continuous but at low amplitude between 1300 on 16 November and 1200 on 18 November. Intermittent low-amplitude tremor and numerous low-frequency (B-type) events recorded after 1200 on 18 November were continuing on 21 November.

An increase in seismic activity had also been recorded in mid-July. Seismicity remained at background levels until 11 July. During the 24-hours beginning at 1500 on the 11th, 6 explosions were recorded at a Lamont-Doherty seismic monitoring station near the volcano. The number of recorded events increased to 55 for the same period on 12-13 July, and to 150 on 13-14 and 14-15 July, then decreased to 120 on the 15-16th, 38 on the 16-17th, and 19 on the 17-18th, returning to back-ground after 1500 on 18 July. During the period of increased seismicity, approximately half of the recorded events were explosions and half low-frequency events.

At 1549 on 15 July, a thermal infrared image from the NOAA 7 polar orbiting satellite showed a bright spot over Pavlof and an elongate plume extending approximately 150 km to the E. This plume was distinctly colder (at higher altitude) than the layer of low clouds that covered the area. No activity was visible on other NOAA 7 images returned approximately every 12 hours 11-18 July.

Information Contacts: S. McNutt, LDGO; T. Miller and M. E. Yount, USGS, Anchorage; M. Matson, NOAA/NESDIS.

December 1983 (SEAN 08:12) Cite this Report

Brief ash emission episodes

Activity continued through December. At 1400 on 15 December, an airline pilot observed a burst of ash from the volcano, producing a plume that drifted NW. Brief periods of ash emission separated by longer quiescent periods were continuing as of 28 December. These short eruptions produced plumes that dissipated after a few hours.

Information Contacts: M. E. Yount and T. Miller, USGS, Anchorage.

January 1984 (SEAN 09:01) Cite this Report

Plumes on satellite imagery; harmonic tremor

Six explosions were recorded between 1600 and 2000 on 15 December by Lamont-Doherty's 5-station seismic net 4.5-10 km from the volcano. One station, about [8.5] km from Pavlof, detected bursts of harmonic tremor 17 December, 1100-18 December, 0330; 18 December, 0530-0615 and 1040-1110; 20 December, 2200-2245; and 21 December, 2035-2048. Seismicity then decreased to the background level of several tens of events per day and remained at that level as of 26 January.

Eruption plumes were observed on three images returned 15-17 December from the NOAA 8 polar orbiting satellite. The images at 2101 on the 15th and 1031 on the 17th showed well-defined, relatively dense plumes extending 225 km E and 400 km NE from Pavlof above the weather cloud layer. A diffuse plume was observed on the image at 2018 on 18 December. No volcanic plumes were observed on other images 15-21 December, but heavy weather clouds obscured the area. There have been no eyewitness reports of eruptive activity since airline pilots last reported eruption clouds from Pavlof at 1400 on 15 December.

Information Contacts: S. McNutt, LDGO; M. E. Yount, USGS, Anchorage; M. Matson and W. Gould, NOAA/NESDIS.

March 1984 (SEAN 09:03) Cite this Report

Vapor plume to 6 km altitude

At 1225 on 16 March, the pilot of Air Pacific flight S27 observed a white vapor plume rising to 6 km altitude from the volcano and drifting NW. There had been no eyewitness reports of activity at Pavlof since 15 December 1983 (8:12). After an increase on 17-21 December, seismicity decreased to the background level of several tens of events per day and remained at that level as of 2 April.

Information Contacts: M. E. Yount, USGS, Anchorage; S. McNutt, LDGO.

March 1986 (SEAN 11:03) Cite this Report

Ash cloud to 4 km after 10 days of increasing seismicity

On 16 April at about 1100, a Reeve Aleutian Airways pilot saw an ash and vapor plume rising from Pavlof to ~4 km asl [see also 11:5]. About 30 minutes later, another pilot reported relatively steady ash emission to ~4.5 km altitude. Similar activity was observed around 1900.

A large eruption column that rose through low weather clouds to 14.5-16 km altitude was observed by airline pilots on 18 April at about 1620. That evening, ~0.3 cm of ash fell on Cold Bay, 55 km WSW. Minor ash emission was seen the next day, but weather conditions limited observations. Increased flow was reported in the Cathedral River, which drains Pavlof's NW flank.

Seismographs recorded a gradual increase in small volcanic events starting on 6 April, and a more rapid increase in seismicity 10-12 April. On 12 and 13 April, frequent discrete volcanic events were accompanied by brief (6-7 minutes or less) episodes of tremor. Preliminary inspection of later records indicated that vigorous seismicity was continuing as of 17 April, and instruments were saturated by events associated with the strong explosive activity on 18 April.

Information Contacts: M.E. Yount, USGS Anchorage; J. Taber, Lamont-Doherty Geological Observatory (LDGO).

April 1986 (SEAN 11:04) Cite this Report

Strong tremor accompanied large 18 April plume

No other reports of activity had been received as of early May [but see 11:05], and no information was available about changes that might have occurred to the active crater.

John Taber provided the following information from seismic stations operated by LDGO. "The number of volcanic events increased from a slightly above normal 20 events on 6 April, to 370 events on 11 April and 750 events on 13 April. The rate of seismicity then stayed relatively constant until the main eruption on 18 April. Continuous tremor began at around 1440 and intensified around 1610, when it was visible at stations 100 km away. The strong tremor continued until 1800 then gradually subsided, ending around 2100. The number and duration of volcanic events dropped abruptly after the tremor ended and continued to decrease until background levels were reached by 26 April."

Information Contacts: J. Taber, LDGO.

May 1986 (SEAN 11:05) Cite this Report

Strombolian activity feeds lava flow; seismicity increases

Airplane pilots reported occasional ash emission through early June. Seismicity had decreased to background levels by 26 April but began to increase again in late May and remained vigorous as of mid-June. On 14-15 June, T. Miller observed Strombolian activity feeding a lava flow from a new vent on the E side of the cone.

J. Reeder provided additional observations of the 16-18 April eruption clouds and reports of continuing minor ash emission. On 16 April at 1117, Reeve Aleutian Airways Captain Edward Livingston noted a white steam plume rising to 4.6 km altitude over Pavlof. About noon the next day, MarkAir Captain Ray Wells reported a dark gray plume over Pavlof that rose to 4.9 km altitude and drifted SW over the Pacific Ocean. On 18 April at about 1130, Reeve Aleutian Captain Lee Goch saw a plume at 3 km altitude that was again drifting over the Pacific Ocean. At 1743, a jetliner crew reported to FAA Flight Control that a plume at 15.2 km altitude was drifting NE.

Weather prevented further observations of the volcano until 6 May, when Goch saw a white steam plume, containing some minor swirls and streaks of gray ash, that reached 2.9 km altitude. The plume drifted NW for 1.5 km, but traces of ash could be detected several kilometers further downwind. A minor ash deposit was visible on the S flank of the volcano. At about noon on 10 May, Reeve Aleutian Flight Engineer George Wooliver observed ash on the N and NW flanks of Pavlof and the SW upper flank of Pavlof Sister, about 4.5 km NE. Pavlof was steaming weakly, emitting a white plume that rose no more than 100 m above the summit.

The USGS received reports from several airplane pilots of renewed activity on 30 May. At 1135, MarkAir and Reeve Aleutian pilots observed an eruption cloud rising to 4.5-5 km and drifting to the west. At 1523, an ash cloud reached about 3.5 km altitude and extended NE over Pavlof Bay. At 1604, the crew of another airliner reported "heavy smoke" that rose to about 6 km altitude and drifted NE. At 1723, white steam was being emitted to <3 km altitude.

John Reeder reports that on 4 June at about 1230, Goch and MarkAir Captain Clint Schoenleber observed a white plume with no visible ash that rose to 4.9 km and drifted N. On 9 June at 1200, Captain Livingston and co-pilot Don Munson saw a gray steam and ash plume drifting 40 km to the NE. Reeder noted that Livingston's 9 June photograph suggested that a pyroclastic flow had just moved down the ESE side of the volcano and that previous pyroclastic flows had advanced down the same flank. Goch's 6 May photograph and other observations suggested to Reeder that April eruptive activity had been confined to N and NE parts of the summit [see below], depositing tephra on the NE, N, and NW sides of the volcano. Goch observed only minor steaming on 10 June.

John Taber reported that seismicity began a gradual increase about 23 May, reached high levels by 28 May, remained vigorous through 15 June, then declined slightly.

During fieldwork near Pavlof 14-15 June, T. Miller observed vigorous Strombolian activity from a new upper E flank vent ~150 m below the summit. Spatter rose 200-250 m above the vent, feeding an E flank lava flow that was ~100 m wide and reached ~1,000 m elevation [see also 11:6]. The activity produced a small ash-poor plume fed at 5-10-second intervals. Adjacent to the new vent, the old crater was emitting steam and had enlarged somewhat since 1983.

While flying past the volcano on 16 June, James Dickson observed a 600-m ash and steam plume that was drifting toward the N. Reddish-brown ash fell from the plume.

Information Contacts: J. Reeder, ADGGS; T. Miller and M.E. Yount, USGS Anchorage; J. Taber, LDGO.

June 1986 (SEAN 11:06) Cite this Report

Strombolian activity feeds lava flow; strong seismicity

USGS personnel observed Strombolian activity from a new vent E of and ~100 m below the summit during fieldwork in the area 10 June-1 July. Activity was similar on each of seven occasions when the volcano was clearly visible. Spatter from the vent fell on the upper E flank, feeding a flow [see 11:4] that had reached ~1,100 m altitude by 15 June and had advanced to ~600 m altitude two weeks later. On 29 June, the flow was ~100 m wide at 750 m altitude. Some mudflow activity (2-3 lobes) was associated with the [spatter-fed] flow. Occasional moderate ash emission occurred but the largest ash columns rose only several hundred meters above the vent. On the NNE side of the summit, the vent that had been active for the past 15 years had enlarged, probably during the April eruption, but was only steaming in June. A flow fed by spatter from the old vent, probably in April, was snow-covered [see 11:7]. Thick April ashfalls were sampled NW of the volcano, and large April mudflows and significant flooding were evident.

When John Reeder flew past Pavlof on 8 July at 1500, ash ejection fed a plume that reached about 3900 m altitude and drifted approximately 15 km WSW. Flows were seen on the SE and N sides, and black ash covered the summit area.

Seismicity declined slightly 17 June, but was still strong as of 4 July. No harmonic tremor was recorded. Seismicity was stronger on GMT days 28 June and 1 July than other days between 17 June and 4 July, but USGS personnel did not observe significant fluctuations in eruptive activity.

Information Contacts: T. Miller, USGS Anchorage; J. Taber, LDGO; J. Reeder, ADGGS.

July 1986 (SEAN 11:07) Cite this Report

Continued Strombolian activity and vigorous seismicity

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 1/2 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).

Table 1. Summary of reports describing activity at Pavlof, 14 June-15 September 1986, compiled by John Reeder. Observers (initials in brackets): FWS; Jean Shaul, Marcia Brown, and Clayton Brown, Cold Bay; Reeve Aleutian Airways; MarkAir; Peninsula Airways; Sand Point Air; NOAA aircraft; Clint Schoenleber and Jerry Chisum, Markair; John Reeder; Mike Whelan; Lee Goch, Don Munson, George Wooliver, Harold Black, Andy Livingston, and Edward Livingston, Reeve Aleutian Airways; Mary Maurer; Adrian Brown; Robert Adams; Steve Hakala; Guy Morgan; Chris Dan; John Sarnis; Mike and Peggy Blenden; Jeff Wilson; Jeff Backlund; Federal Aviation Administration; Wayne Russell; British sailors on the Ashley St. Mary; Theresa Dubber and Robert Senimore, FAA, Cold Bay; Steve Hakala, Sand Point.

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.

Information Contacts: T. Miller, USGS Anchorage; J. Taber, LDGO; J. Reeder, ADGGS.

August 1986 (SEAN 11:08) Cite this Report

Strombolian activity and ash emission ends

Airplane pilots frequently reported vapor and ash rising to 3.5-4 km altitude from Pavlof through 22 August (table 1). Sailors observed lava fountaining during the evening of 13 August, but airplane pilots generally do not fly close enough to the volcano to observe Strombolian explosions and most flights are in daylight when eruptive glow is unlikely to be evident. No pilot reports of activity were received between 22 August and 8 September, although there was some clear weather during that period. On 8 September, vigorous vapor emission was observed from Sand Point, 90 km E. During an overflight on 12 September, only minor steaming was observed.

Information Contacts: T. Miller, USGS Anchorage; J. Taber, LDGO; J. Reeder, ADGGS.

October 1986 (SEAN 11:10) Cite this Report

Ash emission and lava fountaining

Airplane pilots reported that ash emissions and lava fountaining resumed in early November. No activity had been reported since mid-September. Observers named below are Reeve Aleutian Airways pilots; other reports are from the FAA.

The start time of the eruption is not known, but on 30 October a clear view of the snow-covered volcano showed that it remained inactive, without vapor emissions. No other observations of Pavlof are known until 1100 on 6 November when activity was first reported. At 1130 Lee Goch and Don Munson observed a 2750 m-high plume of dark ash and white vapor drifting more than 40 km to the NE. An incandescent lava fountain (about 10 m high) emerged from a vent just SE of the summit. Two fresh-looking, gray-black, linear, flow-like features, 120 m and 50 m wide, extended about 1/3 of the way down the SE and NW flanks. Neither feature emitted any steam, and both were free of snow. The rest of the volcano remained covered with white snow. At 2045 a pilot reported that lava fountaining was continuing at the SE summit vent.

On 10 November at 1135 James Fredenhagen and Ken Gendron observed bubbling incandescent lava and a 10-m-high, 6-m-wide fountain in a crater on the upper SE flank, ~120 m below the summit. A vertical ash and steam plume rose to ~300 m above the vent.

Fredenhagen did not see incandescent lava during a 12 November flight at 1140, but vapor and ash were released from the SE summit-area vent and drifted eastward. Earlier that day he passed through a haze layer at 1,500-1,800 m altitude during his approach to Sand Point, 90 km to the east. A fissure-like feature extending from the summit to at least halfway down the SE flank appeared to be emitting black ash and depositing it nearby. No incandescent material or vapor was visible in the fissure.

On [13] November Edward Livingston reported that grayish-black ash deposits covered the entire SE side of the volcano. At 1200 on 17 November a 3,600-3,900-m-high plume issued from the vent below the SE crater. Black ash pulses occurred at 5-8-minute intervals and winds blew the ash SE in a 70-km-long cloud. By afternoon (around 1400-1630) the FAA reported that the plume had grown to 4,500 m height.

On 18 November the FAA reported that the semi-continuous plume was 4200 m high between 1000 and 1030. At 1512 an image from the NOAA 9 polar orbiting satellite recorded a weak plume extending 50-60 km N. The FAA has warned pilots in the area not to go below 3750 m and to avoid the vicinity of the volcano.

Information Contacts: J. Reeder, ADGGS; M.E. Yount, USGS Anchorage.

November 1986 (SEAN 11:11) Cite this Report

Eruption continues; lava flow reaches ocean

The eruption continued into December with steam plumes from the N-summit vent and ash emissions from a SE flank vent 200-300 m below the summit (table 2). Steam and ash emissions were continuing at 1840 on 10 December when pilots saw an orange-red glow in a cloud above the summit. They did not observe any material moving through the clouds. The next day at 1324, airplane pilots reported that 3-4 lava flows had moved down the SSE flank and that one had reached the Pacific Ocean at Pavlof Bay, 10 km from the summit [but see 11:12]; a 300-m-high steam column rose where the flow entered the water.

Table 2. Summary of reports describing activity at Pavlof, 17 November-16 December 1986, collected by John Reeder. Observers (initials in brackets): Edward Livingston, Lee Goch, James Fredenhagen, Don Munson, and Harold Black (Reeve Aleutian Airways); Clayton Brown (Alaska Fish and Game); Jerry Chisum (MarkAir); Marsha Brown (FAA, Cold Bay); unnamed observers from Reeve Aleutian Airways (RA), MarkAir (MA), Peninsula Airways (PA), and a private plane (Cessna).

Seismicity in early November reached more than 150 events/day, all apparently explosion shocks. Individual events had similar durations but variable amplitudes. Only a few hours of tremor were recorded.

Information Contacts: J. Reeder, ADGGS; J. Taber, LDGO.

December 1986 (SEAN 11:12) Cite this Report

Strong tremor accompanied early December lava flows; steaming from summit and flank fissures

John Taber reported that from 1 to 6 December, 140-200 volcanic events/day were recorded; 30-60 were explosions with a distinct airwave arrival. On 8 and 9 December the number of individual events gradually decreased until replaced by low-amplitude harmonic tremor. Tremor amplitude gradually increased until the record suddenly became saturated on 9 December at 2310, remaining saturated until 13 December at about 1400. Tremor continued to decrease until discrete events were again visible by about 0100 the next morning.

Airplane pilots had observed steam and ash emissions from summit area vents in early December, saw glow on 10 December, and new SSE flank lava flows the next day (SEAN 11:11). Closer examination by Harold Black, James Fredenhagen, and Donald Munson (Reeve Aleutian Airways) revealed that the narrow lava flow that appeared to have reached the ocean on 11 December had in fact stopped in a stream bed about 600 m short of Pavlof Bay. The flow was black and emitting no steam when Fredenhagen observed it on 20 December.

On 18 December, Fredenhagen observed that a fissure, emitting steam plumes to as much as 60 m height at intervals of 6-15 m, extended from the SE summit vent, active in the November eruption (SEAN 11:10-11), to about halfway down the SE flank. There it split at about a 90° angle, with both segments continuing for another 300 m downslope; steam issued from both segments for at least 150 m. On 23 December, he also observed small puffs of steam emerging from the old NE summit vent. During clear weather on 27-28 December, Marcia Brown (FAA, Cold Bay) saw steam issuing from the NE summit vent and moving about 300 m down the NE flank. On the 29th, the plume was blowing horizontally about 300 m to the E. The volcano was snow-covered with no detectable ash deposits. Poor weather prevented further observations.

Information Contacts: J. Reeder, ADGGS; J. Taber, LDGO.

January 1987 (SEAN 12:01) Cite this Report

Steam emission from two near-summit vents

Satellite images at 0514 and 1041 on 9 January show plumes drifting 150 and 100 km ESE from Pavlof. Between 17 January and 11 February, airplane crewmembers and observers at Cold Bay reported steaming from two near-summit vents (table 3).

Table 3. Summary of observations describing activity at Pavlof, 17 January-11 February 1987, collected by John Reeder.Observers (initials in brackets): Marsha Brown and Theresa Dubber, FAA, Cold Bay; Lee Goch, Don Munson, Janice Reeve Ogle, and James Fredenhagen, Reeve Aleutian Airways; Guy Morgan, Peninsula Airways.

Information Contacts: J. Reeder, ADGGS; Steve Shivers, USGS Anchorage.

March 1987 (SEAN 12:03) Cite this Report

Fresh ash on summit

At 1105 on 22 March Marsha Brown (FAA, Cold Bay) saw a steam plume emerging from the upper NE flank vent and drifting NE. The summit appeared to be covered with a thick layer of fresh ash. Minor steam emission occurred at other times during the month.

Information Contacts: J. Reeder, ADGGS.

May 1987 (SEAN 12:05) Cite this Report

Ash emission; flow; seismic amplitudes increase

The 15-month eruption continued in May. The summit was visible on several days from the end of May until 9 June. On about 22 May, Warren Johnson (Kenai Float Plane Air Service) observed a small lava flow and minor ash emission from a vent on the NE flank near the summit. On 28 May at 0627, Marsha Brown (FAA, Cold Bay) observed emission of black ash puffs at 5-minute intervals from 2 vents on the NE flank, one near the summit and the other halfway down the flank. The near-summit vent was emitting more ash. Ash from both vents rose about 800 m above the volcano and drifted about 10 km SE. Ash deposits covered the NE flank. At 1200 that day Lee Goch (Reeve Aleutian Airways) saw only minor steam emission.

On 30 May, Edward Livingston (Reeve Aleutian Airways) observed fairly steady black ash emission from the near-summit vent rising to ~300 m above the summit and drifting 24 km SSE. The SE flank was blackened by ash while the rest of the volcano was snow-covered. Ash emission was continuing at 1956 when a Peninsula Airways aircraft passed.

On 2 June at 1615 Pavlof was again visible and James Gibson (Reeve Aleutian Airways) observed, from more than 25 km away, a black steaming lava or debris flow on the NE slope that originated from the near-summit vent. The flow extended into the saddle between Pavlof and Pavlof Sister, then turned NW. Ash rose about 600 m above the near-summit vent and to about summit altitude from a source in the flow about 1/3 of the way down the NE flank.

On 6 June at 1601, the pilot of a small aircraft saw an ash plume rising to 600 m above the summit and drifting S. The 2 June flow had descended to 120 m elevation and steam was rising where the flow touched snow. On 9 June only steam emission from the upper NE vent was seen but the entire summit area was covered with ash.

Volcanic earthquake amplitudes and tremor were greatly increased from about 5 to 10 June and were much higher than during most eruptions.

Information Contacts: J. Reeder, ADGGS; J. Taber, LDGO; T. Miller, USGS Anchorage.

June 1987 (SEAN 12:06) Cite this Report

Incandescent flow; summit glow; ash emission continues

The eruption . . . continued in June. On 7 June at 1601 pilot Richard Williams (Peninsula Airways) saw an incandescent flow (probably lava) that had descended to 1,200 m elevation on the NNE flank. Ash was rising to 600 m above the volcano from the NE near-summit vent, drifting S. Forty minutes later the ash plume reached 760 m. Residents of Nelson Lagoon, 80 km NE, reported a bright red glow from the summit throughout the night. On 9 June at 1153 a pilot reported only steam emission from the NE near-summit vent.

The following day at 1419 the U.S. Coast Guard reported that gray ash was being emitted from the active vent and 7 hours later pilot Harold Wilson (Peninsula Airways) saw (and photographed) gray then black-gray ash emission. A plume rose to 760 m above the NE vent and drifted 2 km W. The black-gray ash emission was intermittent and lasted for about 15 minutes. Only minor steam emission was reported on 11 June but dark ash emission had resumed on 12 June at 1602 when Chuck Nickerson and George Wooliver (Reeve Aleutian Airways) passed the volcano.

A lava or debris flow seen from 25 km away during the afternoon of 2 June was reported in 12:5. That evening from 2130 to 2200 Marsha Brown (FAA, Cold Bay) saw an incandescent flow move down the NNE flank. Richard Williams observed the incandescent flow from Nelson Lagoon (80 km NE) and estimated that it extended almost 1/3 of the way down the volcano.

Information Contacts: J. Reeder, ADGGS.

July 1987 (SEAN 12:07) Cite this Report

Activity decreases; steam and ash emission

Activity appeared to decrease during July. A 460-m-high steam plume was sighted on 6 July by R. Williams (Peninsula Airways) from Nelson Lagoon, and a small amount of what appeared to be ash on the summit and N flank was reported on 11 July by Marsha Brown (FAA, Cold Bay). The weather was generally poor for the rest of the month but numerous pilots reported that no eruptive activity was occurring on 26 and 28 July.

Information Contacts: J. Reeder, ADGGS.

August 1987 (SEAN 12:08) Cite this Report

Continued ash emission

Eruptive activity was continuing in late August. At about 1130 on 29 August, pilot Chuck Nickerson (Reeve Aleutian Airways) observed a thick dark ash horizon at 1200-1800 m altitude extending 50 km SSW from Pavlof. The ash appeared to originate from a SE flank vent. Below that vent, white steam was rising along a 100 m, narrow, flow-like, feature or crack. At 1550 pilots Andy Livingston and James Fredenhagen (Reeve Aleutian Airways) observed from Sand Point (88 km E), a diffuse ash layer at 1200-1800 m altitude that extended at least 15 km SW. Ben and Lorie Kirker reported that no ash reached King Cove, about 40 km SW. On 30 August at 1400 Theresa Dubber (FAA) saw no ash being emitted during a clear view of the volcano from Cold Bay.

Information Contacts: J. Reeder, ADGGS.

September 1987 (SEAN 12:09) Cite this Report

New ash deposit

Marsha Brown (FAA, Cold Bay) observed "fairly extensive" black ash deposits on Pavlof's flanks during a flight on 4 September. Both the NE and SE summit-area vents were emitting steam. The ash deposits had not been visible when the volcano was observed on 30 August. On 20 September the volcano was inactive and fresh snow covered the flanks.

Information Contacts: J. Reeder, ADGGS.

October 1987 (SEAN 12:10) Cite this Report

Ash-rich explosions; blocks and spatter form flow

Activity increased 17 October when ash-rich explosions from the NW summit vent occurred every 30 seconds to several minutes. Ash plumes, reported through 9 November attained a maximum altitude of 4500-5800 m on 30 October (table 4). Plumes stretched E as much as 200 km and tephra fell at Canoe Bay, 45 km ENE, on 19 October. Blocks and spatter ejected to 30 m from the near-summit vent formed a short spatter flow. Night glow from the summit was visible on several evenings in mid-October. Incandescent rocks and spatter were reportedly continuing in early November. Ash deposits were last reported on 4 September (SEAN 12:09).

Table 4. Reports of activity at Pavlof, 16 October-27 November 1987, compiled by John Reeder and M.E. Yount from the following observers: Steve Hakala (Canoe Bay); Dan Coy (Sand Point Air); Scotty Gibbens and John Sarvis (USFWS); Deedee and Tom O'Malia (Cold Bay); Marsha Brown, Theresa Dubber, Chuck Taylor, Jim Yakal (FAA Flight Service, Cold Bay); Steve Shivers (USGS, Anchorage); James Gibson, George Wooliver, Chuck Nickerson, Gary Lintner, Harding (Reeve Aleutian Airways); Guy Morgan, Bryan Carricaburu, Doug Ruberg (Peninsula Airways); Harold Johnson Sr. (Nelson Lagoon); Will Gould (NOAA/NESDIS); John Sarvis (USFWS); Sand Point Air pilot; James Fredenhagen (Reeve Aleutian Airways);Coast Guard pilot. Unattributed reports were compiled by M.E. Yount.

Information Contacts: J. Reeder, ADGGS; M.E. Yount, USGS Anchorage.

November 1987 (SEAN 12:11) Cite this Report

Dark ash plumes from near-summit vent

The volcano had apparently been quiet for several weeks after new flank ash deposits were seen 4 September; it was inactive and covered with fresh snow on 20 September. A NOAA 9 satellite image on 30 September at 0525 showed a plume extending about 20 km S from Pavlof. Weather clouds obscured later activity.

Eruptions began again in mid-October and continued in November (table 4). Dark ash was steadily emitted to 250 m above the summit on the morning of 5 November. The next day wind blew ash down the SE flank for 120 m; the plume trailed about 30 km SE. Continued ash emission was observed on 6, 7, and 9 November. Plumes reached to 3.6 km altitude (~1 km above the summit) and drifted a maximum of 35 km from the volcano. During observations on 16 and 27 November only white steam was emitted.

Information Contacts: J. Reeder, ADGGS; M. Matson, NOAA/NESDIS.

February 1988 (SEAN 13:02) Cite this Report

Weak steam and ash emission

During observations that were limited by poor weather December-February, only weak steam and ash emission was reported (table 5).

Table 5. Reports of activity at Pavlof, 9 January-2 March 1988, compiled by John Reeder from the following observers: Theresa Dubber, James Yakal, and Marsha Brown (FAA, Cold Bay); Guy Morgan (Peninsula Airways).

Information Contacts: J. Reeder, ADGGS.

July 1988 (SEAN 13:07) Cite this Report

Fresh ash on upper flanks

At 1800 on 20 July, Marsha Brown observed light steam emission from Pavlof's NE summit vent. The volcano's upper flanks appeared black with fresh ash, with more on the N than on the S slope. Six days earlier, at 1519 on 14 July, this ash had not yet been emitted and no steam was evident.

Information Contacts: J. Reeder, ADGGS.

August 1988 (SEAN 13:08) Cite this Report

New ash deposit on summit

At 1000 on 13 August, M. Brown (FAA) observed fairly extensive ash deposits on Pavlof's flanks. From Cold Bay ~20% of the visible edifice appeared dark gray and the remainder was black. The ash had been deposited since the volcano was last observed on 7 August. A wispy dark plume rose <300 m from the NE summit vent. The next day at 0740, Peninsula Airways pilot Gary Joseph reported that the volcano was covered with white snow and no ash or steam was being emitted.

Information Contacts: J. Reeder, ADGGS.

August 1996 (BGVN 21:08) Cite this Report

Vigorous seismicity and intermittent eruptive activity

Residents of the Alaska Peninsula first noticed small glowing plumes from Pavlof on 15 September. During the following week, seismicity was vigorous and eruptions were intermittent. Poor weather in the first week inhibited visual observations.

On the morning of 16 September, AVO received a report from residents of Cold Bay about an unusual plume emanating from the N flank of the volcano. Local pilots reported glow near the summit and large "car-sized" fragments being ejected from the summit vent. Satellite imagery showed a hot spot in the vicinity of the cone. Seismic data from stations on and near the volcano suggested a low-level eruption. By that afternoon eruptive activity had declined somewhat, although seismicity indicative of eruption remained sporadic.

Persistent seismic activity suggested that an episodic, low-level eruption continued during 17-22 September. The summit hot spot was not seen on images obtained on the early morning of 17 September. Several periods of increased seismicity from the afternoon of 17 September to the morning of 18 September suggested that the eruption was episodic in character with ejection of ash and bombs up to 300 m above the summit of the cone.

Intermittent low-level ash clouds were detected on satellite imagery on 18 September, although imagery from that afternoon to the next morning showed no ash cloud. Pilot reports on 18 September confirmed that there was no significant ash venting above the cloud tops at 3,000 m altitude. Satellite imagery from the afternoon of 19 September to the morning of 20 September also detected no ash cloud.

From the afternoon of 20 September to the afternoon of 22 September several periods of increased seismicity were observed and intermittent low-level ash clouds were detected on satellite imagery. On 23 September local observers confirmed Strombolian fountaining 150-200 m above the summit. There were no major explosions.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA, 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.

September 1996 (BGVN 21:09) Cite this Report

Increasing seismicity corresponds to stronger eruptive activity

Residents of the Alaska Peninsula observed small glowing plumes from Pavlof on 15 September. During the next week, seismicity was vigorous and eruptions were intermittent (BGVN 21:08). At 1328 on 24 September seismicity began to increase, suggesting stronger eruptive activity. This increased level of seismicity persisted through the first half of October. Visual observations and satellite imagery verified that increased seismicity correlated with eruptions of ash and bombs up to 1,200 m above the summit.

On 26 September satellite imagery showed a small steam-and-ash plume extending ~45 km SE. A pilot subsequently reported a steam plume to an estimated altitude of 3,700 m. AVO staff doing airborne observations during 27-30 September reported low-level fountaining and occasional small explosions of incandescent material in the summit crater. The small explosions produced sporadic steam-and-ash plumes to 610 m above the vent. The largest plume drifted S for ~110 km and appeared faintly on satellite images. Incandescent spatter was deposited on the NW summit slope or moved down a deep gully on the NW side of the volcano.

During 4-11 October lava fountaining from two vents continued to heights of a few hundred meters above the summit. Incandescent spatter-fed lava flows moved down the steep, snow- and ice-covered slope, widening at the base and extending NW. Occasional water-rich slurries of ash and mud descended the N flank. Diffuse plumes of steam, gas, and ash rose to as high as 6 km above sea level and drifted 160 km downwind. On 15 October eruptive activity increased and seismicity reached the highest levels yet observed. Satellite imagery and pilot reports showed ongoing lava fountaining from two vents near the summit. Pilot reports indicated that diffuse ash layers reached 7,300-m altitude and extended perhaps as far as 50 km SE.

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.

October 1996 (BGVN 21:10) Cite this Report

Eruptions that began on 15 September persist during November

The current episode of eruptive activity, which began on 15 September (BGVN 21:08-21:09), persisted during 15 October-29 November. Seismicity, satellite imagery, and visual observations indicated ongoing lava fountaining from two vents near the summit. Two lava flows advanced down the flanks of the volcano. Intermittent bursts of lava to heights of <300 m above the crater continued. These bursts formed diffuse ash layers that pilots reported to be typically below an altitude of 6,100 m.

From 2200 on 18 October intense seismicity was recorded, but it ceased the next morning (19 October). This increased seismic activity indicated that the eruption strengthened considerably. On 22 October, seismicity showed a short-lived burst at about 1300, and at 1330 pilots reported an ash plume rising to an altitude of 7,600 m. At 1450 satellite observations showed a narrow ash cloud 90 km long drifting NE up the peninsula. During 26 October-1 November, intermittent narrow plumes, extending up to several hundred kilometers downwind, were visible on satellite images. Light ashfall was observed at Nelson Lagoon on the morning of 28 October.

On 4 November, seismicity significantly increased at 0800 and declined several hours later. This seismic increase was followed by an increase in eruptive activity. Satellite imagery showed a plume extending 250 km to the NE. Pilots reported the cloud top at 5,800-m altitude. During 9-15 November, diffuse ash layers occasionally reached altitudes of 7,000-7,300 m. Observers in Sand Point, Nelson Lagoon, and Cold Bay reported a steam-and-ash plume, rising from low on the N flank, to altitudes as high as the plume issuing from the summit vents. Although the source of the plume was not directly observed, the presence of ash suggested the possibility of a new vent low on the N flank. Two half-hour periods of intense seismicity, beginning at 1130 on 22 November and at 0430 on 23 November, resulted in steam-and-ash emissions. The plumes reached altitudes of 9,100 m and extended downwind. A small ash plume from the 23 November event was detected moving several hundred kilometers to the NW. However, no ashfall was reported from local communities.

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.

December 1996 (BGVN 21:12) Cite this Report

Intermittent eruptions from 15 September through [3] January

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.

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.

January 1997 (BGVN 22:01) Cite this Report

Pause in eruptive activity, but continued small intermittent steam plumes

The latest eruption began on 15 September and continued intermittently through 3 January, when a small ash burst was observed just above the summit (BGVN 21:08-21:12). On the morning of 24 January a pilot reported weak emission of steam and possibly minor ash rising to ~4.2 km altitude, mixing with a cloud layer, and drifting SE. Very low levels of seismicity were recorded from early January through late February.

A ground observer who glimpsed the volcano on 4 February reported that the summit vent area, which had been snow-covered the previous week, was now bare. On 6 February the same observer saw a small steam plume rising from the vent area to the level of the summit. U.S. National Weather Service observers in Cold Bay (60 km SW) reported another small steam plume up to 300 m above the summit vents during 15-16 February. A similar steam plume to 600-900 m above the vents was reported by pilots on 19 February.

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.

September 1997 (BGVN 22:09) Cite this Report

Steam plume in June; ash covering ice near summit seen in July

The latest eruption began on 15 September 1996 and continued intermittently through 3 January 1997, when a small ash burst rose just above the summit (BGVN 21:08-21:12). On the morning of 24 January a pilot reported steam and possibly minor ash emissions. A ground observer reported on 4 February that the summit vent area, which had been snow-covered the previous week, was bare. Small steam plumes, varying in height up to 900 m above the summit, were reported on 6, 15-16, and 19 February (BGVN 22:01).

Observers in Cold Bay (60 km SW) reported snow-free conditions at the summit during 8-21 March. Satellite images at the time did not show any thermal anomalies. By 4 April seismicity had returned to a near-background level, after fourteen weeks of eruptive quiescence, and the alert status was lowered from Yellow to Green. A slight increase in the level of seismicity began on 1 June and continued through the 6th. On the afternoon of 2 June U.S. National Weather Service observers in Cold Bay saw a steam plume rising and increasing in vigor to 900 m above the summit. This event prompted an increase in the alert status to Code Yellow. Satellite images recorded a moderate thermal anomaly on 3 June. Seismic activity declined during 7-13 June. Although fluctuations continued, the level was near background, so on 13 June the status returned to Code Green. Clouds obscured the volcano during 4-13 June.

During fieldwork on 8 July, Alaska Volcano Observatory (AVO) scientists had a clear aerial view of the volcano. They circled the volcano in an upward spiral, observing that most of the glacial ice was still intact. At a distance, the Pavlof cone appeared black, which had led to the belief that the glacial ice had been melted by volcanic heat. Closer inspection showed that the dark coloring was due to ash covering the snow; bright blue glacial ice could be seen in crevasses. The adjacent peak of Pavlof Sister showed an ash concentration gradient; higher concentrations made it appear darker towards its base. The only parts where the ice seemed to be missing corresponded to a rust-red region where lava had recently flowed. Approaching the summit of Pavlof, the scientists observed sulfur deposits within and around the perimeter of the jagged crater walls. The crater was filled in with debris and tephra; only remnant steam remained around the crater, and no active degassing could be seen around the buried vent.

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.

August 2007 (BGVN 32:08) Cite this Report

Thermal and seismic data presage August 2007 eruption

The Alaska Volcano Observatory (AVO) reported that Pavlof (figure 1) erupted on 15 August 2007 for the first time since 15 September 1996 (BGVN 22:09 and Waythomas, Miller, and Mangan, 2006) . Thermal anomalies and seismic activity just prior to the eruption prompted scientists at AVO to issue a warning. This report covers events reported through 3 October 2007.

Figure 1. Index map showing the location of Pavlof and other Alaska Peninsula volcanoes. Courtesy of AVO and Alaska Division of Geological & Geophysical Surveys.

According to AVO, an abrupt increase in earthquake activity began at Pavlof early on the morning of 14 August 2007. Based on patterns of unrest leading to past eruptions at Pavlof, AVO elevated the alert level and color code to 'Advisory/Yellow.' Observers from the National Oceanic and Atmospheric Administration's (NOAA) National Weather Service (NWS) in Cold Bay, Alaska, were able to see the volcano on the morning of 14 August and reported no anomalous steaming or other activity; satellite imagery from this morning also showed no obvious signs of surface activity or ash emission.

[AVO detected a strong thermal anomaly at the volcano overnight 14-15 September, and seismic activity continued to increase in both the number of events per hour and duration of individual events. Eyewitnesses aboard a ship reported incandescent blocks tumbling down the ESE flank of the volcano beginning at midnight 14 September (the night of 14-15 September). Satellite data confirmed the presence of lava. Pilot reports indicated that a weak ash plume extended 8 km SW of the summit at a height of ~ 2.6 km. Seismic activity continued at a high level. On 15 August AVO raised the aviation color code from Yellow to Orange and the Alert Level from Advisory to Watch.]

Earthquake intensity continued to increase slowly from 15 to 16 August. Strong signals at a single station SE of the summit suggested local flow activity, probably lahars (or mudflows) on that flank. Satellite images of the volcano overnight and during the morning of 16 August continued to show a strong thermal feature (figure 2). Residents of both Cold Bay and Sand Point, Alaska (105 km and 70 km, respectively, from Pavlof), observed incandescence at the summit during the night.

Figure 2. NOAA Advanced Very High Resolution Radiometer (AVHRR) satellite image showing a strong thermal anomaly at the summit of Pavlof on 16 August 2007 at 0750 local time (1550 UTC). In this image, white represents hot temperatures. Courtesy of the AVO/U.S. Geological Survey (USGS).

Persistent earthquake activity and flow events, probably lahars (mudflows), continued on 17 August 2007. Several discrete explosion earthquakes were also recorded. Though clouds obscured the volcano in most satellite images, one GOES (Geostationary Observational Environmental Satellite) image documented a large thermal feature at the summit, interpreted to be lava at the surface.

Activity at Pavlof continued to increase during 17-24 August 2007, with reports that the steam-and-ash plume sometimes exceeded 3 km altitude. For example, a pilot reported the top of the plume to be 5.5 km in the late afternoon of 23 August, and a plume height of 4 km was estimated using satellite data from 1410 that day. Seismic activity remained elevated, with moderate levels of tremor occurring almost continuously and with occasional bursts of higher amplitude. The average seismic amplitude increased slowly throughout the week of 17-24 August. Many small-to-moderate explosions were recorded in the seismic record, as were events from lahars flowing down the SE flank. [Note: Pilot Jeff Linscott of JL Aviation filmed a lahar front on Pavlof's lower flanks before it hit the ocean on 18 August 2007; the film is available on the AVO website, which is listed in under Information Contacts below.] Satellite data showed strong thermal anomalies at the summit, as well as occasional ash clouds, throughout this week.

An AVO field crew visited Pavlof on 18-19 August to make FLIR (forward looking infrared) thermal observations of the ongoing eruption. These observations confirmed the existence of a new vent ~ 200 m below the summit on the SE flank. The vent, ~ 50 m across, fed a lava flow that, on 18 August, was more than 0.5 km long and ~ 25 m across. The crew also observed a lahar reaching the Pacific coast, incandescent lava, and explosions at the vent that sent 5-m-long blocks flying 50 m through the air. Figure 3 shows the plume from Pavlof on 23 August, and figure 4 shows the plume on 30 August 2007.

Figure 3. An Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite image on 23 August 2007 at 2210 UTC showing Pavlof with a small steam-and-ash plume emitting from the crater on the SE side of the summit. The plume in this image (having a resolution of 15 m/pixel) reached an altitude of ~ 4.0 km based on the plume's temperature. Courtesy of the AVO/USGS.

Figure 4. Pavlof volcano and eruption plume on evening of 30 August at 2120 local time. View is to the S, out of the right side of a PenAir Metro Airline plane en route to Anchorage from Cold Bay; plume height was approximately 5.2-5.5 km. Courtesy of Chris Waythomas and AVO/USGS.

At about 2130 local time on 31 August, NOAA/NWS observers in Cold Bay reported a substantial plume emanating from Pavlof, along with associated lightning. The plume, which rose to an altitude of ~ 6 km, was also visible in images from the Pavlof web camera located in Cold Bay. However, there were no indications in satellite data or ground reports of an ash plume. Seismic activity remained elevated through 31 August.

During 1-19 September 2007 the eruption continued; however, seismicity after 10 September declined markedly from levels recorded earlier. AVO pointed out that typical eruptions at Pavlof were characterized by periods of diminished activity interspersed with periods of renewed eruptive activity. Satellite observations continued to show thermal anomalies even through the clouds, as well as steam plumes up to as high as 6.1 km altitude. Table 6 shows thermal anomalies from the beginning of 2007 through 3 October measured by MODIS satellite infrared detectors and processed by the Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System called MODVOLC. Anomalies measured during 2007 began on 15 August and continued through 11 September, after which none have been reported to present (3 October). Satellite thermal anomalies are frequently masked by cloud cover.

Table 6. MODIS/MODVOLC thermal anomalies measured at Pavlof for 15 August-11 September 2007; no anomalies were during 1 January-14 August or 12 September through 3 October. Courtesy of the HIGP Thermal Alerts System.

A status report on 3 October 2007 stated that "A pause in eruptive activity at Pavlof continues. Seismicity remains at low levels and has been relatively unchanged since about September 13. No sign of renewed volcanic activity was noted in clear satellite and web camera views today." [By 5 October the alert levels were returned to Green and Normal for aviation after more than three weeks without eruptive activity; the eruption was determined to have ended on 13 September 2007.]

References. Waythomas, C.F., Miller, T.P., and Mangan, M.T., 2006, Preliminary Volcano Hazard Assessment for the Emmons Lake Volcanic Center, Alaska: Anchorage, Alaska, U.S. Geological Survey, Scientific Investigations Report 2006-5248, 33 p., 1 sheet (available online at http://www.avo.alaska.edu/pdfs/SIR2006-5248.pdf ).

Linscott, J., 2007, Film of Pavlof lahar front, 18 August 2007 [on AVO website, URL: http://www.avo.alaska.edu/volcanoes/volcimage.php?volcname=Pavlof ).

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667, USA; Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA; and Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://www.avo.alaska.edu/); Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Jeff Linscott, JL Aviation Helicopter Service, 8015 NE Airport Way, Portland, OR 97218 USA.

May 2013 (BGVN 38:05) Cite this Report

Eruption in May-June 2013 with lava flows and ash emissions to ~8.5 km a.s.l.

Pavlof, the most active volcano in the Aleutian arc, erupted on 13 May 2013. Before this, it had most recently erupted on 15 August 2007, following an 11-year period of quiescence. The eruption that began in May 2013 continued through June before slowly subsiding to background levels by 8 August. Pavlof generated several ash plumes during the six-week eruption that disrupted aviation, including an 8-km high plume on 24 June. As in past Pavlof eruptions, the recent eruptions fluctuated in intensity. This report briefly discusses earthquake data during 2007-2011 and, in greater detail, the series of eruptions during May and June 2013.

According to Mangan and others (2009), Pavlof has discharged more than 40 recorded eruptions within the previous 200 years, producing mostly basaltic andesite to andesite products. That work, discussed in a separate subsection near the end of this report, also discusses the adjacent 12x19 km Emmons Lake caldera (a chain of nested calderas) on Pavlof's SW flank (figure 5). The Emmons Lake Volcanic Center (ELCV) is used to collectively describe the entire complex, including the nested caldera, intra-caldera stratovolcanoes, and the adjacent stratovolcanoes (including Pavlof) to the NE.

Figure 5. Maps showing both the location of Pavlof on the lower Alaska Peninsula (upper left) and showing more details of the complex, including the elongate Emmons Lake caldera and six stratovolcanoes. The lake occupies but a small volume of the caldera, which is breached to the SE. Taken from Mangan and others (2009).

According to the Alaska Volcano Observatory (AVO), 48 earthquakes were located beneath Pavlof in 2007, the year of the previous eruption. During the following non-eruptive years, AVO reported 9 earthquakes centered at Pavlof in 2008, 7 earthquakes in 2009, 19 in 2010, and 13 in 2011. As of this writing, AVO has not yet published 2012 earthquake data.

Eruption in May 2013. On 13 May 2013, seismicity increased at 0800 and an intense thermal anomaly was observed at the summit in satellite imagery. Several spikes in seismicity occurred between 0900 and 1000. AVO noted that similar patterns of seismicity and elevated surface temperatures in previous cases had signaled the onset of eruptive activity at Pavlof. The Volcanic Alert Level was increased to Watch (the second highest category of four) and the Aviation Color Code was increased to Orange (the second highest category of four).

On 14 May 2013, pilot reports and satellite images indicated a spatter-fed lava flow that had advanced about 0.5 km down the N flank. The advancing lava had also generated debris-laden deposits, presumably from the interaction of hot lava with snow and ice on the flank. According to AVO, a diffuse ash plume drifted about 160 km NE at an altitude of 4.6 km before dissipating. Minor ashfall was reported the evening of 14 May in a mining camp 80 km NE of the volcano. No other nearby communities reported ashfall. Minor steam-and-ash emissions from the summit were visible from Cold Bay (~58 km SW).

During 14-15 May 2013, elevated seismicity persisted. Steam-and-ash clouds observed with a web camera at Cold Bay (55 km W of the volcano) occasionally rose to an altitude of 6.1 km. Residents in Cold Bay observed incandescence from the summit during the night. On 15 May a pilot reported a dark ash cloud drifting ENE at an altitude of 6.1 km.

On 16 May, AVO observed lava fountaining at the summit and a continuous ash, steam, and gas cloud extending 50-100 km downwind at an altitude of about 6.1 km. Satellite images showed persistent elevated surface temperatures at the summit and on the NW flank, consistent with lava fountaining at the summit and the resulting lava flow.

During 18-19 May 2013, reports noted that a narrow plume of steam, ash, and gas occasionally rising up to an altitude of 6.7 km and drifting SE was visible in satellite and pilot images (figures 6 and 7). Pilots noted that lava fountaining and ash emission continued. Overnight, trace amounts of ash fell on the community of Sand Point (88 km E). During the afternoon on 19 May, pilots reported that ash plumes rose to altitudes of 4.6-6.7 km. Trace amounts of ash fell in Nelson Lagoon (78 km NNE) during 19-20 May.

Figure 6. Photograph of Pavlof taken on 18 May 2013 by astronauts aboard the International Space Station. The space station was ~770 km away and S-SE of the volcano when the photograph was taken. The volcanic plume extended SE over the North Pacific Ocean. Residing next to Pavlof is the white, seemingly ash free stratovolcano Pavlof Sister. Courtesy of NASA Earth Observatory with credit for caption and processing to Robert Simmon, (NASA Earth Observatory) and G. M. Gentry (DB Consulting Group at NASA-JSC).

Figure 7. Photo of Pavlof eruption taken by a commercial pilot on 18 May 2013. Plume direction was not identified, but based on the NASA photo taken the same day (figure 6), the plume is drifting SE and the volcano in the foreground is Pavlof Sister (NE of Pavlof). Courtesy of Brandon Wilson (PenAir) and provided by AVO/Alaska Division of Geological & Geophysical Surveys.

News articles (Associated Press, PRNewswire, Alaska Dispatch) stated that during 19-21 May 2013 two regional airlines canceled flights to several remote communities and delayed or re-routed other flights. On 21 May AVO reported that a low-level plume of steam, gas, and ash occasionally rose to an altitude of 6.1 km and drifted NNE. Trace amounts of ash again fell in Nelson Lagoon.

AVO reported that seismic tremor markedly declined around 1100 on 21 May 2013 and was followed through 23 May by the detection of small discrete events, likely indicative of small explosions, by an infrasonic pressure sensor (Chaparral model 2.5 at site PN7). Although cloud cover prevented satellite observations, elevated surface temperatures at the vent were detected. On 22 May a pilot report and photographs indicated weak steam-and-gas emissions containing little to no ash.

The eruption continued at a lower level during 24-26 May. Neither evidence of elevated surface temperatures nor a plume were observed in partly clear satellite images during 24-25 and 27 May. Clouds obscured views on 26 May. The Volcanic Alert Level was lowered to Advisory and the Aviation Color Code was lowered to Yellow on 28 May.

According to AVO, Pavlof emitted ash on 4 June at about 1100, as observed in satellite images and by pilots. Satellite images showed an ash cloud drifting SE, and pilots estimated that the cloud was at an altitude of 5.8 km. Weak seismicity that began at 1057 accompanied the emissions, and then continued. AVO increased the Volcanic Alert Level to Watch and increased the Aviation Color Code to Orange.

AVO reported that ash emissions continued during 5-11 June 2013, accompanied by tremor and explosion signals. Overnight during 4-8 June, satellite images detected elevated surface temperatures near the vent consistent with lava effusion and fountaining. Elevated surface temperatures persisted until 14 June. On 5 and 6 June, an ash plume drifted 40-45 km W and SW at altitudes of 4.3-5.5 km based on pilot estimates. During 8-10 June, an ash plume drifted 20-53 km SE. During 12-14 June, ash emissions were intermittent and minor; ash plumes remained below an altitude of 6.1 km and mostly drifted SE.

During 14-15 June 2013, seismicity decreased. Minor emissions probably ceased, but web-camera views were partially obscured by clouds. On 17 June no plumes were visible in satellite images, and web camera views showed mostly cloudy conditions.

During 17-18 June, tremor amplitude increased slightly, and elevated surface temperatures were again detected in satellite images. A small ash plume rose from the crater. The eruption continued during 19-25 June, with tremor and occasional explosions. Cloud cover prevented web camera views. Elevated surface temperatures continued to be detected during 19-20 and 24 June. A small ash plume from the summit vent was also detected in a satellite image on 19 June, and possibly during 20-22 June.

On 24 June, seismicity increased to the strongest level to date during 2013 and included continuous intense tremor and frequent small explosions likely associated with lava fountaining and ash production. Seismicity remained high on 25 June. Satellite images and pilot observations indicated that a plume drifted W at altitudes as high as 8.2-8.5 km. Satellite images also detected a strong thermal anomaly at the summit. Trace amounts of ash fell in King Cove (48 km SW). According to a news report (Reuters), regional air traffic was again cancelled or re-routed.

According to AVO, seismicity declined during 25-26 June and consisted of intermittent bursts of tremor and occasional small explosions. Satellite images showed a plume containing small amounts of ash drifting NW, and strong thermal anomalies at the summit. Pilot reports on 26 June indicated that plumes rose to altitudes between 6.1-7.6 km during the morning and then to heights just above the summit later that day. Seismicity during 26 June-1 July continued at low levels and consisted primarily of intervals of continuous, low-level tremor. Thermal anomalies at the summit detected in satellite images were strong during 26-29 June and weak during 30 June-1 July.

AVO reported that activity further declined during 1-2 July; tremor and explosions were no longer detected in seismic and pressure sensor data. Satellite images did not detect elevated surface temperatures, volcanic gas, or ash emissions, and there were no visual observations from pilots or from webcam images of any eruptive activity since 26 June. Consequently, AVO lowered the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory.

On 8 August, AVO reported that no lava or ash emissions had been observed at Pavlof since 26 June and the volcano had exhibited gradually declining levels of unrest. Seismicity was at background levels. Thus, AVO lowered the Aviation Color Code to Green and the Volcano Alert Level to Normal.

Mangan and others (2009) discussion. Mangan and others (2009) cite Power and others (2004) as stating that background (non-eruptive) seismicity at Pavlof occurs as infrequent long-period earthquakes at focal depths between 20-40 km. Mangan and others contend that while only a few of these events at most occur annually, they are a stable feature attributed to quasi-steady fluxing of basaltic magma and exsolved CO₂ in a deep dike and sill complex. According to the article, the seismic network at Pavlof is poorly situated to detect deep seismicity under the Emmons Lake caldera.

Mangan and others state, "All witnessed [Emmons Lake Volcanic Center] ELVC eruptions have occurred outside the caldera [,specifically] at Pavlof, the most active volcano in the entire arc. Pavlof's slopes are extensively mantled with tephra and pyroclastic debris produced during [its] historical strombolian, vulcanian, and lava fountain events (Miller et al., 1998). Limited precursory seismicity herald Pavlof eruptions (McNutt, 1989) and, to the extent studied, negligible precursory ground deformation (Lu et al., 2003; Z. Lu personal communication 2008). Of the 20 eruptions occurring since the installation of Pavlof's seismic network (1973), 13 eruptions have occurred with less than 24 h of warning. Pavlof is essentially an "open vent" volcano with magma rising aseismically through a thermally well-groomed conduit. High-frequency volcano-tectonic earthquakes, characteristic of magma rise through brittle crust, are virtually absent."

Figure 8 presents Mangan and others (2009) conceptualization of the plumbing beneath the ELVC, which includes Pavlof.

Figure 8. Conceptual cross-section through the Emmons Lake Volcanic Center looking at a vertical plane parallel to the volcanic axis. The drawing shows two distinct plumbing systems drawing from a common magmatic source at more than 20 km depth. Courtesy of Mangan and others (2009).

The other volcano of the ELVC considered to have high likelihood of eruption is Mt. Hague (Waythomas and others, 2006). That study also presents a set of hazard maps for the complex.

References. Mangan, M., Miller, T., Waythomas, C., Trusdell, F., Calvert, A., and Layer, P., 2009, Diverse lavas from closely spaced volcanoes drawing from a common parent: Emmons Lake Volcanic Center, Eastern Aleutian Arc, Earth and Planetary Science Letters, Vol. 287, pp. 363-372.

Waythomas, CF; Miller, TP, and Mangan, MT, 2006, Preliminary Volcano Hazard Assessment for the Emmons Lake Volcanic Center, Alaska, U.S. Geological Survey Scientific Investigations Report 2006-5248 (URL: http://pubs.usgs.gov/sir/2006/5248/).

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 (URL: http://www.dggs.alaska.gov/); Associated Press (URL: http://www.ap.org/); PRNewswire (URL: http://www.prnewswire.com); Alaska Dispatch (URL: http://www.alaskadispatch.com/); and Reuters (URL: http://www.reuters.com/).

April 2015 (BGVN 40:04) Cite this Report

Spatter-fed lava interacting with ice, spawning clastogenic lava flows, lahars, and pyroclastic flows

This report discusses Pavlof's behavior during May 2014 through 26 December 2014, a time period with two clear eruptive intervals that included lava fountaining, spatter, fragmental (agglutinate-rich, clastogenic) lava flows, lahars, pyroclastic flows, and diverse plumes. On 30 May 2014, an eruption began that continued intermittently through the first week of June. A thermal image taken from a satellite on 24 June 2014 showed warm areas ~5 km down the N flank interpreted as the signature of an earlier, still-warm lava flow. (This flow was perhaps similar to (fountain- and spatter-fed, fragmental, agglutinate-rich, clastogenic) lava flows and possible associated lahars seen during 2013; Waythomas and others, 2014; Wolf and Sumner, 2000.) Another eruption took placed during 12-16 November 2014. Besides the previously mentioned characteristics, common observations during eruptions included strombolian emissions, multiple-kilometer-long zones of incandescent lava, plumes ranging from those dominated by steam and gas to others that were rich in ash. Diagnostics from distant instruments included acoustical signals of eruption received with infrasonics and lightning from inferred ash plumes detected with a lightning detection array.

Background. In BGVN 38:05 we reported on the then most recent eruption at Pavlof, which occurred during May-June 2013. Waythomas and others (2014) summarized Pavlof's eruptive behavior during 2013. This is relevant, in part, because similar ice-spatter interactions also prevailed during 2014. "The 2013 eruption of Pavlof Volcano, Alaska began on13 May and ended 49 days later on 1 July. The eruption was characterized by persistent lava fountaining from a vent just north of the summit, intermittent strombolian explosions, and ash, gas, and aerosol plumes that reached as high as 8 km above sea level and on several occasions extended as much as 500 km downwind of the volcano. During the first several days of the eruption, accumulations of spatter near the vent periodically collapsed to form small pyroclastic avalanches that eroded and melted snow and ice to form lahars on the lower north flank of the volcano. Continued lava fountaining led to the production of clastogenic lava flows that extended to the base of the volcano, about 3–4 km beyond the vent. The generation of fountain-fed lava flows was a dominant process during the 2013 eruption; however, episodic collapse of spatter accumulations and formation of hot spatter-rich granular avalanches was a more efficient process for melting snow and ice and initiating lahars. The lahars and ash plumes generated during the eruption did not pose any serious hazards for the area. However, numerous local airline flights were cancelled or rerouted, and trace amounts of ash fall occurred at all of the local communities surrounding the volcano, including Cold Bay, Nelson Lagoon, Sand Point, and King Cove."

The reports by the AVO also announced Volcano Alert Levels and Aviation Color Codes. The four Alert Levels apply to conditions in vicinity to the volcano (of greatest concern to residents). The Levels consist of Normal, typical background or noneruptive state; Advisory, exhibiting signs of unrest or possible renewed increase; Watch, exhibiting escalating or heightened unrest; and Warning, hazardous eruption is eminent or underway. The respective Color Codes address risks to aircraft from ash plumes. The Codes consist, in increasing order of concern, Green, Yellow, Orange, and Red.

Pavlof is monitored by satellite imagery, observers, several in-situ and remote instruments, and by a Federal Aviation Administration (FAA) web camera. Figure 9 shows Pavlof as seen from the FAA web camera, which resides in Cold Bay. The photo shows conditions on a clear day when the volcano was quiet. The camera produces still images sometimes used to convey the volcano's behavior ('FAA supplementary weather products').

Figure 9. A NE view that features snow- and ice-clad Pavlof as seen from the FAA web camera in Cold Bay (Alaska) on a clear day, date unknown. MSL stands for elevation (in this case with respect to MSL, mean sea level, here expressed in feet, 1 foot = 0.305 m). SM stands for statue miles, used to describe the distance from the camera to a building and to Pavlof (~56 km away; 1 SM = 1.61 km). Courtesy of FAA (US Federal Aviation Administration).

Eruption of 30 May to 4 June 2014. The AVO weekly report issued on 6 June 2014 summarized conditions during the 30 May-4 June eruption period as follows: "Pavlof Volcano is experiencing a typical Strombolian eruption, characterized by lava fountaining, minor explosions, and the accumulation of spatter on the upper north flank of the volcano. Accumulations of spatter have occasionally built up and collapsed, forming hot, ashy, particle-rich flows that generate high-rising steam plumes on the lower north flank of the volcano. As these flows interact with ice and snow on the volcano, they produce meltwater and steam plumes. Spatter-fed lava flows also are likely forming".

According to AVO's 6 June 2014 weekly summary, Pavlof began erupting on 30 May 2014. On the morning of 31 May 2014 elevated surface temperatures were detected at the summit of Pavlof, suggesting a low-level eruption with extruding lava. Campers near the volcano confirmed this detection, and noted lava flows originating from a vent on the NE flank. As those lava flows interacted with glacier ice, low-altitude ash clouds and plumes were created. The plumes were detected in satellite imagery, as well as by pilots and with the Cold Bay FAA web camera.

On the evening of 31 May 2014, small explosion signals were detected by a distant infrasound sensor. The eruption continued, followed by incandescence. The FAA web camera in Cold Bay detected weak incandescence glowing at the summit on the evenings of 31 May and 1 June. Clouds obscured views of the volcano by web camera although no ash clouds were detected in satellite imagery. Weak seismic activity was detected on the Pavlof network of seismometers near the volcano. An increase of seismic tremor occurred 2 June at 1500, decreasing around 2300 that evening (Alaska Standard Time = UTC - 9 hours; during May-June, Daylight Saving Time = UTC - 8 hours). The Aviation Color Code and Alert Levels on 31 May were Orange and Watch respectively.

On 2 June 2014, AVO reported a plume discharged almost continuously from the vent rising to an altitude of 6.7 km and extending over 75 km E, as seen in figure 10. The AVO daily report for this eruption stated "Hazardous conditions exist on the north flank and north side drainages heading on the volcano due to continued pyroclastic and lahar activity. Ash in the vicinity of the volcano remains a hazard to local air traffic" (figure 10).

Figure 10. Visible image of a Pavlof plume acquired by the MODIS instrument on the Terra satellite on 2 June 2014 (2145 UTC 2 June, which corresponds to local Daylight Saving Time and date of 1345 on 2 June). The plume extended ~75 km E of Pavlof. Courtesy of NASA and AVO/USGS.

The AVO photo archive for 2 June contained over 40 photos with captions. Some were taken from Cold Bay and others from at sea and aircraft, documenting eruptive activity that day. Chris Waythomas (AVO) noted incandescence associated with lava fountaining and low-level ash and steam plume on images caught by the FAA camera. Several photos by Rachael Kremer were captioned by AVO scientists. The caption of one image (ID #591161 written by Game McGimsey, AVO/USGS) not only described incandescence from lava fountaining at the summit vent, it also stated the presence of "spatter-fed lava flowing down the N flank." Further, "ash and steam clouds rising from lower on the north flank were likely generated by pyroclastic flows intermixing with glacier ice."

AVO daily reports issued on 2 and 3 June 2014 described a vigorous continuing eruption. Late on the 2nd, tremor increased again. During the night included observers noted intense lava fountaining and a spatter fed lava flow down the N flank. By the morning of the 3rd, and ash and steam plumes rose up to 7.3 km altitude. The AVO report issued at 1233 on the 3rd noted a wind shift and wind at the time of that report carrying the main plume SSW. Lower winds (below ~3 km altitude) carried a plume that may have contained trace ash to the WSW.

The AVO report issued at 1754 on the 3rd made these statements: "Although the eruption of Pavlof continues, seismic tremor has deceased over the past 12 hours and has remained relatively steady throughout the day at a much lower level than that of yesterday. Recent satellite data and web camera views of the eruption plume indicate that there are now two distinct parts of the plume. The part of the plume that reaches high above the volcano appears to be mainly steam and gas with minor ash present, extending south of the volcano. Additionally, pyroclastic flow activity on the north flank is producing diffuse ash emissions that result in areas of hazy air, with variable concentrations of ash below [~3 km]. Low-level winds are likely to disperse this ash to the west-southwest with no more than trace amounts accumulating. There are no reports of ash falling in nearby communities." The Aviation Color Code was reduced from Red to Orange and the Alert Level to Watch. Ash remained a hazard to local air traffic.

Similar conditions prevailed on 4 June, with plumes containing minor ash but rich in sulfur dioxide extending 30 to 100 km downwind over Cold Bay. Although incandescence was visible in early morning web cam images, seismicity had remained stable for the past 24 hours. Incandescence from lava fountaining was visible in webcam images on 4 June. According to a news article, flights in and out of Cold Bay and Unalaska were canceled on 4 June, affecting about 200 people. At 0205 and 0245 on 5 June 2014, seismic data indicated two distinct explosions. AVO inferred these represented the collapse of spatter built up around the vent, with a possible explosive component. A similar third, less energetic, event occurred at 0844. The explosions generated lightning, which was detected by the World Wide Lightning Location Network (WWLLN, a collaboration of over 50 universities) (Morton, 2014). AVO inferred that hot debris moved down the N flank, resulting in localized low-level clouds of fine ash. There was no ash above the meteorological clouds whose tops reached 8.8 km in height. As of 6 June 2014, elevated surface temperatures persisted but cited that on this morning they had observed greatly diminished ash and lava emissions. Steam or ash plumes were absent in satellite images since 4 June. A weekly summary issued on 6th noted plumes during the eruption that started on the evening of 30 May 2014 had reached about 9.1 km in altitude. Seismic data indicated lahars occurred intermittently.

Comparative quiet. During 7-23 June 2014, Pavlof was comparatively quiet. Although extreme temperatures associated with fountaining were not seen, a thermal image of Pavlof on 24 June 2014 suggested broad areas of warm temperatures from what AVO interpreted as a recent lava flow (figure 11). According to the scientist who prepared the image, David Schneider, "Composite satellite image of Pavlof Volcano showing the extent of the lava flows on the northeast flank. The base image was collected by the Worldview-2 satellite on May 9, 2014 (prior to the onset of eruptive activity) and is overlain (in color) with a Landsat-8 thermal infrared image collected early in the morning on June 24, 2014. The thermal infrared sensor measured the heat given off by the still-warm lava flow. The length of the longest branch of the lava flow is about 5 km (3 miles). Note that the lava flow appears to have traveled under the ice on the upper flank of the volcano."

Figure 11. A thermal image from Landsat 8, with areas of increased infrared radiation, acquired in the early morning of 24 June 2014 showing the path of lava flows down the slopes. For scale, the longest arm of the flow was about 5 km. The lava flow traveled under the ice in an area of the upper flank. For more details, see text. Courtesy of AVO. Caption details and image preparation by D. Schneider (AVO/USGS).

An AVO Notification issued on the 25th indicated that AVO had observed no evidence of ash emission from the volcano since early June. Clear web camera and satellite images of the volcano over the past several days showed no evidence of continued lava fountaining. The Aviation Color Code was reduced to Yellow and the Volcano Alert Level was reduced to Advisory. AVO further added that small discrete seismic events continued. They suggested that the signals may have been related to several processes including, (1) degassing of unerupted magma within the volcano's conduit and (2) periodic collapse of ejecta and other debris down the steep flanks of the volcano. The latter, appears consistent with the lava flow seen on figure 11.

On 30 July 2014 the Color Code was lowered to Green and the Volcano Alert Level to Normal. Since mid-June, levels of unrest had gradually declined. Rockfalls and small avalanches of debris still occurred sporadically on the NNW flank of volcano. The next eruptive event did not occur until 12 November.

Eruption of 12-16 November 2014. As previously mentioned, an eruption occurred during 12-16 November 2014. On 12 November 2014, AVO reported a ground observer in Cold Bay sighted ash emissions from Pavlof rising to an altitude of 2.7 km, signifying a new eruption. Minor ash emissions were visible in the Cold Bay web camera beginning around 1650 Alaska Standard Time (AKST) on 12 November. AVO raised the Aviation Color Code and Volcano Alert Level at 1957 on 12 November. Tremor remained elevated on the 12th, 13th, and 14th, with lava fountaining and ash emissions. On 14 November satellite imagery revealed a narrow ash plume extending ~200 km at 4.8 km altitude.

On 15 November 2014, AVO reported the eruption of had intensified. Thus, the Aviation Color Code was raised to Red and the Volcano Alert Level to Warning. Behavior was characterized by explosive eruptions, lava fountaining from a vent just N of the summit, and flows of rock debris and ash descending the N flank of the volcano. Ash emissions were observed from the ground and in satellite images. The intensity of seismic tremor had increased significantly, and satellite data indicated the ash cloud top at 7.6 km altitude extending 200 km NW from the vent. Figure 12 shows a Landsat 8 image captured on the 15th. The top of an ash plume in the image had reached an altitude of ~9 km. Another satellite image taken the same day showed ash plume above cloud cover and extending ~300 km NW from the volcano.

Figure 12. On 15 November 2015, Pavlof was lofting ash plumes to an altitude of 9 kilometers as shown in the natural-color image, acquired by the Operational Land Imager (OLI) on the Landsat 8 satellite. Pavlof's volcanic plume rises well above the cloud deck. NASA Earth Observatory image by Jesse Allen, using Landsat 8 data from the U.S. Geological Survey. Original image by David Schneider.

Although as mentioned above, on 15 November 2014, the ash plume reached more than 9 km, tremor had abruptly decreased at about 1900 that day. This was accompanied by a large decrease in ash emissions, and the next day no evidence of an ash plume at the volcano was reported. On the 16th, the Aviation Color Code decreased to Orange and the Volcano Alert Level to Watch. During 17-18 November seismicity remained low; surface temperatures on the upper NW flank were elevated. The AVO weekly report issued on 21 November 2014 described the week's activity as still remaining low. Intermittent tremor was detected, and satellite images still showed lava flow on the volcano's NW flank. At that stage it reached ~7 km from the summit. On 25 November 2014, AVO further lowered the Aviation Color Code/Volcano Alert Level to Yellow/Advisory, citing continued low seismicity and lack of any observations to suggest ongoing lava fountaining or ash emission. According to the last AVO weekly report issued on 26 December 2014, the status of Pavlof remained unchanged. Seismicity at Pavlof continued slightly above background levels. Weather conditions continued to be cloudy during the week and no activity was observed in satellite or web camera views of the volcano.

References. Demas, A., (3 June) 2014, Volcano Warning Alert Issued for Alaska's Pavlof Volcano, U.S. Geological Survey [accessed August 2014] (URL: http://www.usgs.gov/blogs/features/usgs_top_story/volcano-warning-alert-issued-for-alaskas-pavlof-volcano/ ). [accessed August 2014]

Morton, M, (6 April) 2014, Volcanic Lightning Generated in a Bottle, Earth Magazine (URL: http://www.earthmagazine.org/article/volcanic-lightning-generated-bottle)

Schwaiger, H.F., Denlinger, R.P., and Mastin, L.G., April 2012, Ash3d: A finite-volume, conservative numerical model for ash transport and tephra deposition. Journal of Geophysical Research, v. 117, Issue B4, 20 p.[accessed August 2014] (URL: http://onlinelibrary.wiley.com/doi/10.1029/2011JB008968/pdf).

Schwartz, D., (11 August) 2013, Ash3D is Federal Answer to Ash Cloud Response, Peninsula Clarion [accessed August 2014] (URL: http://peninsulaclarion.com/news/2013-08-10).

Waythomas, C. F., Haney, M. M., Fee, D., Schneider, D. J., and Wech, A., 2014, The 2013 eruption of Pavlof Volcano, Alaska: a spatter eruption at an ice-and snow-clad volcano. Bulletin of Volcanology, 76(10), pp. 1-12.

Wolff, J. A., & Sumner, J. M. (2000). Lava fountains and their products. Encyclopedia of volcanoes, H Sigurdsson, B Houghton, S McNutt, H Rymer, J Stix (Eds.); pp. 321-329.

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 (URL: http://www.dggs.alaska.gov/); Christopher Waythomas, Game McGimsey, and Cheryl Cameron, AVO; Rachel Kremer (affiliation unknown); Federal Aviation Administration (FAA), 800 Independence Ave, SW, Washington, DC 20591, USA (URL: http://www.faa.gov/); and National Aeronautics and Space Administration (NASA) (URL: http://modis.gsfc.nasa.gov/).

March 2017 (BGVN 42:03) Cite this Report

Ash plume to 11 km on 27 March 2016 that drifted 1,200 km NE; multiple smaller ash events through July 2016

Pavlof volcano, near the end of the Alaska Peninsula 970 km SW of Anchorage, frequently produces explosive eruptions from the summit vents and occasional lava flows. The largest confirmed historical eruption took place in 1911 when a fissure opened on the N flank; it has erupted more than 25 times since then. The last reported eruption in mid-November 2014 included lava fountaining from a vent just N of the summit, and flows of rock debris and ash descending the N flank, along with an ash plume that rose to around 9 km altitude and drifted 300 km NW. Pavlof was quiet in 2015, but then abruptly renewed activity in late March 2016. It is monitored primarily by the Alaska Volcano Observatory (AVO) and the Anchorage Volcanic Ash Advisory Center (VAAC).

A sudden vigorous eruption that began on 27 March 2016 lasted for about 20 hours, sending ash to 11 km altitude, producing a plume dispersed NE for 1,200 km, and a similarly large SO₂ plume. The volcano was then quiet until a short-lived, smaller ash emission occurred in mid-May for eight days. Intermittent low-level ctivity picked up again from late June through late July 2016, characterized by minor emissions of dark-colored ash and steam rising to 4.5 km altitude. Fallout of ash was limited to the flanks of the volcano and the immediate area around Pavlof. The last report of ash emissions was on 30 July, although low-amplitude tremors and steam plumes persisted through August, and intermittent thermal anomalies from the summit continued through the end of 2016.

After a short and intense eruption between 12 and 15 November 2014 (BGVN 40:04), activity decreased quickly to background levels. The AVO had reduced the Aviation Color Code (ACC) from Red (highest) to Orange on 16 November, and from Orange to Yellow on 25 November. Seismicity remained slightly above background levels until early January. On 15 January 2015 the AVO reduced the ACC to the lowest level of Green where it remained for over a year until it was changed abruptly to Red on 28 March 2016 at the start of a new eruption.

AVO reported that seismicity began to increase at 1553 on 27 March 2016, characterized by a quick onset of continuous tremor. An ash plume rose to an altitude of 6.1 km, and by 1618 was drifting N (figure 13). During the night, lava fountaining from the summit crater was observed by mariners, pilots, and residents of nearby Cold Bay (60 km SW).

Figure 13. Pavlof erupts, sending a plume of volcanic ash into the air on the evening of 27 March 2016 (AKDT) as photographed by a passenger on a plane travelling to Anchorage from Dutch Harbor. Courtesy of Colt Snapp.

On 28 March, tremor levels remained high; lightning in the ash plume was detected in the morning, and infrasound data from a sensor network in Dillingham (470 km NE) indicated sustained ash emissions. At 0700 a continuous ash plume was evident in satellite images drifting more than 650 km NE, and a MODIS image captured at midday revealed the extent and substantial thickness of the cloud (figure 14). A SIGMET (significant meteorological information notice) issued by the National Weather Service (NWS) Alaska Aviation Weather Unit indicated that the maximum ash-cloud altitude was approaching 11 km. Strongly elevated surface temperatures also suggested the presence of lava flows.

Figure 14. The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on a NASA satellite acquired this image of the ash plume from Pavlof at 1145 Alaska time (2145 UTC) on 28 March 2016 extending several hundred km to the NE. Courtesy of NASA Earth Observatory.

The energetic ash-producing phase of the eruption lasted from 1600 AKDT (00:00 UTC) on 27 March until about 1230 AKDT (20:30 UTC) on 28 March, and produced an ash cloud that stretched NE over Bristol Bay and interior Alaska for over 1,200 km. As a result, over 40 Alaska Airlines flights to and from Fairbanks, Alaska, were cancelled according to NBC News. Minor ashfall (0.8 to 6.3 mm or 1/32 to 1/4 in) was reported in the nearby community of Nelson Lagoon (80 km NW) and trace ashfall (less than 0.8 mm) was confirmed near Dillingham (470 km NE). A large SO₂ plume also drifted NE from the volcano extending all the way across Alaska to Yukon Territory and British Columbia in Canada (figure 15).

Figure 15. A large SO2 plume trails NE from Pavlof on 28 March 2016 after a substantial explosion sent an ash plume to nearly 12 km altitude. The ash cloud and the SO2 plume both extended for 1,200 km NE across interior Alaska. Courtesy of NASA/GSFC.

Seismicity and infrasound signals had decreased to low enough levels by 1230 on 28 March that the AVO lowered the Aviation Color Code to Orange and the Volcano Alert Level to Watch. However, seismic tremor remained above background levels. Ash emissions decreased through the night and were barely visible in a satellite image acquired at 0625 AKDT on 29 March. Remnant ash continued to drift over Bristol Bay and areas of interior Alaska. The webcam at Cold Bay recorded intermittent, low-level ash plumes rising as high as 4.6 km.

Thermal anomalies, measured by MODIS satellite sensors and analyzed by MODVOLC, appeared from 28 March (0025 UTC) through 29 March 2016 (1360 UTC), with 20 pixels recorded on 28 March. The MIROVA system also recorded an abrupt spike to 'Very High' thermal anomaly levels on 28 March, dropping slightly in the next two days (figure 16) and then disappearing a few days later. Low-power anomalies were detected on 2 and 6 April, and then ceased for several months.

Figure 16. MIROVA Log Radiative Power data for Pavlof between 28 December 2015 and 28 December 2016. Note the 'Very High' level spike in Log Radiative Power during 28-30 March 2016. Values dropped significantly in early April and then disappeared for several months. Low VRP values reappeared in late August and were intermittent for the remainder of 2016. AVO determined that the summit crater was enlarged as a result of the March 2016 explosion; the new crater geometry possibly allowed satellite sensors to more easily detect emissions of hot gases from the vent. Ongoing observations of moderately elevated surface temperatures between August and December 2016 likely reflect this change in the crater, and do not indicate new eruptive activity or rising magma, according to AVO scientists. Courtesy of MIROVA.

The AVO reported that the intensity of the eruption greatly decreased during 29-30 March, although The Canadian Press reported that ash from the eruption had caused flights in and out of Yellowknife and Regina, Canada, to be cancelled on those dates. Elevated surface temperatures identified in satellite data and visual observations of low-level, intermittent ash plumes were noted during brief breaks in poor weather conditions during these days. Airwave signals, indicative of small explosions at the summit, were recorded on 3 April, but tremors had ceased by the next day. On 6 April AVO noted no signs of ash emissions or lava effusion during the previous week, and seismicity was at low levels. Thermal anomalies at the summit were occasionally visible, though likely indicating cooling processes of previously erupted lava. AVO lowered the Aviation Color Code to Yellow and Volcano Alert Level to Advisory on 6 April. After two more weeks of no activity, the ACC was lowered to Green/Normal on 22 April 2016.

On 13 May 2016 the AVO raised the Aviation Color Code back to Orange as a result of increased seismicity typically associated with minor eruptive activity. Four minor ash eruptive episodes were inferred from seismic data between 13 and 16 May. On 14 May, local observers in Cold Bay reported ash emissions below 5 km in the vicinity of the volcano. According to the Anchorage VAAC, on 15 May a minor eruption was noted on the Cold Bay web camera, but volcanic ash was not visible in satellite data. Elevated surface temperatures were detected in satellite data on 15 May. Periods of elevated volcanic tremor and a small explosion associated with minor ash emissions was noted on 17 May; observers in Cold Bay and Sand Point (90 km E) reported ash emissions interspersed with steam emissions. The Anchorage VAAC noted that strong winds caused resuspension of volcanic ash on the lee side of Pavlof on 17 and 18 May. The AVO lowered the ACC to Yellow on 20 May and noted that all volcanic ash clouds produced during the 13-17 May event were below 4.5 km altitude, and that no lava effusion or fountaining was detected. Weak seismic tremor and small explosions were observed on 21 May, after which activity ceased. The AVO lowered the ACC to Green on 17 June.

Seismic activity increased again on 30 June for about a week, prompting the AVO to raise the ACC to Yellow on 1 July 2016; minor steam emissions were also observed in the web camera. AVO technicians installed a new web camera in the Black Hills area north of the volcano near the Bering Sea coast in early July. On 11 July, weakly elevated surface temperatures were observed at the summit in satellite imagery and a steam and gas cloud extended SW for about 80 km. Minor ash emissions reaching a few tens of meters above the summit were observed that afternoon extending a few kilometers to the SW. Small ash emissions were again observed on 18 July along with an increase in seismic tremor for about 48 hours.

On 28 July a low-intensity eruption with vigorous degassing produced a steam-rich plume and minor ash emissions. As a result, the AVO raised the ACC to Orange. The drifting steam and ash cloud was below 4.6 km above sea level and dissipated rapidly. The Anchorage VAAC reported steam and minor ash emissions continuing through 30 July.

A decline in activity led AVO to lower the ACC to Yellow on 4 August. Periods of low-amplitude tremor continued, but no plumes or thermal signals at the summit were detected. Elevated surface temperatures at the summit were observed in satellite data on 8 August, and a low-level but persistent steam plume was visible in web camera images on 11 August. A large steam plume was noted by observers in Sand Point on 15 August. Elevated surface temperatures were detected through cloud cover in satellite data on 20 and 25 August. Low-level unrest continued through the fall with persistent degassing from the summit and elevated surface temperatures detected in satellite data. A robust steam plume on 31 August reached 4.6 km, but there was no evidence of ash and it dissipated rapidly.

Several times during late September during clear views, webcam images showed a persistent steam plume from the summit crater. Elevated surface temperatures in the summit crater were observed in satellite images on 25, 28, and 29 September, and again during 4-6, 13-14, and 16 October. In early November, the AVO determined that the summit crater was larger and more centrally located than before, as a result of the March 2016 explosion. The new crater geometry possibly allowed satellite sensors to more easily detect emissions of hot gases from the vent. Ongoing observations of moderately elevated surface temperatures (figure 16) likely reflect this change in the crater, and do not indicate new eruptive activity or rising magma. Seismicity remained slightly above background levels through the end of 2016, and the ACC remained at Yellow.

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 (URL: http://www.dggs.alaska.gov/); Anchorage Volcanic Ash Advisory Center (VAAC), Alaska Aviation Weather Unit, NWS NOAA US Dept of Commerce, 6930 Sand Lake Road, Anchorage, AK 99502-1845(URL: http://www.ssd.noaa.gov/); Hawai'i Institute of Geophysics and Planetology (HIGP), MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); Colt Snapp (URL: https://twitter.com/colt_snapp/status/714345047173369856); The Canadian Press, via Vancouver Observer (URL: http://www.vancouverobserver.com/news/environment/flights-cancelled-and-out-regina-yellowknife-after-volcano-alaska); NBC News (URL: http://www.nbcnews.com/news/weather/pavlof-volcano-erupts-covering-400-miles-alaska-ash-n546956).

November 2021 (BGVN 46:11) Cite this Report

Intermittent minor explosions and gas-and-ash emissions during August-October 2021

Pavlof is located on the Alaska Peninsula, about 970 km SW of Anchorage, and was constructed along a line of vents extending NE from the Emmons Lake caldera. The volcano is about 7 km in diameter and contains active vents on the N and E flanks near the summit. Historical eruptions have been characterized by Strombolian to Vulcanian explosions from the summit vents, along with occasional lava flows; the largest eruption took place in 1911 when a fissure opened on the N flank and ejected large blocks and effused lava. More recently, there was a short eruption from March to July 2016 that consisted of a strong ash plume and several smaller ash events (BGVN 42:03). This report updates information from December 2017 through October 2021, which includes low-level gas-and-ash emissions, minor explosions, and local ash deposits during the second half of 2021, based on daily and weekly reports from the Alaska Volcano Observatory (AVO) and satellite data.

Summary of activity during 2019. No activity was reported during December 2017 through April 2019, according to the AVO weekly reports, although Sentinel-2 infrared satellite imagery showed occasional high temperatures at the summit crater during December 2017 and through 2018 (figure 17). Low-frequency seismic tremor was initially detected on 16 May 2019 that prompted AVO to raise the Aviation Color Code (ACC) and Volcano Alert Level (VAL) to Yellow and Advisory, respectively. Webcam images showed a gas-and-steam plume rising from the summit and overnight satellite images indicated warm surface temperatures. This activity continued to be reported through 11 June 2019, after which seismicity declined to background levels. As a result, the ACC and VAL were lowered to Green and Normal, respectively. Gas-and-steam emissions continued intermittently, along with elevated surface temperatures, but were described as normal background behavior (figure 18). Small explosion signals were detected on infrasound and seismic networks on 19 October 2019, though no ash cloud was visible due to weather conditions. Clear satellite images and webcam images later that week also did not show any deposits associated with these events, according to AVO. Weakly elevated surface temperatures and small gas-and-steam plumes were visible, however, which continued to be detected through 30 October. On 28 December seismicity increased again, though no eruptive activity or emissions were reported, which caused AVO to raise the ACC and VAL to Yellow and Advisory, respectively.

Figure 17. Sentinel-2 infrared satellite imagers showing a small, yet persistent thermal anomaly in the summit crater of Pavlof on 28 December 2017 (top left), 2 April (top right), 13 June (middle left), 1 July (middle right), 30 August (bottom left), and 9 September (bottom right) 2018. Some anomalies are accompanied by white gas-and-steam emissions. Sentinel-2 images with "Atmospheric penetration" (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Figure 18. Sentinel-2 infrared satellite images showing a continued thermal anomaly in the summit crater of Pavlof on 6 June (top left), 21 July (top right), 15 August (bottom left) 2019 and renewed activity marked with a weaker anomaly on 4 August 2021 (bottom right). Sentinel-2 images with "Atmospheric penetration" (bands 12, 11, 8A) rendering. Courtesy of Sentinel Hub Playground.

Summary of activity during 2020. Slightly elevated seismicity and surface temperatures continued to be reported through 2 March 2020. Little to no activity was detected from March through much of September. Then, increased seismic tremors accompanied by occasional small earthquakes were reported during 21-30 September, which prompted AVO to raise the ACC and VAL to Yellow and Advisory. Clouds often obscured the summit, but occasional gas-and-steam emissions were observed during clear weather.

Activity during July-October 2021. Activity remained low through most of 2021. On 9 July the ACC and VAL were raised to Yellow and Advisory, respectively, due to increased seismic activity and near-continuous tremor that was observed in infrasound and regional seismic data. No eruptive activity or emissions from the summit were observed, however. During the following week, intermittent periods of seismic tremor lasted tens of minutes to several hours and were accompanied by minor gas-and-steam emissions and elevated temperatures in the summit crater, as detected by satellite and webcam images. A thin deposit of ash, likely carried out by gas-and-steam emissions, was visible on the flank that extended a short distance from the summit crater on 18 July. Weakly elevated surface temperatures were detected on 19 July. A pilot on 23 July reported a small, resuspended ash cloud that rose 2 km altitude and extended 32-40 km SE, which was a result of high winds.

Sentinel-2 infrared satellite imagery showed renewed, but weak thermal activity beginning on 4 August (figure 18). During the morning of 5 August minor ash emissions were reported for an hour just above the summit and drifted almost 10 km SE before dissipating; seismic tremor and small explosions were detected by local seismic and infrasound instruments, according to AVO. This prompted AVO to raise the ACC and VAL to Orange and Watch, respectively. Local ash fallout was visible on the SE flank. Intermittent small explosions and low-level ash emissions continued to be reported during most of August, although cloud cover prevented a clear view of the activity. Weakly elevated surface temperatures were reported at night in satellite data. On 25 August an explosion generated an ash plume that rose to 3.7 km altitude, based on a pilot report.

Weather clouds often prevent clear views of the summit during September, but intermittent explosions and low-level ash emissions persisted. Elevated seismicity was detected but remained at low levels. A series of four minor ash emissions were visible in webcam images for a period of five hours on 10 September, which produced minor and diffuse ash emissions that rose from a vent on the E flank. A small explosion occurred early on 12 September that likely produced a minor ash cloud surrounding the vent near the summit, although views were obscured by clouds. During the morning of 18 September clear webcam views showed a recent ash deposit on the upper flanks that was associated with at least one minor ash emission event; periods of increased seismic tremor were also detected over the past day (figure 19). Subsequent ash emission events were noted in webcam images and by local observers at 0900 and 1500 on 19 September. Emissions remained low and dispersed quickly, though satellite images showed deposits on the surrounding flanks. Periods of increased tremor accompanied these ash emissions. During 22-23 September low level ash emissions rose possibly several hundred meters above the summit, according to webcam data. Small explosions were detected in seismic and infrasound data during 25-28 September; on 26 September explosions were visible in webcam images at 2016 and during the next morning at 0716. A small ash emission was observed in webcam images at 1601. Ash deposits resulting from the explosions were visible on the E and SE flanks (figure 20). Regional infrasound data also recorded a small explosion on 28 September.

Figure 19. Photo of a small ash cloud rising above Pavlof on 18 September 2021 at 1513, as seen from Cold Bay, AK. Photo by Robert Nielsen, courtesy of AVO.

Figure 20. Worldview-3 near IR satellite image of Pavlof on 28 September 2021 showing ash deposits on the SE flanks from intermittent explosions and gas-and-steam plumes from the N and SE vents (blue arrows). Photo by Matt Loewen, courtesy of AVO.

Activity during October consisted of minor gas-and-steam and ash emissions and intermittent minor explosions, although clouds often prevented clear views of the summit. As a result, low-level ash emissions and local ash deposits were visible on the flanks only during clear weather. Explosions during the latter half of October were focused at the vent on the upper SE flank, near the location of the 2007 vent. Seismicity remained elevated through the month, as well as slightly elevated surface temperatures.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://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, 3354 College Rd., Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

August 2022 (BGVN 47:08) Cite this Report

Weak ash explosions, small lava flows, and lahars during November 2021-July 2022

A new eruptive period at Pavlof began on 5 August 2021 (BGVN 46:11), with intermittent minor ash explosions, low-level ash emissions, seismic tremor, and weak thermal activity. This activity continued through at least July 2022 along with lava effusion and lahars. The volcano, one of the most consistently active on the Alaskan Peninsula, is monitored remotely by the Alaska Volcano Observatory (AVO) using local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks. During November 2021-July 2022, the Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch, the second highest level for both. Weather clouds often prevented satellite and webcam views.

According to AVO reports, during November 2021-July 2022 there were frequently weak explosions detected in seismic and infrasound data; minor ash emissions rose a few hundred feet above the vent and quickly dissipated near the summit, as observed in satellite and webcam images. Recent ballistic ejecta were sometimes visible on the upper flanks. Consistent with the explosions and lava effusion, seismic tremor was recorded throughout the reporting period. In addition, AVO reported sustained high to moderately elevated surface temperatures near the summit associated with the lava effusion were recorded at least through July.

Based on AVO reports, lava effusion in the form of fountaining, spatter, and small lava flows from the SE vent continued through at least July. AVO reports described the lava flows as short and sometimes sporadic. Lahars that were first reported on 10 November 2021 eventually extended down the ESE flank for several kilometers, not quite to the base of the volcano.

In November 2021 weak intermittent ash explosions generated minor ash emissions that rose a few hundred feet above the vent. On 8 November a small lava flow was first noted in satellite data near a vent on the volcano’s upper SE flank that had by 11 November extended 200 m below the vent (figure 21). Occasional periods of lava fountaining from the vent began on 14 November. By then, a small cone of hot unstable spatter and rock debris had formed. Occasional collapse of parts of the cone sent hot granular flows of rock debris several hundred meters down the flank that melted the snow and ice and produced variable amounts of meltwater, which began to incorporate loose debris on the flank to form lahars less than 2 m deep. During early December numerous explosions were detected, but then explosions became intermittent during the rest of the month. Weak ash emissions rose to an altitude no higher than about 3 km, about 500 m above the summit. Flows were up to 200 m long through December.

Figure 21. WorldView-3 false color short-wave infrared satellite image of Pavlof on 11 November 2021, showing lava flow and steam emission. SWIR rendering (bands 8, 3, 1). Prepared by M. Loewen, courtesy of AVO.

Lava flows were 300 m long in first week of January, after which satellite-based radar showed a widening of the vent. Satellite images on 19 January showed lava extending downslope 1.3 km and a new flow branch developing (figure 22). Explosions were detected again on 19 January (figure 23), 22 January, 24 February, and 28 February 2022. Robust steam emissions were observed in webcam images during mid-January.

Figure 22. WorldView-3 visible (top) and false color short-wave infrared (bottom) satellite images of Pavlof on 19 January 2022. A lava flow extends 1.3 km E from a vent on the E flank. Lava has melted the snow and ice and produced lahars that extend beyond the lava flow up to 4.4 km E of the vent. Minor ash deposits are also visible as dark streaks on the snow. Prepared by H. Dietterich, courtesy of AVO.

Figure 23. WorldView-3 false color near-infrared satellite image of Pavlof on 28 February 2022 showing a small area of active lava near the E vent, previous lava flow and lahar deposits to the E and SE, and a minor ash deposit to the S. Prepared by M. Loewen, courtesy of AVO.

Infrasound instruments detected occasional small explosions in early March, and satellite images sometimes showed minor ash deposits covering snow near the vent. Explosions became more numerous in mid-March, then decreased towards the end of the month, with no ash emissions seen during the latter part of the month. However, steaming from the vent was observed, along with a weak gas plume. Sulfur dioxide emissions were visible in satellite images during 24-26 March. Two small explosions were detected in local and regional infrasound data during 23-24 March and one was recorded during 26-27 March.

Weak explosive activity was detected during 31 March and 2-3 April with low-level plumes visible in the mornings, though satellite and webcam views were mostly obscured by clouds. Seismic tremor and elevated surface temperatures were detected almost daily during April. Possible minor lava effusion was visible in satellite images on 6 April, and a few small explosions were recorded each day during 6-9 April. Low-level ash emissions were visible in webcam and satellite images during 6-7 April, and satellite images captured ash and pyroclastic flow deposits extending at most 1.5 km from the vent and short lava flows on 9 April. Steam emissions from the vent were visible during 8-10 April. A short lava flow had descended the E flank during 26-28 April and was about 500 m long. Steaming from the active vent was visible during 30 April-2 May.

The lava flow from the E vent was 500 m long by 8 May. Daily elevated surface temperatures were identified in satellite images consistent with the effusion of short lava flows on the upper flank through 31 May. An active flow that was 650 m long was visible in satellite images during 28-29 May. No explosions or ash emissions were detected during May and June 2022. Sulfur dioxide emissions were detected in satellite data on 12 June. Lava flows up to 500 m long were observed through 10 June, and then up to 615 m long on 24 June and 1 July. On 7 July three small explosions were detected, although ash emissions were not observed. Thereafter, minor explosions were detected daily, and weak ash emissions rose as high as 500 m above the summit. Sulfur dioxide emissions were detected on 26 July.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://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, 3354 College Rd., Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/).

February 2023 (BGVN 48:02) Cite this Report

Ongoing ash explosions and lava flows until eruptive activity ended on 7 December 2022

Pavlof, one of the most consistently active volcanoes in the Aleutian Islands, has been erupting since mid-November 2021, with frequent weak explosions, periods of lava spatter, fountaining, small lava flows and emissions of steam and gas that sometimes contained ash (BGVN 47:08). This eruption continued through 7 December 2022, when both explosions and significant thermal activity had stopped (figure 24). The current report, which describes activity between August 2022-January 2023, is monitored remotely by the Alaska Volcano Observatory (AVO) using local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks, and the information below is based on AVO weekly reports and satellite data. During August to mid-December 2022, the Aviation Color Code (AVO) remained at Orange and the Volcano Alert Level (VAL) remained at Watch, the second highest level for both on a four-level scale, after which they were lowered. Weather clouds often prevented satellite and webcam views.

Figure 24. Graph of thermal anomalies at Pavlof during April 2022-March 2023, as recorded by the MIROVA system. Frequent mostly moderate power hotspots were identified during August through early December 2022. Courtesy of MIROVA.

During August, the minor eruption from the active vent on the E flank, just below the summit, continued, with occasional explosions that probably produced minor ash emissions (figure 24) that usually rose no higher than an altitude of 3 km and dissipated quickly. According to a pilot report, a small ash emission rose to an altitude of about 4 km on 24 August. A small pyroclastic debris flow was observed in satellite data on 26 August, extending about 590 m SE from the active vent. Seismic tremor persisted, and elevated surface temperatures were identified almost daily in satellite images. No active lava flows extending down the flank were detected.

Figure 25. A typical small ash plume is visible rising from Pavlof in this aerial photo at 1123 on 26 August. Photo by Pavel Izbekov; courtesy of AVO/University of Alaska Fairbanks, Geophysical Institute.

By 2 September a second small vent had re-opened ESE of the main vent. Elevated surface temperatures in the active vent area were identified in satellite images on most days, consistent with minor lava effusion. Strong incandescence at the vent and from an area within 200 m downslope was visible in webcam images starting on 14 September, indicating a short lava flow. Small explosions were detected during 18-21 September, and steam and gas plumes often containing minor amounts of ash were frequently visible. New lahar and minor ash deposits extending less than 900 m from the vent were visible during 11-13 September. Seismic tremor was nearly continuous in September.

Small explosions and persistent seismic tremor continued during October except for the first week, multiple explosions were recorded almost daily. Strongly elevated surface temperatures were often identified in satellite images, incandescence and glowing ejected rock fragments were frequently visible in nighttime webcam images, and diffuse steam plumes were sometimes visible in webcam views. Discolored snow near the vent was visible in webcam images on the morning of 7 October, suggesting low-level explosive activity; a small explosion was recorded at 1503 that same day. Webcam images on 20 October showed a new dark flow of lava and debris extending about a third of the way down the E flank. During 31 October-1 November, a diffuse steam-and-ash plume rose less than 3 km in altitude and deposited minor amounts of tephra on the E flank.

During November, multiple explosions were detected almost daily until mid-month, after which none were clearly recorded, but seismic tremor was nearly continuous. Webcam images taken on 12 and 15 November showed a lava flow and ash deposits on the upper flanks, and nighttime crater incandescence was sometimes visible. Elevated surface temperatures were observed in satellite data on most days, and occasional clear webcam views showed recent flow deposits on the volcano’s flank as well as weak steaming and light ash emissions and deposits, probably associated with lava spattering or fountaining. On 28 November, a flowage event likely resulting from the interaction of hot rocks with ice and snow on the flank was observed in seismic data and a series of webcam images, and satellite and webcam images showed that the deposit extended about 5 km down the SE flank, the largest flowage event during the current eruptive period. This event produced an ash and gas plume that reached an altitude as high as 6 km, before dissipating within tens of minutes.

Multiple small explosions were detected on 1-3 December that resulted in a narrow gully being eroded in the ice below the vent, extending about 2 km down the flank. Elevated surface temperatures and incandescent lava were not observed after 2 December. Activity significantly decreased after 3 December, with AVO reporting that the last explosion was detected on 7 December. Occasional small steam and gas plumes were visible in webcam images. Tremor had become more intermittent by mid-month, but overall seismicity remained above background levels through the end of December. On 17 December, due to the reduced activity, AVO decreased the ACC to Yellow and the VAL to Advisory, both the second lowest on a four-level scale. Weakly elevated temperatures were detected the last week of December, and a high-resolution satellite image on 27 December showed a small spot of strongly elevated temperatures within the recently active vent on the volcano’s upper E flank, but there was no sign of eruptive activity.

During the first two weeks of January 2023, seismicity remained above background levels and weakly elevated surface temperatures and minor steaming from the active vent were observed that AVO suggested might have been associated with the cooling of lava around the vent. However, no explosions or other eruptive activity was noted. By mid-January seismicity had decreased to background levels. On 19 January, AVO decreased the ACC and VAL to Green and Normal, respectively, stating that the eruption had ended. However, weakly elevated surface temperatures and minor steaming from the recently active vent continued to be observed intermittently in satellite and web camera imagery, consistent with cooling of previously erupted lava.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://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, 3354 College Rd., Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).