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According to AVO satellite images acquired on 23 November showed that lava effusion at Great Sitkin continued, though at a low rate. Lava continued to fill the summit crater and the flows on the flanks advanced short distances. During 24-30 November seismicity remained slightly above background levels. Elevated surface temperatures were periodically detected. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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Explosion on 25 May 2021 and new summit crater lava dome in July 2021
No major eruption has occurred at Great Sitkin since 1974, but intermittent steam emissions and small steam explosions, accompanied by slightly elevated seismicity, were observed during July 2016-December 2017, and most recently during June-December 2018 and in June 2019 (BGVN 43:09, 44:07). The volcano, located in the Aleutian Islands (figure 9) is monitored by the Alaska Volcano Observatory (AVO) using local real-time seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks. Events from July 2019 through July 2021 are summarized in this report.
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Figure 9. Location map showing Great Sitkin and other nearby volcanoes in the Aleutian Islands. Courtesy of AVO. |
On 15 July 2019, AVO reported that the seismicity recorded in June 2019 had decreased to background levels over the previous few weeks, with no evidence of eruptive activity in geophysical or satellite data. The Aviation Color Code (ACC) was lowered to Green and the Volcano Alert Level (VAL) was lowered to Normal. On 26 February 2020 AVO noted that seismicity had increased during the previous month, but by 21 October it had returned to background levels and no eruptive activity or unusual surface temperatures had been observed in satellite images. A Volcano Observatory Notice for Aviation on 13 May 2021 noted that elevated surface temperatures identified by AVO in satellite images since January had increased in frequency during the previous two months. A minor increase in seismicity began on 9 May and volcanic gas emissions increased on 10 May.
An increase in local earthquakes during 24-25 May prompted AVO to raise the ACC to Orange and the VAL to Watch. An explosive eruption began on 25 May at 2104; the ACC and the VAL were raised to Red/Warning, respectively. The explosions lasted for 1-2 minutes and produced an ash plume that rose to an altitude of 4.6 km (figures 10 and 11). Witnesses 42 km away heard “a very loud explosion.” After the explosive period, seismicity decreased and satellite image images showed a detached plume drifting E. Around mid-morning on 26 May, AVO lowered the ACC and VAL to Orange/Watch, respectively.
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Figure 10. Eruption plume of Great Sitkin on 25 May 2021 at 2106 (local time) as seen from the RV Tiglax. Courtesy of Lauren Flynn (U.S. Fish and Wildlife Service). |
Seismicity was low on 27 May 2021 and satellite images showed only minor steaming along with slightly elevated surface temperatures. Seismicity remained low through 1 June; moderately elevated surface temperatures were detected during 29-30 May. Aerial images on 30 May and 6 June showed ash deposits from the 25 May explosion (figure 11).
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Figure 11. Two aerial images of the summit crater of Great Sitkin from different angles, one taken on 30 May 2021 at 1745 (top) and the other taken on 6 June 2021 (bottom). According to AVO, the black deposit in the summit crater is likely fresh ash deposits from the 25 May explosion. The black linear feature trailing down the flank in the middle foreground is a lahar (mudflow) deposit. Minor ashfall can be observed on the right flank. Courtesy of Steve Rhodes (AVO) (top) and Max Zelenevich (bottom). |
According to AVO, a satellite image acquired at 0932 on 22 July 2021 showed a small area of uplift, about 50 m in diameter, and elevated surface temperatures associated with this feature. A Sentinel-2 satellite image suggested that the elevated surface temperature had begun on 12 July or earlier (figure 12). A 26 July satellite image confirmed that the feature was a lava dome that it had grown to 130 m in diameter (figure 13). Seismic data suggested that the dome probably emerged sometime during 14-22 July. Unrest continued during 26-31 July, with small earthquakes but no explosive activity. Elevated surface temperatures were detected on 27 and 28 July, and on 28 July a steam plume was observed drifting N in satellite imagery.
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Figure 12. Sentinel-2 satellite image of Great Sitkin in false color (bands 12, 11, 4) on 12 July 2021, showing a hot spot in the summit crater with a gas emission. Courtesy of Sentinel Hub Playground. |
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Figure 13. GeoEye-1 satellite image of Great Sitkin showing the new circular lava dome in the summit crater on 26 July 2021. Courtesy of AVO. |
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, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).
2021: May
| July
| August
| September
| October
| November
2020: February
| October
2019: February
| May
| June
| July
2018: January
| June
| August
2017: November
2002: May
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According to AVO satellite images acquired on 23 November showed that lava effusion at Great Sitkin continued, though at a low rate. Lava continued to fill the summit crater and the flows on the flanks advanced short distances. During 24-30 November seismicity remained slightly above background levels. Elevated surface temperatures were periodically detected. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that lava effusion at Great Sitkin had slowed or paused based on a 17 November satellite image that showed no advancement of the lava flows since 10 November. Seismicity remained elevated during 17-23 November and elevated surface temperatures were visible in occasionally clear satellite images. Steam emissions were sometimes visible in webcam images. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that lava effusion at Great Sitkin continued during 10-16 November. Elevated surface temperatures were visible in clear satellite images during 11 and 15-16 November. Two days later effusion was confirmed in satellite images; the N lava flow had not progressed but the W and S flank flows had advanced to 680 and 650 m, respectively. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that lava flows at Great Sitkin were approximately 680 m long on the W flank, 560 m long on the S flank, and 90 m long on the N flank by 30 October based on satellite images. Lava effusion probably continued during 3-9 November, though weather clouds prevented visual confirmation. Elevated surface temperatures were occasionally detected in satellite images during 2-3 and 5-6 November. Seismicity was low overall, though slightly increased during 6-7 November. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that lava flows on Great Sitkin’s S and W flanks were 600 m long by 27 October based on satellite images, and lava effusion likely continued during 27 October-2 November. Seismicity remained elevated and was characterized by small earthquakes consistent with ongoing lava flows. Elevated surface temperatures were occasionally detected in satellite images. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that lava effusion continued at Great Sitkin during 20-26 October. Lava advanced down the S and W flanks and were 500 m long by 20 October based on satellite images. Geophysical and web camera data streams returned back online during 22-23 October; seismicity remained slightly elevated and no explosive activity was recorded in both seismic and infrasound data through 26 October. Elevated surface temperatures were visible in satellite images during 25-26 October. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that lava effusion continued at Great Sitkin during 13-19 October. Seismicity remained elevated and was characterized by small earthquakes consistent with lava effusion, though an outage affected geophysical data streams during 16-18 October. Satellite images acquired on 11 October showed that lava filled more than half of the summit crater, flowing onto the S and W flanks, and had recently reached the N crater rim. Lava traveled 330 m down the S flank, 350 m down the W flank, descended small valleys, and in some areas, advanced over snow and ice. Blocks that had detached from the end of the W flow descended 450 m. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that lava effusion continued at Great Sitkin during 29 September-5 October. Seismicity remained elevated and was characterized by small earthquakes consistent with lava effusion. Elevated surface temperatures were detected in satellite images. By 3 October the dome had grown to 1,200 m E-W and 1,000 m N-S. Lava flows that continued to advance down the S and SW flanks were about 300-350 m long. The SW lobe was descending two drainages and produced hot avalanches that traveled 450 m downslope on top of a snow field. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that lava effusion continued at Great Sitkin during 22-28 September, though weather clouds sometimes prevented webcam and satellite views. Seismicity remained elevated and was characterized by small earthquakes consistent with lava effusion. By 24 September the dome had overtopped the S and W crater rims and flowed 305 m down the S flank and 195 m down the W flank. The dome was about 25 m thick and had grown to 1,170 m E to W and 925 m N to S in dimension. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that lava effusion continued at Great Sitkin during 15-21 September, though weather clouds often prevented webcam and satellite views. Seismicity remained elevated and was characterized by small earthquakes consistent with lava effusion. A radar image from 16 September indicated that the lava dome had grown to 1,130 m E-W and 910 m N-S, and was about 30 m thick. The edges of the dome touched the S and W rims of the crater. Elevated surface temperatures were identified in satellite data during 20-21 September. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that lava effusion continued at Great Sitkin during 7-14 September, though weather clouds often prevented webcam and satellite views. Seismicity remained elevated and was characterized by small earthquakes consistent with lava effusion. A radar image from 9 September indicated that the lava dome had grown to 1,100 m E to W and 860 m N to S, and was 25-30 m thick. Lava began to advance though a gap in the S rim of the summit crater. Elevated surface temperatures were visible in satellite data on 14 September. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that elevated surface temperatures at Great Sitkin and daily small earthquakes were detected during 31 August-7 September, consistent with the growing lava dome. Gas plumes were observed almost daily in satellite data. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that the lava dome at Great Sitkin continued to grow, reaching 880 m in diameter by 25 August and 1,090 m during 28-29 August. Elevated surface temperatures and small earthquakes were detected during 25-31 August, consistent with the growing dome. Daily steam-and-gas plumes were observed in satellite data and by local observers. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that the lava dome at Great Sitkin remained active during 17-24 August; satellite imagery showed changes from a diameter of 800 m on 17 August to 850-860 m throughout 18-21 August. Elevated surface temperatures and daily small earthquakes were consistent with an active dome. Gas-and-steam plumes were visible to local ground observers and in satellite imagery during 20-22 and 24 August. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that the lava dome at Great Sitkin continued to be active, growing in diameter from 250 m on 6 August, to 380 m three days later, and to 700 m by 16 August. Steam-and-gas plumes were periodically observed in webcam images, in satellite data, and by observers on the ground during 11-17 August. Elevated surface temperatures and small earthquakes detected daily were consistent with the growing dome. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that unrest continued at Great Sitkin during 4-10 August with elevated seismicity, elevated surface temperatures, and explosions associated with a growing lava dome. Numerous earthquakes and small explosions were recorded on local infrasound and seismic stations during 4-5 August. During the morning of 5 August observers reported possible low-level lava fountaining from the active vent; the activity was also visible from Adak Island. Throughout the same day a volcanic plume comprised mostly of gas and steam (and possibly ash) was visible in webcam images rising to 3 km (10,000 ft) a.s.l. By 6 August the lava dome had grown to 250 m in diameter and had an approximate volume of about 1 million cubic meters. AVO noted that most of the crater was full of lava erupted in 1974 and that the current lava covered only 4-5 percent of the total summit crater area. Elevated surface temperatures were identified in satellite images during 6-10 August, though weather clouds mostly prevented satellite and webcam views. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that unrest continued at Great Sitkin during 28 July-3 August. Seismicity remained elevated and a small swarm was recorded during 1-2 August. Elevated surface temperatures and a north-drifting steam plume were identified in satellite images during 27-28 July. Minor steam emissions were visible during 31 July-3 August. The circular lava dome in the crater had grown to 180 m in diameter based on measurements taken using a 3 August satellite image. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that a satellite image of Great Sitkin acquired at 0932 on 22 July showed a small area of uplift, about 50 m in diameter, and elevated surface temperatures associated with the feature. These observations suggested magma rising near the surface, prompting AVO to raise the Aviation Color Code and the Volcano Alert Level to Orange and Watch, respectively. Small earthquakes were recorded during 23-25 July. A 26 July satellite image confirmed that the feature was a lava dome, and that it had grown to 130 m in diameter. Seismic data suggested that the dome probably emerged sometime during 14-22 July.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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On 27 May AVO reported that seismicity at Great Sitkin was low following a 25 May explosion, and satellite images showed minor steaming and slightly elevated surface temperatures. The Aviation Color Code and the Volcano Alert Level were lowered to Yellow and Advisory, respectively. Seismicity remained low through 1 June; moderately elevated surface temperatures were detected during 29-30 May.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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An increase in local earthquake activity at Great Sitkin during 24-25 May prompted AVO to raise the Aviation Color Code to Orange and the Volcano Alert Level to Watch on 25 May. An explosive eruption began later that day at 2104; the Aviation Color Code and the Volcano Alert Level were raised to Red/Warning, respectively. The explosions lasted for 1-2 minutes and produced an ash plume rising to 4.6 km (15,000 ft) a.s.l. that was observed by local mariners. Witnesses 42 km away heard “a very loud explosion.” After the explosive period seismicity decreased and satellite image images showed a detached plume drifting E. Around mid-morning on 26 May AVO lowered the Aviation Color Code and the Volcano Alert Level to Orange/ Watch, respectively.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that elevated surface temperatures at Great Sitkin had been identified in satellite images since January and had been increasing in frequency during the previous two months. A minor increase in seismicity began to be recorded on 9 May and volcanic gas emissions increased on 10 May. AVO raised the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory on 12 May.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that seismicity at Great Sitkin had decreased during the previous several months and had returned to background levels by 21 October. Additionally, eruptive activity or unusual surface temperatures had not been observed in clear satellite images. The Aviation Color Code was lowered to Green and the Volcano Alert Level was lowered to Normal.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that seismicity at Great Sitkin had increased during the previous month and by 26 February was above background levels; the Aviation Color Code was raised to Yellow and the Volcano Alert Level was raised to Advisory. No eruptive activity was evident in geophysical or satellite data.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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On 15 July AVO reported that seismicity at Great Sitkin had decreased to background levels during the past few weeks with no evidence of eruptive activity in geophysical or satellite data. The Aviation Color Code was lowered to Green and the Volcano Alert Level was lowered to Normal.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that a small steam explosion at Great Sitkin was detected in seismic data at 1318 on 7 June. The Aviation Color Code remained at Yellow and the Volcano Alert Level remained at Advisory.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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A small steam explosion at Great Sitkin was detected in seismic data at 2140 on 1 June, prompting AVO to raise the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory. Low-level seismic activity was elevated just before and after the event.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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On 25 February AVO reported that seismicity at Great Sitkin had decreased to background levels during the past month and there was no evidence of explosive activity. The Aviation Color Code was lowered to Green and the Volcano Alert Level was lowered to Normal.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that a small phreatic explosion at Great Sitkin was recorded by the seismic network at 1105 on 11 August. The event was preceded by small local earthquakes. Cloudy satellite images prevented views of the volcano during 12-14 August. The Aviation Color Code remained at Yellow and the Volcano Alert Level remained at Advisory.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that after the brief explosion at Great Sitkin on 10 June seismicity gradually declined to background levels. On 27 June AVO lowered the Volcano Alert Level to Normal and the Aviation Color Code to Green.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported continuing low-level unrest at Great Sitkin during 20-26 June; seismic activity remained at or near background levels. A recently analyzed satellite image acquired on 11 June, one day after short-duration explosive event was recorded, showed a minor ash deposit on the snow extending 2 km from a vent in the summit crater. The Aviation Color Code remained at Yellow and the Volcano Alert Level remained at Advisory.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that satellite images captured after the short-lived steam explosion at Great Sitkin on 10 June indicated minor changes in the summit crater, characterized by possible new fumaroles in the N part of the main crater and slightly more vigorous steaming at pre-existing fumaroles. Seismicity declined to background levels during 15-16 June.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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Seismicity at Great Sitkin was elevated during the previous five days, though at 1139 on 10 June a seismic signal indicating a possible short-lived steam explosion prompted AVO to raise the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory. No infrasound signal associated with the event was detected, and no volcanic clouds rose about the meteorological cloud deck at 2.4 km (8,000 ft) a.s.l.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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On 18 January AVO reported that seismicity at Great Sitkin had declined over the past two months to near background levels. In addition, no significant activity was observed in satellite data during this time period and no steam plumes were noted. AVO lowered the Aviation Color Code and Volcano Alert Level to Green/Normal.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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AVO reported that during 28 November-5 December low-level unrest continued at Great Sitkin. Nothing noteworthy was identified in seismic data nor in partly cloudy to cloudy satellite images. The Aviation Color Code remained at Yellow and the Volcano Alert Level remained at Advisory.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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Recent observations of a robust steam plume and a period of gradually increasing seismicity over several months at Great Sitkin prompted AVO to raise the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory on 22 November. On 19 November local observers photographed a light-colored vapor plume rising about 300 m above the vent and drifting 15-20 km S. A satellite image acquired on 21 November showed steam continuously jetting from a small fumarole on the W side of the 1974 lava flow within the summit crater, and at least one area where snow and ice had been melted.
Seismicity had fluctuated but increased overall since July 2016, most notably in June 2017. The seismic activity was characterized by earthquakes less than M 1, and occurred either just below the summit or just offshore the NW cost of the island, 30 km below sea level. Possible explosion signals were recorded in seismic data on 10 January and 21 July 2017, but there were no confirmed emissions.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
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On 27 and 28 May, AVO recorded anomalous seismicity at Great Sitkin. The seismicity consisted of two periods of seismic tremor on 27 May (lasting for 20 and 55 minutes) and two earthquake swarms on 28 May (beginning at 0406 and 1328). The earthquake swarms each began with a relatively large event (ML (local magnitude) 2.2 and ML 4.3) followed by tens to hundreds of smaller aftershocks, most located 5-6 km SE of the crater at depths of 0-5 km. Both the tremor and the earthquake swarms represent significant changes from what is considered to be normal, "background" seismicity at Great Sitkin. However, aftershocks declined significantly overnight, and no tremor episodes were detected after the 27th. There were neither signs of surface volcanic activity on satellite imagery nor ground-level reports of anomalous activity. Great Sitkin remained at Concern Color Code Green.
Source: US Geological Survey Alaska Volcano Observatory (AVO)
Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.
Explosive activity and light-colored plume
Card 1804 (22 February 1974) Explosive activity and light-colored plume
The following was cabled from the Geophysical Institute on 22 February 1974. "Explosive activity was observed at 1855 local time from Adak, 48 km WSW of Great Sitkin volcano. An earthquake originating at the volcano of Richter magnitude 2.6 occurred at the time of onset. An estimated 10,000-foot light-colored plume was reported over the summit at dusk. Since that time bad weather has obscured the island."
Information Contacts: J. Kienle and D.B. Stone, Geophysical Institute, University of Alaska, Fairbanks AK; Dale Glover, NOAA, Adak AK.
Lava dome still growing, overspilling crater rim
Card 1938 (30 September 1974) Lava dome still growing, overspilling crater rim
D. Glover reported that, on a helicopter inspection trip to the volcano on 22 February, he ". . . found that a large lava dome had been emplaced in the crater, with mostly steam and gas being emitted." Foul weather prevented observations on all but three occasions between then and 29 March. About one week after the eruption, he noted, through a high-power telescope, ". . . that the dome had been extruded a considerable extent, with some ash being emitted. Since then activity has decreased to steam and gas emissions."
In mid-September, her reported that he had ". . . only been able to observe Great Sitkin volcano by high-power telescope and a few times from aircraft. The lava dome appears to continue to extrude with minor lava flows spilling over the lip of the crater. The size of the dome is hard to estimate but it is probably close to 700 m in diameter and 200-300 m high. We have recently installed seismic instrumentation on the volcano but we have not noticed any unusual activity." In summary, it appears that, following the initial release of the pressure head of volatiles on 19 February a dome has been extruded in the caldera of Great Sitkin. The new dome appears to be about the size of that extruded in 1945. Further, the dome is still active, overspilling the lip of the caldera as it grows. The extrusion has been quiet, with little of no associated explosiveness.
Information Contacts: Dale Glover, NOAA Adak Observatory, FPO Seattle WA; Dan Shackleford, Villa Park CA.
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Abnormal tremor and earthquake swarms in May 2002
On 27 and 28 May the Alaska Volcano Observatory (AVO) detected anomalous seismicity at Great Sitkin, a volcano located 1,895 km SW of Anchorage, Alaska. On 27 May two periods of seismic tremor lasted for 20 and 55 minutes and on 28 May earthquake swarms began at 0306 and 1228. The earthquake swarms each began with a relatively large event (ML 2.2 and ML 4.3) followed by tens to hundreds of smaller aftershocks, most located 5-6 km SE of the crater at depths of 0-5 km. Both the tremor and earthquake swarms represent significant changes from background seismicity at Great Sitkin. However, aftershocks declined significantly overnight, and seismicity returned to background levels with a lack of recorded tremor since 27 May. Satellite imagery showed no signs of surface volcanic activity, and no reports of anomalous activity were received by AVO.
Information Contacts: Tom Murray and John Eichelberger, Alaska Volcano Observatory (AVO) (URL: http://www.avo.alaska.edu/).
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Small phreatic explosions in June and August 2018; ash deposit on snow near summit
Episodic recent and historic volcanic activity has been reported at Great Sitkin, located about 40 km NE of the community of Adak in the Aleutian Islands. Prior to the recent 2018 activity, the last confirmed eruption in 1974 produced at least one ash cloud that likely exceeded an altitude of 3 km (figures 1 and 2). This eruption extruded a lava dome that partially destroyed an existing dome from a 1945 eruption. Most recently, a small steam explosion was reported on 10 June 2018. In response, the Alaska Volcano Observatory (AVO) raised the Aviation Color Code (ACC) to Yellow (Advisory) from the previous Green (Normal).
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Figure 1. Eruption of Great Sitkin volcano in 1974. Photo taken from Adak Island, Alaska, located 40 km SW of the volcano. Photographer/Creator: Paul W. Roberts; courtesy of AVO/USGS (color corrected). |
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Figure 2. Worldview-3 satellite image of Great Sitkin on 21 November 2017 showing the crater, areas of 1974 and 1945 lava flows, and steam (indicated by the red arrow) from the reported seismic swarm and steam event ending in 2017. Photographer/Creator: Chris Waytomas; image courtesy of AVO/USGS. |
AVO had previously reported that a seismic swarm had been detected beginning in late July 2016 and continuing through December 2017. Steam from the crater was also observed during this time period, in late November 2017 (figure 2). The seismicity was characterized by earthquakes typically less than magnitude 1.0 and at depths from near the summit to 30 km below sea level. Most earthquakes were in one of two clusters, beneath the volcano's summit or just offshore the NW coast of the island. Possible explosion signals were observed in seismic data on 10 January and 21 July 2017, but no confirmed emissions were observed locally or detected in infrasound data or satellite imagery.
The most recent eruption at Great Sitkin produced a small steam explosion which was detected in seismic data at 1139 local time on 10 June 2018 (figure 3). The explosion was followed by seismic activity which began diminishing after 24 hours, and by 15-16 June had returned to background levels.
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Figure 3. View of Great Sitkin steaming on 10 July 2018. Photographed from Adak Island, Alaska, approximately 40 km SW. Photo by Alain Beauparlant; image courtesy of AVO/USGS (color corrected). |
Due to heavy cloud cover on 10 June 2018, satellite views were obscured. Subsequent satellite data collected on 11 June showed an ash deposit on the surface of the snow extending to about 2 km SW from a vent in the summit crater (figure 4). Minor changes in the vicinity of the summit crater were observed from satellite data, including possible fumaroles north of the main crater. On 17 June an aerial photograph showed minor steaming at the vent (figure 5).
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Figure 4. Satellite view of the Great Sitkin crater at 2300 UTC on 11 June 2018 showing an ash deposit extending for about 2 km to the SW. Ash was likely deposited during the brief explosion on 10 June 2018. Minor steaming from a vent through the 1974 lava flow is also visible in this image. View is from the southwest. Photographer/Creator: David Schneider; image courtesy of AVO/USGS. |
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Figure 5. Aerial photo showing minor steaming at the summit of Great Sitkin, 17 June 2018. A small ash deposit extends SW from the vent. Photographer: Alaska Airlines Captain Dave Clum; image courtesy of AVO/USGS. |
Another small phreatic explosion was observed in seismic data at 1105 local time on 11 August. Small local earthquakes preceded the event but were not recorded following the explosion. The event is similar to three other phreatic explosions that have occurred over the past 2 years.
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, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/).
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Small steam explosions in early June 2019
The Great Sitkin volcano is located about 40 km NE of Adak Island in the Aleutian Islands and has had a few short-lived eruptions over the past 100 years. Prior to the latest activity in early June 2019 described below, small phreatic explosions occurred in June and August 2018 (BGVN 43:09). An eruption in 1974 produced a lava dome in the center of the crater. The Alaska Volcano Observatory (AVO) is the primary source of information for this September 2018-June 2019 reporting period.
Low-level unrest occurred from September 2018 through February 2019 with slightly elevated seismic activity (figure 6). Small explosions were seismically detected by AVO on 30 October, 5 and 16 November, and 11 December 2018, but they were not seen in regional infrasound data and satellite data did not show an ash cloud.
On 1, 7, and 9 June 2019, AVO reported small steam explosions as well as slightly elevated seismic activity. Steam plumes and surficial evidence of an explosion were not observed during these events. On 18 June 2019 weakly elevated surface temperatures were recorded, field crews working on Adak observed some steam emissions, and a gas flight was conducted. Elevated concentrations of carbon dioxide detected above the lava dome were likely associated with the steam explosions earlier in the month (figures 7 and 8). From 23 June through the end of the month seismicity began to decline back to background levels.
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Figure 6. A steam plume was seen at the summit of Great Sitkin on 7 December 2018. Photo by Andy Lewis and Bob Boyd; courtesy of AVO/USGS. |
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Figure 7. Some degassing was observed on the southern flank of the Great Sitkin during an overflight on 18 June 2019. Photo by Laura Clor; image courtesy of AVO/USGS. |
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Figure 8. View of Great Sitkin with white plumes rising from the summit on 20 June 2019. Photo by Laura Clor, courtesy of AVO/USGS. |
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, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/).
Cite this Report
Explosion on 25 May 2021 and new summit crater lava dome in July 2021
No major eruption has occurred at Great Sitkin since 1974, but intermittent steam emissions and small steam explosions, accompanied by slightly elevated seismicity, were observed during July 2016-December 2017, and most recently during June-December 2018 and in June 2019 (BGVN 43:09, 44:07). The volcano, located in the Aleutian Islands (figure 9) is monitored by the Alaska Volcano Observatory (AVO) using local real-time seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks. Events from July 2019 through July 2021 are summarized in this report.
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Figure 9. Location map showing Great Sitkin and other nearby volcanoes in the Aleutian Islands. Courtesy of AVO. |
On 15 July 2019, AVO reported that the seismicity recorded in June 2019 had decreased to background levels over the previous few weeks, with no evidence of eruptive activity in geophysical or satellite data. The Aviation Color Code (ACC) was lowered to Green and the Volcano Alert Level (VAL) was lowered to Normal. On 26 February 2020 AVO noted that seismicity had increased during the previous month, but by 21 October it had returned to background levels and no eruptive activity or unusual surface temperatures had been observed in satellite images. A Volcano Observatory Notice for Aviation on 13 May 2021 noted that elevated surface temperatures identified by AVO in satellite images since January had increased in frequency during the previous two months. A minor increase in seismicity began on 9 May and volcanic gas emissions increased on 10 May.
An increase in local earthquakes during 24-25 May prompted AVO to raise the ACC to Orange and the VAL to Watch. An explosive eruption began on 25 May at 2104; the ACC and the VAL were raised to Red/Warning, respectively. The explosions lasted for 1-2 minutes and produced an ash plume that rose to an altitude of 4.6 km (figures 10 and 11). Witnesses 42 km away heard “a very loud explosion.” After the explosive period, seismicity decreased and satellite image images showed a detached plume drifting E. Around mid-morning on 26 May, AVO lowered the ACC and VAL to Orange/Watch, respectively.
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Figure 10. Eruption plume of Great Sitkin on 25 May 2021 at 2106 (local time) as seen from the RV Tiglax. Courtesy of Lauren Flynn (U.S. Fish and Wildlife Service). |
Seismicity was low on 27 May 2021 and satellite images showed only minor steaming along with slightly elevated surface temperatures. Seismicity remained low through 1 June; moderately elevated surface temperatures were detected during 29-30 May. Aerial images on 30 May and 6 June showed ash deposits from the 25 May explosion (figure 11).
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Figure 11. Two aerial images of the summit crater of Great Sitkin from different angles, one taken on 30 May 2021 at 1745 (top) and the other taken on 6 June 2021 (bottom). According to AVO, the black deposit in the summit crater is likely fresh ash deposits from the 25 May explosion. The black linear feature trailing down the flank in the middle foreground is a lahar (mudflow) deposit. Minor ashfall can be observed on the right flank. Courtesy of Steve Rhodes (AVO) (top) and Max Zelenevich (bottom). |
According to AVO, a satellite image acquired at 0932 on 22 July 2021 showed a small area of uplift, about 50 m in diameter, and elevated surface temperatures associated with this feature. A Sentinel-2 satellite image suggested that the elevated surface temperature had begun on 12 July or earlier (figure 12). A 26 July satellite image confirmed that the feature was a lava dome that it had grown to 130 m in diameter (figure 13). Seismic data suggested that the dome probably emerged sometime during 14-22 July. Unrest continued during 26-31 July, with small earthquakes but no explosive activity. Elevated surface temperatures were detected on 27 and 28 July, and on 28 July a steam plume was observed drifting N in satellite imagery.
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Figure 12. Sentinel-2 satellite image of Great Sitkin in false color (bands 12, 11, 4) on 12 July 2021, showing a hot spot in the summit crater with a gas emission. Courtesy of Sentinel Hub Playground. |
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Figure 13. GeoEye-1 satellite image of Great Sitkin showing the new circular lava dome in the summit crater on 26 July 2021. Courtesy of AVO. |
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, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).
This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.
Synonyms |
| Great Sitchin | Sitchin | Chetkin | Belaya | Bolshoi Sitkin |
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There is data available for 17 Holocene eruptive periods.
| Start Date | Stop Date | Eruption Certainty | VEI | Evidence | Activity Area or Unit |
|---|---|---|---|---|---|
| 2021 May 25 | 2021 Oct 14 (continuing) | Confirmed | Historical Observations | ||
| 2019 Jun 1 | 2019 Jun 7 | Confirmed | 1 | Historical Observations | |
| 2018 Jun 10 | 2018 Aug 11 | Confirmed | 1 | Historical Observations | |
| [ 1987 Mar 18 ] | [ Unknown ] | Uncertain | |||
| 1974 Feb 19 | 1974 Sep 16 ± 15 days | Confirmed | 2 | Historical Observations | |
| [ 1953 May 11 ] | [ 1953 May 14 ] | Discredited | |||
| 1950 Nov 5 | 1950 Nov 29 | Confirmed | Historical Observations | ||
| 1949 Dec 30 | 1950 Jan 7 | Confirmed | 1 | Historical Observations | |
| [ 1946 Aug 14 ] | [ Unknown ] | Uncertain | |||
| 1945 Mar | Unknown | Confirmed | 2 | Historical Observations | |
| 1933 Nov | Unknown | Confirmed | 2 | Historical Observations | |
| [ 1904 ] | [ Unknown ] | Uncertain | |||
| [ 1829 ] | [ Unknown ] | Uncertain | |||
| [ 1828 ] | [ Unknown ] | Uncertain | |||
| 1792 May 26 ± 5 days | Unknown | Confirmed | Historical Observations | ||
| [ 1784 ] | [ Unknown ] | Uncertain | |||
| [ 1760 ] | [ Unknown ] | Uncertain |
There is no Deformation History data available for Great Sitkin.
There is no Emissions History data available for Great Sitkin.
Great Sitkin in the Andreanof Islands is seen here from Finger Bay on Adak Island. The volcano was constructed within the caldera of an older edifice, part of which forms the high snow-covered peak to the right. The post-caldera cone contains a small, 0.8 x 1.2 km ice-filled summit caldera. The rim forms the irregular summit on the left horizon. In 1945 a lava dome400-600 m wide was emplaced through a glacier filling the caldera.
This September 1946 photo of Great Sitkin from the E rim of the summit crater shows a plume rising above a lava dome emplaced in the crater in March 1945. The fissures in the ice were produced as the dome grew through the summit glacier. The maps shown below have been scanned from the GVP map archives and include the volcano on this page. Clicking on the small images will load the full 300 dpi map. Very small-scale maps (such as world maps) are not included. The maps database originated over 30 years ago, but was only recently updated and connected to our main database. We welcome users to tell us if they see incorrect information or other problems with the maps; please use the Contact GVP link at the bottom of the page to send us email.
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Title: Atka Publisher: US Geological Survey Country: United States Year: 1982 Series: Radar Image Mosaic Map Type: Photo (Radar) Scale: 1:250,000 |
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Title: Magnetic Map of N Pacific Ocean Publisher: US Department of Commerce, NOAA National Ocean Service Country: United States Year: 1973 Series: Seamap Map Type: Geophysical (Magnetic) Scale: 1:1,000,000 |
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Title: Gravity Map of N Pacific Ocean Publisher: US Department of Commerce, NOAA National Ocean Service Country: United States Year: 1973 Series: Seamap Map Type: Geophysical (Gravity) Scale: 1:1,000,000 |
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Title: Adak Publisher: US Geological Survey Country: United States Year: 1957 Series: AK Topo 250 Map Type: Topographic Scale: 1:250,000 |
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Title: Atka Publisher: US Geological Survey Country: United States Year: 1952 Series: AK Recon Topo 250 Map Type: Topographic Scale: 1:250,000 |
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There are no samples for Great Sitkin in the Smithsonian's NMNH Department of Mineral Sciences Rock and Ore collection.
| Alaska Volcano Observatory (Link to Great Sitkin) | The Alaska Volcano Observatory (AVO) is a joint program of the U.S. Geological Survey (USGS), the Geophysical Institute of the University of Alaska Fairbanks (UAFGI), and the State of Alaska Division of Geological and Geophysical Surveys (ADGGS). AVO was formed in 1988, and uses federal, state, and university resources to monitor and study Alaska's hazardous volcanoes, to predict and record eruptive activity, and to mitigate volcanic hazards to life and property. |
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WOVOdat
Single Volcano View Temporal Evolution of Unrest Side by Side Volcanoes |
WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore.
GVMID Data on Volcano Monitoring Infrastructure The Global Volcano Monitoring Infrastructure Database GVMID, is aimed at documenting and improving capabilities of volcano monitoring from the ground and space. GVMID should provide a snapshot and baseline view of the techniques and instrumentation that are in place at various volcanoes, which can be use by volcano observatories as reference to setup new monitoring system or improving networks at a specific volcano. These data will allow identification of what monitoring gaps exist, which can be then targeted by remote sensing infrastructure and future instrument deployments. |
| Volcanic Hazard Maps | The IAVCEI Commission on Volcanic Hazards and Risk has a Volcanic Hazard Maps database designed to serve as a resource for hazard mappers (or other interested parties) to explore how common issues in hazard map development have been addressed at different volcanoes, in different countries, for different hazards, and for different intended audiences. In addition to the comprehensive, searchable Volcanic Hazard Maps Database, this website contains information about diversity of volcanic hazard maps, illustrated using examples from the database. This site is for educational purposes related to volcanic hazard maps. Hazard maps found on this website should not be used for emergency purposes. For the most recent, official hazard map for a particular volcano, please seek out the proper institutional authorities on the matter. |
| MIROVA | Middle InfraRed Observation of Volcanic Activity (MIROVA) is a near real time volcanic hot-spot detection system based on the analysis of MODIS (Moderate Resolution Imaging Spectroradiometer) data. In particular, MIROVA uses the Middle InfraRed Radiation (MIR), measured over target volcanoes, in order to detect, locate and measure the heat radiation sourced from volcanic activity. |
| MODVOLC Thermal Alerts | Using infrared satellite Moderate Resolution Imaging Spectroradiometer (MODIS) data, scientists at the Hawai'i Institute of Geophysics and Planetology, University of Hawai'i, developed an automated system called MODVOLC to map thermal hot-spots in near real time. For each MODIS image, the algorithm automatically scans each 1 km pixel within it to check for high-temperature hot-spots. When one is found the date, time, location, and intensity are recorded. MODIS looks at every square km of the Earth every 48 hours, once during the day and once during the night, and the presence of two MODIS sensors in space allows at least four hot-spot observations every two days. Each day updated global maps are compiled to display the locations of all hot spots detected in the previous 24 hours. There is a drop-down list with volcano names which allow users to 'zoom-in' and examine the distribution of hot-spots at a variety of spatial scales. |
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Sentinel Hub Playground
Sentinel Hub EO Browser |
The Sentinel Hub Playground provides a quick look at any Sentinel-2 image in any combination of the bands and enhanced with image effects; Landsat 8, DEM and MODIS are also available. Sentinel Hub is an engine for processing of petabytes of satellite data. It is opening the doors for machine learning and helping hundreds of application developers worldwide. It makes Sentinel, Landsat, and other Earth observation imagery easily accessible for browsing, visualization and analysis. Sentinel Hub is operated by Sinergise |
| IRIS seismic stations/networks | Incorporated Research Institutions for Seismology (IRIS) Data Services map showing the location of seismic stations from all available networks (permanent or temporary) within a radius of 0.18° (about 20 km at mid-latitudes) from the given location of Great Sitkin. Users can customize a variety of filters and options in the left panel. Note that if there are no stations are known the map will default to show the entire world with a "No data matched request" error notice. |
| UNAVCO GPS/GNSS stations | Geodetic Data Services map from UNAVCO showing the location of GPS/GNSS stations from all available networks (permanent or temporary) within a radius of 20 km from the given location of Great Sitkin. Users can customize the data search based on station or network names, location, and time window. Requires Adobe Flash Player. |
| DECADE Data | The DECADE portal, still in the developmental stage, serves as an example of the proposed interoperability between The Smithsonian Institution's Global Volcanism Program, the Mapping Gas Emissions (MaGa) Database, and the EarthChem Geochemical Portal. The Deep Earth Carbon Degassing (DECADE) initiative seeks to use new and established technologies to determine accurate global fluxes of volcanic CO2 to the atmosphere, but installing CO2 monitoring networks on 20 of the world's 150 most actively degassing volcanoes. The group uses related laboratory-based studies (direct gas sampling and analysis, melt inclusions) to provide new data for direct degassing of deep earth carbon to the atmosphere. |
| Large Eruptions of Great Sitkin | Information about large Quaternary eruptions (VEI >= 4) is cataloged in the Large Magnitude Explosive Volcanic Eruptions (LaMEVE) database of the Volcano Global Risk Identification and Analysis Project (VOGRIPA). |
| EarthChem | EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS). |
