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U.S. Geological Survey
Friday, September 10, 2021, 12:57 PM AKDT (Friday, September 10, 2021, 20:57 UTC)

52°4'35" N 176°6'39" W, Summit Elevation 5709 ft (1740 m)
Current Volcano Alert Level: WATCH
Current Aviation Color Code: ORANGE

Summary of Current Activity

The ongoing eruption at Great Sitkin Volcano has been growing a lava dome in the summit crater of the volcano since mid-July. This lava eruption follows an explosion on May 25 that generated an ash cloud that rose approximately 15,000 ft above sea level and likely provided a pathway for magma to move from depth to reach the surface.

The dome has grown consistently over the past six weeks as shown in satellite radar and visual imagery, and has now mostly covered the previous summit lava dome from the 1974 eruption. A satellite radar image from September 9 shows that the lava is starting to advance through a gap in the southern rim of the summit crater. The lava dome currently has dimensions of about 1100 m (3600 ft) east-west and about 860 m (2820 ft) north-south, and is 25–30 m (82–98 ft) thick.

Concurrent with lava effusion, sulfur dioxide (SO2) emissions have been routinely detected by the TROPOMI satellite sensor, with estimated SO2 emission rates of a few hundred (metric) tons per day. Small earthquakes, consistent with the growth of the lava dome, continue to be observed, as well as occasional deeper events located beneath the volcano. No ash or explosive activity has been detected during this effusive phase since a small ash deposit was observed on July 26.

Prognosis and hazards

The most likely outcome of the current eruption of Great Sitkin is continued lava effusion. Because lava is nearing a low-point in the crater rim, it is likely to begin flowing downslope on the upper southern flank of the volcano. As lava flows down steep slopes, it is possible that portions of the lava flow front could collapse, generating block-and-ash flows and related small volume ash clouds. However, if the lava is fluid enough it will flow downslope passively and not generate ash clouds. We will continue to analyze satellite data and monitor geophysical data to watch for any increase in activity that could produce ash clouds.

Renewed explosive activity from the vent is also possible if changes in magma flux or composition occur, or if the vent area becomes blocked, leading to increased pressure build up. Such renewed explosive activity could occur without warning; however, continued effusion suggests the vent is current open and allowing effusive (non-explosive) eruption of lava.

AVO will continue to provide timely warnings of activity and will issue Volcanic Activity Notices (VANs) and Volcanic Observatory Notices for Aviation (VONAs) as needed. Based on the current lava eruption and possibility of explosive activity, Great Sitkin remains at Aviation Color Code ORANGE and Volcano Alert Level WATCH.

In the event of renewed explosive activity, or ash clouds generated by block-and-ash flows, AVO would work closely with the National Weather Service to track ash clouds and ash fall. For official forecasts of ash cloud movement or ash fall, please visit the Alaska Aviation Weather Unit or Anchorage Forecast Office, respectively (http://www.weather.gov.gov/aawu and http://www.weather.gov/afc/). Ash clouds are the main risk from this eruption because Great Sitkin Island is uninhabited. The closest community is in Adak, located 43 km (26 miles) to the west.

Monitoring status

Great Sitkin Volcano is monitored by a local digital network that includes seismic and infrasound sensors and a web camera.

In addition to local monitoring, AVO maintains a Great Sitkin web camera in Adak, as well as a regional infrasound array, which can detect explosive eruptions from Great Sitkin. Because the air pressure waves move at the speed of sound, there is a delay of minutes between eruption onset and detection. In addition, atmospheric conditions can affect movement of the infrasound waves through the atmosphere, and sometimes prevent them from reaching more distant arrays such as the one at Adak.

Radar satellite data and high-resolution visual satellite data have been extremely useful to track the growing lava dome, but these data are collected only once every few days. Near-real-time satellite data are used to detect and characterize ash clouds and track the thermal emissions from the lava. Sulfur dioxide emissions from Great Sitkin have been routinely detected from space during the current episode of lava effusion and changes in satellite-derived SO2 emission rates may indicate or precede changes in volcanic activity. Data from the World Wide Lightning Location Network (http://wwlln.net/) and Earth Networks (https://www.earthnetworks.com/) provide near-real-time automated alerts of lightning strikes and could help confirm larger eruptions were they to occur.

Background with more detailed chronology of recent activity

In late July of 2016 and continuing through 2020 Great Sitkin volcano entered a period of increased unrest that was characterized by elevated rates of earthquake activity, anomalous steaming from the summit crater, and small, likely steam-driven explosive events.

Beginning in January 2021, AVO began detecting elevated surface temperatures in satellite images of Great Sitkin Volcano with detections increasing in frequency over time. In early May the first satellite detections of SO2 emissions from Great Sitkin were observed, with detections and derived emissions increasing over the coming weeks. A slight increase in local seismicity was also observed on May 10. These collective observations suggested the potential for increased eruptive activity at Great Sitkin and prompted AVO to raise the Aviation Color Code to YELLOW and the Volcano Alert Level to ADVISORY on May 13.

A further increase in seismic activity started on May 25, 2021 and increased over several hours, prompting AVO to raise the Aviation Color Code to ORANGE and the Volcano Alert Level to WATCH. A short-duration (1-2 minutes) explosive eruption at 21:04 AKDT (5:04 UTC 26 May) resulted in an ash cloud up to 15,000 ft asl, prompting AVO to raise the Aviation Color Code to RED. The explosion produced strong infrasound and seismic signals, generated pyroclastic flows and lahars that extended up to 2 km (1.2 miles) from the vent, and deposited ash to more than 5 km (3.1 miles) southeast. Following the explosion, seismicity and infrasound decreased and satellite images showed an ash cloud had detached from the vent and drifted east. The aviation color code was lowered to ORANGE at 8:31 AKDT May 25 (16: 31 UTC 26 May). Seismicity, gas, and thermal emissions declined to low levels following the explosive event on May 25 and the Aviation Color Code was lowered to YELLOW and the Volcano Alert Level to ADVISORY on May 27.

On July 22, a satellite radar image showed a small ~50 m (~150 ft) diameter lava dome in the center of the May explosion crater at Great Sitkin. Previous imagery indicates that the lava eruption began sometime between July 14 and 22. As a result AVO raised the Aviation Color Code to ORANGE and the Volcano Alert Level to WATCH on July 23.

Lava effusion has continued since July, and the dome in the summit crater now measures about 1100 m (3600 ft) east-west and about 860 m (2820 ft) north-south, and is 25–30 m (82–98 ft) thick.

Reference Information

Great Sitkin Volcano is a basaltic andesite volcano that occupies most of the northern half of Great Sitkin Island, a member of the Andreanof Islands group in the central Aleutian Islands. It is located 43 km (26 miles) east of the community of Adak. The volcano is a composite structure consisting of an older dissected volcano and a younger parasitic cone with a 3-km-diameter summit crater. A steep-sided lava dome, emplaced during an eruption in 1974, occupies the center of the crater. Great Sitkin erupted at least three times in the 20th century, most recently in 1974. That eruption produced at least one ash cloud that likely exceeded an altitude of 25,000 ft above sea level. A poorly documented eruption occurred in 1945, also producing a lava dome that was partially destroyed in the 1974 eruption. Within the past 280 years a large explosive eruption produced pyroclastic flows that partially filled the Glacier Creek valley on the southwest flank.


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Michelle Coombs, Scientist-in-Charge, USGS mcoombs@usgs.gov (907) 786-7497

David Fee, Coordinating Scientist, UAF dfee1@alaska.edu (907) 322-4085

The Alaska Volcano Observatory is a cooperative program of the U.S. Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys.
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