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ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, September 24, 2021, 1:26 PM AKDT (Friday, September 24, 2021, 21:26 UTC)


SEMISOPOCHNOI VOLCANO (VNUM #311060)
51°55'44" N 179°35'52" E, Summit Elevation 2625 ft (800 m)
Current Volcano Alert Level: WATCH
Current Aviation Color Code: ORANGE

The north crater of Mount Cerberus on Semisopochnoi Island remained active last week with numerous explosions detected each day and low-level ash emissions below 15,000 ft. asl observed in clear web camera views. The frequency and intensity of the explosions increased on Sunday, September 19, resulting in a continuous ash plume that extended approximately 330 km to the northwest. Do to this increase in activity, the Aviation Color Code/Alert Level was increased to RED/WARNING. After activity decreased over the following day, the Aviation Color Code/Alert Level was lowered back to ORANGE/WATCH on Monday, September 20. Elevated sulfur dioxide emissions were detected in satellite images on the 19th and 20th with lower emissions in the subsequent days

Small eruptions producing minor ash deposits within the vicinity of the active north crater of Mount Cerberus and ash clouds under 10,000 feet above sea level have characterized the recent activity and show no signs of abating.

Semisopochnoi is monitored by local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.


Remote Semisopochnoi volcano occupies the largest, young volcanic island in the western Aleutians. The volcano is dominated by a 5-mile (8-km) diameter caldera that contains a small lake and several post-caldera cones and craters. The age of the caldera is not known with certainty but is likely early Holocene. Prior to 2018, the previous known historical eruption of Semisopochnoi occurred in 1987, probably from Sugarloaf Peak on the south coast of the island, but details are lacking. Another prominent, young post-caldera landform is Mount Cerberus, a three-peaked cone cluster in the southwest part of the caldera. The island is uninhabited and part of the Alaska Maritime National Wildlife Refuge. It is located 40 miles (65 km) northeast of Amchitka Island and 130 miles (200 km) west of Adak.

GREAT SITKIN VOLCANO (VNUM #311120)
52°4'35" N 176°6'39" W, Summit Elevation 5709 ft (1740 m)
Current Volcano Alert Level: WATCH
Current Aviation Color Code: ORANGE

Lava effusion continued at Great Sitkin this week. The growing lava dome has now overtopped the south and west rims of the summit crater with lobes extending down to the west and southwest. Satellite radar images show that the dome currently has dimensions of about 1170 m (3840 ft) east-west and about 925 m (3030 ft) north-south, and is about 25 m (82 ft) thick. Seismicity remains slightly elevated with small earthquakes consistent with lava effusion continuing. Elevated surface temperatures were observed in clear satellite images over the past week. No explosive activity or ash emissions have been detected in seismic, infrasound and satellite data.

There is no indication of how long lava effusion will continue during the current eruption, and it is possible that explosive activity could occur with little or no warning.

Great Sitkin Volcano is monitored by local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.


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 the most recent significant eruption in 1974, occupies the center of the crater. 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.

PAVLOF VOLCANO (VNUM #312030)
55°25'2" N 161°53'37" W, Summit Elevation 8261 ft (2518 m)
Current Volcano Alert Level: WATCH
Current Aviation Color Code: ORANGE

Small ash emissions and explosions were detected in seismic and infrasound data at Pavlof on September 18, 19, 22, and 23. These ash producing explosions could be seen in web camera images and by local observers with ash clouds rising to several hundred meters above the summit and occasionally being pushed by wind down the flanks of the volcano. On other days elevated seismic tremor was detected but no explosions, and most web camera and satellite images were obscured by clouds.

The explosions that have occurred during Pavlof Volcano's current period of activity have been short-lived, with ash deposits confined to the flanks of the volcano. Eruptive activity is focused at a vent on the upper southeast flank of the volcano, near the location of the eruptive vent in 2007. The level of unrest at Pavlof can change quickly and the progression to more significant eruptive activity can occur with little or no warning.

Pavlof Volcano is monitored by local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.


Pavlof Volcano is a snow- and ice-covered stratovolcano located on the southwestern end of the Alaska Peninsula about 953 km (592 mi) southwest of Anchorage. The volcano is about 7 km (4.4 mi) in diameter and has active vents on the north and east sides close to the summit. With over 40 historic eruptions, it is one of the most consistently active volcanoes in the Aleutian arc. Eruptive activity is generally characterized by sporadic Strombolian lava fountaining continuing for a several-month period. Ash plumes as high as 49,000 ft ASL have been generated by past eruptions of Pavlof, and during the March 2016 eruption, ash plumes as high as 40,000 feet above sea level were generated and the ash was tracked in satellite data as distant as eastern Canada. The nearest community, King Cove, is located 48 km (30 miles) to the southwest of Pavlof.

CLEVELAND VOLCANO (VNUM #311240)
52°49'20" N 169°56'42" W, Summit Elevation 5676 ft (1730 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

Elevated surface temperatures were detected in the summit crater of Cleveland in clear overnight satellite images. On most days, however, cloudy conditions obscured satellite and web camera images. Earthquake activity has remained low and no explosions were detected in seismic or infrasound data.

Episodes of lava effusion and explosions can occur without advance warning. Explosions from Cleveland are normally short duration and only present a hazard to aviation in the immediate vicinity of the volcano. Larger explosions that present a more widespread hazard to aviation are possible but occur less frequently.

Cleveland volcano is monitored by only one seismic station, which restricts AVO's ability to precisely locate earthquakes and detect precursory unrest that may lead to an explosive eruption. Rapid detection of an ash-producing eruption may be possible using a combination of seismic, local and regional infrasound, lightning, and satellite data.


Cleveland volcano forms the western portion of Chuginadak Island, a remote and uninhabited island in the east central Aleutians. The volcano is located about 45 miles (75 km) west of the community of Nikolski, and 940 miles (1500 km) southwest of Anchorage. The most recent significant period of eruption began in February 2001 and produced 3 explosive events that generated ash clouds as high as 39,000 ft (11.8 km) above sea level. The 2001 eruption also produced a lava flow and hot avalanche that reached the sea. Since then, Cleveland has been intermittently active producing small lava flows, often followed by explosions that generate small ash clouds generally below 20,000 ft (6 km) above sea level. These explosions also launch debris onto the slopes of the cone producing hot pyroclastic avalanches and lahars that sometimes reach the coastline.

KATMAI VOLCANO (VNUM #312170)
58°16'44" N 154°57'12" W, Summit Elevation 6716 ft (2047 m)
Current Volcano Alert Level: NORMAL
Current Aviation Color Code: GREEN

On September 20-21 and September 23-24 strong winds in the vicinity of Katmai and the Valley of Ten Thousand Smokes picked up loose volcanic ash erupted during the 1912 Novarupta-Katmai eruption. The resulting resuspended ash cloud extended over Shelikof Strait and Kodiak Island.

This phenomenon is not the result of recent volcanic activity and occurs seasonally in the spring and fall during times of high winds and dry snow-free conditions in the Katmai area and other young volcanic areas of Alaska. All of the volcanoes of the Katmai area (Snowy, Griggs, Katmai, Novarupta, Trident, Mageik, Martin) remain at color code GREEN.

Resuspended volcanic ash should be considered hazardous and could be damaging to aircraft and health. For more information on volcanic ash and human health, visit the following website: http://volcanoes.usgs.gov/ash/. Official warnings about these ash resuspension events are issued by the National Weather Service: http://www.weather.gov/afc. Forecasts of airborne ash hazard to aircraft: http://www.weather.gov/aawu . Volcanic Ash Advisories: http://vaac.arh.noaa.gov/ . Forecasts of ash fall: http://www.weather.gov/afc. Air quality hazards and guidance from Alaska Department of Environmental Conservation, Division of Air Quality: http://dec.alaska.gov/Applications/Air/airtoolsweb/Advisories/Index


OTHER ALASKA VOLCANOES

Information on all Alaska volcanoes is available at : http://www.avo.alaska.edu.

For definitions of Aviation Color Codes and Volcano Alert Levels, see: http://www.avo.alaska.edu/color_codes.php

SUBSCRIBE TO VOLCANO ALERT MESSAGES by email: http://volcanoes.usgs.gov/vns/

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CONTACT INFORMATION:

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.