ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, August 6, 2021, 1:27 PM AKDT (Friday, August 6, 2021, 21:27 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

Intermittent explosive eruptive activity has characterized the unrest at Semispochnoi volcano over the past week. Occasional explosions sometimes associated with periods of seismic and infrasonic tremor have occurred almost daily during the week. The explosions have generated small ash clouds detectable in satellite data that have reached 5,000 to 10,000 feet above sea level. Most of these ash clouds have drifted 50-100 miles beyond Semisopochnoi Island before they were no longer evident in satellite data. Ash emissions from north crater, the active vent on Semisopochnoi volcano, have been occasionally visible in web camera images and indicate that light ash fallout is likely occurring on much of Semisopochnoi Island. Sulfur dioxide emissions were detected in satellite data on multiple days this week.

Small eruptions producing minor ash deposits within the vicinity of the active north crater of Mount Cerberus and ash clouds under 10,000 ft above sea level are typical of recent activity at Semisopochnoi. New explosions could occur at any time with no warning.

Semisopochnoi is monitored by local seismic and infrasound sensors, satellite data, web cameras, and distant 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.

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

Low-level unrest continued at Cleveland volcano over the past week. Earthquake activity has remained low but above background. Clouds obscured views of the volcano for most of the week although elevated surface temperatures were detected on August 2 and 5. Minor steam emissions were observed in web camera views of the summit on August 5. No explosive activity was observed in seismic, infrasound, or satellite 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, 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.

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

Effusion of lava at the active vent on Great Sitkin volcano has characterized activity over the past week. As of today, the circular lava accumulation in the summit crater of the volcano is roughly 250 m in diameter and has an approximate volume of about 1 million cubic meters. Yesterday morning, local observers witnessed possible lava fountaining near the active vent accompanied by sustained emissions of gas (most likely water vapor and sulfur dioxide) that could have contained small amounts of fine ash. Seismicity has remained above background and occasional small explosions have been detected in infrasound and seismic data this week.

It is uncertain how long the ongoing period of lava effusion will last. Most of the summit crater is filled with lava emplaced in 1974 and the present lava flow covers roughly 4-5 percent of the summit crater area. At the present rates of effusion it would take many months for the lava to fill the summit crater. Under the present conditions, continued lava effusion accompanied by minor explosions and gas emissions are likely. AVO is monitoring the activity closely and will continue to provide information as we receive it.

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

A brief period of minor ash emission occurred at Pavlof volcano yesterday but otherwise the volcano has been mostly quiet. Intermittent periods of low-level seismic tremor and small explosions were detected by local seismic and infrasound sensors this week. Over the past day, some small explosions also registered on a regional infrasound array. Occasional satellite views of the volcano throughout the week revealed nothing unusual occurring at the summit and web camera views from yesterday morning (August 5, 2021) captured minor ash emissions over a period of about 1 hour. Since yesterday there have been no additional observations or reports of ash emissions. A few pilot reports from yesterday mention only steam emissions at the summit.

The volcano is in a minor period of unrest and it remains possible for conditions to change rapidly and for more significant eruptive activity to occur with little to no warning. AVO is monitoring Pavlof closely and local seismic and infrasound sensors, satellite data, web cameras, and distant infrasound and lightning networks are all fully operational and providing monitoring data.


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.

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/

FOLLOW AVO ON FACEBOOK: https://facebook.com/alaska.avo

FOLLOW AVO ON TWITTER: https://twitter.com/alaska_avo

CONTACT INFORMATION:

Matt Haney, Acting Scientist-in-Charge, USGS mhaney@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.

---