ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, July 23, 2021, 12:04 PM AKDT (Friday, July 23, 2021, 20:04 UTC)

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

Low-level unrest continued at Semispochnoi volcano over the past week. AVO decreased the Aviation Color Code and Alert level to YELLOW/ADVISORY on July 21 (local) based on a marked decline in seismicity and no evidence of explosive activity since July 12. Seismicity remains above background but no explosive activity was detected in seismic, infrasound, or satellite data this week. Clouds obscured views of the volcano most of the week although sulfur dioxide emissions were detected in satellite data on July 17 and 19 and robust steaming from the active vent was observed in web camera images on July 22 and today.

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.

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

Unrest continued at Great Sitkin volcano over the past week. Small earthquakes were detected this week with an overall decline in earthquake activity compared to last week. No explosive activity was observed in seismic or local and regional infrasound data. Clouds obscured views of the volcano by satellite and web camera most of this week although moderately elevated surface temperatures were detected on July 22. A satellite radar image from last night showed a small ~50 m (164 ft) diameter area of uplift in the center of the crater suggestive of rising magma near the surface. This feature appears to have been emplaced sometime between July 14 and today. AVO will continue to closely monitor this new uplift feature.

Renewed explosive activity or lava effusion remain possible outcomes of the current period of unrest. This is, however, uncertain and a decline in unrest to background levels of activity is also possible.

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.

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 declined during the last week. Clouds obscured views of the volcano for most of the week although weakly elevated surface temperatures were detected on July 17 and 19 and today. No explosive activity was observed in seismic, regional 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 two seismic stations, 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.

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

Unrest continued at Pavlof volcano over the past week. Intermittent periods of low-level seismic tremor were detected throughout the week. On July 18 a thin layer of volcanic ash was deposited on the flank of the volcano extending a short distance from summit crater and may indicate minor explosive activity or material that was carried out of the vent by robust steaming. Clouds obscured views of the volcano much of the week although weakly elevated surface temperatures were detected on July 19. No other eruptive activity was observed this week. Today a pilot reported a small ash cloud from sea level to 6,500 feet above sea level (~2000 m) and extending 20–25 mi (32–40 km) to the southeast. This ash cloud is not from an active eruption but rather from ash deposited on the flanks of the volcano and resuspended by high winds.

There have been no observations of gas jetting or significant explosive activity detected in infrasound data. Minor steaming and warm temperatures in the summit crater were observed in web camera and satellite images. The volcano remains restless and past eruptions of Pavlof have occurred with little to no warning.

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

GARELOI VOLCANO (VNUM #311070)
51°47'21" N 178°47'46" W, Summit Elevation 5161 ft (1573 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

Unrest continued at Gareloi volcano over the past week. Seismicity was above background levels but no explosive activity was detected in seismic or infrasound data. Cloudy conditions obscured views of the volcano by satellite and web camera all week.

Gareloi volcano persistently emits magmatic gases from a fumarole field on the south crater and commonly exhibits low-level seismic activity. These observations suggest the presence of shallow magma and potential interaction with a hydrothermal system. The current increase in seismicity likely reflects a change to the magmatic-hydrothermal system, but it is not clear that the likelihood of a volcanic eruption has increased. AVO will continue to monitor activity to determine if the recent changes are related to influx of new magma or other changes to the magma system.

Gareloi is monitored by local seismic and infrasound sensors, satellite data, a web camera, and distant infrasound and lightning networks.


Mount Gareloi, which makes up all of Gareloi Island, is a stratovolcano located in the Delarof Islands group of the Aleutian Islands, about 2,000 km (1,242 mi) west-southwest of Anchorage and about 150 km (93 mi) west of Adak, the westernmost community in Alaska. This small volcano is 10 × 8 km (6.2 × 5.0 mi) in diameter at its base with two summits, separated by a narrow saddle. The northern, slightly higher peak contains crater about 300 m (1,000 ft) across. The southern summit has a crater open to the south and a persistent degassing vent (fumarole) on its western rim. Gareloi has been one of the most active in the Aleutians since the 1740s, with 16 reports of eruptive activity at Gareloi since 1760. In 1929, its largest historical eruption produced sixteen small south- to southeast-trending craters that extend from the southern summit to the coast, as well as lava flows and pyroclastic deposits on the southeastern flank of the volcano. Eruptions of Gareloi commonly produce ash clouds and lava flows, and the primary hazard is airborne clouds of ash that could affect aircraft. Since seismic instruments were installed in 2003, they have detected small but consistent seismic signals from beneath Mount Gareloi’s edifice.

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

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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.

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