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U.S. Geological Survey
Friday, December 10, 2021, 2:02 PM AKST (Friday, December 10, 2021, 23:02 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

Lava effusion within the summit crater of Great Sitkin Volcano likely continues at a slow rate, though viewing conditions were too poor to confirm this over the past week. The overall level of seismicity is very low. Elevated surface temperatures consistent with lava effusion were detected in satellite data last weekend, when the volcano was not obscured by cloud cover. No ash emissions were observed and no local observations have been reported to AVO this week.

Erupted lava has overtopped the summit crater rim and is flowing into small valleys on the south, west, and north flank of the volcano. The terrain is steep in these areas, and blocks of lava and lava rubble could detach from the terminus of the flow lobes without warning and form small rock avalanches in these valleys. Such avalanches may liberate ash and gas and could travel several hundred meters beyond the lava flows; they would be hazardous to anyone in those areas.

Great Sitkin 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.

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 eruption at Semisopochnoi volcano continues. Small explosions producing low-level ash clouds from the north crater of Mount Cerberus on Semisopochnoi Island occurred multiple times per day throughout the last week. These explosions were detected by local seismic sensors, local and regional infrasound sensors, and, when weather conditions allowed, in webcam and satellite images. Clouds obscured the volcano most of the past week, although small ash clouds drifting at least 185 km (115 miles) southeast of Semisopochnoi were detected in satellite data on December 8 and 9.

Small explosions that result in minor ash deposits within the vicinity of the active north crater of Mount Cerberus, and ash clouds typically under 10,000 ft 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.

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

Eruptive activity at Pavlof Volcano continues. Elevated seismicity, consisting of periods of sustained tremor and discrete low-frequency events, was detected throughout the past week. Numerous explosions were detected in infrasound data, and these may have produced localized fallout of ballistic ejecta around the active vent. Weak ash emissions that rose to an altitude of about 3,000 m (~10,000 ft) – about as high as the volcano itself – were observed on December 4 in web camera views and by a passing aircraft. Satellite and web camera views were cloudy for the rest of the week and no other ash emissions were seen.

Occasional periods of lava fountaining from the vent on the upper southeast flank has been occurring since mid-November. This activity has built a small cone of hot unstable spatter and rock debris. Occasionally, gravitational collapse of parts of the cone results in the development of hot granular flows of rock debris that flow down the flank, erode snow and ice, and can produce variable amounts of meltwater. The meltwater typically incorporates loose debris on the flank of the volcano and forms lahars. The lahar deposits observed so far appear relatively thin (<2 m thick) and were likely emplaced by water-rich flows. The lahar deposits extend down the east-southeast flank for several kilometers, not quite to the base of the volcano.

Previous eruptions of Pavlof indicate that the level of unrest can change quickly and the progression to more significant eruptive activity can occur with little or no warning.

Pavlof 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 above sea level have been generated by past eruptions of Pavlof, and during the March 2016 eruption, ash plumes as high as 40,000 ft 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.

51°57'15" N 178°19'34" E, Summit Elevation 1076 ft (328 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

A swarm of earthquakes occurred in the vicinity of Davidof volcano during the latter part of the week and is ongoing. The largest earthquake to date happened this morning (December 10) at about 19:45 UTC (10:45 am AKST) and had a magnitude of 4.2. This swarm may be associated with volcanic unrest or it could also be due to regional tectonic activity. Due to the possibility of escalating volcanic unrest, AVO raised the Aviation Color Code and Volcano Alert Level for Davidof to YELLOW/ADVISORY. AVO continues to monitor the situation with seismometers deployed on nearby islands since there is no real-time seismic monitoring network at Davidof volcano. The closest seismometers to Davidof are approximately 13 km (8 miles) to the east of the volcano on Little Sitkin Island.

Davidof volcano is a mostly submerged stratovolcano in the Rat Islands group in the western Aleutian Islands, about 350 km (218 miles) west of Adak. The subaerial part of the volcano comprises Davidof, Khvostof, Pyramid, and Lopy islands, which encircle Crater Bay, a 2.5 km (1.5 mile) diameter caldera. The islands are built up from interbedded lava flows and explosive deposits. The volcano has been sparsely studied, but visits by Alaska Volcano Observatory geologists in 2021 documented thick sequences of rhyolite to dacite pyroclastic flow and fall deposits that represent the most recent explosive eruptions. The age of these deposits is unknown, but they appear older than Holocene deposits from nearby Segula and Little Sitkin. There are no known historical eruptions from Davidof.

53°23'49" N 168°9'58" W, Summit Elevation 3520 ft (1073 m)
Current Volcano Alert Level: NORMAL
Current Aviation Color Code: GREEN

AVO released an Information Statement for Okmok volcano on December 8 in response to a change in the long-term deformation pattern between September and November 2021. The deformation has since returned to background levels. There have been no notable changes in seismic activity during this time period, and satellite images have shown no surface changes.

The deformation observed recently suggests a pressure increase at a shallow depth—less than 1 km (0.6 miles) below the caldera floor. This is consistent with a small intrusion of magma at that depth, but other non-magmatic processes such as hydrothermal activity cannot be ruled out. If the deformation was caused by a shallow magma intrusion, this may increase the likelihood of an eruption. Previous eruptions at Okmok have occurred with little precursory seismic activity, and AVO continues to monitor the volcano closely. The Aviation Color Code and the Volcanic Alert Level at Okmok remain at GREEN/Normal.

Ground deformation at Okmok is monitored with 4 continuous GNSS instruments, two inside the caldera and two outside. Data for these stations are downloaded remotely and analyzed daily. The volcano has had a local real-time seismic monitoring network since 2003. Currently, the network consists of 7 three-component seismometers on the eastern portion of Umnak Island, including 4 within the caldera. Okmok is also monitored by satellite data, a web camera, and remote infrasound and lightning networks.

Okmok volcano is a 10-kilometer (6-mile) -wide caldera that occupies most of the eastern end of Umnak Island, located 120 kilometers (75 miles) southwest of Unalaska/Dutch Harbor in the eastern Aleutian Islands. Okmok has had several eruptions in historical time typically consisting of lava flows crossing the caldera floor, such as in 1945, 1958, and 1997. However, in 1817, Okmok produced an explosive eruption that resulted in ash fall in the Dutch Harbor area. The volcano last erupted in July–August 2008, producing continuous ash emissions that fluctuated between about 1,500 and 9,000 meters (5,000 and 30,000 feet) above sea level for the 5-week duration of the eruption, with an initial plume height of 15,000 meters (50,000 feet) during the first 12 hours. This eruption resulted in the construction of a new cone (Ahmanilix) within the caldera.

The nearest settlement is Nikolski, about 70 kilometers (45 miles) west of the volcano. A ranch caretaker family lives at Fort Glenn on the flank of the volcano about 10 kilometers (6 miles) east of the caldera rim during the summer months.


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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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