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USGS Volcano Notice - DOI-USGS-AVO-2023-09-29T08:13:19-08:00


U.S. Geological Survey
Friday, September 29, 2023, 11:49 AM AKDT (Friday, September 29, 2023, 19:49 UTC)

54°45'19" N 163°58'16" W, Summit Elevation 9373 ft (2857 m)
Current Volcano Alert Level: WATCH
Current Aviation Color Code: ORANGE

A significant explosive eruption of Shishaldin Volcano occurred early in the morning of September 25, the eleventh such event that has occurred at the volcano since July 14, 2023. This event was among the most impactful of the eruptive sequence, resulting in ashfall on several communities and flight cancellations in the region.

The explosive eruption was preceded by about 72 hours of increasing seismic unrest, and in the 12 hours ahead of the paroxysm, increasing lava fountaining, low-level ash emissions, and flows on the flanks of the volcano. During this ramp-up, AVO issued a series of 6 volcanic activity notices (VANs), providing updates about the level of activity and likelihood of explosive activity. The last such VAN was issued at 5:25 AM AKDT (13:25 UTC) on the 25th, less than 20 minutes before the explosive eruption commenced. During this phase, the Aviation Color Code remained at ORANGE and the Alert Level at WATCH.

A change in the seismicity and infrasound signals occurred around 5:35 am AKDT (13:25 UTC). The explosive eruption then commenced at 5:42 am AKDT (13:42 UTC), signaled by volcanic lightning and an increase in infrasound. As a result of the eruption, a significant ash cloud formed that quickly reached 40,000 to 45,000 ft above sea level and drifted east along the Alaska Peninsula. The cloud generated at least 150 lightning strokes with thunder that could be heard by people in False Pass, about 23 mi from the volcano. In response, AVO issued a volcanic activity notice at 6:02 AM AKDT (14:02 UTC), raising the Aviation Color Code to RED and the Alert Level to WARNING. Trace to minor amounts of ashfall were reported by the communities of False Pass (http://www.avo.alaska.edu/images/image.php?id=194929), Cold Bay (http://www.avo.alaska.edu/images/image.php?id=194928), King Cove and Sand Point. The ash cloud detached from the volcano around 7:00 am AKDT (15:00 UTC) and drifted east-southeast at an altitude of 38,000 ft above sea level. After the eruption of the ash cloud, seismicity declined rapidly around 06:00 am AKDT (14:00 UTC) but stayed elevated. Ash emissions continued at a lower altitude of about 20,000–25,000 ft above sea level until about 08:20 am AKDT (16:20 UTC). AVO lowered the Aviation Color Code to ORANGE and the Alert Level to WATCH at 12:34 AKDT (20:34 UTC).

Since the event, seismic activity has quieted over the past couple days. Synthetic aperture radar data show that significant collapses of the crater area occurred during the event. Satellite data also show significant hot, steaming pyroclastic and lahar (mud flow) deposits on all flanks of the volcano including more extensive flows on the ENE and WSW sectors below two new collapse scarps. Web camera views from September 27 showed small ash clouds most likely generated from collapses of the summit cone and/or small explosions at the summit. At least one such event was detected in infrasound data. Collapse of accumulated lava near the summit crater can occur without warning and generate hot mass flows on the upper flanks and small volcanic ash clouds that dissipate quickly.

There have been eleven periods of elevated eruptive activity resulting in significant ash emissions and mass flows of volcanic debris on the volcano's flanks since the onset of the current eruption on July 12. The last five periods of elevated eruptive activity have occurred approximately every 10 days and all eleven have been preceded by increases in seismicity in the hours before they occur. It unknown how long this eruptive episode will last. However, previous eruptions of Shishaldin Volcano have lasted weeks to months with repeated cycles of activity like those seen over the last month. Before the current activity, the 2019–2020 eruption of Shishaldin was the first to result in lava flows outside the crater area since 1976. Minor eruptions in 2004 and 2014 erupted lava confined to the summit crater. Previous eruptions of Shishaldin have produced ash clouds like those seen during the current eruption, most recently in January 2020. Routine evaluations of satellite, seismic, and infrasound data provide warning of unrest associated with the production of ash clouds. In addition, ashfall forecast models are kept up to date on the public activity page (https://avo.alaska.edu/activity/Shishaldin.php). 

Local seismic and infrasound sensors, web cameras, and a geodetic network monitor Shishaldin Volcano. In addition to the local monitoring network, AVO uses nearby geophysical networks, regional infrasound and lightning data, and satellite observations to detect eruptions. 

Shishaldin volcano, located near the center of Unimak Island in the eastern Aleutian Islands, is a spectacular symmetric cone with a base diameter of approximately 10 miles (16 km). A 660 ft-wide (200 m) funnel-shaped summit crater typically emits a steam plume and occasional small amounts of ash. Shishaldin is one of the most active volcanoes in the Aleutian volcanic arc, with at least 54 episodes of unrest including over 24 confirmed eruptions since 1775. Most eruptions are relatively small, although the April-May 1999 event generated an ash column that reached 45,000 ft. above sea level.

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

Slow eruption of lava is continuing at Great Sitkin Volcano. Weakly elevated surface temperatures were observed from the eruptive vent region in satellite images and earthquake activity remained slightly elevated this week. Satellite imagery and radar observations showed continued growth of the eastern branch of the summit lava flow. 

The current lava flow at Great Sitkin Volcano began erupting in July 2021. No explosive events have occurred since a single event in May 2021.    

The volcano is monitored by local seismic, geodetic, and infrasound sensors, web cameras, regional infrasound and lightning networks, and satellite data. 

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 26 miles (43 km) 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 ~1 mile (1.5 km)-diameter summit crater. A steep-sided lava dome, emplaced during the 1974 eruption, occupies the center of the crater. That eruption produced at least one ash cloud that likely exceeded an altitude of 25,000 ft (7.6 km) 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.

TRIDENT (VNUM #312160)
58°14'3" N 155°6'9" W, Summit Elevation 3599 ft (1097 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

Earthquake activity beneath Trident Volcano continued over the past week, with volcano-tectonic earthquakes noted most days and a few deep, low-frequency earthquakes and tremor observed throughout the week. No significant activity was observed in satellite and web camera data this week.

The current period of unrest began with a swarm of earthquakes on August 24, 2022. Earthquake depths at the beginning of the swarm were mainly deep, around 16 miles (25 km) below sea level and became progressively shallower to around 3 miles (5 km) over the following four days. Since late August 2022, most earthquakes have occurred within the shallow crust, with depths less than 4 miles (6 km) below sea level; however, an increasing number of earthquakes have been occurring deeper (greater than 9 miles or 15 km depth).  

Starting in May 2023 an increase in low-frequency earthquakes and tremor has been observed—in addition to the regular earthquakes—near Trident Volcano. Such low-frequency events are often associated with the movement of magma or volcano-related fluids within the ground.  

Ground uplift at Trident Volcano has also been detected in satellite radar data. Snow cover prohibits winter observations, which limits our ability to provide precise timing, but data from June 3, 2023, indicates that about 2 in (5 cm) of ground uplift has occurred since October 6, 2022. Uplift is most significant on the volcano’s south flank.    

Increases in seismic activity have been detected previously at Trident Volcano and other similar volcanoes and did not result in eruptions. We expect additional shallow seismicity and other signs of unrest, such as gas emissions, elevated surface temperatures, and ground movement, to precede any future eruption if one were to occur.         

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

Trident is one of the Katmai group of volcanoes located within Katmai National Park and Preserve on the Alaska Peninsula. Trident consists of a complex of four cones and numerous lava domes, all andesite and dacite in composition, that reach as high as 6,115 ft. (1,864 m) above sea level. An eruption beginning in 1953 constructed the newest cone, Southwest Trident, and four lava flows on the flank of the older complex. This eruption continued through 1974 and produced ash (an initial plume rose to 30,000 ft. or 9 km above sea level), bombs, and lava at various times. Fumaroles remain active on the summit of Southwest Trident and on the southeast flank of the oldest, central cone. Trident is located 92 miles (148 km) southeast of King Salmon and 273 miles (440 km) southwest of Anchorage.

KATMAI (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 Wednesday September 27, strong northwesterly 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 and carried it to the northwest beginning about 02:00 am AKDT (10:00 UTC). The National Weather Service has issued a SIGMET for this low-level event and suggests that the maximum cloud height is 6,000 ft (1.8 km) above sea level. A Special Weather Statement has also been issued by the National Weather Service warning of possible trace ashfall on communities downwind on Kodiak Island.  A hazy skyline and a spectacular a sunset on September 27, and small amounts of ash were reported to have fallen over night in the City of Kodiak on September 28.

This phenomenon is not the result of recent volcanic activity and occurs during times of high winds and dry snow-free conditions in the Katmai area and other young volcanic areas of Alaska. No eruption is in progress. Trident Volcano remains at color code YELLOW due to elevated earthquake activity, while other volcanoes of the Katmai area (Snowy, Griggs, Katmai, Novarupta, 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 


Matt Haney, Scientist-in-Charge, USGS mhaney@usgs.gov (907) 786-7497

David Fee, Coordinating Scientist, UAFGI dfee1@alaska.edu (907) 378-5460

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.