ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, July 21, 2023, 12:24 PM AKDT (Friday, July 21, 2023, 20:24 UTC)
Two significant episodes of elevated eruptive activity occurred at Shishaldin Volcano during the past week.
On July 15, ash emissions began at 9:50 pm AKDT (04:50 UTC on July 16) and continued until 11:30 pm (07:30 UTC on July 16) reaching 16,000 ft. (5 km) above sea level and extending for over 80 miles (125 km) south of the volcano. On July 18, ash emissions began around 7:00 am AKDT (15:00 UTC) initially reaching around 30,000 ft. (7 km) above sea level (http://www.avo.alaska.edu/images/image.php?id=194432) and continued until 8:30 am AKDT (16:30 UTC). Lower-level ash emissions continued for several hours but stayed below 10,000 ft. (3 km) above sea level. The Aviation Color Code and Volcano Alert Level were raised to RED and WARNING for both episodes.
Significant ash emissions on both days were preceded by around 6 hours of heightened eruptive activity, including elevated seismic tremor, infrasound signals, and increased surface temperatures visible in satellite images. Incandescence was visible in clear web camera images, but no lava flow activity was observed outside of the summit crater (http://www.avo.alaska.edu/images/image.php?id=194428).
High-resolution satellite images acquired after the activity on July 16 and July 18 showed pyroclastic deposits from the eruption extending as far as 1.9 miles (3 km) from the vent (http://www.avo.alaska.edu/images/image.php?id=194444). These deposits generated mudflows that extended further down drainages on the volcano's flanks. Ash deposits were focused to the south-southeast, downwind from the vent, and extended to the shore of Unimak Island.
Following the July 18 episode, clear satellite images showed elevated surface temperatures consistent with low-level eruptive activity in the summit crater. No significant ash emissions were observed in clear web camera images or satellite data, although mostly cloudy conditions obscured views of the volcano in the last few days. Seismicity has remained at low levels and no infrasound signals of explosive activity have been detected.
Eruptive activity could increase again rapidly with little or no warning. Before the current activity, the 2019–2020 eruption of Shishaldin was the first to result in lava flows outside of the crater area since 1976. Minor eruptions in 2004 and 2014 erupted lava confined to the summit crater. Eruptions from Shishaldin have produced ash clouds in the past similar to those seen during the current eruption, most recently in January 2020. Satellite, seismic, and infrasound data is routinely monitored for signs of explosions that might produce ash clouds. In addition, ashfall forecast models are kept up to date on the public activity page (https://avo.alaska.edu/activity/Shishaldin.php).
Shishaldin Volcano is monitored by local seismic and infrasound sensors, web cameras, and a telemetered geodetic network. The local monitoring network has been partially impaired over the last few weeks due to telecommunications issues but seismic stations and web cameras south of the volcano were brought back online on July 19. In addition to the local monitoring network, AVO uses nearby geophysical networks, regional infrasound and lighting data, and satellite images 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.
Slow eruption of lava likely continued at Great Sitkin over the past week, adding to a thick lava flow in the summit crater. Three to ten small local earthquakes were detected each day. Most satellite and web camera images were obscured by persistent cloud cover, but clear images on July 19 showed light steaming from the active lava flows and minimal change in the overall lava flow extent, consistent with recent observations.
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 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.
The Aviation Color Code and Volcano Alert Level were raised to YELLOW and ADVISORY on July 19 following an increase in earthquake activity over the past two weeks. Many of the most recent earthquakes are located less than 4 miles (6 km) below the surface. While these earthquakes are small (less than magnitude 2), the frequency of events is unusual for Cleveland.
Web camera images over the last week showed normal steam and gas emissions that are typically seen in clear views of Cleveland’s summit crater (http://www.avo.alaska.edu/images/image.php?id=194442) as well as slightly elevated temperatures within the summit crater detected in clear satellite images. A high-resolution satellite image on July 19 showed no significant surface changes within the summit crater (http://www.avo.alaska.edu/images/image.php?id=194447).
Episodes of lava effusion and explosions can occur without advance warning. Explosions from Mount 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 are less likely and occur less frequently.
Cleveland volcano is currently monitored with a five-station real-time seismic network and three nearby web cameras. Based on past events, explosive eruptions of Cleveland may occur with little or no warning. Rapid detection of an ash-producing eruption may be possible using a combination of seismic, infrasound, web camera, 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.
Earthquake activity beneath Trident Volcano remained elevated over the past week. Most of the earthquakes were small with magnitudes below 1.0, but there were a few larger earthquakes including a magnitude 3.3. Sequences of 14–24 mile-deep (23–38 km) low-frequency earthquakes and tremor were noted each day. No eruptive activity or other signs of unrest were observed in satellite and web camera images.
The current period of seismic unrest began 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.
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.
Earthquake activity beneath Aniakchak volcano was low over the past week with one to four small events detected each day. Satellite and web camera images were mostly obscured by clouds but clear views indicated no unusual activity. No new satellite radar imagery was available to analyze ground deformation.
The current period of seismic unrest began in October 2022. Increases in seismic activity have been detected previously at other similar volcanoes, with no subsequent eruptions. We expect additional shallow seismicity and other signs of unrest, such as gas emissions, elevated surface temperatures, and additional surface deformation to precede any future eruption, if one were to occur.
Aniakchak volcano is monitored by local seismic and infrasound sensors, web cameras, regional infrasound and lightning networks, and satellite data.
Aniakchak volcano, located in the central portion of the Alaska Peninsula, consists of a stratovolcano edifice with a 6 mile (10 km)-diameter summit caldera. The caldera-forming eruption occurred around 3,500 years ago. Postcaldera eruptions have produced lava domes, tuff cones, and larger spatter and scoria cone structures including Half-Cone and Vent Mountain all within the caldera. The most recent eruption occurred in 1931 and created a new vent and lava flows on the western caldera floor while spreading ash over much of southwestern Alaska. Aniakchak volcano is 15 miles (25 km) southeast of the nearest community, Port Heiden, and 416 miles (670 km) southwest of Anchorage, Alaska.
Earthquake activity beneath Semisopochnoi volcano was low over the past week with only one or two small events detected each day. Weak tremor was observed on July 15 and 16. Satellite and web camera views of the volcano throughout the week were mostly obscured by clouds, but a small steam plume from the north crater of Mount Young was observed during clear views on July 16.
The last evidence of ash emission from the volcano was on May 5, 2023, and consisted of a minor dusting of ash on the northwest flank of the north crater of Mount Young.
Small eruptions producing minor ash deposits within the vicinity of the active north crater of Mount Young and ash clouds usually under 10,000 ft. (3 km) above sea level have characterized recent periods of eruption since 2018. Additional ash-producing events could occur again with little or no warning.
Semisopochnoi volcano is monitored by local seismic and infrasound sensors, web cameras, regional infrasound and lightning networks, and satellite data.
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 volcano 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 Young, a three-peaked cone cluster in the southwest part of the caldera. Mount Young has been intermittently active since 2018. 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.
The Aviation Color Code and Volcano Alert Level were lowered to GREEN and NORMAL this week following a gradual decline of earthquake activity and absence of other signs of volcanic unrest. During the peak of activity on March 9–11 over 150 earthquakes per day were located on Tanaga Island. The rate has decreased to less than 3 earthquakes per day over the past few weeks. Clouds have obscured most satellite and web camera images over the last week.
Takawangha is monitored with a local seismic network, a single local infrasound sensor, a web camera, regional infrasound and lightning sensors, and satellite imagery.
Takawangha is a remote, 4,754 ft. (1,449 m)-high stratovolcano located on the northeast portion of Tanaga Island, roughly 59 miles (95 km) west of Adak in the Andreanof Islands. Takawangha's summit is mostly ice-covered, except for four young craters that have erupted ash and lava flows in the last few thousand years. Parts of Takawangha's edifice are hydrothermally altered and may be unstable, possibly leading to localized debris avalanches from its flanks. Takawangha lies across a saddle from historically active Tanaga volcano to the west. No historical eruptions are known from Takawangha; however, field work shows that recent eruptions have occurred, and it is possible that historic eruptions attributed to Tanaga may instead have come from Takawangha.
The Aviation Color Code and Volcano Alert Level were lowered to GREEN and NORMAL this week following a gradual decline of earthquake activity and absence of other signs of volcanic unrest. During the peak of activity on March 9–11 over 150 earthquakes per day were located on Tanaga Island. The rate has decreased to less than 3 earthquakes per day over the past few weeks. Clouds have obscured most satellite and web camera images over the last week.
Tanaga is monitored with a local seismic network, a single local infrasound sensor, a web camera, regional infrasound and lightning sensors, and satellite imagery.
Tanaga Island lies in the Andreanof Islands approximately 62 miles (100 km) west of the community of Adak and 1260 miles (2025 km) SW of Anchorage. The northern half of the island is home to the Tanaga volcanic complex, comprising three main volcanic edifices. Tanaga Volcano is the tallest of these (5,925 ft. or 1,806 m) and lies in the center of the complex. The last reported eruption of Tanaga occurred in 1914 and earlier eruptions were reported in 1763-1770, 1791, and 1829. Reports of these eruptions are vague, but deposits on the flanks of the volcano show that typical eruptions produce blocky lava flows and occasional ash clouds. Eruptions have occurred both from the summit vent and a 5,197 ft. (1,584 m)-high satellite vent on the volcano's northeast flank. Immediately west of Tanaga volcano lies Sajaka, a 4,443 ft. (1,354 m)-high compound edifice with an older cone to the east that collapsed into the sea within the last few thousand years, and a new cone that has grown in the breach. The new cone is 4,305 ft. (1,312 m) high and consists of steeply dipping, interbedded cinders and thin, spatter-fed lava flows. To the east of Tanaga lies Takawangha, which is separated from the other active volcanic vents by a ridge of older rock. No historical eruptions are known from Sajaka or Takawangha; however, field work shows that recent eruptions have occurred, and it is possible that historic eruptions attributed only to Tanaga may instead have come from these other vents.
Chris Waythomas, Acting Scientist-in-Charge, USGS, cwaythomas@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.