ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, August 11, 2023, 1:38 PM AKDT (Friday, August 11, 2023, 21:38 UTC)
The most recent significant explosive event ended last week on the evening of August 4, when the Aviation Color Code and Volcano Alert Level was lowered to ORANGE/WATCH, where it currently remains. Following the eruption, satellite imagery revealed changes to the summit crater and flanks of the volcano. Many of the drainages on the upper flank of the volcano have been filled in with pyroclastic deposits extending 0.6–1.2 miles (1-2 km) in length, while mud and debris flows have scoured some drainages further down the flanks. The summit crater was also mostly filled with deposits from the eruption, and a small lava flow was observed extending from the crater rim to the upper northeast flank of the volcano. This flow advanced to about 180 ft (55 m) in length by August 9, but appears stalled in imagery from August 11.
Elevated surface temperatures were observed in satellite data throughout the week, which are caused by the small lava flow at the summit, and steaming was observed in web camera images on August 11. Seismicity remains slightly elevated, but no significant explosive activity was observed in seismic or infrasound data.
Shishaldin has now had six periods of elevated eruptive activity resulting in significant ash emissions during the current eruption which started on July 12. It is unknown how long this eruption will last, but previous eruptions of Shishaldin Volcano have lasted weeks to months with repeated cycles of activity similar to those seen in the last month. 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 like 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 geodetic network. In addition to the local monitoring network, AVO uses nearby geophysical networks, regional infrasound and lighting 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.
Slow eruption of lava continued at Great Sitkin over the past week, adding to a thick lava flow in the summit crater. Satellite radar data through August 10 show the eastern lobe of the lava flow continuing to advance into glacial ice surrounding the crater, causing it to deform and crack. Earthquake activity remained slightly elevated this week. Weak steaming was observed in web camera images on August 6.
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.
Earthquake activity continued at Cleveland volcano this week, although at a low level with occasional local earthquakes detected. Clouds obscured satellite images of the volcano over much of the week, but weakly elevated surface temperatures were observed in satellite data from August 7, and normal steam and gas emissions from the summit crater were observed in web camera images on August 7, 8, and 9.
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 by a local seismic network, infrasound sensors and web cameras. In addition to the local monitoring network, AVO uses nearby geophysical networks, regional infrasound and lighting data, and satellite observations to detect eruptions. 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 with multiple local low-frequency earthquakes and tremor noted each day. No eruptive activity or other signs of unrest were observed in clear 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. 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.
No significant volcanic activity has occurred over the past week. Strong southeasterly winds resuspended ash from the Aniakchak region on the afternoon of August 4. This phenomenon resulted in an ash cloud that drifted to the northwest, but it is not the result of volcanic activity and is unrelated to ongoing unrest at Aniakchak volcano. Earthquake activity beneath Aniakchak volcano was low over the past week. Several small earthquakes were detected daily. No unusual activity was observed in webcam and satellite images of the volcano, and satellite radar data from August 2 showed no detectable surface changes.
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, geodetic, 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.
Matt Loewen, Acting Scientist-in-Charge, USGS, firstname.lastname@example.org, (907) 786-7497
Ronni Grapenthin, Acting Coordinating Scientist, UAFGI, email@example.com, (907) 378-5460
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