Alaska Volcano Observatory

From Orr and others, 2024: "Great Sitkin Volcano erupted in 2021 after a period of seismic unrest and minor steam explosions that began in 2016 (for example, Dixon and others, 2020).
"Leading up to 2021, activity at Great Sitkin Volcano was characterized by years of precursory seismicity, elevated surface temperatures, and gas emissions. This unrest culminated with a Vulcanian explosion on May 25, 2021, an event successfully forecasted by AVO in the hours prior. An effusive eruption then began in mid-July, gradually filling both the 2021 explosion crater and much of the summit crater with lava, which then spilled down the volcano’s flanks. Lava effusion persisted at the volcano throughout the rest of the year. The 2021 eruption followed the pattern of the 1974 eruption: an explosive event followed by lava effusion in the summit crater. AVO crews visited the volcano in June 2021 to sample the explosive eruption deposits and to carry out a gas and airborne imaging survey.
"2016-2021 Precursory Unrest
"Volcanic unrest began at Great Sitkin Volcano in July 2016 and was characterized by elevated seismicity, anomalous steam emissions from the summit crater, and a few small explosive events. The thousands of small earthquakes detected between 2016 and 2021 were located primarily in the shallow crust (between the surface and ~10 km [~6 mi] depth) and had ML values of less than 3 (for example, Dixon and others, 2020). The elevated seismicity at Great Sitkin Volcano began waning in early 2020 and had declined to background levels before the end of that year, leading AVO to lower the Aviation Color Code and Volcano Alert Level to GREEN and NORMAL on October 21, 2020.
"In January 2021, AVO observed a slight elevation of surface temperatures within the summit crater. More signs of activity began appearing that spring: earthquakes were recorded at an increasing frequency, satellite observations increasingly showed weakly to moderately elevated surface temperatures, and TROPOMI sensors began detecting SO2 emissions. This increasing unrest led AVO to raise the Aviation Color Code and Volcano Alert Level to YELLOW and ADVISORY on May 12.
"May 25 Explosion
"An earthquake swarm began on May 24, and the earthquake rate steadily increased, which indicated an increased potential for eruptive activity. This led AVO to raise the Aviation Color Code and Volcano Alert Level to ORANGE and WATCH on May 25. Great Sitkin Volcano produced an explosion one hour and 39 minutes later, at 20:04 HADT on May 25 (05:04 UTC on May 26), sending an ash and gas plume northeastward at an elevation of ~15,000 ft (~4,600 m) ASL. The ~2-minute-long explosion was detected in seismic, infrasound, and satellite data, as well as by local observers. The eruption and resulting ash cloud led AVO to raise the Aviation Color Code and Volcano Alert Level to RED and WARNING at 20:30 HADT on May 25 (05:30 UTC on May 26). After a decline in seismic activity and cessation of ash emissions, AVO lowered the Aviation Color Code and Volcano Alert Level to ORANGE and WATCH at 07:31 HADT (16:31 UTC) on May 26. A continued lack of eruptive activity accompanied by a decrease in seismicity, surface temperatures, and steam emissions led AVO to further reduce the Aviation Color Code and Volcano Alert Level to YELLOW and ADVISORY on May 27.
"Characterization of the May 25 Eruption
"Analysis of geophysical, geological, and remote sensing data suggest that the May 25 explosion of Great Sitkin Volcano was a Vulcanian eruption. The event produced high-amplitude infrasound (~190 pascals [0.0019 bar] at 6 km [4 mi] distance) and was preceded by a slow pressure rise, a phenomenon that is consistent with other Vulcanian explosions and is caused by pre-explosion inflation of a dome or plug feature (for example, Yokoo and others, 2009; Iezzi and others, 2020). The explosion infrasound signal also showed two peaks in pressure that were likely related to a multi-part failure of the dome. After the main explosion, lower-amplitude infrasound emissions were recorded for tens of seconds. The explosion produced an ash cloud that quickly detached and drifted northeastward before dispersing. The next day, TROPOMI data showed an SO2 plume over the Bering Sea consistent with the eruptive cloud’s trajectory. The explosion also widened the pre-existing explosion crater in the 1974 dome, blasting large blocks (wider than 2 m [6.6 ft]) of old, altered lavas into a radial ballistic field ~1.5 km [~1 mi] in diameter. Some likely landed warm because they were found in deep melt pits when observed later by AVO geologists. Other eruptive deposits were a trace tephra deposit extending 5 km [3 mi] east-southeast, dark pyroclastic surge deposits ~1 km [0.6 mi] long, and a lahar that extended 2 km [1.2 mi] downslope to the south. Extending between the lahar starting point at the southeast crater rim and the vent within the crater was a 600-m-long [2,000-ft-long], pyroclastic flow deposit made of large, altered blocks, some more than 3 m [10 ft] in diameter. The deposits of the May 25 explosion were mapped with high-resolution optical and thermal imagery and sampled by an AVO field team on June 11.
"The eruption samples showed that the tephra deposit was primarily lithic and coarse-grained with rare juvenile breadcrust bombs (less than 1 percent of the deposit). The breadcrust bombs have a bulk andesite composition comprising a matrix of high-silica (rhyolitic?) glass and phenocrysts of highly zoned plagioclase, clinopyroxene, orthopyroxene, magnetite, and apatite. The high crystallinity, presence of both apatite and a silica groundmass phase, and evolved interstitial melt composition all suggest that the erupted magma was a shallow, low-pressure, and near-solidus magma plug. The coarse-grained, poorly sorted, and lithic-rich deposit characteristics also support a Vulcanian eruption mechanism, which is consistent with geophysical observations and photographs of the eruption event.
"2021 Effusive Phase
"After the May 25 explosion, seismicity and elevated surface temperatures were regularly recorded at the volcano, consistent with post-explosion fumarolic activity observed in a thermal survey on June 11. Steaming, elevated surface temperatures, and SO2 emissions were observed through mid-July. On July 23, high-resolution TerraSAR-X spotlight SAR imagery showed a small, new lava dome in the center of the explosion crater. No lava was present in previous imagery from July 14, so lava effusion began sometime during the period of July 14-23. In response to the onset of effusion, AVO raised the Aviation Color Code and Volcano Alert Level to ORANGE and WATCH on July 23.
"Lava overflowed the explosion crater by August 4, spreading radially over the 1974 and 1945 lava domes and onto the ice that filled the east part of the summit crater. The lava effusion in July and August was accompanied by steam and SO2 emissions; incandescence; saturated SWIR and midinfrared satellite data; and elevated seismicity. Lava effusion rates were 3-7 cubic meters per second (m^3/s) [100-250 cubic feet per second (ft^3/s)] in August as estimated by mapping the flow extent and thickness. This mapping was produced using high-resolution optical and radar satellite imagery, as well as the thermal radiative power measured by the Moderate Resolution Imaging Spectroradiometer and Visible Infrared Imaging Radiometer Suite (satellite sensors). The effusion rate declined after August, dropping to ~2 m^3/s [70 ft^3/s] in September and falling below 1 m^3/s [35 ft^3/s] in November. Seismicity and SO2 emissions also waned when effusion rates declined in September.
"Lava began to overflow the summit crater in September, eventually forming three flow lobes that advanced down the steep flanks of the volcano: a west lobe that started by September 19, a south lobe that started by September 29, and a north lobe that started by November 12. The flow fronts had minor rockfall activity, depositing blocks as far as ~50 m [~150 ft] below the flow front. The east flow margin advanced onto the ice, melting it and causing fracturing and collapse of the ice near the flow front, but only producing minor steaming. By the end of 2021, the lava flow covered 1.3 square kilometers [0.5 square miles] and had an estimated total volume of 0.031 km^3 [0.0074 mi^3]. The dome within the summit crater was ~1,230 m [~4030 ft] wide (in its east-west direction), whereas the west, south, and north flow lobes were ~830 [2700], ~865 [2840], and ~180 m [590 ft] long, respectively. Slow effusion continued into 2022.
"During 2021, AVO located 966 earthquakes near Great Sitkin Volcano, ranging in ML from −1.1 to 2.1 and in depth from −1.8 to 32.04 km below sea level (negative depths reflect height above sea level). Most hypocenters clustered between 0 and 10 km [0-6 mi] depth beneath the Great Sitkin Volcano edifice. The most notable seismicity during 2021 was a ~24-hour-long swarm of LP [long period] earthquakes that immediately preceded the explosive eruption on May 25."
Slow lava emissions continued over the next several months, with weak seismicity and elevated surface temperatures observed in clear weather. Steam and gas plumes were occasionally observed. By mid-February 2022, the southern lava flow was 3400 ft long, the western lava flow was 3000 ft long, and the northern lava flow was 720 ft long. After this, the southern and western lava flows were occasionally observed to grow longer. In May and June, most new lava was confined to the crater rather than the lava flows.
In mid-2022, the lava began slowly advancing eastward in the crater as well as piling up on top of the vent. This marked the start of a new phase of the eruption, as the previous lava flows on the flanks stopped growing and new lava flows were established within the crater. Initially these new flows travelled south and east of the vent. By November 4, 2022, the new flows had extended 2000 feet to the east and 1400 feet to the south. Weak seismicity and sometimes elevated surface temperatures continued through this period.
Into the beginning of 2023, the lava flow field continued to grow in the eastern direction, interacting with the inter-crater icefield. Some flows also traveled south on top of the cooler lava flows from the first phase of the eruption, but by April 2023, the only growth direction was eastward. As the lava flow plowed into the inter-crater glacier, the ice deformed and cracked. Low-level seismicity, somewhat elevated surface temperatures, and steaming from around the vent location all continued throughout the rest of the year.
AVO scientists visited the lava flow in September 2023 and took samples. They found the flows were warm and steaming. At the time, the active part of the flow was advancing about a foot every day.
In December 2023, new uplift of the lava on top of the vent location was noted, which caused radial cracks to form on the surface of the flow.
In early January 2023, the upward movement over the vent continued, which pushed some material to the north. Lava began flowing in a northward direction from the vent, reaching the northern margin of the previous 2021 flow by January 12. Lava activity remained restricted to the center of the active summit lava dome, with little advancement of its outer margins in the next week.
On January 2, the data streams from all instruments on Great Sitkin went down due to weather, and the outage continued until January 28. AVO used satellite data and regional infrasound to track eruptive activity during this time.
On January 24, satellite radar data showed that the new northwestward-moving lobe of the lava flow had extended about 600 feet from the northernmost of the radial cracks that had been noted in December.
Extrusion of the northwest lobe continued throughout February.
On March 6, satellite radar data showed some movement of the eastern lobe, but other observations during the rest of March showed extrusion had returned to the newer northwest lobe, along with additional uplift over the vent region. High-resolution satellite images showed the northwest lobe mostly snow-free, indicating its warmth, and steaming at the vent.
In the first half of April, both the northwestern and eastern lobes grew, but in the latter half of April through mid-May only the northwestern lobe continued to grow. By April 12, it had reached 850 ft (260 m) in length. The active part of the lava flow was observed to be snow-free and steaming when clear satellite views allowed.
In the later part of May northward flow continued, but more lava also uplifted the surface above the location of the vent. This pattern continued through the early part of July, with occasional detections of elevated temperatures over the surface of the flow and observations of steaming when the weather was clear.
In mid-July flow direction moved to the east and south, with new lava covering flows from 2021-2024. Rock falls occurred as cooled lava blocks fell down the steep margins of the flow, creating seismic signals.
This activity continued into early August. Lava flow direction was mainly to the east-northeast, with elevated surface temperatures and steaming observed in clear weather. Late in August the flow direction became more to the northeast, but in mid-September was more to the east. Weak thermal anomalies continued to be observed through mid-October.
The alert levels have remained at ORANGE/WATCH since July 23, 2021.