From Associated Press (1974): "A spokesman for the Adak Seismological Observatory, a mere 23 miles away from the volcano, said the eruption was reported at 6:58 p.m. BDT and created 'a perfect mushroom cloud.' He said the clouds arose to at least 10,000 to 12,000 feet within four minutes of the eruption and was colored a dirty white to a light beige." AVO has several photographs of a Great Sitkin eruption plume, taken from Adak.. . .
"'It was a very spectacular explosion,' he said, 'but we don't know its type yet. It could be a steam explosion or a genuine volcanic eruption.'"
"The spokesman said witnesses saw a number of bright flashes as the mushroom cloud boiled its way up lending credence to the possibility of volcanic origin."
"He said it was not known if anyone actually heard the blast but he noted a number of witnesses said they heard a deep rumble shortly before the eruption."
From Smithsonian Institution, 1974, CSLP Report 1804: (22 February 1974): Explosive activity and light-colored plume
The following was cabled from the Geophysical Institute on 22 February 1974. "Explosive activity was observed at 1855 local time from Adak, 48 km WSW of Great Sitkin volcano. An earthquake originating at the volcano of Richter magnitude 2.6 occurred at the time of onset. An estimated 10,000-foot light-colored plume was reported over the summit at dusk. Since that time bad weather has obscured the island."

Smithsonian Institution, 1974, CSLP Report 1938: (30 September 1974): Lava dome still growing, overspilling crater rim
"D. Glover reported that, on a helicopter inspection trip to the volcano on 22 February, he '. . . Found that a large lava dome had been emplaced in the crater, with mostly steam and gas being emitted.' Foul weather prevented observations on all but three occasions between then and 29 March. About one week after the eruption, he noted, through a high-power telescope, '. . . That the dome had been extruded a considerable extent, with some ash being emitted. Since then activity has decreased to steam and gas emissions.'
"In mid-September, he reported that he had '. . . Only been able to observe Great Sitkin volcano by high-power telescope and a few times from aircraft. The lava dome appears to continue to extrude with minor lava flows spilling over the lip of the crater. The size of the dome is hard to estimate but it is probably close to 700 m in diameter and 200-300 m high. We have recently installed seismic instrumentation on the volcano but we have not noticed any unusual activity.' In summary, it appears that, following the initial release of the pressure head of volatiles on 19 February a dome has been extruded in the caldera of Great Sitkin. The new dome appears to be about the size of that extruded in 1945. Further, the dome is still active, overspilling the lip of the caldera as it grows. The extrusion has been quiet, with little of no associated explosiveness."
Newhall and Melson (1983) estimate the size of this dome at 96x10^6 cubic meters.

From the AVO Volcano Activity Notice of November 22, 2017: Recent observations of a robust steam plume and a period of gradually increasing seismicity over several months indicate that Great Sitkin Volcano has become restless and is exhibiting behavior that is above background levels. AVO is thus raising the aviation color code and volcano alert level to YELLOW/ADVISORY.

Photographs of the volcano taken by local observers on Sunday, November 19 show a light-colored vapor plume rising about 300 m (1,000 ft) above the vent area and extending about 15-20 km (9 -12 mi) to the south. Nothing unusual was observed in seismic or infrasound data around the time the photographs were taken and nothing noteworthy has been observed in satellite data since the emissions were observed.

An increased number of small earthquakes was evident as early as late July 2016, and since then the level of seismic activity has fluctuated at low levels but has exhibited a gradual overall increase most notable since June 2017. Seismic activity to date has been characterized by earthquakes that are typically less than magnitude 1.0 and range in depth from near the summit of the volcano to 30 km below sea level. Most earthquakes are in one of two clusters, beneath the volcano's summit or just offshore the northwest coast of the island. The largest earthquake so far was a magnitude 2.8 on September 29, 2017.

Possible explosion signals were observed in seismic data on January 10 and July 21 of [2017] but no confirmed emissions were observed locally or detected in infrasound data or satellite imagery.
After two months of declining seismicity, AVO lowered Great Sitkin to Green/Normal on January 18, 2018.

From Cameron and others, 2023: "During 2018, AVO located more than 2,300 earthquakes at Great Sitkin Volcano, principally clustered in the shallow crust (extending from the summit to roughly 10 km [6.2 mi] below sea level). Additional earthquakes were also located between 15 and 35 km [9 and 22 mi] depth, with waveforms and frequency contents suggesting both VT and deep LP earthquakes. The magnitudes of these located events ranged from an ML of less than −1.0 to 2.48 - the largest event took place on August 31, 2018. Unfortunately, the Great Sitkin Volcano seismic network experienced several station failures in 2018, most importantly at stations GSTD and GSSP, which impaired AVO’s ability to locate earthquakes and resulted in the data gaps. The most notable of these failure periods spanned early November 2017 to mid-January 2018. Shorter-term failures of station GSSP also compromised AVO’s earthquake locating capabilities during the winter of 2018-2019. AVO identified several tremor bursts associated with this unrest during 2018, with most taking place between June and December. Interpreted as small explosion events, the tremor bursts contained a variety of waveforms with impulsive to emergent onsets, extended codas, and frequency between 1 and 15 hertz. The bursts were commonly associated with increased earthquake activity, but none produced infrasound signals identifiable by sensors in the nearby City of Adak, Alaska. The explosion of June 10, 2018, proved especially noteworthy when a Sentinel-2 satellite image acquired on June 11 at 23:00 UTC (14:00 HADT) showed a 2-kilometer [1.2 mile] long ash deposit extending southwest from the summit of Great Sitkin Volcano. Additional photographs of the summit area taken from a passing aircraft a week later showed an ash deposit on the snow. This deposit presumably came from the June 10 explosion.
To characterize the size and progression of the Great Sitkin Volcano explosions, AVO measured the durations of their signals using methodology described by Searcy and Power (2020). Tremor events lasting less than 2 minutes were excluded, though several took place during the unrest. For many of the signals, some uncertainty remains on whether they reflect explosions, short volcanic tremor episodes, or more minor steam bursts. Except for the event on June 10, which produced an identifiable ash deposit, any of these events could have been produced by any of the previously listed mechanisms.
"In response to the volcanic activity, AVO made four changes to Great Sitkin Volcano’s Aviation Color Code and Volcano Alert Level during 2018. The volcano began the year at YELLOW and ADVISORY, but on January 18, after the number of earthquakes had fallen to background levels, AVO lowered the Aviation Color Code and Volcano Alert Level to GREEN and NORMAL. It raised them again to YELLOW and ADVISORY on June 10 in response to the explosion signal that took place that day. The Aviation Color Code and Volcano Alert Level were lowered back to GREEN and NORMAL on June 27 after declining earthquake activity, then raised to YELLOW and ADVISORY again on July 1 as earthquake activity increased. Great Sitkin Volcano remained at YELLOW and ADVISORY for the rest of 2018."
From Orr and others, 2023: "On February 2 [2019], because of declining earthquake activity, the Aviation Color Code and Volcano Alert Level were lowered from YELLOW and ADVISORY, where they had been since July 1, 2018, to GREEN and NORMAL."

From Orr and others, 2023: "AVO identified a single small explosion associated with Great Sitkin Volcano’s seismic unrest [in 2019]. It took place at 05:40 UTC on June 2 (June 1 at 20:40 HADT) and produced an emergent waveform with most of its energy between 1 and 5 hertz, similar to other small explosions recorded at Great Sitkin Volcano since January 2017. The event had a duration of 2 minutes and 17 seconds, determined using the methodology described in Searcy and Power (2020) for calculating the duration of explosions. No associated infrasound signal was observed on the instruments AVO operates in the City of Adak, indicating that the explosion was small...
"The Aviation Color Code and Volcano Alert Level were elevated...to YELLOW and ADVISORY on June 2, after the identification of the explosion signal recorded June 1. The Aviation Color Code and Volcano Alert Level were lowered again to GREEN and NORMAL on July 15, on the basis of declining seismicity. The Aviation Color Code and Volcano Alert Level remained there through the end of the year...
"During 2019, AVO located 629 earthquakes at Great Sitkin Volcano, principally clustered within the shallow crust extending from the summit to roughly 10 km [6 mi] deep. Additional shocks were also located between 10 and 35 km [6-22 mi] deep, with waveforms and frequency contents indicating both VT and deep LP events. Local magnitudes of located events ranged from −1.37 to 2.22. The largest event was located roughly 20 km [12 mi] southwest of the summit of Great Sitkin Volcano at a depth of 8.6 km [5.3 mi]. This hypocenter was deeper than those for earthquakes typically associated with volcanic processes beneath the volcano.
"The Great Sitkin Volcano seismic network experienced several station failures in 2019, impairing AVO’s ability to locate earthquakes. The most notable failure period spanned January to mid-June. These failures are likely the cause of a reduction in the number of located earthquakes in early 2019 relative to 2018, and the cause of an absence of shallow hypocenters detected in the first half of 2019. In response to the failures, AVO carried out a major upgrade to the seismic network in June 2019. The upgrades involved changing most of the older analog stations (installed in 1999) to broadband digital stations, although the analog stations GSSP and GSCK were left in operation for continuity. These network upgrades resulted in significantly improved station performance for the remainder of 2019."

From Orr and others, 2024: "In late July 2016, Great Sitkin Volcano entered a period of increased unrest characterized by an elevated frequency of earthquakes, anomalous steaming from its summit crater, and small explosive events (Dixon and others, 2020). Similar small earthquakes and steaming from the summit crater continued throughout 2020.
"AVO located 3,393 earthquakes at Great Sitkin Volcano during 2020, a large increase from earlier years. This increase may, in part, reflect network upgrades made during the 2019 field season that replaced older analog sensors with broadband sensors capable of digital telemetry. This new instrumentation has proven more reliable at Great Sitkin Volcano than the older equipment, improving AVO’s ability to detect and locate earthquakes in the area.
"In late January 2020, Great Sitkin Volcano began to experience an increase in earthquake activity that was interpreted as unrelated to the recently improved seismic detection capabilities, prompting AVO to raise the Aviation Color Code and Volcano Alert Level to YELLOW and ADVISORY on February 26. This activity peaked in late March and then slowly declined throughout the remainder of 2020. In response to the declining rates of seismicity, the Aviation Color Code and Volcano Alert Level were lowered back to GREEN and NORMAL on October 21. No seismic or infrasound signals indicative of explosive events were detected at the volcano during 2020.
"The hypocenters of earthquakes at Great Sitkin Volcano in 2020 were principally clustered within the shallow crust, extending from the summit to roughly 10 km [6 mi] depth below sea level. Earthquakes were also recorded at depths between 10 and 35 km [6 and 22 mi]; these had waveforms and frequency contents suggesting both volcano-tectonic (VT) and deep LP events. Located events ranged in magnitude from ML -1.2 to ML 3.5. The ML 3.5 event occurred on March 6 (at 15:31 UTC; 05:31 HAST) under the east rim of the summit crater at a depth of 0.2 km [0.1 mi]. This earthquake was the strongest event recorded beneath Great Sitkin Volcano since its period of unrest began in 2016. For comparison, the largest earthquake recorded beneath the volcano’s edifice since monitoring began in 1999 was ML 4.3. That event took place on May 28, 2002, and was located beneath the southeast flank of the volcano (Pesicek and others, 2008).
"The other notable earthquake activity at Great Sitkin Volcano in 2020 consisted of a cluster of seven earthquakes that occurred between January 31 and February 1. These seven events ranged from ML -0.6 to 0.27 and had depths ranging from 22.6 to 27.0 km [14.0 to 16.8 mi].
"AVO noted no major changes or unusual activity at the summit crater and dome during 2020, although the degree of visible steaming decreased compared to the 2016-2019 period (Dixon and others, 2020; Cameron and others, 2023; Orr and others, 2023). Photographs taken by passing airplanes in March, shortly after the ML 3.47 earthquake of March 6, show small snow-free areas and minor steaming at the summit of the volcano. Satellite imagery of Great Sitkin Volcano acquired in June and July indicated weakly elevated surface temperatures."

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.
In November, lava activity was mainly near the vent location, with a small amount of activity at the northern margins of the flow. Occasional volcanic earthquakes continued, with occasional sights of steaming or thermal anomalies when clear weather was present.
Lava extrusion continued without much change through mid-February 2025.
The alert levels have remained at ORANGE/WATCH since July 23, 2021.