On May 13, 2021, AVO raised the Aviation Color Code and Volcano Alert Level at Great Sitkin from GREEN/NORMAL to YELLOW/ADVISORY, citing elevated surface temperatures in satellite images, beginning in January, and a slight increase in seismicity over the previous two days.
At 7:43 pm, May 25, 2021 (AKDT), AVO raised the Aviation Color Code and Volcano Alert Level to ORANGE/WATCH, noting an increase in local earthquakes. Explosive eruption began at 21:04 AKDT (5:04 UTC May 26), confirmed by geophysical data and mariner observations. Initial mariner reports suggested the cloud was as high as 15,000 ft asl. At 9:30 pm AKDT, AVO raised the Aviation Color Code and Volcano Alert Level to RED/WARNING. The explosion produced strong infrasound and seismic signals, generated pyroclastic flows and lahars that extended up to 2 km (1.2 miles) from the vent, and deposited ash to more than 5 km (3.1 miles) southeast.
With no further immediate explosions and declining seismicity, AVO lowered the Aviation Color Code and Volcano Alert Level back to ORANGE/WATCH later in the day on May 26. On May 27, 2021, AVO lowered the Aviation Color Code and Volcano Alert Level further to YELLOW/ADVISORY, citing very low seismicity levels and satellite observations of only minor steaming and slightly elevated surface temperatures. Seismicity remained at generally low levels for much of June.
A satellite radar image acquired July 22, 2021, 9:32 pm local time (July 23, 5:32 UTC) showed a small, about 50 m (150 ft) diameter area of uplift in the center of the crater at Great Sitkin, suggestive of rising magma near the surface. This lava dome-like feature appeared to have been emplaced sometime between July 14 and 22. As a result, AVO raised the Aviation Color Code and Volcano Alert Level to ORANGE/WATCH. Seismicity was at relatively low levels compared to the previous week and AVO suspects the lava dome was emplaced the prior week.
By July 30, the lava dome had grown to approximately 130 m in diameter and 8 m high. Satellite data from July 27 suggested minor fresh ash deposits. Great Sitkin's lava dome continued to grow throughout August, and reached about 1090 m diameter by August 30. Steam and gas plumes were visible to local observers, and AVO seismic instruments detect small earthquakes likely related to lava effusion.
By the end of July, the lava dome in the summit crater was ~425 ft in diameter. By September 9 it had begun to advance through a small gap in the southern rim of the crater wall. By September 17, the lava dome had also advanced through a small gap in the west rim. By late November it filled more than half of the summit crater, with two lobes of lava flowing over low points in the crater rim and extending downslope ~2000 ft to the south and west. Lava also overtopped the crater rim on its northern edge.
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 into February.
The alert levels have remained at ORANGE/WATCH since July 23, 2021.