Great Sitkin reported activity

Event Name : Great Sitkin 2021/05

Start:

May 25, 2021

Observed

Lava dome:
Tephra plume:

Description: 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 rased the Aviation Color Code and Volcano Alert Level to RED/WARNING. 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, 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. As of this writing (late November, 2021) it currently fills 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 has also reached the crater rim on its northern edge. Seismicity remains above background levels.