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Report on Great Sitkin (United States) — 29 September-5 October 2021


Great Sitkin

Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
29 September-5 October 2021
Managing Editor: Sally Kuhn Sennert

Please cite this report as:

Global Volcanism Program, 2021. Report on Great Sitkin (United States). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 29 September-5 October 2021. Smithsonian Institution and US Geological Survey.

Weekly Report (29 September-5 October 2021)

Great Sitkin

United States

52.076°N, 176.13°W; summit elev. 1740 m

All times are local (unless otherwise noted)


AVO reported that lava effusion continued at Great Sitkin during 29 September-5 October. Seismicity remained elevated and was characterized by small earthquakes consistent with lava effusion. Elevated surface temperatures were detected in satellite images. By 3 October the dome had grown to 1,200 m E-W and 1,000 m N-S. Lava flows that continued to advance down the S and SW flanks were about 300-350 m long. The SW lobe was descending two drainages and produced hot avalanches that traveled 450 m downslope on top of a snow field. The Aviation Color Code and the Volcano Alert Level remained at Orange and Watch, respectively.

Geological Summary. The Great Sitkin volcano forms much of the northern side of Great Sitkin Island. A younger parasitic volcano capped by a small, 0.8 x 1.2 km ice-filled summit caldera was constructed within a large late-Pleistocene or early Holocene scarp formed by massive edifice failure that truncated an ancestral volcano and produced a submarine debris avalanche. Deposits from this and an older debris avalanche from a source to the south cover a broad area of the ocean floor north of the volcano. The summit lies along the eastern rim of the younger collapse scarp. Deposits from an earlier caldera-forming eruption of unknown age cover the flanks of the island to a depth up to 6 m. The small younger caldera was partially filled by lava domes emplaced in 1945 and 1974, and five small older flank lava domes, two of which lie on the coastline, were constructed along northwest- and NNW-trending lines. Hot springs, mud pots, and fumaroles occur near the head of Big Fox Creek, south of the volcano. Historical eruptions have been recorded since the late-19th century.

Source: US Geological Survey Alaska Volcano Observatory (AVO)