Report on Great Sitkin (United States) — 4 August-10 August 2021
Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 4 August-10 August 2021
Managing Editor: Sally Sennert.
Please cite this report as:
Global Volcanism Program, 2021. Report on Great Sitkin (United States) (Sennert, S, ed.). Weekly Volcanic Activity Report, 4 August-10 August 2021. Smithsonian Institution and US Geological Survey.
52.076°N, 176.13°W; summit elev. 1740 m
All times are local (unless otherwise noted)
AVO reported that unrest continued at Great Sitkin during 4-10 August with elevated seismicity, elevated surface temperatures, and explosions associated with a growing lava dome. Numerous earthquakes and small explosions were recorded on local infrasound and seismic stations during 4-5 August. During the morning of 5 August observers reported possible low-level lava fountaining from the active vent; the activity was also visible from Adak Island. Throughout the same day a volcanic plume comprised mostly of gas and steam (and possibly ash) was visible in webcam images rising to 3 km (10,000 ft) a.s.l. By 6 August the lava dome had grown to 250 m in diameter and had an approximate volume of about 1 million cubic meters. AVO noted that most of the crater was full of lava erupted in 1974 and that the current lava covered only 4-5 percent of the total summit crater area. Elevated surface temperatures were identified in satellite images during 6-10 August, though weather clouds mostly prevented satellite and webcam views. 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.