Recent observations of a robust steam plume and a period of gradually increasing seismicity over several months at Great Sitkin prompted AVO to raise the Aviation Color Code to Yellow and the Volcano Alert Level to Advisory on 22 November. On 19 November local observers photographed a light-colored vapor plume rising about 300 m above the vent and drifting 15-20 km S. A satellite image acquired on 21 November showed steam continuously jetting from a small fumarole on the W side of the 1974 lava flow within the summit crater, and at least one area where snow and ice had been melted.

Seismicity had fluctuated but increased overall since July 2016, most notably in June 2017. The seismic activity was characterized by earthquakes less than M 1, and occurred either just below the summit or just offshore the NW cost of the island, 30 km below sea level. Possible explosion signals were recorded in seismic data on 10 January and 21 July 2017, but there were no confirmed emissions.

Geological Summary. The Great Sitkin volcano forms much of the northern side of Great Sitkin Island. A younger 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 older edifice and produced a submarine debris avalanche. Deposits from this and an even 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. Eruptions have been recorded since the late-19th century.

Source: US Geological Survey Alaska Volcano Observatory (AVO)