AVO reported that a satellite image of Great Sitkin acquired at 0932 on 22 July showed a small area of uplift, about 50 m in diameter, and elevated surface temperatures associated with the feature. These observations suggested magma rising near the surface, prompting AVO to raise the Aviation Color Code and the Volcano Alert Level to Orange and Watch, respectively. Small earthquakes were recorded during 23-25 July. A 26 July satellite image confirmed that the feature was a lava dome, and that it had grown to 130 m in diameter. Seismic data suggested that the dome probably emerged sometime during 14-22 July.

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)