The Alaska Volcano Observatory (AVO) reported that signs of volcanic unrest at Shishaldin had been gradually increasing during the previous month. Specifically, sulfur dioxide emissions had increased, observers noted a more vigorous steam plume rising from the summit crater, and seismic and infrasound data showed moderate increases in both the number and amplitude of events. At 1148 on 25 August the Volcano Alert Level was raised to Advisory (the second lowest level on a four-level scale) and the Aviation Color Code was raised Yellow (the second lowest color on a four-color scale).

Geological Summary. The symmetrical glacier-covered Shishaldin in the Aleutian Islands is the westernmost of three large stratovolcanoes in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." Constructed atop an older glacially dissected edifice, it is largely basaltic in composition. Remnants of an older edifice are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is covered by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. A steam plume often rises from the summit crater.

Source: US Geological Survey Alaska Volcano Observatory (AVO)