AVO increased the Aviation Color Code for Shishaldin to Yellow and the Volcano Alert Level to Advisory on 6 December following several weeks of increasing seismicity and pressure waves recorded by infrasound sensors. Continuous infrasound waves were detected for more than 10 hours on instruments located in Sand Point, ~230 km E. Steam emissions visible in satellite and webcam images during 5 and 8-12 December were rising hundreds of feet above the summit crater. The steam emissions were occasionally accompanied by infrasound signals indicating episodes of short-duration energetic gas emissions and/or small explosions.

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)