AVO reported that seismicity at Shishaldin remained elevated during 10-11 December; low-level tremor was detected along with three small explosions. Elevated surface temperatures were identified in satellite images and a steam plume drifting from the summit was visible in webcam images. A short-lived explosion began at 0710 on 12 December and lasted about three minutes, coincident with a three-minute long period of elevated tremor. The event generated an ash plume that rose to altitudes of 6.1-7.6 km (20,000-25,000 ft) a.s.l., drifted almost 85 km/hour WNW, and then dissipated a few hours later. Three lightning strokes were detected between 0715 and 0717. The explosion may have collapsed the summit spatter cone. Highly elevated surface temperatures were visible in satellite images during 12-13 December, and the webcam showed nighttime incandescence and a robust steam plume emanating from the summit. Seismicity remained elevated through 16 December and elevated surface temperatures continued to be detected. A plume appearing to contain ash drifted from the summit on 14 December. A lava flow was reported by a pilot on 16 December; the next day satellite images showed a 2-km-long flow on the NW flank. The Aviation Color Code remained at Orange and the Volcano Alert Level remained at Watch.

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)