AVO reported that after almost a week of relatively quiet conditions at Shishaldin, during 16-17 January seismicity began to climb and the temperature of the thermal anomaly slightly increased. Activity intensified at 0030 on 19 January and by around 0630 the plume became more ash-rich. By around 0828 the ash plume rose to 6.1 km (20,000 ft) a.s.l. and drifted 150 km E, prompting AVO to raise the Aviation Color Code to Red and the Volcano Alert Level to Warning. Lava flows descended the NE and N flanks and generated lahars. By 1530 seismicity abruptly decreased, though around the same time the robust steam-and-ash plume (visible to pilots and in webcam and satellite images) rose as high as 9.1 km (30,000 ft) a.s.l. and continued to drift 150 km SSE. Minor amounts of ash fell in False Pass. Ash emissions had significantly declined by 2200 and seismicity was low; the Aviation Color Code was lowered to Orange and the Volcano Alert Level was lowered to Watch just after midnight the next morning. A detached volcanic cloud was identified in satellite images drifting ESE over the Pacific Ocean. During 20-21 January elevated surface temperatures were identified in satellite images, though the N-flank flow was not active. Seismicity remained above background levels, and coincided with detections in infrasound data that suggested small explosions at the vent. Steaming from the summit was visible in webcam images.

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)