AVO reported that the eruption at Shishaldin continued during 20-26 September with activity increasing during the week and culminating in a notable eruption during 24-25 September. Seismicity remained elevated during 19-22 September, with tremor and small earthquakes detected by the seismic network. Elevated surface temperatures were identified in satellite images. Minor steaming and small volcanic debris flows on the upper flanks were visible in webcam images on 20 September, a small ash deposit on the upper flank was visible in images the next day.

Seismic tremor increased significantly during 22-23 September. Regional infrasound sensors suggested that low-level eruptive activity was likely occurring within the summit crater by around 1800 on 23 September. Even though seismicity was at high levels, strongly elevated surface temperatures indicating lava at the surface were absent and no ash emissions were detected, though weather clouds at 0.6-4.6 km a.s.l. obscured views. At 0025 on 24 September AVO noted that seismicity was continuing at high levels and nearly continuous small infrasound signals had begun to be detected, likely from low-level eruptive activity. Strongly elevated surface temperatures were identified in satellite images at least by 0900 and persisted throughout the day; the higher temperatures along with infrasound and seismic data were consistent with lava erupting at the summit. Highly elevated surface temperatures detected at around 1700 suggested the start of more vigorous lava fountaining. Beginning at around 1800 low-level ash emissions rose to altitudes less than 4.6 km (15,000 ft) a.s.l. and quickly dissipated. At around midnight seismic data indicated that lava flows were active on the N flank, and lava fountaining over the crater rim was visible during early morning hours on 25 September. At 0540 a significant ash plume began to rise from the summit prompting AVO to raise the Aviation Color Code to Red (the highest color on a four-color scale) and the Volcano Alert Level to Warning (the highest level on a four-level scale). The ash cloud produced lightning, quickly rose to 14 km (45,000 ft) a.s.l., and drifted E along the Alaska Peninsula. Seismicity dropped rapidly to near-background levels at around 0600. The ash plume detached from the summit at around 0700 and drifted ESE at an altitude of 11.6 km (38,000 ft) a.s.l. Ash emissions continued until about 0820, rising to 6.1-7.6 km (20,000-25,000 ft) a.s.l. Small explosions at the vent area continued to be detected in infrasound data. At noon the Volcano Alert Level was lowered to Watch and the Aviation Color Code was lowered to Orange. Trace to minor amounts of ashfall were reported by the communities of False Pass, King Cove, Cold Bay, and Sand Point.

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)