AVO reported that the significant explosive eruption at Shishaldin that occurred on 25 September was the eleventh that had occurred since 14 July, and among the most impactful. Ashfall was reported in several communities downwind and flight cancellations occurred in the region. A large ash cloud quickly rose to 14 km (45,000 ft) a.s.l., produced at least 150 lightning strokes with thunder heard in False Pass, and drifted E along the Alaska Peninsula. Seismicity decreased notably near the end of the strong activity on 25 September and continued to decrease during 26 September-3 October. Satellite data from 26 September showed that significant collapses had occurred at the summit crater and hot, steaming, deposits from pyroclastic flows and lahars were present on all flanks, particularly to the ENE and WSW. A small ash cloud was visible in webcam images on 27 September, likely from a collapse at the summit cone. Elevated surface temperatures were intense during 27-28 September. Minor steaming at the summit crater and from an area on the upper flanks was visible in webcam images on 28 September. Weakly elevated surface temperatures were identified in satellite data during 30 September-1 October and a pilot reported steaming from the vent on 1 October.

Seismicity significantly increased starting at around 2100 on 2 October and around the same time satellite images showed an increase in surface temperature consistent with lava fountaining. Small hot avalanches of rock and lava descended the flank. A distinct increase in infrasound, seismicity, and lighting detections was followed by the identification of an ash plume at 12 km (40,000 ft) a.s.l. at 0520 on 3 October in satellite images. The Aviation Color Code was raised to Red (the highest color on a four-color scale) and the Volcano Alert Level was raised to Warning (the highest level on a four-level scale). By 0931 ash plumes were only rising to altitudes of 6.1-7.6 km (20,000-25,000 ft) a.s.l. and both seismicity and infrasound levels had decreased. At 1036 the Aviation Color Code was lowered to Orange and the Volcano Alert Level was lowered to Watch; seismic and infrasound activity were slightly above background levels and steaming at the summit was observed 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)