AVO reported that the eruption at Shishaldin continued during 23-29 August. Low-level explosive activity likely continued during 23-24 August based on seismic and infrasound data. Elevated surface temperatures at the summit were identified in satellite data; webcam images were obscured by weather clouds. Seismic tremor began increasing at around 0300 on 25 August and was followed by elevated surface temperatures identified in satellite images, consistent with lava erupting at the summit. Small explosions were recorded in infrasound data. At 1204 AVO raised 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) in response to a pilot report of an ash plume at 8.5 km (28,000 ft) a.s.l. Seismicity peaked at 1630 and then began to rapidly decline at around 1730. Ash plumes rose as high as 10 km (32,000 ft) a.s.l. and drifted as far as 400 km NE. Ash plume altitudes began to decrease and by 2020 the plumes were rising as high 6.4 km (21,000 ft) a.s.l. and drifting NE. Ash emissions ended at 0000 on 26 August and seismicity was at low levels; AVO lowered the Aviation Color Code to Orange and the Volcano Alert Level to Watch. This large, ash-producing explosive event was the eighth to occur since the eruption started. Minor explosive activity within the summit crater was detected during 26-28 August and strongly elevated surface temperatures visible in satellite imagery. On 26 August a gas plume drifted NE. An AVO field crew working on Unimak Island observed a mass flow that descended the upper flanks beginning around 1720 on 27 August. The flow produced a short-lived ash cloud that rose to around 4.5 km (15,000 ft) a.s.l. and rapidly dissipated. The mass flow was likely caused by the collapse of lava spatter that had accumulated on the summit crater rim.

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)