AVO reported that a vigorous eruption at Shishaldin on 15 August produced ash plumes that rose 9.1-11 km (30,000-36,000 ft) a.s.l. and drifted 100 km NE. Seismicity declined by 1322. An associated sulfur dioxide cloud that drifted over parts of Alaska and western Canada had mostly dissipated by 16 August, though remnants continued to be identified in satellite images at least through 18 August.

Seismicity was low during 16-22 August. Elevated surface temperatures observed daily in satellite images indicated hot material on the upper parts of the volcano. Small steam plumes with minor amounts of ash were visible in webcam images during 16-19 August. Small explosions were detected in infrasound data on the morning of 19 August and were consistent with pilot reports of small, short-lived ash plumes rising about 4.3 km (14,000 ft) a.s.l. Low-level explosive activity continued to be recorded during 20-21 August, though weather clouds sometimes prevented views; no emissions were visible in clear webcam images on the morning of 20 August. A billowing white plume was observed by an AVO field crew working nearby on 21 August. The Volcano Alert Level remained at Watch (the third level on a four-level scale) and the Aviation Color Code remained at Orange (the third color on a four-color scale).

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)