AVO reported that the effusive and explosive eruption at Shishaldin was continuing during 18-25 July. Satellite images acquired on 18 July, after a period of activity where ash plumes rose to 9.1 km (30,000 ft) a.s.l., showed ash deposits extending to the SW as well as to the SSE where they reached the shore of Unimak Island. Pyroclastic flow deposits up to 3 km long were visible on the N, E, and S flanks, and deposits from lahars triggered by those events extended farther down the flanks. Weather clouds often obscured webcam and satellite views of the summit during 19-21 July, though elevated surface temperatures consistent with low-level eruptive activity in the summit crater were visible in some clear images. Seismicity was low and no infrasound signals indicating explosive activity were detected.

Elevated surface temperatures detected in satellite images overnight during 21-22 July, despite weather cloud cover, were consistent with increased lava effusion. Sulfur dioxide emissions were detected in satellite data midday on 22 July. In a special notice issued at 1653 on 22 July AVO noted that eruptive activity had intensified over the previous six hours, characterized by a steady increase in seismic tremor and intermittent infrasound signals consistent with small explosions. Pilots first reported low-level ash plumes at around 1900. By 2330 the ash plume had risen to 9 km (30,000 ft) a.s.l. based on additional pilot reports and 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) at 2343. Ash emissions were sustained for just over an hour (from 2320 on 22 July to 0030 on 23 July) and rose as high as 11 km (36,100 ft) a.s.l.; by the end of the period ash plume altitudes had declined to 4.6 km (15,000 ft) a.s.l. Seismic tremor peaked at 0030 on 23 July and then began to rapidly decline at 0109. Activity had significantly declined, and tremor levels were low, by 0418; AVO lowered the Volcano Alert Level to Watch and the Aviation Color Code to Orange. The ash plume had detached and was drifting NE along the Alaska Peninsula. Bursts of increased seismicity were detected by the seismic network throughout the morning but overall remained at low levels. Elevated surface temperatures were identified in satellite images until about 0600. During 23-24 July pilots reported seeing vigorous steam-and-gas plumes rising to about 3 km (10,000 ft) a.s.l.; the plumes may have contained minor amounts of ash. Seismicity was low.

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)