AVO reported that the eruption at Shishaldin continued during 30 August-5 September. Weather conditions sometimes prevented observations. Daily, small, repetitive explosions were recorded in seismic and infrasound data, though high winds occasionally masked the signals. Elevated surface temperatures at the summit were identified in satellite data on most days. Hot deposits on the NE flank, emplaced during 25-26 August, were visible in a 1 September satellite image, along with steam emissions obscuring the summit vent. Possible incandescence at the summit was visible in nighttime webcam images during 3-4 September, and small steam emissions were visible in daytime images.

Seismicity began to gradually increase at around 0300 on 5 September and activity escalated around 0830. A pilot in the vicinity of the volcano reported an ash plume at about 7.6 km (25,000 ft) a.s.l. at 0842 that was continuing to rise. The ash plume was large and may have risen as high as 9.7 km (32,000 ft) a.s.l. and drifted SSE based on satellite images. 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) at 0901. Seismic amplitude decreased rapidly at around 1100, and remained low, and the altitude of ash emissions observed in satellite images also decreased to an estimated 4.5 km (15,000 ft) a.s.l. By 1200 the lower-altitude portion of the ash plume had drifted 125 km E. Significant ash emissions ended by 1330 based on webcam images. At 1440 AVO lowered the Aviation Color Code to Orange and the Volcano Alert Level to Watch. This event marks the ninth period of elevated eruptive activity resulting in significant ash emissions and mass flows of volcanic debris on the volcano's flanks since the onset of the current eruption.

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)