AVO reported that the effusive and explosive eruption at Shishaldin continued during 1-8 August. Low-level ash emissions rose to below 3 km (10,000 ft) a.s.l. and drifted over 90 km N during 1-2 August. Seismicity was low but punctuated by a few episodes of volcanic tremor associated with the ash emissions. Elevated surface temperatures were consistent with cooling lava over a few days but had begun to increase and were visible though moderate weather cloud cover during 2-3 August; the increasing temperatures were consistent with lava effusion. Seismic tremor began to steadily increase at around 1036 on 3 August. Explosive activity about 20 hours later was detected using infrasound and seismic data. An ash cloud was visible in a satellite image at 0520 on 4 August rising to 7.6 km (25,000 ft) a.s.l. and drifting 60-75 km NE. Pilots saw and reported the plume at 0836. By 0900 the plume had risen to 9 km (30,000 ft) a.s.l., prompting AVO to raise 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 1017. The National Weather Service issued a Significant Meteorological Information (SIGMET) statement for the ash cloud and a Special Weather Statement warned of possible trace amounts of ash on marine waters downwind of Shishaldin. Seismic tremor levels peaked at 1400 and then sharply declined at 1500 to slightly elevated levels; the plume was sustained during the period of high tremor and drifted N and NE. AVO lowered the Volcano Alert Level to Watch and the Aviation Color Code to Orange at 1955. Seismic tremor levels were low. Ash emissions were possibly continuing but they could not be confirmed due to meteorological clouds cover. Elevated surface temperatures observed in satellite data during 5-8 August were consistent with cooling lava. Seismicity has remained low with a few daily local earthquakes.

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)