AVO reported that on 23 July, seismicity from Okmok changed from episodic volcanic tremor to nearly continuous mid-level volcanic tremor. Although cloud cover obscured views of Okmok, previously emitted ash plumes were observed to the ESE. On 24 July, a thermal anomaly was possibly present on satellite imagery. On 25 July, seismic amplitude increased. Based on pilot reports and observations of satellite imagery, AVO reported that ash plumes rose to altitudes of 10.7-12.2 km (35,000-40,000 ft) a.s.l. The Volcano Alert Level was raised to Warning and the Aviation Color Code was raised to Red.

On 26 July, seismic activity decreased and satellite imagery indicated that ash plumes rose to altitudes of 6.1-6.7 km (20,000-22,000 ft) a.s.l. The Volcano Alert Level was lowered to Watch and the Aviation Color Code was lowered to Orange. Seismicity increased again on 27 July. Satellite imagery possibly indicated another thermal anomaly; a possible plume at an altitude of less than 3 km (10,000 ft) a.s.l. was also noted. On 28 July, seismic tremor decreased. An ash plume at a possible altitude of 8.2 km (27,000 ft) a.s.l. drifted 90 km SE. Seismicity changed from nearly continuous volcanic tremor to episodic. Later that day and on 29 July, ash plumes possibly rose to an altitude of 10.7 km (35,000 ft) a.s.l. and drifted E to SE. The Volcano Alert Level was raised to Warning and the Aviation Color Code was raised to Red.

Geological Summary. The broad, basaltic Okmok shield volcano, which forms the NE end of Umnak Island, has a dramatically different profile than most other Aleutian volcanoes. The summit of the low, 35-km-wide volcano is cut by two overlapping 10-km-wide calderas formed during eruptions about 12,000 and 2050 years ago that produced dacitic pyroclastic flows that reached the coast. More than 60 tephra layers from Okmok have been found overlying the 12,000-year-old caldera-forming tephra layer. Numerous satellitic cones and lava domes dot the flanks of the volcano down to the coast, including 1253-m Mount Tulik on the SE flank, which is almost 200 m higher than the caldera rim. Some of the post-caldera cones show evidence of wave-cut lake terraces; the more recent cones, some of which have been active historically, were formed after the caldera lake, once 150 m deep, disappeared. Hot springs and fumaroles are found within the caldera. Historical eruptions have occurred since 1805 from cinder cones within the caldera.

Source: US Geological Survey Alaska Volcano Observatory (AVO)