USGS Volcano Notice for Aviation - DOI-USGS-AVO-2023-08-03T09:36:51-08:00

(1) VOLCANO OBSERVATORY NOTICE FOR AVIATION (VONA)
(2) Issued:	(20230803/2342Z)
(3) Volcano:	Semisopochnoi (VNUM #311060)
(4) Current Color Code:	GREEN
(5) Previous Color Code:	YELLOW
(6) Source:	Alaska Volcano Observatory
(7) Notice Number:	2023/A1198
(8) Volcano Location:	N 51 deg 55 min E 179 deg 35 min
(9) Area:	Aleutians
(10) Summit Elevation:	2625 ft (800 m)
(11) Volcanic Activity Summary:	No eruptive activity has been observed at Semisopochnoi volcano since May 5, 2023, and there are no signs of heighted volcanic unrest. Reflecting these conditions, the Aviation Color Code is being lowered to GREEN and Volcano Alert Level to NORMAL. The last evidence of ash emissions from the volcano was observed on May 5 and consisted of a light dusting of ash on the northwest flank of Mount Young. The last detection of possible explosions in monitoring data was April 25, 2023. Since this time, earthquake activity has been at low levels. Steam plumes continue from the north crater of Mount Young. Although steaming was not observed from this vent prior the start of this eruption, it is common for steam plumes to persist at volcanoes for years following an eruption as the volcano slowly cools. The current eruption sequence at Semisopochnoi began in September 2018 and has consisted of small eruptions producing minor ash deposits within the vicinity of the active north crater of Mount Young and ash clouds usually under 10,000 ft (3 km) above sea level with occasional periods of continuous ash production. While current observations indicate that this eruption may be over, it is also possible activity could resume, and small ash-producing events could occur with little warning. The Alaska Volcano Observatory will continue to closely monitor Semisopochnoi and report on any changes in activity. Semisopochnoi is monitored by a local seismic and infrasound network, local web cameras, regional lightning and infrasound sensors, and satellite data.
(12) Volcanic cloud height:	NA
(13) Other volcanic cloud information:	none
(14) Remarks:	The remote Semisopochnoi volcano occupies a young volcanic island in the western Aleutians. The uninhabited island is part of the Alaska Maritime National Wildlife Refuge. It is located 65 km (40 mi) northeast of Amchitka Island and 200 km (130 mi) west of Adak, Alaska. Semisopochnoi volcano is dominated by an 8-km (5-mile) diameter caldera that contains a small lake and several post-caldera cones and craters. The age of the caldera is not known with certainty but is likely early Holocene. The north cone of Mount Young, in the southwest part of the caldera, is the site of the current eruption, which began in 2018. The last known eruption prior to 2018 was in 1987 from Sugarloaf Peak on the south coast of the island.
(15) Contacts:	Matt Loewen, Acting Scientist-in-Charge, USGS, mloewen@usgs.gov, (907) 786-7497 Ronni Grapenthin, Acting Coordinating Scientist, UAFGI, rgrapenthin@alaska.edu, (907) 378-5460
(16) Next Notice:	A new VAN will be issued if conditions change significantly or alert levels are modified. While a VAN is in effect, regularly scheduled updates are posted at http://www.avo.alaska.edu.