(1) VOLCANO OBSERVATORY NOTICE FOR AVIATION (VONA)
(2) Issued:	(20201021/2321Z)
(3) Volcano:	Great Sitkin (VNUM #311120)
(4) Current Color Code:	GREEN
(5) Previous Color Code:	YELLOW
(6) Source:	Alaska Volcano Observatory
(7) Notice Number:
(8) Volcano Location:	N 52 deg 4 min W 176 deg 6 min
(9) Area:	Aleutians
(10) Summit Elevation:	5709 ft (1740 m)
(11) Volcanic Activity Summary:	Seismicity at Great Sitkin has been steadily decreasing for the past several months and is now at background levels. No eruptive activity or unusual surface temperatures have been observed in clear satellite imagery. The Alaska Volcano Observatory is returning the Aviation Color Code and Alert Level to GREEN/NORMAL to reflect these background conditions. AVO will continue to monitor the volcano using seismic, infrasound, web camera, and satellite observations. Great Sitkin is monitored with a local real-time seismic network, which will typically allow AVO to detect changes in unrest that may lead to an explosive eruption. Rapid detection of an ash-producing eruption would be accomplished using a combination of seismic, infrasound, lightning, and satellite data.
(12) Volcanic cloud height:	n/a
(13) Other volcanic cloud information:	n/a
(14) Remarks:	Great Sitkin Volcano is a basaltic andesite volcano that occupies most of the northern half of Great Sitkin Island, a member of the Andreanof Islands group in the central Aleutian Islands. It is located 43 km (26 miles) east of the community of Adak. The volcano is a composite structure consisting of an older dissected volcano and a younger parasitic cone with a 3-km-diameter summit crater. A steep-sided lava dome, emplaced during an eruption in 1974, occupies the center of the crater. Great Sitkin erupted at least three times in the 20th century, most recently in 1974. That eruption produced at least one ash cloud that likely exceeded an altitude of 25,000 ft above sea level. A poorly documented eruption occurred in 1945, also producing a lava dome that was partially destroyed in the 1974 eruption. Within the past 280 years a large explosive eruption produced pyroclastic flows that partially filled the Glacier Creek valley on the southwest flank.
(15) Contacts:	Michelle Coombs, Scientist-in-Charge, USGS mcoombs@usgs.gov (907) 786-7497 David Fee, Coordinating Scientist, UAFGI dfee1@alaska.edu (907) 322-4085
(16) Next Notice:

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AVO/USGS Volcanic Activity Notice

Volcano: Great Sitkin (VNUM #311120)

Current Volcano Alert Level: NORMAL
Previous Volcano Alert Level: ADVISORY

Current Aviation Color Code: GREEN
Previous Aviation Color Code: YELLOW

Issued: Wednesday, October 21, 2020, 3:21 PM AKDT
Source: Alaska Volcano Observatory
Notice Number:
Location: N 52 deg 4 min W 176 deg 6 min
Elevation: 5709 ft (1740 m)
Area: Aleutians

Volcanic Activity Summary: Seismicity at Great Sitkin has been steadily decreasing for the past several months and is now at background levels. No eruptive activity or unusual surface temperatures have been observed in clear satellite imagery. The Alaska Volcano Observatory is returning the Aviation Color Code and Alert Level to GREEN/NORMAL to reflect these background conditions. AVO will continue to monitor the volcano using seismic, infrasound, web camera, and satellite observations.

Great Sitkin is monitored with a local real-time seismic network, which will typically allow AVO to detect changes in unrest that may lead to an explosive eruption. Rapid detection of an ash-producing eruption would be accomplished using a combination of seismic, infrasound, lightning, and satellite data.

Recent Observations:
[Volcanic cloud height] n/a
[Other volcanic cloud information] n/a

Remarks: Great Sitkin Volcano is a basaltic andesite volcano that occupies most of the northern half of Great Sitkin Island, a member of the Andreanof Islands group in the central Aleutian Islands. It is located 43 km (26 miles) east of the community of Adak. The volcano is a composite structure consisting of an older dissected volcano and a younger parasitic cone with a 3-km-diameter summit crater. A steep-sided lava dome, emplaced during an eruption in 1974, occupies the center of the crater. Great Sitkin erupted at least three times in the 20th century, most recently in 1974. That eruption produced at least one ash cloud that likely exceeded an altitude of 25,000 ft above sea level. A poorly documented eruption occurred in 1945, also producing a lava dome that was partially destroyed in the 1974 eruption. Within the past 280 years a large explosive eruption produced pyroclastic flows that partially filled the Glacier Creek valley on the southwest flank.

Contacts: Michelle Coombs, Scientist-in-Charge, USGS
mcoombs@usgs.gov (907) 786-7497

David Fee, Coordinating Scientist, UAFGI
dfee1@alaska.edu (907) 322-4085

The Alaska Volcano Observatory is a cooperative program of the U.S. Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys.

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