ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, November 24, 2017, 12:08 PM AKST (Friday, November 24, 2017, 21:08 UTC)

BOGOSLOF VOLCANO (VNUM #311300)
53°55'38" N 168°2'4" W, Summit Elevation 492 ft (150 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

No significant eruptive activity at Bogoslof was detected in seismic or infrasound data obtained from our networks on neighboring islands over the past week. Weather conditions prevented clear views of the volcano in satellite data during the week. Weakly elevated surface temperatures consistent with a hot crater lake were last observed in clear nighttime satellite images on November 10. The last explosive eruption from Bogoslof occurred on August 30th.

Volcanic explosions producing high-altitude (>15,000 ft asl) volcanic clouds remain possible with little or no warning. Some previous explosions have been preceded by an increase in earthquake activity that allowed for short-term forecasts of imminent significant explosive activity. Although we are able to detect energetic explosive activity in real-time, there can be a lag of tens of minutes until we can characterize the magnitude of the event and the altitude of the volcanic cloud.

With existing data sources, AVO may not detect low-level unrest, including minor explosive activity. Such low-level periods of unrest and possible explosions could pose hazards near the volcano.

AVO has no ground-based volcano monitoring equipment on Bogoslof volcano. We continue to monitor volcanic activity with satellite images, seismic and infrasound instruments on nearby islands, and lightning data from the Worldwide Lightning Location Network.

Bogoslof Island is the largest of a cluster of small, low-lying islands making up the emergent summit of a large submarine stratovolcano. The highest point above sea level prior to this eruption was about 100 m (300 ft); however, the volcano is frequently altered by both eruptions and wave erosion and has undergone dramatic changes in historical time. The two main islands currently above sea level are Fire Island and Bogoslof Island, both located about 98 km (61 mi) northwest of Unalaska/Dutch Harbor, 123 km (76 mi) northeast of Nikolski, and 149 km (93 mi) northeast of Akutan. The volcano is situated slightly north (behind) the main Aleutian volcanic front. Bogoslof volcano is within the USFWS Aleutian Maritime Wildlife Refuge and is habitat for marine mammals and seabirds.

At least 8 historical eruptions have been documented at Bogoslof prior to the current unrest, most recently in July 1992. Previous eruptions of the volcano have lasted weeks to months, and have on occasion produced ash fall on Unalaska. These eruptions are often characterized by multiple explosive, ash-producing events as well as the growth of lava domes, such as we have seen in the ongoing 2016-2017 eruption.

CLEVELAND VOLCANO (VNUM #311240)
52°49'20" N 169°56'42" W, Summit Elevation 5676 ft (1730 m)
Current Volcano Alert Level: WATCH
Current Aviation Color Code: ORANGE

Low-level unrest at Cleveland volcano continues. Nothing noteworthy was observed in satellite data this week, although the volcano was obscured by cloud cover for much of the week. No significant seismic activity or other signs of unrest were detected this week.

Cleveland volcano is monitored with a limited real-time seismic network, which inhibits AVO's ability to detect precursory unrest that may lead to an explosive eruption. Rapid detection of an ash-producing eruption may be possible using a combination of seismic, infrasound, lightning, and satellite data.

Cleveland volcano forms the western portion of Chuginadak Island, a remote and uninhabited island in the east central Aleutians. The volcano is located about 75 km (45 mi) west of the community of Nikolski, and 1500 km (940 mi) southwest of Anchorage. The most recent significant period of eruption began in February, 2001 and produced 3 explosive events that generated ash clouds as high as 39,000 ft above sea level. The 2001 eruption also produced a lava flow and hot avalanche that reached the sea. Since then, Cleveland has been intermittently active producing small lava flows, often followed by explosions that generate small ash clouds generally below 20,000 ft above sea level. These explosions also launch debris onto the slopes of the cone producing hot pyroclastic avalanches and lahars that sometimes reach the coastline.

GREAT SITKIN VOLCANO (VNUM #311120)
52°4'35" N 176°6'39" W, Summit Elevation 5709 ft (1740 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

Earlier this week, AVO raised the aviation color code and volcano alert level at Great Sitkin volcano to yellow based on reports of robust steaming on November 19, and a gradual increase in earthquake activity that has been evident for a number of months. Anomalous seismicity, suggestive of minor explosions, occurred on January 10 (January 11 UTC) and July 21 (July 22 UTC) of this year. These events, together with the increased level of seismic activity and the emissions observed on November 19, suggest that there has likely been an intrusion of new magma beneath the volcano, which increases the possibility of a volcanic eruption.

A high-resolution satellite image of the volcano obtained on November 21, 2017 shows a small fumarole on the west side of the 1974 lava flow within the summit crater emitting a continuous jet of steam and at least one area where snow and ice had been melted. So far no thermal signals or volcanic emissions have been observed in other types of satellite data and there have been no significant changes in the level of seismic activity over the past week.

Great Sitkin volcano 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. The last explosive eruption of Great Sitkin volcano occurred in February, 1974 and resulted in at least one ash cloud that reached about 7.6 km (25,000 feet) above sea level. The 1974 eruptive period also resulted in a lava flow of about 600,000 square meters that was emplaced on the floor of the snow-and-ice filled summit crater.

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 decapitated volcano and a younger parasitic cone with a 2-3 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 a lava dome and 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.

OTHER ALASKA VOLCANOES

Information on all Alaska volcanoes is available at : http://www.avo.alaska.edu.

For definitions of Aviation Color Codes and Volcano Alert Levels, see: http://www.avo.alaska.edu/color_codes.php

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CONTACT INFORMATION:

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

Jeff Freymueller, Coordinating Scientist, UAFGI
jfreymueller@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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