ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, February 10, 2017, 12:20 PM AKST (Friday, February 10, 2017, 21:20 UTC)

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

Volcanic unrest at Bogoslof continues. One low-level eruptive event was detected at Bogoslof since our last Weekly Update. It occurred last Friday, February 3, at 16:41 AKST (1:41 February 4 UTC), and produced a small volcanic ash plume below 25,000 ft asl, as seen in satellite observations and confirmed by pilot reports. Weakly elevated surface temperatures were observed in satellite imagery on February 7 and 8, possibly related to hot eruptive deposits from last Friday's explosive activity.

It has been 7 days since the last detected explosive activity at Bogoslof. This is the longest repose time between explosive eruptive events since the eruption began on Dec 12, 2016. The longer repose, along with the fact that the last two eruptive events were less energetic and produced relatively low-level ash clouds (<25,000 feet asl), may indicate that the eruption is waning. It is also possible that significant explosive eruptive activity could resume with little or no warning. A satellite image from February 8 shows the vent location at Bogoslof remains underwater, suggesting that if future explosive eruptive activity occurs, it will likely be similar in character to that observed over the past 2 months.

Bogoslof is not monitored by a local geophysical network, which limits our ability to forecast and closely track activity at this volcano. AVO uses seismic and infrasound (pressure) sensors on neighboring Umnak and Unalaska Islands to monitor activity. In addition, we use satellite imagery to track ash clouds and information from the Worldwide Lightning Location Network to identify volcanic lightning; lightning strikes in the erupted plume have been detected during the current eruptive sequence.

Some explosions at Bogoslof have been preceded by an increase in earthquake activity, recorded on seismic networks on adjacent volcanoes, which allowed for short-term forecasts of imminent significant explosive activity. It is likely that lower-level explosive activity is occurring that is below our ability to detect in our data sources. These low-level explosions could pose a hazard in the immediate vicinity of the volcano. Although we are able to detect significant explosive activity in real-time, there is typically a lag of tens of minutes until we can characterize the magnitude of the event and the altitude of the volcanic cloud. 

For more details about the current eruption, monitoring efforts, and information about the volcano's previous eruptions and possible hazards, please refer to the latest Information Statement (http://avo.alaska.edu/activity/report.php?need=current&id=350341&type=1&mode=hans ).

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. The most recent prior to 2016 occurred from July 6-24, 1992, and produced episodic steam and ash emissions including an ash cloud up to 26,000 ft (8 km) asl on July 20, followed the next day by extrusion of a new 150 m (500 ft) by 275 m (900 ft) lava dome on the north end of the island. Previous eruptions of the volcano have lasted weeks to months, and have on occasion produced ash fall on Unalaska. Eruptions of the volcano are often characterized by multiple explosive, ash-producing events such as we have seen in 2016, as well as the growth of lava domes.

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

Weakly elevated surface temperatures were observed at the summit of Cleveland in satellite imagery on three days this week (February 7-9) and minor steaming was observed in clear web camera images on February 8. These observations are consistent with the presence of a lava dome that began extruding in the summit crater in late January. No significant activity was observed in seismic or pressure sensor data this week. The presence of the recently emplaced lava dome within the summit crater can block the vent, resulting in reduced or restricted gas emissions which may lead to explosive activity with little to no warning.

Cleveland volcano is not monitored with a real-time seismic network and this inhibits AVO's ability to detect unrest that may lead to an explosive eruption. Rapid detection of an ash-producing eruption may be possible using a combination of satellite, infrasound, lightning data and local observations. AVO is monitoring the unrest at Cleveland volcano as closely as possible and will release additional information if or when it becomes available.

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.

TAKAWANGHA VOLCANO (VNUM #311090)
51°52'1" N 178°1'37" W, Summit Elevation 4754 ft (1449 m)
Current Volcano Alert Level: NORMAL
Current Aviation Color Code: GREEN

The energetic earthquake swarm on Tanaga Island that began on January 23, 2017, declined significantly this week and seismicity is near background levels. As a result, AVO lowered the Aviation Color Code to GREEN and Alert Level to NORMAL for Takawangha volcano on Wednesday, February 8.

Takawangha is a remote, 1,449 m (4,754 ft)-high stratovolcano located on the northeast portion of Tanaga Island, roughly 95 km (59 miles) west of Adak in the Andreanof Islands. Takawangha's summit is mostly ice-covered, except for four young craters that have erupted ash and lava flows in the last few thousand years. Parts of Takawangha's edifice are hydrothermally altered and may be unstable, possibly leading to localized debris avalanches from its flanks. Takawangha lies across a saddle from historically active Tanaga volcano to the west. No historical eruptions are known from Takawangha; however, field work shows that recent eruptions have occurred and it is possible that historic eruptions attributed to Tanaga may instead have come from Takawangha.

OTHER ALASKA VOLCANOES

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

AVO scientists conduct daily checks of earthquake activity at all seismically-monitored volcanoes, examine web camera and satellite images for evidence of airborne ash and elevated surface temperatures, and consult other monitoring data as needed.

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

SUBSCRIBE TO VOLCANO ALERT MESSAGES by email: http://volcanoes.usgs.gov/vns/

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CONTACT INFORMATION:

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

Jessica Larsen, Acting Coordinating Scientist, UAFGI
jflarsen@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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