ALASKA VOLCANO OBSERVATORY INFORMATION STATEMENT U.S. Geological Survey Wednesday, September 27, 2017, 3:07 PM AKDT (Wednesday, September 27, 2017, 23:07 UTC)
53°55'38" N 168°2'4" W,
Summit Elevation 492 ft (150 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW
Current status of the volcano
AVO lowered the Aviation Color Code to YELLOW and the Volcano Alert Level to ADVISORY today following 4 weeks of relative quiet since the last explosion on August 30, 2017. It is uncertain whether the 2016-17 eruptive period has ended, or if the current quiet period is only a pause in a longer eruptive episode. Thus far in the 2016-17 unrest, there have been pauses in activity of up to two months between explosions, so it remains possible for the volcano to reawaken and erupt explosively in the coming weeks to months. If eruptive activity resumes, we expect a similar style of behavior as observed since mid-December 2016, including: explosively generated ash clouds, pyroclastic base surges, and possibly the effusion of subaerial lava domes.
AVO’s ability to detect unrest and forecast eruptions at Bogoslof remains limited because it is not monitored by a local, on-island geophysical network. AVO is using seismic sensors from Okmok (32 mi, or 50 km) and Makushin (45 mi, or 72 km) volcanoes on neighboring Umnak and Unalaska Islands to monitor activity. Some significant explosive events have been preceded by unrest observed on the Okmok and Makushin networks, allowing AVO to provide some forecasts, whereas other explosions have occurred with no detectable precursors.
While many of the explosive events in the current episode have not been preceded by detectable unrest, AVO has in all cases been able to detect the onset of an explosive eruption within minutes, using seismicity as well as infrasound signals (pressure waves) that we can detect on sensors at Okmok volcano. The World Wide Lightning Location Network (http://wwlln.net/) provides near-real-time (within minutes) automated alerts of lightning flashes near Bogoslof that have been shown to be indicative of explosive activity at the volcano.
AVO also uses near-real-time satellite data to detect explosive eruptions, to estimate volcanic cloud height and to track the dispersion of the resulting volcanic clouds. Although we can detect energetic 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. These data can also detect highly elevated surface temperatures from lava effusion, the presence of a lava dome, or hot ash deposits. Extensive and persistent cloud cover can obscure observations of volcanic activity in satellite data.
Past eruptions of Bogoslof volcano
Bogoslof volcano has had at least eight historical eruptive periods since 1795–96. Most of the eruptive episodes were characterized by ash emission, intermittent periods of lava fountaining, ballistic ejecta, and the effusion of lava in the form of conically shaped accumulations known as lava domes. Drifting ash clouds from Bogoslof eruptions have produced measurable ash fall on Umnak and Unalaska Islands to the south and east in 1796–1806, 1883, 1907 and 2017. The duration of individual eruptive periods is not well constrained although historical records indicate that Bogoslof can remain restless and erupt intermittently for periods of a few weeks to three or more years. Many eruptions have culminated with the subaerial emplacement of a lava dome. During some eruptive periods, including the current activity, multiple lava domes were emplaced, but were subsequently destroyed by explosive activity. Many of the documented Bogoslof eruptions occurred from shallow submarine vents and the interaction of magma and seawater has been a common eruptive process. As a result, volcanic clouds are typically water rich and appear lighter in color relative to ash clouds produced by subaerial eruptions. The violent nature of magma-water interaction can lead to the generation of pyroclastic base surges which may travel 5–6 km or more beyond Bogoslof Island.
Bogoslof Island is the largest of a cluster of small, low-lying islands making up the 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 U.S. Fish and Wildlife Service Alaska Maritime National Wildlife Refuge and is habitat for marine mammals and seabirds (https://www.fws.gov/refuge/alaska_maritime/).
Jeff Freymueller, Coordinating Scientist, UAFGI
email@example.com (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.