ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, December 8, 2017, 1:30 PM AKST (Friday, December 8, 2017, 22:30 UTC)

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, but it is unclear whether lava effusion has stopped or paused. The last clear satellite image from November 26 shows the summit dome has been mostly reduced to pock-marked rubble in the bottom of a shallow crater. There were no satellite-based observations of elevated surface temperatures in the summit crater (that are typically observed during periods of lava effusion) during periods of clear weather . There was no significant seismic or infrasound activity 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.

SHISHALDIN VOLCANO (VNUM #311360)
54°45'19" N 163°58'16" W, Summit Elevation 9373 ft (2857 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

The status of Shishaldin Volcano was increased to Aviation Color Code YELLOW and Volcano Alert Level ADVISORY on December 6 following a several week-long increase in the amplitude of seismic activity and of pressure waves recorded by infrasound sensors. The uptick in seismic and infrasound amplitudes is likely the result of more energetic degassing at this open vent volcano and may indicate that the magma level has risen closer to the surface in the summit crater.

Continuous robust steaming rising hundreds of feet above the summit crater was observed in clear web cam images throughout the week. At times, these steam emissions were accompanied by infrasound signals indicating episodes of short-duration energetic gas emissions and/or small explosions. Current observations represent a departure from normal background activity at Shishaldin, but do not necessarily indicate that an eruption will occur. Shishaldin is monitored by local seismic and infrasound sensors, satellite data, a web camera, a telemetered geodetic network, and distant infrasound networks.

Shishaldin volcano, located near the center of Unimak Island in the eastern Aleutian Islands, is a spectacular symmetric cone with a base diameter of approximately 16 km (10 mi). A 200-m-wide (660 ft) funnel-shaped summit crater typically emits a steam plume and occasional small amounts of ash. Shishaldin is one of the most active volcanoes in the Aleutian volcanic arc, with at least 54 episodes of unrest including over 24 confirmed eruptions since 1775. Most eruptions are relatively small, although the April-May 1999 event generated an ash column that reached 45,000 ft above sea level.

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

Low-level unrest at Great Sitkin volcano continued. An increased number of small earthquakes was evident as early as late July 2016, and since then the level of seismic activity has fluctuated at low levels but exhibited a gradual overall increase most notable since June 2017. Seismic activity decreased over the past month, and remained at a low rate over the past week. These types of fluctuations in the number and size of volcanic earthquakes are typical during periods of unrest, and do not mean that unrest has ended. There have been no addition reports of anomalous steaming from summit and no activity has been observed in satellite data 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.

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.

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

The 2016-17 eruption of Bogoslof volcano may have come to an end, and the Aviation Color Code and Volcano Alert Level were both decreased to UNASSIGNED on December 6. AVO only assigns Aviation Color Codes and Alert Levels to volcanoes that have a local seismic monitoring network. Since Bogoslof does not have a local network the alert status is now UNASSIGNED. We cannot say for certain that the volcano has returned to its normal background state, however we are no longer detecting any activity that would be considered to be indicative of unrest.

The eruption that began in mid-December 2016 produced more than sixty explosive events, the last of which occurred on August 30, 2017. The most energetic of these explosive events sent water-rich volcanic ash clouds to altitudes exceeding 35,000 feet. The resulting dispersed volcanic clouds impacted local and international aviation operations over portions of the North Pacific and Alaska. Although most of the volcanic ash fell into the ocean, trace amounts were twice deposited on the community of Unalaska and the Port of Dutch Harbor. The 2016-17 eruption greatly changed the morphology of Bogoslof Island. At its greatest extent, the area of the island increased to about three times of its pre-eruption size. Nearly all of the new material on the island is unconsolidated pyroclastic fall and flow (surge) deposits. The deposits are highly susceptible to wave erosion and additional changes in the configuration of the island are likely. A satellite image from December 3 shows significant erosion of the island with the vent lagoon now open to the ocean on the north shore of the island.

Although the eruption may be over, it is possible that activity could resume without warning. We would likely detect the resumption of volcanic activity should it occur 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.

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

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

FOLLOW AVO ON FACEBOOK: https://facebook.com/alaska.avo

FOLLOW AVO ON TWITTER: https://twitter.com/alaska_avo

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.

---