ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, December 6, 2019, 3:45 PM AKST (Saturday, December 7, 2019, 00:45 UTC)

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

The low-level eruption of Shishaldin volcano that began in late July continues. This past week was marked by episodic effusion of lava flows and low-level Strombolian explosive activity.

Thus far, the 2019 eruption has produced lava flows, pyroclastic flows, and lahars (mudflows) on the flanks of the volcano. Ash emissions have been very minor and restricted to the immediate vicinity of the volcano and altitudes of less than 12,000 ft. asl. During this heightened level of activity, however, more powerful explosions could occur with little warning and produce higher ash clouds that may pose a hazard to aircraft.

From Saturday, November 31 to Monday, December 2 seismicity was elevated and high surface temperatures were detected in satellite data each day. A high-resolution satellite image confirmed the emplacement of a 1.5 km lava flow on the NW flank on December 1. By December 3 seismicity and surface temperatures in satellite decreased.

On Wednesday, December 4, seismicity and surface temperatures once again increased, and a pilot report that day confirmed renewed lava flows down the volcano’s flank. Minor steam or ash emissions were observed in clear web camera views early Thursday, December 5 from the summit and north flank. Also on Thursday, very strongly elevated surface temperatures were detected in satellite imagery, and seismicity, including detection of Strombolian explosions, continued to increase until around 21:00 AKST on December 5 when the continuous tremor ended and was replaced by episodic tremor bursts and occasional detections of Strombolian vent activity.

Shishaldin is monitored by local seismic and infrasound sensors, satellite data, web cameras, a telemetered geodetic and tilt network, and distant infrasound and lightning 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.

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

Slightly elevated surface temperatures were occasionally observed during mostly cloudy satellite and web camera images over the past week. Seismicity remains low and no significant activity was observed in local or regional infrasound data during the week.

Additional episodes of lava effusion and explosions are likely and will occur without advance warning. The most recent explosion of Cleveland occurred on January 9, 2019. These explosions are normally short duration and only present a hazard to aviation in the immediate vicinity of the volcano. Larger explosions that present a more widespread hazard to aviation are possible, but less likely and/or frequent.

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.

SEMISOPOCHNOI VOLCANO (VNUM #311060)
51°55'44" N 179°35'52" E, Summit Elevation 2625 ft (800 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

Slightly elevated seismicity was recorded throughout the week in the form of weak tremor. This tremor increased slightly December 5 and 6. No eruptive activity was detected in regional infrasound data and clear satellite images showed no indication of surface activity.

Semisopochnoi is monitored with an on-island seismic network and remotely by satellite and lightning sensors. Furthermore, an infrasound array on Adak Island may detect explosive emissions from Semisopochnoi with a 13 minute delay if atmospheric conditions permit.

Remote Semisopochnoi volcano occupies the largest, young volcanic island in the western Aleutians. The volcano is dominated by an 8-km (5-mile) diameter caldera that contains a small lake and a number of post-caldera cones and craters. The age of the caldera is not known with certainty but is likely early Holocene. The last known eruption of Semisopochnoi occurred in 1987, probably from Sugarloaf Peak on the south coast of the island, but details are lacking. Another prominent, young post-caldera landform is Mount Cerberus, a three-peaked cone cluster in the southwest part of the caldera. The island is uninhabited and part of the Alaska Maritime National Wildlife Refuge. It is located 65 km (40 mi) northeast of Amchitka Island and 200 km (130 mi) west of Adak.

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

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.

---