ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, September 25, 2009, 12:54 PM AKDT (Friday, September 25, 2009, 20:54 UTC)

REDOUBT VOLCANO (VNUM #313030)
60°29'7" N 152°44'38" W, Summit Elevation 10197 ft (3108 m)
Current Volcano Alert Level: ADVISORY
Current Aviation Color Code: YELLOW

Field crews visited Redoubt twice this week, collecting new samples of dome material, repairing the web camera at the AVO observation hut, conducting airborne gas measurements, taking temperature readings of the cooling lava dome, and making extensive geologic observations of the terrain alterations from the recent eruption. These data, along with those from Redoubt's continuous geophysical monitoring network all suggest diminishing levels of unrest. Gas emissions from the volcano continue to decline, as do temperatures measured at the new lava dome. Seismicity and ground deformation rates are down and approaching background levels. All indications point toward decreased dome instability and a reduced likelihood of dome collapse.

Favorable weather conditions at the beginning of the week resulted in a widely reported, conspicuous steam plume from Redoubt's summit. Such plumes are not unusual and can arise as new precipitation evaporates while percolating down through fractures in the still-hot lava dome. Prominent steam plumes from Redoubt are to be expected over the coming months to years as the lava dome slowly cools.

Along with steam plumes, weak thermal anomalies were seen a few times in satellite data this week at Redoubt. Again, these come as no surprise and can result from sun shining off the brilliant white steam or from the radiant heat of the cooling dome.

Though increasingly unlikely, some threat of a major dome collapse remains. Such a collapse would cause significant ash production, hot block-and-ash flows, and flooding and lahars in the Drift River Valley.

Heavily ice-mantled Redoubt volcano is located on the western side of Cook Inlet, 170 km (106 mi) southwest of Anchorage and 82 km (51 mi) west of Kenai, within Lake Clark National Park. Redoubt is a stratovolcano which rises to 10,197 feet above sea level. Recent eruptions occurred in 1902, 1966-68, 1989-90, and 2009. The 1989-90 and 2009 eruptions produced mudflows, or lahars, that traveled down the Drift River and partially flooded the Drift River Oil Terminal facility. The ash plumes produced by the 1989-90 and 2009 eruptions significantly disrupted air traffic and resulted in minor or trace amounts of ash in the city of Anchorage and other communities in south-central and interior Alaska.


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

Seismic activity at Shishaldin remained low all week. Several clear to partly cloudy satellite and web cam views showed nothing of note.

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 10 miles (16 km). A small summit crater typically emits a noticeable steam plume with occasional small amounts of ash. Shishaldin is one of the most active volcanoes in the Aleutian volcanic arc, erupting atleast 28 times since 1775. Most of Shishaldin's eruptions have consisted of small ash and steam plumes, although the most recent eruption in April-May 1999 produced an ash column that reached a height of 45,000 ft above sea level.

OTHER ALASKA VOLCANOES

Seismic activity is monitored in real time at 31 volcanoes in Alaska. Satellite images of all Alaskan volcanoes are analyzed daily for evidence of ash plumes and elevated surface temperatures. Some volcanoes may currently display anomalous behavior but are not considered to be at a dangerous level of unrest. Akutan, Aniakchak, Augustine, Dutton, Fisher, Fourpeaked, Gareloi, Great Sitkin, Griggs, Iliamna, Isanotski, Kanaga, Katmai, Korovin, Mageik, Makushin, Martin, Novarupta, Okmok, Pavlof, Snowy, Spurr, Tanaga, Trident, Ugashik-Peulik, Ukinrek Maars, Veniaminof, Westdahl, and Wrangell volcanoes are in color code GREEN and volcano alert level Normal. All are at or near normal levels of background seismicity. AVO did not detect ash plumes or significant elevated surface temperatures in the vicinity of any of these volcanoes.

Please see http://www.avo.alaska.edu/color_codes.php for complete definitions of Aviation color codes and Volcano alert levels.

VOLCANO INFORMATION ON THE INTERNET: http://www.avo.alaska.edu
RECORDING ON THE STATUS OF ALASKA'S VOLCANOES (907) 786-7478

CONTACT INFORMATION:

John Power, Acting Scientist-in-Charge, USGS
jpower@usgs.gov (907) 786-7497

Steve McNutt, Coordinating Scientist, UAF
steve@giseis.alaska.edu (907) 474-7131

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.

---