ALASKA VOLCANO OBSERVATORY WEEKLY UPDATE
U.S. Geological Survey
Friday, October 2, 2009, 12:00 PM AKDT (Friday, October 2, 2009, 20:00 UTC)

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

AVO lowered the Aviation Color Code to GREEN and the Volcano Alert Code to NORMAL at 10:44 AKDT Tuesday, September 29. Over the past several months, seismic activity, volcanic gas output, ground deformation, lava dome temperatures, and outward signs of lava dome instability at Redoubt Volcano have been declining. The volcano appears to have returned to its normal background condition and for now poses no immediate threat of eruptive activity.

Since it began growing in early April 2009, the lava dome in the summit crater at the top of the Drift glacier canyon has reached a volume of approximately 70 million cubic meters (91 million cubic yards). Lava domes, particularly those built on steep slopes, are typically unstable and subject to sudden collapse. A dome collapse at Redoubt would likely produce ash clouds and falls, hot block-and-ash flows, and downstream flooding and lahars. However, based on the length of time since the cessation of dome growth (more than 60 days), AVO believes the possibility for dome collapse is small and declining toward a background level of danger typical of many active stratovolcanoes with steep unstable rock slopes.

Redoubt's new lava dome remains hot and gas-rich and will continue to pose local hazards for some time. High levels of sulfur dioxide, carbon dioxide, and hydrogen sulfide may occur near the dome, as will rock falls and abundant steaming. Depending on wind and cloud conditions, sulfur smells could reach populated areas and steam plumes will be visible on occasion. Small outbursts of impounded water, not associated with a dome collapse, are possible, but these are unlikely to pose a threat much beyond the upper Drift River. As temperatures in the area begin to decline and winter freeze up occurs, the likelihood of flooding of the valleys of the lower Drift River, Rust Slough, Cannery Creek, and Montana Bill Creek will also decline.

At this time it is not known if the volcano is entering a prolonged period of quiet, as it did after the 1966-68 and 1989-90 eruptions when the volcano exhibited little to no unrest for periods of about 20 years.

A resumption of eruptive activity at Redoubt in the near future is possible, but AVO considers this scenario unlikely. Hours to days of precursory seismicity would likely precede resumed activity, providing AVO ample time to issue prompt forecasts of possible volcanic hazard. AVO continuously monitors Redoubt with an alarm system that alerts on-call staff of potential unrest.

No further updates will be issued for Redoubt Volcano unless a significant change in activity is detected.

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 remains low. Partly cloudy webcam and satellite views show no unusual activity this week.

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.



CONTACT INFORMATION:

Stephanie Prejean, Acting Scientist-in-Charge, USGS
sprejean@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.

---