Printer friendly versionALASKA VOLCANO OBSERVATORY
Tuesday, February 21, 2006 3:15 PM AKST (015 UTC)
61°17'58"N 152°15'4"W , Summit Elevation 11070 ft (3374 m)
Current Level of Concern Color Code: GREEN
Previous Level of Concern Color Code: YELLOW
The purpose of this Information Release is to summarize the recent period of volcanic unrest at Mt. Spurr Volcano and present AVO's rationale for downgrading the Level of Concern Color Code from Yellow to GREEN.
AVO identified an increase in earthquake activity beneath the summit of Mount Spurr Volcano beginning in February 2004. The increase marked a notable departure from the normal background seismicity of the volcano, and AVO raised the level of concern color code to YELLOW on July 26, 2004. By this time, the seismicity had increased to a rate greater than that observed since the last eruptive period in 1992. Seismicity remained elevated until about April 5, 2005, declined gradually since about June 25, 2005, and has remained relatively stable since then. The majority of located earthquakes have been smaller than magnitude 1.5 and range in depth between 1 km above and 6 km (0.6 and 4 miles) below sea level. Relatively few earthquakes have been located beneath the Crater Peak vent, the site of the 1953 and 1992 eruptions. In response to the elevated seismicity, AVO geologists first observed Mt. Spurr Volcano by fixed-wing aircraft on July 15, 2004 and observed no indications of obvious volcanic activity, but did observe several fresh-looking, dark debris-flow deposits on the southeast and south face of the summit edifice suggesting water flow at the surface. On August 3, 2004, AVO scientists observed a circular melt depression; about 50 meters (165 feet) in diameter and about 25 meters (82 feet) deep in the snow and ice cover just northeast of the summit of Mt. Spurr. A small ice-rich pond on the floor of the depression also was observed. Since then, the melt depression and pond have increased in size although the volume of water in the depression has fluctuated somewhat. Occasionally a small steam plume has been visible above the depression from Anchorage and by passing pilots. Intermittent observations made from August 2004 through September 2005 chronicled the enlargement of the melt depression, documented areas of warm rock on the floor of the depression and in other areas on the edifice with an infrared camera, and noted at least two additional debris flows. Localized but vigorous roiling within the medium-grey colored lake was observed on numerous occasions and is consistent with emission of hot volcanic gases, whereas the persistent grey color usually indicates high acidity. Intermittent airborne measurements of volcanic gases (CO2, H2S and S02) from August 2004 to September 2005 indicated small but distinctly measurable quantities, although by September 2005 gas emissions had declined to low levels. As of the last AVO over flight of Mt. Spurr in January, 2006, the melt depression persists, the lake remains open, but had a distinctly greenish cast.
The declining seismicity, the reduced gas emissions, and the changing summit lake color, all suggest a reduced level of activity at the volcano. Although the overall level of seismicity has remained higher than it was prior to the recent period of unrest, it now appears that this level denotes a new background condition for Mt. Spurr Volcano. Thus, based on available information and an increased level of monitoring of Mt. Spurr, AVO concludes that the current episode of unrest at Mt. Spurr has ended and the likelihood of eruption in the near term has diminished significantly. Thus the level of concern color code for Mt. Spurr is returning to GREEN.
Although the overall level of unrest at Mt. Spurr Volcano has declined to a new background level, conditions on the mountain could remain hazardous to skiers, snowboarders, mountaineers, pilots (especially those landing near the summit area) and other visitors for some time. It is possible for unstable snow and ice conditions to persist in and around the summit area, especially the steep and unstable walls of the melt pit, and for low-level gas emissions to continue. Thus, visitors to the area may encounter foul-smelling and possibly dangerous levels of volcanic gas that might be impossible to detect from a distance. Occasional felt earthquakes may occur and these could be large enough to disturb steep or unstable areas of ice and snow. Water still impounded in the summit melt depression could be released quickly and without warning and this could lead to hazardous fast-moving floods of debris or water on the upper flanks of the summit.
Additional information about Mt. Spurr hazards can be found on the AVO web site at www.avo.alaska.edu/activity/Spurr.php
ABBREVIATED COLOR CODE KEY
(contact AVO for complete description):
GREEN volcano is dormant; normal seismicity and fumarolic activity occurring
YELLOW volcano is restless; eruption may occur
ORANGE volcano is in eruption or eruption may occur at any time
RED significant eruption is occurring or explosive eruption expected at any time
VOLCANO INFORMATION ON THE INTERNET: http://www.avo.alaska.edu
RECORDING ON THE STATUS OF ALASKA'S VOLCANOES (907) 786-7478
Chris Waythomas, Acting Scientist-in-Charge, USGS
firstname.lastname@example.org (907) 786-7497
Steve McNutt, Acting Coordinating Scientist, UAF-GI
email@example.com (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.