Cascade Range Current Update

Monday, April 11, 2005 10:45 a.m. PDT (1745 UTC)

MOUNT ST. HELENS UPDATE

Current status is Volcano Advisory (Alert Level 2); aviation color code ORANGE

Growth of the new lava dome inside the crater of Mount St. Helens continues, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. During such eruptions, episodic changes in the level of activity can occur over days to months. The eruption could also intensify suddenly or with little warning and produce explosions that cause hazardous conditions within several miles of the crater and farther downwind. Small lahars could suddenly descend the Toutle River if triggered by heavy rain or by interaction of hot rocks with snow and ice. These lahars pose a negligible hazard below the Sediment Retention Structure (SRS) but could pose a hazard along the river channel upstream.

Potential ash hazards: Wind forecasts from the National Oceanic and Atmospheric Administration (NOAA), coupled with eruption models, show that any ash clouds that rise above the crater rim today would drift northeastward.

Potential ash hazards to aviation: Under current eruptive conditions, small, short-lived explosions may produce ash clouds that exceed 30,000 feet in altitude. Ash from such events can travel 100 miles or more downwind.

Recent observations: The volcano is blanketed with new snow, which mantles the region to below the 3000-ft altitude, judging from the clear views in the US Forest Service Volcano Cam this morning. Seismicity has decreased steadily in the past week, both by reduced number of larger earthquakes (M2.5-3.0) and decreasing size of the mid-range earthquakes. A GPS receiver located 200 m north of the new dome creeps steadily north-northwest at about 10 cm per day. Observations yesterday by helicopter in the crater and from a fixed station on the crater’s eastern rim suggest the new dome continues to fracture and spread laterally. The process has been likened to “pancaking,” as the molten core yields and its solid carapace shatters. As a consequence, the dome’s summit has been lowered broadly by a few tens of meters in the past two to three weeks, except for isolated high-standing remnants. Rockfalls and their associated rising dust clouds accompany this process. Virtually all these effects are limited to the dome and adjacent areas within 600 m radius, well within the crater. The combination of the GPS measurements adjacent to the dome and the qualitative estimate of lateral spreading suggest that extrusion of new lava continues. We still lack, however, an instrument on the new dome itself that might specify the rate and direction of propagation.

Also yesterday, two nonoperating GPS and sesmic monitoring stations in the crater were grappled by a helicopter sling line and pulled from the crater—scientific housekeeping, so to speak. A cover glass on our Sugarbowl dome cam (2.3 km from vent) was again replaced, owing to sandblasting by windborne sand and silt.

For additional information, background, images, and other graphics: http://vulcan.wr.usgs.gov/Volcanoes/MSH/Eruption04/

For seismic information: http://www.pnsn.org/HELENS/welcome.html

For a definition of alert levels: http://vulcan.wr.usgs.gov/Volcanoes/Cascades/CurrentActivity/volcano_warning_scheme.html

For a webcam view of the volcano: http://www.fs.fed.us/gpnf/volcanocams/msh/

Telephone recordings with the latest update on Mount St. Helens and phone contacts for additional information can be heard by calling:

Media (360) 891-5180

General public (360) 891-5202

OTHER CASCADE VOLCANOES

All other volcanoes in the Cascade Range are all at normal levels of background seismicity. These include Mount Baker, Glacier Peak, Mount Rainier, and Mount Adams in Washington State; Mount Hood, Mount Jefferson, Three Sisters, Newberry, and Crater Lake, in Oregon; and Medicine Lake, Mount Shasta, and Lassen Peak in northern California.

USGS Cascades Volcano Observatory, the Pacific Northwest Seismograph Network at the University of Washington, and the USGS Northern California Seismic Network and Volcano Hazards Team in Menlo Park, California, monitor the major volcanoes in the Cascade Range of northern California, Oregon, and Washington.



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