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U.S. Geological Survey, Vancouver, Washington
University of Washington, Pacific Northwest Seismograph Network, Seattle, Washington
October 25, 2004 10:00 am PDT (17:00 UTC)
MOUNT ST. HELENS VOLCANO
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. As long as this eruption is in progress, episodic changes in the level of activity can occur over days, weeks, or even months. Increase in the intensity of eruption could occur suddenly or with very little warning and may include explosive events that produce hazardous conditions within several miles of the volcano. Small lahars (volcanic debris flows) could suddenly descend the Toutle River valley if triggered by heavy rain or by interaction of hot rocks with snow or glacier ice. These lahars pose a negligible hazard below the Sediment Retention Structure (SRS), but could pose a hazard to people along the river channel upstream of the SRS. At this time of year, it is not unusual for rivers draining the volcano to contain high concentrations of sediment that turn the water murky.
Although considered less likely at this time, the current eruptive activity could evolve into a more explosive phase that affects areas farther from the volcano and sends significant ash thousands of feet above the crater where it could be a hazard to aircraft and to downwind communities.
Wind forecasts from the National Oceanic and Atmospheric Administration (NOAA), coupled with eruption models, show that ash clouds that rise above the crater rim today would drift northeastward from the volcano.
Seismicity remains at a low level compared to that observed early in this unrest. The current seismicity is consistent with a continuing, slow rise of magma driving uplift of the crater floor and feeding a surface extrusion of lava. The overall low rates of seismicity and gas emission suggest that the lava reaching the surface is gas poor, thereby reducing the probability of highly explosive eruptions in the near term.
Yesterday field crews were able to make brief geological and thermal-imaging observations and to install an additional GPS receiver. Thick steam clouds obscured much of the new lava dome, but some changes were apparent since the last observations on 21 October. A small apron of warm ash and blocks that fell from the dome lies on newly fallen snow on its west side and a small stream of melt water flows from a nearby area of intensely deformed glacier ice. A new GPS instrument was slung by helicopter and placed on a large boulder just north of the 1980-86 lava dome. It will aid in detecting any crater-wide deformation. Initial analyses were made over the weekend of samples of the new lava dome that were collected on 20 October. They show that, since lava first appeared on 11 October, it has been rising more easily from depth and not spending much more than a few days at shallow levels before being extruded onto the surface. Reviews of several lines of evidence confirm that the average rate of volume change between late September and mid-October of the deformed area and new lava dome was about 8 cubic meters (10 cubic yards, or a typical dump truck full) per second. A substantial part of that change must be magma, which suggests a rate similar to that of many other lava-dome-building eruptions.
The U.S. Geological Survey and the University of Washington continue to monitor the situation closely and will issue additional updates and changes in alert level as warranted.
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