International Glaciospeleological Survey
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U.S. Geological Survey, Vancouver, Washington October 27, 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 southwest 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. Samples of the new dacite lava collected on 20 October are mostly similar chemically and texturally to lava erupted during the late period of dome growth in the 1980’s (1984-1986). This is consistent with slow rise, degassing, and cooling of magma within the volcano’s conduit. However, a minor component of the 10/20/04 sample has textures and mineral compositions indicative of rapid ascent of magma from a region at greater depth and greater temperature (900º C). Crews did not go to the field yesterday, but field work including geologic and thermal-imaging observations, a gas-measurement flight, and telemetry maintenance is under way. 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. |
