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U.S. Geological Survey, Vancouver, Washington
University of Washington, Pacific Northwest Seismograph Network, Seattle, Washington
October 28, 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 southward 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.
Preliminary reduction of LIDAR data from October 14 indicates that the volume increase of the new dome (uplifted glacial ice, rock debris, and new lava) was approximately 11 million cubic meters at that time, giving a growth rate of about 7 cubic meters per second.
Crews had a productive day in the field yesterday. Work included installation of two new GPS stations to measure ground deformation on the surface of the growing dome, geologic observations and sampling, collection of oblique stereophotos for tracking growth of the new dome, thermal-infrared mapping to determine temperature distribution in the new and old (1980-86) domes, a gas-measurement flight, and telemetry maintenance. Results include the following: the new GPS station on the southern part of the new dome shows motion downward and to the southeast; a station near the summit of the old dome has moved northward about 7 cm since October 20; thermal imaging showed an elongate band of elevated surface temperature, locally as great as 775ยบ C along the west face of the new dome coincident with the area of exposed newly extruded lava; gas-emission rates measured yesterday are similar to recent previous measurements (SO2 about 250 tons per day, CO2 about 300 tons per day, H2S about 2 tons per day); samples of dome rock similar in appearance to the rock of the older dome were collected from two localities in the vicinity of the exposed new lava. In the aggregate, the above results indicate that the character and rise of magma is continuing as it has over tha past few weeks. The visible steam plume is caused by condensation of moisture in the cold air above the hot dome.
No field work is planned for today.
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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