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U.S. Geological Survey, Vancouver, Washington
University of Washington, Pacific Northwest Seismograph Network, Seattle, Washington
October 21, 2004 10:45 am PDT
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 southeastward from the volcano.
In the past 24 hours, seismicity has decreased somewhat and 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. Last night, glow from this new lava was intermittently visible on the U.S. Forest Service web camera. The overall low rates of seismicity and gas emission suggest that the lava reaching the surface is gas poor.
Yesterday was a busy day in the field. Geological and thermal-imaging observations confirmed that both the area of uplift and the new lava extrusion have increased in size noticeably since last seen on October 14. The area of uplift and intense deformation continues to move southward and is nearing the crater wall. About 1 foot of new snow with a light dusting of ash covers much of the uplift, except for the new lava extrusion, which is steaming heavily. The new lava extrusion, which occupies the western part of the uplift, is now about 900 ft long by 250 ft wide and 230 ft high and has a volume of almost 2 million cubic yards. Its maximum temperature is about 600 degrees C (1100 degrees F). Rock samples from the new lava extrusion were collected from a helicopter by using a bucket slung on a 100-ft line. The samples look like typical Mount St. Helens lava, called dacite. Further detailed analyses of the samples will help to answer questions about the character of the magma driving the eruption and how it relates to lava erupted in the 1980s.
A gas-sensing flight detected low levels of the volcanic gases carbon dioxide, sulfur dioxide, and hydrogen sulfide, which is consistent magma continuing to rise from depth.
Test flights of the Unmanned Aerial Vehicles (UAVs), which will hopefully be used for monitoring the crater area in the future, were conducted yesterday close to the Johnston Ridge Observatory. More tests are scheduled for today if weather conditions permit.
Field crews will attempt to obtain additional geological and thermal-imaging observations today. Other work will continue on maintenance of instrumentation and improving our telemetry systems.
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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