Cascade Range Current Update

November 5, 2004 10:00 am PST (1800 UT)

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 westward to southwestward.

Visibility is excellent and likely will remain so throughout the day. A steam plume is rising passively and drifting south and southwestward over the crater rim. The plume occasionally contains minor ash, which falls out in the crater and on the southern flank of the volcano, darkening the new snow.

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.

Crews were in the field yesterday, the first opportunity following a spate of inclement weather. Their findings:

Sulfur dioxide (SO2) emission rates continue to be low and consistent with previous measurements. No hydrogen sulfide (H2S) was detected. Ash in the steam plume prevented an accurate measurement of carbon dioxide (CO2).

The elongated new dome, which extends southward from the 1980-1986 dome, has undergone substantial vertical growth since October 27. A new mass of dacite has extruded upward by as much as 100 m. Exposed rock faces have temperatures in the range 400-500 degrees Celsius, creating the incandescence that may be seen from the north on clear nights. Field crews conducted geologic observation flights in the crater. To collect samples they landed a helicopter on the new dome for the second time in two weeks. The new dacite lava contains visible crystals of plagioclase, hornblende and hypersthene. These samples are similar to those collected on October 27 and also to lava erupted at Mount St. Helens in the 1980s.

The steep new faces on the dome are generating small hot rockfalls and avalanches within the crater. The finer particulate from these deposits roils upward within the steam plume, rising to about 11,000 ft altitude, or about 2,600 ft above the crater rim. Consequently the south and southwest flank of the volcano have received a notable dusting of ash. This localized ash poses no threat beyond the near slopes of the volcano.

Most dome growth has been vertical, with only about 30 m of outward growth in some directions. The thick glacial ice that forms a buttress on the south and east sides of the dome remains largely intact. All dome growth is contained within the Mount St. Helens crater.

A continuous GPS station north of the volcano at Johnston Ridge Observatory has moved to the south by about 2 cm since late September or early October. This slow shift may reflect a depletion of magma in the subsurface at 5-10 km depth. To confirm this result, five new GPS receivers were positioned around the volcano’s flanks 5-10 km from the crater to better track changes in the deeper parts of the magmatic system. Two additional units will be deployed 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.

For additional information, background, images, and other graphics: http://vulcan.wr.usgs.gov/News/framework.html

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

For a definition of alert levels: http://vulcan.wr.usgs.gov/News/framework.html

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.



Past Updates

News - Mount St. Helens Current Information