Report on St. Helens (United States) — 2 March-8 March 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 2 March-8 March 2005
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2005. Report on St. Helens (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 2 March-8 March 2005. Smithsonian Institution and US Geological Survey.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
A small explosive event began at St. Helens at approximately 1725 on 8 March. Pilot reports indicated that the resulting steam-and-ash plume reached a height of about 11 km a.s.l. within a few minutes and drifted downwind ENE. The principal event lasted about 30 minutes with intensity gradually declining throughout. There were no reports of deaths or injuries. This was one of the largest steam-and-ash emissions to occur since renewed activity began at St. Helens in October 2004. CVO lost radio signals from three monitoring stations in the crater soon after the event started. The event followed a few hours of slightly increased seismic activity that was noted, but was not interpreted as precursory. There were no other indications of an imminent change in activity.
Prior to the explosion, during 2-7 March, growth of the new lava dome inside the crater of St. Helens continued, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. Seismic data indicated that parts of the growing lava dome continued to crumble, forming rockfalls and generating small ash clouds that drifted out of the crater. The bulging ice on the deformed E arm of the glacier in the volcano's crater continued to rapidly move northward about 1.2 m per day.
CVO stated that the current hazard assessment for the ongoing eruption mentions the possibility of events like the 8 March explosion occurring without warning, and the assessment remained unchanged. The eruption could intensify suddenly and with little warning and produce explosions that cause hazardous conditions within several miles of the crater and farther downwind. Small lahars could suddenly descend the Toutle River if triggered by heavy rain or by interaction of hot rocks with snow and ice. These lahars pose a negligible hazard below the Sediment Retention Structure (SRS) but could pose a hazard along the river channel upstream.
Geologic Background. Prior to 1980, Mount St. Helens formed a conical, youthful volcano sometimes known as the Fuji-san of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km horseshoe-shaped crater now partially filled by a lava dome. Mount St. Helens was formed during nine eruptive periods beginning about 40-50,000 years ago and has been the most active volcano in the Cascade Range during the Holocene. Prior to 2200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older St. Helens edifice, but few lava flows extended beyond the base of the volcano. The modern edifice was constructed during the last 2200 years, when the volcano produced basaltic as well as andesitic and dacitic products from summit and flank vents. Historical eruptions in the 19th century originated from the Goat Rocks area on the north flank, and were witnessed by early settlers.