Report on St. Helens (United States) — 7 December-13 December 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 7 December-13 December 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, 7 December-13 December 2005. Smithsonian Institution and US Geological Survey.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Growth of the new lava dome inside the crater of Mount St. Helens continued during 30 November to 12 December, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. There were no significant changes in seismicity or deformation during the report period. The well-established pattern of tiny "drumbeat" earthquakes continued at a rate of one every 1-2 minutes and other monitoring data remained in typical ranges. Despite the continuing procession of earthquakes, the overall seismic energy release was very low compared to that during early phases of the eruption. Small rockfalls continued from the growing lava dome, with larger ones producing ash plumes that were visible above the crater rim. The volume of the lava dome measured on 24 October was 70 million cubic meters-about 90% of the volume of the 1980-to-1986 dome. St Helens remained at Volcano Advisory (Alert Level 2); aviation color code Orange.
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.