Report on St. Helens (United States) — 15 December-21 December 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 15 December-21 December 2004
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2004. Report on St. Helens (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 15 December-21 December 2004. Smithsonian Institution and US Geological Survey.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
During 16-20 December, 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. Observations on 16 December revealed that the lava dome had noticeably widened, the prominent fracture system along its top continued to widen, and ash was intermittently emitted from hot cracks.
Geologic Background. Prior to 1980, Mount St. Helens formed a conical, youthful volcano sometimes known as the Fujisan 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 2,200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older edifice, but few lava flows extended beyond the base of the volcano. The modern edifice consists of 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.