Report on St. Helens (United States) — 29 December-4 January 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 29 December-4 January 2005
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, 29 December-4 January 2005. Smithsonian Institution and US Geological Survey.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Lava-dome growth continued at St. Helens during 28 December to 4 January. Observations on 3-4 January indicated that the new dome, with the exception of the northern-most portion, was becoming heavily fractured and faulted. GPS data showed that expansion of the rear part of the dome had slowed to a rate of only a few meters per day. Seismicity decreased dramatically during 29-30 December, reaching the lowest levels recorded since dome building began. This lull continued through 4 January. 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.