Report on St. Helens (United States) — 23 February-1 March 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 23 February-1 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, 23 February-1 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)
During 23 February to 1 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. Photographs taken on 28 February showed that the W and E margins of the new lava dome were crumbling and that the smooth whaleback form of the lava dome was disintegrating. According to CVO, this process also occurred in December 2004 and in that case several weeks later lava-dome extrusion again manifested a whaleback form. GPS measurements on the bulging E arm of the glacier within St. Helens' crater showed that rapid northward movement of 1.2 m per day continued. St. Helens remained at Volcano Advisory (Alert Level 2); aviation 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.