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Report on St. Helens (United States) — 21 September-27 September 2005

Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 21 September-27 September 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, 21 September-27 September 2005. Smithsonian Institution and US Geological Survey.

Volcano Profile |  Weekly Report (21 September-27 September 2005)


St. Helens

United States

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 21-26 September, 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. Time-series images showed that the active northwestern portion of the new lava dome continued to move westward into the W arm of a glacier, spawning rockfalls. 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.

Source: US Geological Survey Cascades Volcano Observatory (CVO)