Report on St. Helens (United States) — 9 March-15 March 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 9 March-15 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, 9 March-15 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)
A small but significant explosion that occurred at St Helens on 8 March at 1725 produced a fine dusting of ash in Ellensberg, Yakima, and Toppenish, Washington during 1900-2100. By 0200 on 9 March, the leading edge of the faint, diffuse plume had reached western Montana. Scientists found that St. Helens' lava dome was intact after the explosion and that ballistics up to ~1 m in diameter were hurled as far as the northern flank of the old lava dome. No ballistics were found along, or beyond the crater rim. The source of the explosion was from the NNW side of the new lava dome and was very near the source of the 1 October 2004 and 16 January 2005 explosions.
After the 8 March explosion, St. Helens only emitted steam, and seismicity returned to a level similar to that during the several hours prior to the explosion. Gas emissions were very low, essentially unchanged from those measured in late February. During 9-15 March, 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.