Report on St. Helens (United States) — 27 April-3 May 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 27 April-3 May 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, 27 April-3 May 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 27 April to 3 May, growth of the new lava dome inside the crater of Mount St. Helens continued, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. On the morning of 28 April there were reports of minor amounts of ashfall in the eastern part of the Portland metropolitan area. There was no evidence of a new explosion. CVO scientists determined that large convective storms over the Cascades on 27 April entrained ash generated by the frequent hot rockfalls from the growing lava dome and kept it in suspension to fall out as far away as Portland.
CVO corrected the volume of the new lava dome measured on 10 March, with the new estimate at 45 million cubic meters. 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.