Report on St. Helens (United States) — 7 September-13 September 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report,
7 September-13 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, 7 September-13 September 2005. Smithsonian Institution and US Geological Survey.
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 7-13 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. Dry conditions and rockfalls continued to stir ash within the crater, and a persistent gas-and-vapor plume rose from the lava dome. As has happened in the past, large rockfalls from the dome generated conspicuous ash plumes that occasionally rose above the crater rim and drifted downwind. Camera images showed continued slumping of the central part of the dome and westward motion of the presently active area. St Helens remained at Volcano Advisory (Alert Level 2); aviation color code Orange.
Geological Summary. Prior to 1980, Mount St. Helens formed a conical, youthful volcano sometimes known as the Fujisan 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 2,200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older edifice, but few lava flows extended beyond the base of the volcano. The modern edifice consists of 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.