Report on St. Helens (United States) — 27 July-2 August 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 27 July-2 August 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 July-2 August 2005. Smithsonian Institution and US Geological Survey.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Rockfalls and associated minor ash plumes continued to be generated from Mount St Helen's growing lava dome during 27 July to 2 August. Measurements taken on 27 July revealed that emission rates of volcanic gases remained fairly constant, with 60 tons per day of sulfur dioxide and 880 tons per day of carbon dioxide. Analysis of a rock sample collected from the dome on 13 July showed no change in the composition of lava being extruded. 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.