Report on St. Helens (United States) — 20 July-26 July 2005
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 20 July-26 July 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, 20 July-26 July 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 20-26 July, several rockfalls at Mount St. Helens were triggered by earthquakes around M 3. Some of the rockfalls produced ash plumes that rose to low altitudes. Analysis of a digital elevation model created from aerial photographs taken on 15 June showed that the volume of the new lava dome was about 54 million cubic meters, or about 60% of the volume of the lava dome that grew in the crater from 1980 to 1986. The rate of addition of lava to the dome from mid-May to mid-June remained at ~1.5 cubic meters per second. Images of the summit taken around 25 July showed that the lava spine at the N end of the dome continued to extrude at a rate similar to that of the previous few weeks, but the rate of rockfalls was greater and the height of the top of the spine was decreasing. 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.