Report on St. Helens (United States) — September 1985
Scientific Event Alert Network Bulletin, vol. 10, no. 9 (September 1985)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Activity at background levels
Please cite this report as:
Global Volcanism Program, 1985. Report on St. Helens (United States). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 10:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198509-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Through early October, maximum displacement rates were 1-2 mm/day on the S side of the dome, the region where deformation has been concentrated for the past 6 months. The period includes several weeks preceding the May/June lobe extrusion on the S side of the dome (SEAN 10:05). Three measurements during September showed an average rate of 30 ± 15 t/d of SO2 emission. There were no tephra or energetic gas emissions during the month.
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.
Information Contacts: D. Swanson; S. Brantley, USGS CVO, Vancouver, WA; S. Malone, University of Washington.