Report on St. Helens (United States) — March 1985
Scientific Event Alert Network Bulletin, vol. 10, no. 3 (March 1985)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Activity remains at background levels
Please cite this report as:
Global Volcanism Program, 1985. Report on St. Helens (United States) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 10:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198503-321050
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
SO2 emission averaged 50 ± 10 t/d in March, identical to the February rate, and remained low in early April. Maximum displacement rates were 2-3 mm/day. No gas-and-ash emissions from the dome were observed, or detected by seismographs.
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.
Information Contacts: D. Swanson; J. Sutton, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.