Report on St. Helens (United States) — February 1986
Scientific Event Alert Network Bulletin, vol. 11, no. 2 (February 1986)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Longest period of repose continues
Please cite this report as:
Global Volcanism Program, 1986. Report on St. Helens (United States). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 11:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198602-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Activity remained at background levels for the 8th consecutive month, the longest repose period since activity began in 1980. February's single successful SO2 flight (on the 10th) measured 45 plus or minus 10 t/d. Maximum deformation rates were ~1 mm/day. Seismicity also remained low, with no evidence of energetic gas emission.
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, E. Endo, and S. Brantley, CVO; C. Jonientz-Trisler, University of Washington.