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Report on St. Helens (United States) — November 1986

Scientific Event Alert Network Bulletin, vol. 11, no. 11 (November 1986)
Managing Editor: Lindsay McClelland.

St. Helens (United States) Seismicity and SO2 emissions at background

Please cite this report as:

Global Volcanism Program, 1986. Report on St. Helens (United States) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 11:11. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198611-321050.

Volcano Profile |  Complete Bulletin

St. Helens

United States

46.2°N, 122.18°W; summit elev. 2549 m

All times are local (unless otherwise noted)

Since the dome-building activity in late October, the volcano has remained quiet. From November through early December, seismicity and SO2 emissions were at background levels. SO2 could only be measured on three days: 3 November, 60 plus or minus 20 t/d; 2 December, 33 plus or minus 5 t/d; and 9 December, 80 plus or minus 15 t/d. No significant deformation was recorded.

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 and J. Sutton, CVO; C. Jonientz-Trisler, University of Washington.