Report on St. Helens (United States) — December 1985
Scientific Event Alert Network Bulletin, vol. 10, no. 12 (December 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). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 10:12. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198512-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Activity at Mt. St. Helens remained at background levels in December. For the 7th consecutive month, only minor seismicity, geodetic changes, and SO2 emissions were detected. Displacement rates on the dome were approximately 2 mm/day, while the continuously recording strainmeter just N of the May-June 1985 graben showed step-like extension across cracks averaging 0.2 mm/day. Seismicity consisted mostly of surface events (primarily rockfalls). SO2 emissions continued to be low and variable.
Further References. Keller, S.A.H. (ed.), 1986, Mount St. Helens, Five Years Later: Proceedings of a Symposium at Eastern Washington University May 16-18, 1985; Eastern Washington University Press, 448 p. (47 papers).
Manson, C.J., Messick, C.H., and Sinnott, G.M., 1987, Mount St. Helens-A Bibliography of Geoscience Literature, 1882-1986: USGS Open-File Report 87-292 (1600+ references).
Special Section: Mount St. Helens: JGR, 1987, v. 92, no. B10, p. 10,149-10,334 (12 papers).
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; C. Jonientz-Trisler, University of Washington.