Report on St. Helens (United States) — November 1982
Scientific Event Alert Network Bulletin, vol. 7, no. 11 (November 1982)
Managing Editor: Lindsay McClelland.
St. Helens (United States) No deformation; seismicity and SO2 emission low
Please cite this report as:
Global Volcanism Program, 1982. Report on St. Helens (United States) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 7:11. Smithsonian Institution.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
As of early December, no significant deformation had been measured. Snow conditions in the crater limited vertical angle and distance measurements between the dome and the crater floor, but comparison of 24 November and 4 December data showed no change. A continuously recording tiltmeter was installed 19 October on the E crater floor, within 25 m of the talus at the base of the dome. This instrument measured daily oscillations in tilt, but had recorded no deformation exceeding the diurnal variation as of 6 December. The rate of SO2 emission declined to an average of about 35 ± 10 t/d in November, the lowest measured since 18 May 1980. Before the August extrusion episode, the average rate of SO2 emission between eruptions had been roughly 100 t/d. Seismicity remained at background levels through November.
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: T. Casadevall, D. Dzurisin, D. Swanson, USGS CVO, Vancouver, WA; R. Norris, University of Washington.