Report on St. Helens (United States) — December 1984
Scientific Event Alert Network Bulletin, vol. 9, no. 12 (December 1984)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Activity remains at background levels
Please cite this report as:
Global Volcanism Program, 1984. Report on St. Helens (United States). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 9:12. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198412-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Deformation of the composite lava dome, seismicity, and rates of SO2 emission remained at background levels through early January. The maximum rate of outward movement on the N and S sides of the dome in December and early January was 5 mm/day. The rate of SO2 emission, 10 t/d or less on 3 and 5 December, had increased slightly to 50 ± 10 t/d by the next measurement on 2 January and 25 ± 4 t/d 5 January. No tephra emissions have been observed by geologists working in the crater, nor have fresh tephra layers been found in the snowpack accumulating in the crater. Unusually good weather during the early winter has allowed the emplacement of two telemetering tiltmeters on the dome itself (tiltmeters had previously been installed on the crater floor near the dome), a telemetering strainmeter on a crack near the dome's summit, and a small trilateration network consisting of two benchmarks and five targets in the dome's summit area.
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; J. Sutton, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.