Report on St. Helens (United States) — June 1985
Scientific Event Alert Network Bulletin, vol. 10, no. 6 (June 1985)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Activity returns to 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, 10:6. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198506-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
After the extrusion of a new lobe and endogenous growth of the composite lava dome in late May-early June, seismicity, deformation, and SO2 emission returned to background levels. Displacement rates began to decline on 10 June and had reached background levels by late June. As of early July the dome was stable, with maximum displacement rates of a few millimeters per day. After reaching a high of 220 ± 10 t/d on 8 June, SO2 emission had decreased to a background level of 60 ± 5 t/d by the 12th, and remained between 25 ± 5 and 45 ± 5 t/d through early July.
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.