Report on St. Helens (United States) — July 1982
Scientific Event Alert Network Bulletin, vol. 7, no. 7 (July 1982)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Dome growth begins 17 August
Please cite this report as:
Global Volcanism Program, 1982. Report on St. Helens (United States). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 7:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198207-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Electronic distance measurements from sites on the crater floor to targets on the composite dome registered [swelling] beginning in mid-June. The rate of [swelling] remained a few millimeters per day until the end of July, when it accelerated to a few centimeters per day. On 27 July local seismicity increased to 3-4 events per day, then stabilized on 2-3 August to 1-2 events per day. By 13 August, 3-4 events per day were again occurring. The rate of SO2 emission has remained at the inter-eruption background level of about 100 t/d.
The seismic pattern and the rise in displacement rate prompted the USGS and University of Washington to issue an extended outlook advisory on 30 July, predicting an eruption within 3 weeks. Seismicity and displacement rates continued to increase. On 16 August an eruption was predicted within 4 days, then within 24 hours on 17 August.
The dome began to grow endogenously on 17 August. The W and SW sides were expanding at a rate of about 10 m/day. Numerous rockfalls were occurring, but without explosions or changes in rates of deformation, displacement, seismicity, or gas emission. Extrusion of a new lobe on top of the dome began during the day on 18 August, with lava flowing slowly onto the dome's W and S sides.
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: W. Chadwick, C. Newhall, USGS CVO, Vancouver; S. Malone, University of Washington.