Report on St. Helens (United States) — September 1981
Scientific Event Alert Network Bulletin, vol. 6, no. 9 (September 1981)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Minor ash emission; slow deformation
Please cite this report as:
Global Volcanism Program, 1981. Report on St. Helens (United States). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 6:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198109-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
When USGS personnel arrived in the crater on the morning of 11 September, there was a characteristic area of smoother lava on the top of the new lobe. Similar features had marked the end of the December 1980-January 1981 and June 1981 extrusion episodes. No further growth was observed. The new lobe had a volume of about 5 x 106 m3, comparable in size to previous lobes, and brought the total volume of the dome to about 30 x 106 m3.
Poor weather plagued monitoring efforts after the extrusion episode. At 1559 on 10 September, just before extrusion ended, gas and fine ash rose to about 3 km altitude in a 15-minute eruptive episode accompanied by seismicity. Other gas emissions, all accompanied by seismicity, occurred at 0705 on 13 September, 1426 on 14 September, and 1028 on 16 September. No additional gas emissions were observed through the end of September.
Deformation within the crater showed a pattern similar to that of previous post-extrusion periods. The rate of thrust fault movement, which had accelerated to nearly 50 cm/day on the most active fault just prior to the September extrusion, decreased rapidly before stabilizing on 10 September. After the September extrusion ended, continued slow movement (about 0.5-1 cm/day) was measured on some thrust faults around the dome, while other thrusts remained inactive. Similarly, outward movement of one station on the N crater rampart reached more than 10 cm/day before extrusion began; after the extrusion episode ended, rates of outward movement had dropped to 0.25-0.6 cm/day.
The volume of SO2 emission peaked at 660 t/d during the afternoon of 6 September, just prior to the beginning of lava extrusion. During the extrusion episode, emission rates varied from 190 to 310 t/d, then dropped on 11 September to 70 t/d, the lowest measured rate for the month. SO2 emission increased sharply in mid-September to 530 t/d on the 17th and dropped to 340 t/d on the 18th; then poor weather stopped data collection until the end of the month, when two days of measurements showed a rate of about 250 t/d.
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, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.