Report on St. Helens (United States) — July 1984
Scientific Event Alert Network Bulletin, vol. 9, no. 7 (July 1984)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Deformation, seismicity, and SO2 emission drop
Please cite this report as:
Global Volcanism Program, 1984. Report on St. Helens (United States). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 9:7. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198407-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Seismicity, gas emission, and deformation of the composite lava dome declined after the extrusion of a new lobe in June and an intrusive event at the end of the month. The quoted material is from Peter Otway.
"Following late June's record deformation of up to 0.8 m/day at the upper stations on the N side of the dome, rates decreased quickly after 2 July and were down to background levels at a steady 10 mm/day on 16 July. No changes in the rates had been recorded as of early August. On the SE side, displacement rates maintained a constant 6 mm/day throughout July and on the S side remained steady at 1.2 mm/day.
"A number of new survey phototheodolite targets were installed across the top of the dome in July, resulting in a total of 32 targets being surveyed by theodolite several times a month, in addition to the 12 EDM targets tracked 2-3 times a week.
"During the installation of targets on the top of the dome on 30 July, several small steam emissions (accompanying a minor seismic signal) were witnessed at close range. They came from two separate locations: the top of the March lobe, and the old summit crater 80 m to the W. Some ash was present in the emission from the W side. The fume cloud rose less than 300 m from the dome and dispersed slowly. Such emissions appear to occur from time to time even when the general activity is low."
Rates of SO2 emission increased before the lava extrusion in mid-June, then declined gradually, to 55 t/d on 13 July, and only 15 t/d on the 20th and 12 t/d on the 27th. Seismicity was elevated through early July as dome growth continued, then declined to background levels by the second week of the month and remained low through early August.
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: P. Otway, C. Mullins, USGS CVO, Vancouver, WA; R. Norris, University of Washington.