Report on St. Helens (United States) — February 1985
Scientific Event Alert Network Bulletin, vol. 10, no. 2 (February 1985)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Activity at background levels; longest quiet phase since activity began in 1980
Please cite this report as:
Global Volcanism Program, 1985. Report on St. Helens (United States) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 10:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198502-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Seismicity and rates of displacement and SO2 emission remained at background levels in February and early March. Maximum displacement rates on the dome were 2-3 mm/day. SO2 emission averaged 50 ± 10 t/d in February and did not change significantly in early March. The highest temperature yet measured on the dome, 912°C, was recorded 18 February in a rubble-filled crack high on the N side of the dome. Glow at this site has been seen in night photos since early fall 1984, but no temperature measurements had previously been made there.
No tephra emission has occurred from the lava dome since 24 September, and the last extrusion episode was in early September (SEAN 09:08 and 09:09). This is the longest quiet interval since the volcano became active in 1980.
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.