Report on St. Helens (United States) — November 1990
Bulletin of the Global Volcanism Network, vol. 15, no. 11 (November 1990)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Small explosion from lava dome
Please cite this report as:
Global Volcanism Program, 1990. Report on St. Helens (United States). In: McClelland, L. (ed.), Bulletin of the Global Volcanism Network, 15:11. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199011-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
A small explosion occurred from the lava dome on 20 December at 1259. The explosion was marked by a small seismic signal that decreased to low levels after several minutes, but continued for several hours. Airplane pilots reported a light gray plume to as much as 7.5 km altitude that was carried SSW by strong winds. A diffuse plume was first evident on satellite images at 1320, moving SSW at ~30 km/hour. By 1600, the plume could no longer be detected on satellite imagery. Light ashfall was reported to 15 km SW of the volcano. No mudflow or water flow event was detected.
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: CVO; SAB.