Report on St. Helens (United States) — May 1983
Scientific Event Alert Network Bulletin, vol. 8, no. 5 (May 1983)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Lava dome growth continues
Please cite this report as:
Global Volcanism Program, 1983. Report on St. Helens (United States). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 8:5. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198305-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
New lava began to emerge high on the NE flank of the February lobe between 29 April and 4 May. Slow extrusion continued for about the next 3 weeks, but little new material appeared to have emerged onto the surface since then. Deformation measurements indicated that intrusive activity continued after extrusion stopped, as the new lobe continued to move outward at an average rate of 15-20 cm/day through early June. Little movement was noted on the N and W sides of the composite dome. The spine that grew on the February lobe from 15 April to about the end of the month and had formed the highest point on the composite dome toppled between visits to the crater 24 and 26 May.
The rate of SO2 emission averaged 95 ± 35 t/d in May and remained similar in early June. This represented a decline from the April average of roughly 150 t/d, but remained substantially above the September 1982-mid January 1983 quiet rate of 25-50 t/d.
The number of earthquakes with locatable foci dropped from 243 in April to 155 in May. Nearly all were low-frequency events with emergent onsets, as has been observed since the early February explosions. However, strain release for surface events was nearly twice as high as for earthquakes, and nearly twice as many were recorded in May as in April. Most of the rockfalls observed by geologists in the crater occurred from the dome's active NE flank. Large . . . avalanche events were recorded 5, 15, and 18 May.
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, E. Iwatsubo, USGS CVO, Vancouver, WA; C. Boyko, University of Washington.