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Report on St. Helens (United States) — December 1983

St. Helens

Scientific Event Alert Network Bulletin, vol. 8, no. 12 (December 1983)
Managing Editor: Lindsay McClelland.

St. Helens (United States) Renewed extrusion on NE flank; breadcrust bombs

Please cite this report as:

Global Volcanism Program, 1983. Report on St. Helens (United States) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 8:12. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198312-321050

St. Helens

United States

46.2°N, 122.18°W; summit elev. 2549 m

All times are local (unless otherwise noted)

Numerous rockfalls occurred through December as the lobe front broke up at the top of the S and SE flanks. Some rockfalls were hot enough to become fluidized, moving down a chute and spreading over a small area of the crater floor at the base of the dome. The fluidized rockfalls generated from the front of the NE flank lobe in August (SEAN 08:08) were larger and had stronger seismic signatures. Maximum displacement rates measured on the S and SE sides of the dome dropped from about 50 cm/day in late November to about 20 cm/day by late December. Between 28 December and 4 January, values increased sharply, to as much as 60 cm/day.

New lava appeared on the NE flank in mid-December, forming jagged spires and ridges, but little downslope advance was observed. Instead, the lava appeared to have broken through the crust of the dome in a zone that extended about 200 m NE from the summit spine and was about 100 m wide (in a NW-SE direction). Night overflights showed an increased number of glowing cracks on the dome's NE flank. Deformation of the NE flank also began to accelerate in mid-December, increasing from 0.5-1 cm/day to a few centimeters per day by the end of the month.

The dome's summit spine crumbled rapidly, but continued to receive some new lava. As of early January, it remained the high point on the dome, but several meters of net height loss appeared to have occurred. Snow had accumulated in the depression that formed in November near the top of the dome. However, its NW rim tilted dramatically away from the remainder of the depression, and a new mound had grown in that area.

Gas-and-ash ejection from vents high on the dome continued to occur several times a day. On 16 December, a substantial number of breadcrust bombs with a maximum diameter of about 4 cm were found in the crater on snow that had probably fallen 2 days earlier. The bombs were more vesicular than any material ejected since the strong explosive activity of 19 March 1982 (SEAN 07:03). December SO2 emission averaged 105 ± 25 t/d, a 30-40% increase over October and November values, but similar to rates measured April-September.

Seismic events were slightly more numerous in December, but only minor growth was observed in the rate of energy release. Seismicity increased slightly in early December, dropped a little at mid-month, then showed a minor increase at month's end. There were no obvious changes in seismicity that could be correlated with changes in activity on the dome. The number of surface events decreased seasonally because winter conditions inhibit rock avalanching from the crater walls.

Further Reference. Swanson, D.A., Dzurisin, D., Holcomb, R.T., Iwatsubo, E.Y., Chadwick, W.W., Jr., Casadevall, T.J., Ewert, J.W., and Heliker, C.C., 1987, Growth of the Lava Dome at Mount St. Helens, Washington (USA), 1981-1983 in Fink, J. (ed.) The Emplacement of Silicic Domes and Lava Flows; Geological Society of America Special paper 212, p. 1-16.

Geological Summary. Prior to 1980, Mount St. Helens formed a conical, youthful volcano sometimes known as the Fujisan 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 2,200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older edifice, but few lava flows extended beyond the base of the volcano. The modern edifice consists of 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, T. Casadevall, USGS CVO, Vancouver WA; R. Norris, University of Washington.