Report on St. Helens (United States) — May 1982
Scientific Event Alert Network Bulletin, vol. 7, no. 5 (May 1982)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Frequent gas and ash emission; more on May lobe
Please cite this report as:
Global Volcanism Program, 1982. Report on St. Helens (United States). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 7:5. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198205-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Lava began to flow down the NE side of the dome 14 May, but the bulk of the new lava formed a lobe on the dome's NW flank 15-19 May. Since then, ejections of steam and ash, similar to those of July and August 1981, have occurred about once a day. Two of these, on 7 and 8 June, caused light ashfalls on Portland. Gas-emission rates remained high through early June.
The crater-floor deformation before the extrusion was blocky and incoherent, as during previous pre-extrusion periods. While a continuously recording tiltmeter at a new site on the W crater floor recorded increasingly rapid subsidence (as did a dry tilt station at the same location), reoccupation of dry tilt stations less than 100 m away showed accelerating uplift. The development of a small thrust fault was observed between the dome and the continuously recording tiltmeter, leading Dan Dzurisin to suspect that thrusting was responsible for the different tilt directions at nearby sites.
Local seismicity had begun to increase on 8 May (SEAN 07:04). In the 24 hours starting at 0700 on 13 May, 63 earthquakes were recorded (about twice the previous day's number) and some were felt by geologists working in the crater that day. Three radiating fractures, trending NE, N, and NW, were seen in the April lobe, on the N side of the dome. SO2 emission remained at background levels of about 100 t/d. The rapid subsidence measured by the continuously recording tiltmeter stopped about midnight. Harmonic tremor started shortly thereafter, at 0055 on 14 May, and continued until about 0600. Bursts of seismic energy could be seen within the tremor. During an overflight at 0415, spectacular, nearly continuous cascades of incandescent material could be seen on the NE flank of the dome, but an hour later the rockfalls had ceased almost entirely. After dawn, a jumbled, blocky area could be seen on the dome's summit and upper NE flank, and there was a rockfall apron on the NE side of the dome. The jumbled area was larger by afternoon, but it was not certain whether it was new lava or scoriaceous older material being uplifted by endogenous dome growth. Episodic gas emission was observed on 14 May, and by afternoon the rate of SO2 release had increased fourfold from the previous day, to about 400 t/d. The number of earthquakes decreased to twenty in the 24 hours starting at 0700 on 14 May.
On 15 May, rockfall activity continued on the dome's NE flank and rockfalls began on its NW side. The surface morphology of the N side of the April lobe was changed, but no movement was visible, and growth appeared to be occurring within the April lobe. However, by afternoon, flow texture had developed on a tiny lobe on the NE flank and a dominant lobe on the NNW flank. The number of recorded earthquakes declined to eleven between 0700 on 15 May and 0700 on the 16th. Downslope movement of the NNW flank lobe continued on 16 May, but the tiny NE flank lobe was stagnant. The number of earthquakes had dropped to only 1-2/day. Poor weather prevented observations 17-18 May, but seismographs detected numerous rockfalls. When geologists returned to the crater 19 May, rockfalls and some downslope movement of the NNW flank lobe were continuing, but little extrusion appeared to have occurred since the 16th. Only minor rockfalls were observed 20 May and these had stopped by evening.
The rate of SO2 emission remained high 15-19 May, at 250-650 t/d. On 19 May, several fissures, surrounded by a small tephra blanket, were observed in the top of the March lobe. That day, a plume from these fissures quickly increased the SO2 emission rate from 340 to 2,600 t/d, but the rate dropped back to 500 t/d after about 50 minutes. An average of one large gas and ash ejection per day has occurred through early June. Many caused light dustings of ash near the volcano. The ash consisted of abraded and rounded lithic fragments and crystals, but included no fresh magma, although some of the larger fragments were hot. The highest observed plume rose to 5.5 km altitude late 6 June. Light ashfalls occurred in Portland early 7 and 8 June. SO2 emission continued at an elevated level of 200-300 t/d through early June.
USGS analyses showed no significant chemical differences between the lobes extruded in 1981 and in March, April, and May 1982. All contain 62-63% SiO2 and 40-42% phenocrysts (table 3).
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, K. Cashman, D. Dzurisin, C. Newhall, USGS CVO, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington.