Report on St. Helens (United States) — September 1983
Scientific Event Alert Network Bulletin, vol. 8, no. 9 (September 1983)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Lava from new vent added to composite dome
Please cite this report as:
Global Volcanism Program, 1983. Report on St. Helens (United States) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 8:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198309-321050
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Much of the NE-flank lobe has been intruded just below the surface, carrying a thin rubbly carapace of older material downslope. New lava with visible flow structures has occasionally broken through to the surface. Expansion of the S and SE flanks continued to increase slowly until late September but accelerated more rapidly after the 28th and had reached three to four times early September values by 1 October. A USGS-University of Washington advisory notice issued on that date predicted a new extrusion within 10 days.
Lava began to emerge from a new vent about 50 m S of the source of the NE-flank lobe by 7 October. As with the NE-flank lobe, little lava reached the surface, instead lifting a thin layer of older material to form a ridge roughly 300-400 m in E-W dimension, 100 m in N-S dimension, and 20 m thick, at the S edge of the NE flank lobe. The vent area was surmounted by a large pile of material, some of which appeared to be new lava, that by 12 October was 12-13 m higher than the previous summit of the dome. The advance of the NE portion of the May lobe slowed to less than 10% of its previous rate between 5 and 10 October. Deformation of the S portion of the dome stopped accelerating 5-6 October, also approximately coincident with the appearance of the new lava, but remained at rates as high as 105 cm/day. A small thrust fault began to form just beyond the talus pile at the dome's S side about 4-5 October and moved at an approximately constant rate of 2.5-3 cm/day 7-12 October.
Plumes of hot gas (a smell of H2S was often noted by geologists in the crater) continued to be ejected 2-4 times per day. During a period of very clear weather and strong winds 20-22 September, plumes were observed on satellite imagery, rising roughly 2 km above the summit and extending 100-150 km. Most plumes contained little ash. No significant change in plume size, frequency, or density accompanied the appearance of the new lava. Rates of SO2 emission averaged 100 ± 50 t/d in September, a slight increase over August values. Rates decreased slightly in mid-September to 70-90 t/d and usually remained in that range in early October. Since early September, seismic energy release has varied only slightly and there was no change in seismicity associated with the emergence of the new lava.
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, C. Newhall, S. Brantley, USGS CVO, Vancouver, WA; S. Malone, University of Washington; M. Matson, S. Kusselson, E. Legg, NOAA/NESDIS.