Report on St. Helens (United States) — April 1981
Scientific Event Alert Network Bulletin, vol. 6, no. 4 (April 1981)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Steam and ash emission; more data on dome extrusion
Please cite this report as:
Global Volcanism Program, 1981. Report on St. Helens (United States). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 6:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198104-321050.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
About 5 x 106 m3 of lava were added to the composite dome in the crater of Mt. St. Helens in early April. Because of poor visibility, no precise time for the start of extrusion could be determined, but a small explosion that ejected an ash-bearing plume to nearly 5 km altitude on 10 April at 0821 may have marked the beginning of the episode. Much of the lava had already been emplaced by the time geologists had their first view of the crater early 12 April, and extrusion was essentially complete by evening. Most of the associated seismicity had ended by late 11 April, but occasional discrete low-frequency events continued to be registered by the USGS-University of Washington net through 17 April.
Although deformation measurements showed that the magma rose through a conduit beneath the central collapse pit of the pre-existing dome, the April lava emerged from a vent somewhat N of the central pit, covered roughly the N quarter of the older material, and extended about 160 m NNW from it previous margin. After the April event, the dome had a volume of about 15 x 106 m3, maximum and minimum lateral dimensions of 630 m (NNW-SSE) and 310 m (E-W), and a maximum height above the crater floor of 110 m. A substantial but uncertain amount of uplift of the entire crater floor was associated with the April extrusion, and some points on the crater floor spread away from the dome as much as 1.5 m, with most of the movement occurring during extrusion. One radial fissure exhibited about 55 cm of strike-slip movement during the episode. As of 5 May, only a few mm of additional deformation had taken place within the crater. No net deformation of the volcano as a whole has been associated with any of the extrusion episodes.
In the weeks following the April extrusion, characteristic low-level seismicity was sometimes correlated with witnessed bursts of steam emission. Simultaneous seismicity and ejection of steam containing a little ash occurred on 13 April at 0842; 14 April at 0950, 0953, and 1021; 17 April at 0958; and 24 April at 1018. Seismicity accompanied ejection of plumes of steam (without ash) on 25 April at 0921 and 26 April at 0821. A small amount of ash that fell about 50 km SE of Mt. St. Helens on 6 May between 1500 and 1530 may have been ejected during a period of seismicity at 1415.
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: D. Swanson, C. Newhall, USGS, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.