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Report on St. Helens (United States) — November 1980

St. Helens

Scientific Event Alert Network Bulletin, vol. 5, no. 11 (November 1980)
Managing Editor: David Squires.

St. Helens (United States) Harmonic tremor and vapor emission; chemical data summarized

Please cite this report as:

Global Volcanism Program, 1980. Report on St. Helens (United States) (Squires, D., ed.). Scientific Event Alert Network Bulletin, 5:11. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198011-321050

St. Helens

United States

46.2°N, 122.18°W; summit elev. 2549 m

All times are local (unless otherwise noted)

Activity was limited to vapor emission and occasional seismic activity through early December. Most early November seismic events were caused by rockslides from the crater walls. No significant local earthquakes or harmonic tremor were recorded until mid-November, when brief episodes of harmonic tremor began, barely within the detection limits of sensitive seismographs on and near the volcano. Intermittent low-level tremor continued through early December. Stronger tremor started on 25 November at 2054, gradually fading into background noise about 35 minutes later. Observers in a USFS aircraft reported a slightly brighter glow in the dome area after this event. A second burst of stronger tremor began 27 November at 2034, continuing for about an hour, and several more such episodes, lasting only a few minutes each, were detected through 30 November.

USGS monitoring of the N crater rampart revealed a maximum net outward movement of about 23 cm between the October explosions and 26 November. A major reversal to inward movement had occurred in late October before an outward trend resumed in November. Outward growth accelerated in mid-November to slightly more than 1.5 cm/day at times, a rate similar to that recorded during the summer. About 20 cm of expansion was measured between 12 and 26 November.

No major changes have taken place in the volume or ratio of gases emitted by the mountain. Two large fumaroles opened in the crater floor, very close to the margin of the lava dome, one on 18 or 19 November, the other on the 25th. As they opened, both ejected mud (containing no fresh magma) that coated snow on the flank. As of early December, the new fumaroles were 2-3 m across, glowed cherry red, and puffed noisily at half-second intervals.

The following is a report from W.G. Melson: "A small but definite trend toward andesite compositions is revealed by major element analyses of the 18 May-7 August eruptives. A total of 46 samples of probable essential ejecta have been analyzed (table 2 and figure 13), a minimum of five such samples from each eruptive episode. The trend is an irregular one and is most pronounced with regard to MgO and CaO when plotted against time of eruption."

Table 2. Ejecta composition for each eruptive episode of Mt. St. Helens, 18 May-7 August 1980. Electron microprobe analyses of fused powders by W. Melson, T. O'Hearn, and J. Nelen, SI. Samples collected by D. Swanson, C. Hopson, W. Melson, R. Fiske, and C. Kienle. Total Iron calculated as FeO. Values are averages of the total analyses.

Date Episode Analyses SiO2 Al2O3 FeO* MgO CaO K2O Na2O TiO2 P2O5 Total
18 May 1980 1 9 64.13 17.61 4.04 1.88 4.90 1.26 4.63 0.58 0.15 99.18
25 May 1980 2 11 64.19 17.92 3.99 1.91 5.06 1.29 4.83 0.60 0.15 99.94
12 Jun 1980 3 9 63.72 18.04 4.24 1.99 5.16 1.25 4.70 0.64 0.15 99.89
22 Jul 1980 4 7 63.49 17.87 4.44 2.20 5.22 1.26 4.97 0.57 0.15 100.17
07 Aug 1980 5 10 63.28 17.51 4.39 2.17 5.30 1.23 4.89 0.64 0.16 99.57
Figure (see Caption) Figure 13. Average of MgO, left, and CaO, right, concentration in rocks from each eruptive episode at Mt. St. Helens, 18 May-7 August 1980, plotted against time of each episode. Analyses are of fused powders by the electron microprobe. Numbers in parentheses are the number of samples analyzed and included for average value. Total iron calculated as FeO.

Geological Summary. Prior to 1980, Mount St. Helens was a conical 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 breached crater now partially filled by a lava dome. There have been nine major eruptive periods beginning about 40-50,000 years ago, and it 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. Eruptions in the 19th century originated from the Goat Rocks area on the N flank, and were witnessed by early settlers.

Information Contacts: T. Casadevall, C. Newhall, D. Swanson, USGS, Vancouver, WA; R. Tilling, USGS, Reston, VA; S. Malone, R. Crosson, E. Endo, University of Washington; W. Melson, SI.