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

St. Helens

Scientific Event Alert Network Bulletin, vol. 5, no. 8 (August 1980)
Managing Editor: David Squires.

St. Helens (United States) One small explosion; lava dome growth stops

Please cite this report as:

Global Volcanism Program, 1980. Report on St. Helens (United States) (Squires, D., ed.). Scientific Event Alert Network Bulletin, 5:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198008-321050

St. Helens

United States

46.2°N, 122.18°W; summit elev. 2549 m

All times are local (unless otherwise noted)

USGS personnel measured temperatures in the 7 August pyroclastic-flow deposits of 647°C near the crater and 639°C near their distal margin the day after emplacement. By 8 August a lava dome had filled the inner crater formed during the 22 July explosions to about half its former depth of almost 100 m. More than 20 m of additional dome growth had taken place by the morning of 9 August. Occasional bursts of vapor and ash rose to 3-6 km altitude on 8 August, accompanied by small seismic events. SO2 emission increased from about 900 t/d on 8 August to at least 2,000 t/d on the 9th, but explosive bursts had ended and seismic activity was very low.

During the next several days, the rates of CO2 and SO2 emission fluctuated substantially (table 1). The dome appeared to have risen slightly between 9 and 11 August, but no growth has been observed since. The deformation monitoring line between the crater and a ridge W of Spirit Lake shortened about 3 cm/day from 8-13 August, a rate typical of previous inter-eruption periods since measurements resumed in mid-June. No significant changes in ground tilt around the base of the volcano have been recorded since June.

Table 1. Daily rates of CO2 and SO2 emissions from Mt. St. Helens during 10-29 August, measured in t/d by remote sensing equipment. No data are available for 18, 27, and 28 August. Data for 15 August does not include gas released by the explosion.

Date CO2 SO2 CO2/SO2
10 Aug 1980 3,100 600 5.2
11 Aug 1980 5,100 900 5.4
12 Aug 1980 2,100 650 3.2
13 Aug 1980 19,000 3,400 5.6
14 Aug 1980 8,700 1,600 5.4
15 Aug 1980 2,400 800 3.2
16 Aug 1980 less than 3,000 ~1,000 less than 3.0
17 Aug 1980 4,200 1,500 2.8
18 Aug 1980 -- -- --
19 Aug 1980 3,700 1,300 2.8
20 Aug 1980 3,300 1,900 1.7
21 Aug 1980 6,900 2,600 2.7
22 Aug 1980 11,000 2,000 5.5
23 Aug 1980 5,500 1,800 3.1
24 Aug 1980 6,800 1,250 5.4
25 Aug 1980 2,100 520 4.0
26 Aug 1980 3,900 1,400 2.8
27 Aug 1980 -- -- --
28 Aug 1980 -- -- --
29 Aug 1980 6,000 1,000 6.0

On 15 August at 1437, an ash-rich cloud rose to about 1 km above the volcano. The cloud became gradually less ash-laden, and dissipated after less than 15 minutes. Volcanic tremor was recorded during the eruption, declining as the eruption waned, but there was no premonitory seismicity. The activity blasted a small crater in the W side of the dome but did not destroy it. A similar eruption had occurred 28 July.

The surface of the lava dome was about 7 m lower on 17 August than it had been before the 15 August explosion. No shortening of the deformation monitoring line occurred between 13 and 17 August, but shortening resumed 19 August, indicating renewed inflation. Seismicity was limited to infrequent very small shallow events, many of which probably represented rockfalls in the crater.

On 22 August, a small quantity of water from Maratta Creek, a tributary of the North Fork of the Toutle River, breached a portion of the nearly 30 km-long debris dam left in the Toutle by the 18 May eruption. Water and debris flowed about 3 km downstream where it formed a small lake with an estimated volume of 3.8 x 105 m3. Five days later this lake overflowed, moving nearly 10 km down the Toutle valley to Camp Baker, site of a partially completed dam project designed to control the much larger floods that could occur in the debris-clogged valley when heavy rains resume in the autumn.

Some equipment was damaged at the construction site. Much of the water was contained by the unfinished check dam, but some continued about 15 km farther to the town of Toutle, damaging or destroying some temporary bridges and access roads. No casualties were reported.

Mt. St. Helens remained quiet through early September. Gas emission continued to fluctuate but no explosions occurred. Deformation monitoring lines began to lengthen slightly on 25 August, indicating slight deflation of the volcano, but an average of 2 cm/day of contraction by early September showed a return to more typical gradual inflation. On 30 August, the lava dome was the same size as on 9 August. Incandescence could be seen through deep cracks in its surface and through the small crater formed by the 15 August explosion. Cracks in the walls and floor of the inner crater containing the dome also revealed incandescent material. Seismicity remained generally weak in late August and early September. A brief earthquake swarm on 4 September began with a M 2.5 event at 2046, followed by four shocks within the next 9 minutes. All were centered 2-4 km beneath an area about 8 km NNW of Mt. St. Helens and were believed by the USGS to be of tectonic origin.

Monitoring of material ejected into the stratosphere by the 18 May eruption continues. Results from NASA's lidar at Wallops Island, Virginia show that the layer at 18 km had become more diffuse by August, occupying a zone between 16 and 20 km. In Tucson, Arizona, Aden and Marjorie Meinel continued to observe a weak layer at this altitude during August sunrises and sunsets. A NASA P-3 aircraft will collect data on the 18 May material during a mid-September cross-country flight, timed to coincide with information-gathering by NASA's SAGE satellite as it passes over the Northern Hemisphere.

Two reports in the 5 September issue of Science present data on the petrology, chemistry, and size distribution of the 18 May tephra. Hooper and others emphasize the bimodal character of ash deposited about 400 km ENE of Mt. St. Helens. The ash changed abruptly from a relatively dark, glass-poor silicic andesite to a lighter-colored glass-rich rhyodacite 3.25-3.5 hours after ashfall began. This interval corresponds quite closely to the timing of changes observed in the character of the eruption column at the volcano SEAN 05:05). Fruchter and others present bulk analyses of ash collected at numerous locations in Washington. In addition to petrology and major and trace element chemical analyses, the report focuses on the tephra's toxic and radioactive components, which do not appear to have been abundant enough to have a significant effect on animal or plant life.

The July 1980 issue of the Washington Geologic Newsletter lists Mt. St. Helens research projects being carried out by 25 groups at 20 institutions and government agencies, in addition to giving sources for pre- and post-eruption maps and airphotos of the volcano. In addition, names and addresses of USGS personnel with the particular Mt. St. Helens study on which each was working as of 20 June are listed. An eruption chronology, and reports on ashfall distribution and petrography are also presented in this publication.

Pre- and post- 18 May photographs and satellite images of Mt. St. Helens and vicinity are available from the EROS Data Center, Sioux Falls SD 57198. Among these are about 1,500 photographs taken by USGS personnel in low-altitude aircraft; color infrared photographs taken at 18 km altitude from NASA's U-2 aircraft on 1 May and 19 June; and cloud-free LANDSAT images.

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. Peterson, USGS, Vancouver, WA; R.Tilling, USGS, Reston, VA; S. Malone, R. Crosson, E. Endo, University of Washington; M.P. McCormick, NASA Langley Research Center; A. and M. Meinel, University of Arizona; UPI.