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Report on St. Helens (United States) — January 1991

St. Helens

Bulletin of the Global Volcanism Network, vol. 16, no. 1 (January 1991)
Managing Editor: Lindsay McClelland.

St. Helens (United States) Explosions, avalanche, and flows from lava dome

Please cite this report as:

Global Volcanism Program, 1991. Report on St. Helens (United States) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 16:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199101-321050

St. Helens

United States

46.2°N, 122.18°W; summit elev. 2549 m

All times are local (unless otherwise noted)

After more than 6 weeks of quiet, two episodes of dome activity occurred in early February.

Explosive episode, 5 February. A small explosion from the lava dome occurred on 5 February at 0747 without any apparent premonitory activity. The seismic signal, typical of previous episodes of gas/ash emission, remained strong for ~10 minutes, then continued at lesser amplitude for another 20 minutes. Data from the crater seismic station were lost ~20 minutes after the onset of activity. The eruption plume was initially white, then turned a darker color after ~3 minutes. Ground observations suggested that the eruption cloud reached a maximum of slightly > 4 km above sea level, but airplane pilots reported the top of the cloud at 6-7.5 km altitude, with material drifting ENE at low altitude and ESE at higher elevations by 0800. Images from the GOES satellite first showed a very small plume at 0831. By the next image at 0900, the cloud was clearly detached from the volcano (~15 km to the E) and had a N-S length of ~50 km. The cloud was no longer visible on satellite imagery after 0930.

Deposits in the crater indicated that a fairly large explosion or series of explosions had occurred from the dome, roughly comparable to the 20 December episode, but distinctly smaller than that of 5 November (BGVN 15:10 and 15:11). Most of the tephra was ejected from a cone-shaped upper N flank vent (20-30 m across at its rim) that had been the source of the 5 November episode. A steep wall ~100 m high butressed the S side of the vent, apparently directing ejecta to the N. Ballistic fragments, some as large as 1 m across, formed a very dense deposit extending 1 km NNE, and sparse ballistics were found to 1.5 km from the vent, reaching the lower NE crater wall. A slush flow mixed with volcanic debris, emplaced after the ballistics, formed a long dark ribbon extending N across the crater floor from the dome. The slush flow was followed by a small watery flow. Fine ash found to 0.5 km NW, N, and NE of the dome appears to have been warm upon deposition, but did not melt much snow. Another ash deposit darkened snow on the volcano's outer E flank to 1.5 km from the crater rim. This deposit had a very sharp margin and may have formed from a cold density current. Very light ashfall was reported at Goldendale, ~110 km ESE of the volcano. As in previous episodes, most of the tephra seemed to consist of fragmented dome rock. The vent on the dome's upper E flank that was active in the 20 December episode also ejected a few ballistics on 5 February, and the surrounding area was blackened by ash. This vent, roughly 250-300 m from the N-flank vent, was obscured by steam, but appeared to be roughly 50 m in diameter. Geologists noted that the phreatic eruptive activity is apparently driven by groundwater and may have a geyser-like mechanism.

Avalanche and flow episode, 14 February. Strong seismicity associated with a second episode began on 14 February at 0524. Transmission from the station on the lava dome was lost after < 2 minutes, but vigorous seismicity was detected for ~10 minutes on other nearby instruments; numerous small shallow events then persisted for 3-4 hours before declining to background. In contrast to most previous episodes, the 14 February activity was preceded by an increased number of very shallow earthquakes below the dome. Seismologists noted that these events had low stress drops and were characteristic of cracking. Periods of increased seismicity have previously occurred beneath the dome without an associated eruptive episode.

Field data indicated that the 14 February activity was significantly different from the episode 9 days earlier. A hot rock avalanche, accompanied by little if any explosive activity, generated a lithic pyroclastic flow and a granular snow flow. Where the deposit was essentially snow- and water-free near the dome, it was composed entirely of ash- to block-sized lithic material. The snow flow was apparently inflated and relatively fast-moving. Although no more than 10 cm thick in most places, it left granular snow on top of lithic blocks that were decimeters in size and comprised <50% of the deposit. The flow entered Loowit Channel and continued downslope, rising as much as 5 m on the outsides of bends. As it continued downstream, more of the snow melted, and the flow evolved into a dirty flood that reached Spirit Lake.

The area around the N-flank vent had been significantly modified since 5 February, and the large scarp S of the vent had retreated substantially. No obvious rockfall scar was visible on the dome, but a large bare area on the N flank of the otherwise snow-covered dome apparently fed the granular snow flow. There was no evidence of new ballistic ejecta and no new impact craters were found. A thin blanket of pyroclastic-flow material covered Sugarbowl dome, NNE of the active dome, beyond which were deposits from a surge elutriation cloud. Geologists found no airfall ash beyond the base of the volcano, although there were unconfirmed reports of ash mixed with rain in Wapato, WA, roughly 150 km ENE of the volcano, perhaps transported by very strong winds during the activity. An airplane pilot near the volcano at about 0600 was unable to determine whether a plume was present because of clouds and darkness.

No significant deformation has been detected since the last dome-building episode in 1986, although some apparent gravitational adjustments have been detected. Electronic distance measurements have documented changes in line lengths in the crater, but only after explosive episodes.

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: Edward Wolfe, Richard Waitt, D. Dzurisin, and S. Brantley, CVO; C. Jonientz-Trisler, University of Washington; S. Hamerla Harner, SAB.