Report on St. Helens (United States) — October 1990
Bulletin of the Global Volcanism Network, vol. 15, no. 10 (October 1990)
Managing Editor: Lindsay McClelland.
St. Helens (United States) Explosion from N side of lava dome; ash plume and small mudflow
Please cite this report as:
Global Volcanism Program, 1990. Report on St. Helens (United States) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 15:10. Smithsonian Institution.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
At 0207 on 5 November, the start of a brief explosive episode and ash emission was signalled by 2 minutes of low-amplitude seismicity, followed by an increase to high-amplitude seismicity and the failure of several sensors on the summit dome. Pilots reported the plume at altitudes of ~7.5-9 km traveling SE at 90-110 km/hr; ash was reported as far as Fossil, Oregon (~200 km SE). Strong seismicity lasted for 6 minutes, then decreased to normal levels over the following 2-3 hours.
Geologists visiting the crater that day found that the explosive activity took place at a vent on the N side of the lava dome. Two seismic stations and a steel tower were destroyed, but others continued to function. Hot dome blocks and finer-grained material blanketed the snow on the crater floor, NW and N of the dome; blocks up to 2 m in diameter were scattered on the lower part of the W crater wall (NW of the dome). Rock avalanches and hot debris from the explosions moved down the N side of the dome and across the crater floor, abrading and melting snow and ice. The resultant small debris flow traveled out of the crater into the North Fork of the Toutle River, where it formed a small mudflow that extended 16-19 km.
Fine tephra was collected from the extreme limit of deposition, but had not yet been analyzed at press time. Small quantities of fresh-appearing glass had been found in tephra emitted on 6 January (SEAN 14:12).
No precursory events to the 5 November activity have been identified, although two distinctive "cigar-shaped" events (closely spaced, small, shallow earthquakes with concurrent tremor) lasting several hours were recorded 25 and 26 October. Similar signals were associated with the 6 January ash emission, and were recorded 24 September, when no ash was emitted. These signals have been identified at Old Faithful Geyser (Yellowstone Caldera, USA), and Ruiz (Colombia), where they are thought to represent hydrothermal venting or near-surface movement of fluids.
The 5 November ash emission was very similar to the previous explosive events on  December and 6 January (SEAN 14:11 and 14:12). An event on 25 April produced similar explosion-type seismic signals, but bad weather prevented observations and no ash or eruption plume was reported (BGVN 15:04). Each of the events was short-lived (up to 18 hours) and produced little ash. Although the January episode also caused rock and snow avalanches, the November activity was the first to produce a mudflow in the last two years.
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: W. Scott and S. Brantley, CVO; SAB.