Report on St. Helens (United States) — 29 September-5 October 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 29 September-5 October 2004
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2004. Report on St. Helens (United States). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 29 September-5 October 2004. Smithsonian Institution and US Geological Survey.
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
Shortly before noon on 1 October, Mount St. Helens emitted a plume of steam and minor ash from an area of new crevasses that had opened in a portion of the crater glacier between the headwall of the 1980 crater and the lava dome. This marked the first eruption from Mount St. Helens since a series of phreatic explosions during 1989-1991. The area of the new vent, located at the southern base of the lava dome, had become increasingly crevassed and uplifted over the previous few days. The event lasted from 11:57 to 12:21 PDT and created a pale-gray cloud that reached an altitude of about 9700 ft (from pilot reports) and drifted SW. USGS scientists making thermal measurements witnessed the emission and noted that the clouds were not particularly hot. Blocks of rock and ice ejected by the event fell in the crater and rim areas. The emission was accompanied by an abrupt drop in seismicity, which remained at low levels.
Prior to the eruption, on 29 September CVO raised the Alert level to 2 (out of 3) due to a significant increase in seismicity overnight. The Volcano Alert was raised to the highest level on 2 October due to a change in the type of seismic signals (50-minute-long tremor) that occurred immediately after a small steam emission at 1215 that day. A small 2-minute-long eruption occurred around noon on 4 October from the vent just S of the lava dome, sending a steam and minor ash plume to an altitude of about 3 km. It drifted SW accompanied by minor ashfall in areas close to the volcano.
During the evening of 3 October, seismicity increased until a steam (and possibly ash) emission around 2240. The plume barely rose to the crater rim. On 4 October, there were 30- and 10-minute-long steam-and-ash emissions at 0943 and 1410, respectively. The larger emission dusted roads SE of the volcano with ash. The maximum thickness of the ash at 8 km distance was 0.2 mm. Neither event generated earthquakes or an explosion signal. CVO scientists inferred that the eruption occurred because hot rock was pushed up into the glacier, melted ice, and generated the steam. On 5 October earthquake energy slowly increased to previous high values.
Shortly after 9:00 a.m. PDT on 5 October, the most vigorous steam and ash emission of the current period of activity began. The emission originated from the same vent as have others this past week, as well as from another nearby new vent in the intensely deforming area on the south side of the 1980-86 lava dome. Steam clouds billowed from the crater for more than one hour. Ash content varied with intensity of steam jetting from the vent, and ash plumes at times billowed above the 1980 crater rim. For the first time, ash content was sufficient that it was detected by National Weather Service Doppler Radar. Steam and ash clouds reached about 12,000 feet and drifted NNE. Media reports indicated that a light dusting of ash fell in Morton, Randle, and Packwood, Washington, towns ~30 miles N of the volcano. There were no reports of ash falling at greater distances.
Following the 5 October steam-and-ash eruption, seismicity dropped to a low level and remained low. Low-level tremor observed following the eruption also gradually declined. Lack of earthquake and rockfall signals suggested that deformation of the uplift area on the south side of the 1980-86 lava dome had slowed. Brief visual observations the morning of 6 October from Coldwater Visitor Center showed weak steam emissions from the crater. Because the USGS inferred that the vigorous unrest of the past few days had lessened and that the probability of an imminent eruption that would endanger life and property was significantly less than at any time since 2 October, the alert level was lowered to Volcano Advisory (Alert Level 2).
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.