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Report on St. Helens (United States) — December 2006

St. Helens

Bulletin of the Global Volcanism Network, vol. 31, no. 12 (December 2006)
Managing Editor: Richard Wunderman.

St. Helens (United States) Continued lava-dome growth through 2006

Please cite this report as:

Global Volcanism Program, 2006. Report on St. Helens (United States) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 31:12. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200612-321050

St. Helens

United States

46.2°N, 122.18°W; summit elev. 2549 m

All times are local (unless otherwise noted)

The current and ongoing eruption of the St. Helens started on 11 October 2004. Extrusion of the growing dacitic lava dome has continued in the same quiescent but sustained mode exhibited throughout the first half of 2006 (BGVN 31:07) . Levels of seismicity have remained generally low, with low emissions of steam and volcanic gases and minor production of ash.

From 26 July through 3 October 2006, the lava dome continued to grow and produce small rockfalls accompanied by minor earthquakes. M 3-3.6 earthquakes occurred on 26, 28, and 31 July. Resulting dust plumes rose well above the crater rim. A steam plume was observed rising from the growing lava dome on 13 August. During 16-22 August, based on interpretations of seismic data, spine extrusion from the dome continued in conjunction with small earthquakes and rockfalls. By mid-August 2006 the dome's volume was about 85 million cubic meters growing at an average rate of less than 1 m3/s. The lava dome's height above the 1986-crater floor started at 396 m. On 9 and 10 September, five shallow earthquakes greater than M 2 occurred in association with the growing dome. A period of relatively low seismic activity followed.

From 20 September through 3 October, lava extruded slowly from the vent onto the S crater floor; there was only low seismicity that generated occasional rockfalls as talus sloughed off the flanks of the growing dome. The rate of dome deformation was low. There was no change in rock chemistry, suggesting little to no change in eruptive style. The lack of explosive activity coupled with continuing low number of earthquakes and small quantities of volcanic gas indicate that the risks posed by the hazards are currently relatively low.

During October, lava continued to extrude onto the S crater floor of St. Helens and observations and data from deformation-monitoring instruments showed the dome continued to grow. Low seismicity and slight tilting of the crater floor produced small rockfalls. A small steam plume was visible on 9 October. On 22 October, an M 3.5 earthquake triggered the collapse of material from the largest of the lava-dome spines. The resulting ash plume rose to about 3.2 km and quickly dissipated to the W. On 29 October, a M 3.2 earthquake was accompanied by a rockfall that produced a small plume. The plume filled the crater to just above the rim and quickly dissipated.

Throughout November and December, data from deformation-monitoring instruments showed that during 1-7 November, the lava dome continued to grow. Inclement weather prohibited visual observation during most of the reporting period. On 5 and 6 November, acoustic flow monitors recorded rain-induced debris flows within the crater and in the upper part of the North and South Fork Toutle River valleys. Seismicity continued at low levels, punctuated by M 1.5-2.5, and occasionally larger, earthquakes. On 21 November, views from an aircraft and a crater camera showed that an active spine continued to extrude. On 18 December, a steam plume rose several hundred meters above the rim and was visible from the Portland area, about 80 km away.

Scientists working on the "old part" of the new lava dome found evidence to suggest that the lava dome was essentially solidified within several hundred meters beneath the crater floor. The outer 2-3 m of the lava dome was composed of ground rock that transitions to solid rock with numerous fractures. These findings support the stick-slip model of lava dome extrusion. If the model is correct, it may help explain the origin of many of the million plus small, shallow earthquakes as the result of numerous sub-surface slips that created the ground and fractured rock. Scientists have also noted that for short periods (hours to perhaps a day) part of the growing lava dome appears to stick (no movement detected in photographs) and then restarts again after high-M 2 to low-M 3 earthquakes.

Alert Level terminology. On October 1, the alert-level system for all volcanoes monitored by the USGS was changed to a descriptive system (table 9). In the new system, "Normal" indicates background conditions are stable; this is equivalent to aviation color-code Green. The previous alert levels of Volcanic Unrest (Alert Level 1), Volcano Advisory (Alert Level 2) and Volcano Alert (Alert Level 3) have changed to "Advisory," "Watch," and "Warning," respectively. There is a minor additional change for the aviation color-code definitions in that there is no longer an ash-plume threshold given for either Orange or Red. The ash-plume height threshold of 25,000 ft. or less for aviation warning condition "Orange" is no longer mandatory; condition "Red" was for ash above 25,000 ft. Now the height threshold can be adjusted for each case.

Table 9. Revised nomenclature and descriptions of volcano Alert Levels for all volcanos in the Cascade range. Courtesy USGS-CVO.

Old Numerical Level New Descriptor Aviation Color Code
Background conditions are stable Green
1 "Unrest" "Advisory" Yellow
2 "Advisory" "Watch" Orange
3 "Alert" "Warning" Red

Throughout the period covered by this report, the hazard status for St. Helens remained at Volcano Advisory Alert Level (2) "Watch;" aviation color code Orange. The alert-level "Watch" is used for two different situations: (1) heightened or escalating unrest indicating a higher potential that an eruption is likely, but still not certain; or (2) an eruption that poses only limited hazard. Descriptor definition "Watch" fits the current lava-dome eruption at St. Helens.

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: Cascades Volcano Observatory (CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/).