Mount St. Helens
- Current Update, last updated Feb 14, 2011 17:48 PST:
The 40-month-long lava-dome eruption of Mount St. Helens that began in autumn 2004 ended in late January of 2008. Earthquakes, volcanic gas emissions, and ground deformation are all at pre-eruptive background levels. The alert level and aviation color code were reduced to NORMAL/GREEN on July 10, 2008, following five months with no sign of renewed activity.
Even with the end of lava dome growth, some hazards persist. The new lava dome remains hot in places and capable of producing small hot avalanches or minor explosions that could dust areas with ash up to tens of miles downwind. Rock fall from the crater walls can produce clouds of dust that rise above the crater rim, especially during dry, windy days, as has happened in the past. Also, heavy rainfall or rapid snowmelt can send small debris flows onto the Pumice Plain north of the crater.
An M4.3 earthquake struck the Mount St. Helens region this morning, 14 February 2011, at 10:35 a.m. PST (18:35 UTC) and was felt widely through southwestern Washington and Northwestern Oregon (http://earthquake.usgs.gov/earthquakes/dyfi/events/uw/02141835/us/index.html). Its exact magnitude may change by a few tenths from this value as records are further analyzed. The earthquake was followed by several aftershocks up to M2.8 over the next few hours (http://www.pnsn.org/recenteqs/latest.htm), the three largest of which were also reported felt. All of the earthquakes are located in an area about 8 kilometers (5 miles) north of the crater of Mount St. Helens, near the Johnston Ridge Observatory, at a depth of about 4 to 6 kilometers (2.5 to 4 miles).
Today's earthquakes are in the same place as a small swarm that took place about two weeks earlier, on 29 January. These earthquakes are reminiscent of a swarm that took place about 30 years ago, when a swarm of small earthquakes began in August 1980, a few miles northwest of today's activity. The 1980-1981 sequence climaxed with an M5.5 earthquake on 14 February 1981. Analysis of the 1981 events suggested that they occurred along existing faults in the Mount St. Helens seismic zone, a northwest to southeast trending system of faults in which Mount St. Helens lies. The Mount St. Helens seismic zone exhibits strike-slip motion, with the southwestern rocks slipping horizontally northwest relative to the rocks northeast of the fault zone. The fault zone likely exerts control on the location of Mount St. Helens volcano. Studies following the 1980 eruption suggested that the magma removed during the May 1980 eruption and subsequent lava-dome building caused faults along the seismic zone to slip in response to the magma withdrawal. Similar interaction of volcanic activity and tectonic fault movement is possible in the case of today's earthquakes, but at present there appears to be no signs of unrest in the volcanic system.
The USGS and the Pacific Northwest Seismic Network at University of Washington continue to watch conditions at Mount St. Helens closely.
- Volcanic History Overview: 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.
- Location: Western US, WA
Elevation: 2549 m
Recent Eruption: Ongoing current eruption cycle.
- Hazard Assessments: Wolfe and Pierson, 1995, Volcanic-Hazard Zonation for Mount St. Helens, Washington, USGS Open-File Report 95-497.
- Link to monitoring data: CVO Web Site Volcano Monitoring
- Link to more information: CVO Mount St. Helens Page
Volcanic Alert Level: NORMAL Aviation Color Code: GREEN