Location: Skamania County, WA
Elevation: 2,549 m (8,363 ft)
Nearby towns: Castle Rock, Olympia, Vancouver, Portland (OR)
Mount St. Helens produces dacitic to andesitic lava flows, pumice, and lahars. Like Glacier Peak, the composition of its magma makes it erupt more explosively than other Cascade volcanoes that erupt andesitic lava.
The areas prone to lahars are determined in part by figuring out where lahars traveled in the past. The map shows the distribution of lava flows and lahars mapped at the surface compared to hazard zones (gray shaded areas). Evidence of repeated eruptions can be found in many of the valleys that drain Mount St. Helens.
ERUPTION AND LAHAR HISTORY
Mount St. Helens was formed from four eruptive stages starting ~275,000 years ago, and intermittent eruptions occur to this day. During one such eruption around 2,000 years ago, lava flowed down the side of the volcano in streams, one stream formed the Ape Caves, a spectacular 13,042-foot-long lava tube on the southeastern flank of the volcano.
From March 16 to May 18 in 1980, a series of earthquakes, steam explosions, and small eruptions at the summit signaled a new eruptive phase of the volcano. By mid-April of 1980, a large bulge of new volcanic material had formed on the north flank of the mountain and moved outward at an average rate of ~5 feet per day.
On May 18th, the cataclysmic eruption was triggered by a magnitude 5.1 earthquake. The bulge collapsed in a series of three massive slide blocks. This bulge collapse generated a chain reaction, starting with the largest avalanche in recorded history (0.6 cubic miles of material, reaching speeds of 60 miles per hour). The removal of this material decreased the pressure holding back the magma and caused the release of gas, large rocks, and smaller particles that moved across the landscape and destroyed most vegetation at an astounding speed of 650 miles per hour. This initial blast caused major lahar flows, pyroclastic flows, and an ash eruption that formed a eruption column 12 miles high and 45 miles across.
In addition to ash, pyroclastic flows and lahars traveled swiftly across the Pumice Plain and down the North Fork Toutle and Cowlitz Rivers, destroying houses and bridges along the way.
Time-lapse reconstruction of the lateral blast, courtesy of Joseph Friedman.
2004 Dome Building
In September of 2004, earthquake swarms, minor explosions, and lava dome growth were observed in the summit crater. For the next 3+ years, lava continued to build up in the crater and generated a lava dome that is 1,500 feet high. This activity continued steadily until late January of 2008.
Mount St. Helens is located along a 65-mile-long zone of intense earthquake activity called the St. Helens seismic zone. Many small to moderate (up to 5.5) magnitude earthquakes occur in this area. Geologists monitor the earthquake activity very closely to look for signals of another eruption.
Sequential lidar imagery of dome building at the summit of Mount St. Helens. Imagery collected between 2002 and 2004.
Time-lapse images of Mount St. Helens dome growth (2004–2008).
Are You Volcano Ready?
- Get to know your local volcano’s hazards
- Register for notifications about the volcano’s activity
- Make a plan to prepare your entire family for an emergency
Visit the USGS website for more information on how to be volcano ready, view interpretive signs, and find lahar evacuation routes.
Click on the image (left) for a link to the poster.