Mt. St. Helens before the 1980 eruption …	and in late May, 1980, from the same view.

May 18, 2005, marks the 25^th anniversary of the dramatic 1980 eruption of Mt. St. Helens, near Portland, Oregon. The volcano began spewing ash and steam around noon on March 27, 1980, in the first volcanic eruption in the coterminous United States since the eruption of Lassen Peak in 1917, and erupted in a devastating blast almost two months later. The 1980 eruption marked an awakening for many residents of the Pacific Northwest to the risks associated with living in the shadow of a volcano, but it was not the first or the last time that they will see nearby geologic events.

The eruption was not a complete surprise, by any means. Scientists have long recognized that Mt. St.  Helens is one of a chain of volcanoes that make up the Cascade Range. This volcanic mountain range stretches approximately 600 miles from southern British Columbia to northern California and contains 15 active volcanoes, including Mt. Rainier in Washington and Mt. Shasta in California. The volcanic activity in the region is related to the nearby Cascadia subduction zone, where the Juan de Fuca plate is being subducted beneath the North American plate, releasing volatile fluids into the crust to cause melting. In fact, there has been volcanic activity in the Pacific Northwest for at least 80 million years.

Mt. St. Helens itself began erupting 40,000 years ago, but its current cone is only about 2200 years old, making it the youngest volcano in the Cascades. Its youth was evident in the smooth, symmetrical peak prior to the 1980 eruption (see before picture). Unlike Mt. Rainier, Mt. St. Helens bore little evidence of glacial erosion. The lack of glacial scarring and a series of minor eruptions throughout the 19^th century led a group of USGS scientists to publish a paper in Science in 1975 predicting that Mt. St. Helens was the most likely to erupt before the end of the century (Crandell, D. R., Mullineaux, D. R., and Rubin, Meyer, 1975, Mount St. Helens Volcano: Recent and future behavior: Science, v. 187, no. 4175, p. 438-441).

By 1980, the entire northwest region, Mt. St. Helens included, was heavily monitored with instruments to detect sulfur-dioxide gas emissions and ground temperature, and with seismometers to detect earthquakes. Changes in gas emissions and ground temperatures can signal forthcoming eruptions, and shallow earthquakes are usually precursors to eruptive episodes, indicating that magma is moving up to shallower levels in the crust. The 1980 eruption was no exception. On March 20, 1980, the seismometers measured a magnitude 4.2 earthquake beneath the volcano, followed by hundreds of smaller tremors. Starting March 27, ash began to erupt out of the summit. Small ash eruptions continued (along with minor earthquakes) until April 21. At that point, visible activity ceased.

Steam and ash eruptions resumed with increased vigor on May 7. Clear evidence for magmatic intrusion now showed on the surface of the mountain, where a large bulge was forming and moving on the north side of the summit. The bulge continued to grow until 8:32 am, May 18, when a shallow magnitude 5.1 earthquake triggered its devastating collapse, followed by a lateral blast. Despite all of the monitoring equipment, no one could have predicted that the volcano could produce such an enormous eruption out of its side. The blast removed 1300 feet from the top of the volcano, flattened trees as far as 19 miles away, and produced a scene of complete devastation within an 8-mile radius (see after picture). The ash column reached a height of 16 miles and was dispersed by high-altitude winds, darkening skies in Yakima and Spokane to nighttime conditions. Within 24 hours, the eruption ended, but ash from the eruption circled the globe for weeks.

Fortunately, the region surrounding Mt. St. Helens is sparsely populated and hazard warnings had been issued to evacuate people who did live in the vicinity, and casualties were limited to 57 people near the volcano at the time of the eruption. Eruptions in South America, like the 1985 eruption of Nevado del Ruiz, resulted in the deaths of tens of thousands of people who were living and farming on its slopes. Other volcanoes in the Cascade Range could have much deadlier consequences when they erupt,  for example Mt. Rainier towers outside of Seattle, Washington.

In September, 2004, Mt. St. Helens again began to display signs of activity. Ash and steam began to erupt out of the new cone, and since October, a lava dome higher than the Empire State Building has been built. The ongoing activity continues to remind nearby residents that they live in a geologically active region, where earthquakes and volcanic eruptions are a part of their everyday lives.

For more information about where volcanoes occur, see our module Plate Tectonics II: Plates, plate boundaries, and driving forces.

For more information about past, current, and potential future activity at Mt. St. Helens, see the USGS Cascade Volcano Observatory (http://vulcan.wr.usgs.gov/).

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