NATURE : Scientists See Cause for St. Helens to Blow Steam

An increase in seismic activity this year and the heaviest rainfall in the Northwest since the big 1980 eruptions at Mt. St. Helens in Washington state have generated concern among scientists that there could be a renewal of volcanic activity there for the first time since a series of steam explosions in 1991.

Dan Dzurisin, the scientist in charge of the Cascades Volcano Observatory at Vancouver, Wash., explained that water percolating down onto hot rocks below the surface could cause such explosions, even if no new molten rock were to rise to the surface.

And Dzurisin said water-resource experts of the U.S. Geological Survey, which operates the observatory, believe that so much rain has fallen that it constitutes “something between a 100- and a 500-year flood.” In other words, such flood conditions can be expected to recur in the region less frequently than every 100 years.

Flash-flooding downstream has been a considerable worry since May 18, 1980, when the most powerful eruption in the Northwest in centuries loosened masses of sediment and created two large lakes behind unstable debris dams, which could yet give way. But from 1980 to the fall of 1995, rainfall was comparatively light for the region.

The latest issue of the Smithsonian Institution’s Bulletin of the Global Volcanism Network, which covers volcanic activity around the world, reports that the number of tiny earthquakes (about magnitude 1) occurring underneath Mt. St. Helens increased from about 10 in January to about 100 in September.

This increase of quakes between one and seven miles below the surface is small compared to the shaking about two miles down that occurred before each of the explosive, dome-building eruptions at the mountain from 1980 to 1986, the bulletin said.

Still, the bulletin notes, the present zone of seismicity “presumably marks the approximate location of the magma conduit system” underneath the volcano, and it may indicate that volcanic gas has become concentrated around that channel, increasing the pressure. This in turn could lead to fracturing of rocks and sudden buildups of gas, which can cause steam explosions.

The seismicity has declined slightly since September, Dzurisin said, noting that the U.S. Forest Service, taking heed of the scientists’ warnings, decided this fall to close two trails to hikers in the Spirit Lake region just north of the volcanic crater.

The scientists had said it was possible that a steam explosion could occur without ample warning, posing dangers to hikers along the much-used trails.

Sediments indicate that Mt. St. Helens has erupted twice as often in the last 4,000 years as has any other volcano in the Cascade Mountain range. The eruptions of 1980, the first in 123 years, blew the top 1,300 feet of the mountain away, and subsequent extrusions of lava created a 920-foot dome inside the remaining crater.