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Taking the Measure of a Deadly Mountain
16 December 1999 7:00 pm
SAN FRANCISCO--New geologic research has exposed which parts of Washington state's dangerous Mount Rainier are most likely to collapse. The studies may help public safety officials improve plans for the inevitable flows of rock and mud that will cascade down the mountain and into populated valleys--deadly events that may occur without warning.
Gracing the skyline southeast of Seattle, Mount Rainier, a 4400-meter volcano, has lain quiet for more than a century. A titanic eruption that would dwarf the Mount St. Helens blast of 1980 is possible within the next century, although rumblings of ascending magma would provide a warning. An equally hazardous but more insidious threat arises from collapses of unstable rock slopes.
Hot acidic water coursed through the flanks of the volcano during its last major eruptive era about 200,000 years ago, creating deep networks of crumbly rock and weak clays, some of which lie under thick shields of hardened lava. Those sections of the volcano, held together loosely by gravity, can fall apart in response to stresses, such as shaking from a mild earthquake. One such event 5600 years ago spawned a rush of rock and mud that overran 550 square kilometers of surrounding land--including the current sites of the towns of Buckley and Enumclaw and the outskirts of Tacoma. However, researchers have had a poor grasp of which parts of Rainier are most precarious and how much debris they might unleash.
Three teams of geologists from the U.S. Geological Survey (USGS) now have some answers. Thomas Sisson of the survey's division in Menlo Park, California, mapped subtle variations in the composition of rock formations and divined their ages by dating radioactive minerals. He found that the youngest and most unstable rocks lie on the western and eastern flanks, while steep walls to the north and south are surprisingly stable. Carol Finn of the USGS branch in Denver, Colorado, studied the electrical and magnetic properties of the volcano by hovering 50 meters above its slopes in a helicopter bearing remote-sensing tools. The signals exposed the depth to which fluids have eaten away at the rocks, allowing Finn to estimate the volume of material that a collapse might disgorge. For instance, the large "Sunset Amphitheater" on the volcano's western face contains about 1.5 cubic kilometers of weak rock, rivaling the size of the debris flow 5600 years ago.
Finally, Mark Reid, also at Menlo Park, plugged Sisson's and Finn's results into a computer model that uses the three-dimensional shape of the mountain and the strength of its rocks to predict the areas and sizes of the most likely slope failures. Reid foresees debris flows to the west that could threaten the 3600 residents of the low-lying town of Orting.
But the researchers cannot forecast the timing of such events. That's unsettling, says geologist David Zimbelman of G. O. Logic in White Salmon, Washington. "We may get no warning whatsoever, so we must try to mitigate the hazard in advance," he says. Safety measures include forbidding development or camping along scenic rivers and building a costly array of barriers to retain mudflows--politically unpopular proposals, Zimbelman notes. "It may take a disaster to make people enforce these measures, and that's sad."