The mountain: take a look at Mount Rainier, America's most dangerous volcano

Science News,  Nov 24, 2001  by Sid Perkins

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The relentless growth of populated subdivisions shows that residents-to-be either don't realize they're in an ashflow-prone zone or aren't worried about it.

Some members of the emergency-response community have a different view. "I wouldn't build a home or a school there," says Ed Reed, a program manager with the Pierce County Department of Emergency Management in Tacoma.

To assess the risk from Mount Rainier, scientists have combined remote sensing, geologic mapping, and computer modeling into an evaluation of materials that might be swept up into a lahar.

Volcanic rocks are poor conductors of electricity when they are newly formed, but as they weather and become saturated with water, they conduct electricity up to 300 times better, explains Thomas W. Sisson, a volcanologist with the U.S. Geological Survey in Menlo Park, Calif. Also, fresh volcanic rocks are slightly more magnetic than ones that have been weathered and weakened, he notes. Sisson and his colleagues have flown helicopters equipped with electromagnetic detectors at low altitude over the mountain to locate such degraded rocks, which might crumble and contribute to a mudslide.

By broadcasting radio waves along flight paths about 250 m apart, the researchers mapped the mineral degradation on Mount Rainier. They found that only the upper, west slope of the volcano has an appreciable thickness of weakened rock. Most material of this kind fell off the mountain 5,600 years ago in the Osceola Mudflow, says Sisson. The horseshoe-shape crater left behind--similar to the one left by Mount St. Helens' 1980 eruption--faces east, he notes, and it filled up with fresh lavas that today form a relatively strong, stable core. Sisson and his team reported their findings in the Feb. 1 NATURE.

He and another group of USGS colleagues recently extended that research. By combining the distribution of weakened rocks with geologic maps that show the steepness of the terrain, they constructed a computer model of the mountain. Then they sliced the model at nearly 30 million different combinations of angle and depth and calculated the capability of the rocks below the slices to resist the force of gravity and hold up the mass of rocks above. In other words, they estimated whether the rocks could prevent a landslide.

Even though the mountain's north face is the steepest, Sisson and his team found that the upper, west side of the volcano would be most likely to produce lahars that contained more than 0.1 cubic kilometer of material, an amount about one-third the size of the Electron Mudflow. The team published its results in the September GEOLOGY. These findings may allow emergency-response officials and scientists to concentrate their monitoring on the portions of Mount Rainier that most threaten the surrounding population.

Sisson and his colleagues are now turning their remote sensors to Mount Adams and Mount Baker, two other Washington State volcanoes in the Cascade range. Some of the rocks on Mount Adams are about 30,000 years old, and some of those on the slopes of Mount Baker were deposited there about 18,000 years ago. By comparing the depths to which the minerals are weathered, it may be possible to accurately estimate the rate at which rocks lose their strength and capability to support the material upslope.