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

Just after dusk on Aug. 14, at an amphitheater in Mount Rainier National Park's Cougar Rock campground, a deep grumbling sound began to drown out a park ranger who was regaling visitors with an interpretive lecture about the park's natural wonders. The rumble quickly grew to a freight-train-like roar. That's when the ranger ran to a creek near the amphitheater and saw a large flow of mud and debris surging down the normally placid channel.

"It rumbled up on the ridge until about 10:30 [p.m.]," says Jill Hawk, chief ranger at the national park. Although the camp-ground was never threatened, large pulses of rocky mud continued to sweep down the mountainside for more than 5 hours that evening, and smaller clumps followed for the next 5 days. It was a natural wonder all right.

Park rangers and scientists flew over the area on Aug. 15 and determined that the event was simply a landslide, not something worse--like an earthquake or volcanic eruption. A week or more of hot, sunny weather had accelerated the melting of a glacier on the south side of the mountain. The excess water had saturated a steep slope of earth and rock. The soggy mix eventually broke away and raced downhill.

The landslide was small in geologic terms, says Hawk, but it was plenty big enough to scare campers. She notes that the ranger's interpretive lecture that balmy evening turned into an opportunity to discuss the hazards of living in the shadow of a glacier-covered volcano.

Mount Rainier, locally known simply as "the mountain," is the tallest peak in the Cascades, a chain of mountains that parallels the Pacific Coast from British Columbia to northern California. Mount Rainier's summit bears the largest crest of glacier ice on any mountain in the lower 48 U.S. states.

Scientists say the height, steepness, and cubic mile of ice make the steep-sloped peak worth watching. However, they're most concerned about the rapidly growing population in the picturesque valleys of the region, which earns the currently dormant Mount Rainier the title of most dangerous volcano in America.

The landslide of Aug. 14 provides an example of the many powerful surprises that mountains like Rainier can drop on those nearby. The slump of debris began at an altitude of about 2,740 meters above sea level and didn't halt until it had dropped to an altitude of about 762 m, says Patrick T. Pringle, a geologist with Washington State's Department of Natural Resources.

Similar but larger phenomena, called glacial outburst floods, also strike from on high. These torrents, which are sudden releases of water stored within or at the base of glaciers, can contain about 100,000 cubic meters of water, says Joseph S. Walder, a hydrologist with the U.S. Geological Survey's Cascades Volcano Observatory in Vancouver, Wash. That's the volume of several dozen Olympic-size swimming pools. At peak discharge, these glacial outbursts often match stream flow rates experienced only in the worst of floods.

At least three dozen glacial outburst floods have occurred in Mount Rainier National Park during the past century. Bridges, roads, and park facilities have been damaged or destroyed on at least 10 occasions. Even so, Walder notes, the effects of such floods don't normally reach beyond the boundaries of the park.

Mount Rainier's most far-reaching and therefore most dangerous threats derive not from landslides and glacial outburst floods but from its volcanism.

As with most volcanoes, the mountain's past behavior gives a preview to its future hazards. Written history in the area goes back only about 180 years--a period much too short to adequately represent the activity of a volcano that's hundreds of thousands of years old. Indeed, the documentary evidence includes a record of only one eruption, in the 1840s. But the sedimentary evidence--including deposits rife with pumice and volcanic ash, or tephra--suggests that Mount Rainier has erupted at least 11 times in the past 10,000 years.

The 1980 eruptions of southwestern Washington's Mount St. Helens showed that even relatively thin accumulations of tephra can disrupt social and economic activity over a broad region. Downwind, in the eastern part of the state, the communities of Yakima, Ritzville, and Spokane received between 1 and 8 centimeters of ash and came to a near standstill for up to 2 weeks.

More dangerous than tephra are so-called pyroclastic flows, which roll down a volcano rather than towering above it (SN: 1/13/01, p. 21). The hot gases, ash, and rock particles form a dense fluid that travels at 10 to 100 m per second and typically hosts temperatures above 300 [degrees] C. The flows' high densities, velocities, and temperatures blow down, bury, or incinerate everything in their path.

Scientists have found only a few deposits near Mount Rainier that resulted from pyroclastic flows. One such layer that's about 2,500 years old shows up about 12 kilometers southwest of the volcano's summit, and another, 1,000 or so years old, appears about 11 km northeast of the mountain. However, pyroclastic deposits near the mountain may be rare only because the ash flows were often converted into something more dangerous before they left the mountainside, Walder says.

When the hot ash in ground-hugging pyroclastic flows sweeps across glaciers, it can melt prodigious amounts of ice and snow. This water mixes with the ash and other debris to form a lahar, which looks and flows like wet concrete. Lahars can travel at speeds up to 100 km per hour on steep slopes near the volcano and can reach much farther from the volcano than pyroclastic flows can. In 1998, the U.S. Geological Survey issued a report stating that lahars pose a greater threat to communities around Mount Rainier than any other volcanic phenomenon.

More than a dozen volcanic lahars have spewed from Mount Rainier in the past 6,000 years. About 1,200 years ago, a lahar that spilled down valleys on the northeastern slopes of the volcano filled both forks of the White River with 20 to 30 m of debris. The lahar's front edge flowed more than 100 km to reach the spot where the city of Auburn sits today. About 1,000 years before that, a similar lahar filled the Nisqually River southwest of the mountain to depths as great as 40 m and flowed all the way to Puget Sound.

Scientists have discovered the deposits from more than 60 lahars that occurred in the past 10,000 years. Many of these have been so-called cohesive lahars, which form avalanches that consist primarily of ancient volcanic rocks weakened by exposure to the elements. In particular, sulfur gases spewed by the volcano react with rainwater to form sulfuric acids, which gradually break the rocks down into clay.

The largest of Mount Rainier's post--Ice Age lahars was the Osceola Mudflow, which struck more than 5,600 years ago and inundated the White River valley with more than 3.8 cubic kilometers of material. Its leading edge reached all the way to Puget Sound. Deposits from this event now cover about 550 square kilometers and extend as far as the Seattle suburb of Kent.

Another cohesive lahar, dubbed the Electron Mudflow, was spawned by the collapse on the west flank of the volcano about 600 years ago. This lahar was more than 30 m deep when it entered the Puget Sound lowlands near the present-day town of Electron, more than 60 km away.

All of the major river valleys that drain Mount Rainier have been inundated with lahars, says Pringle. Many of the communities northwest of the volcano have been built in whole or in part atop these sediments. The town of Orting, northwest of the volcano, sits on deposits from both the Osceola and the Electron Mudflows.

Evidence of lahars' potential to destroy is directly underfoot. When the debris from these lahars comes to rest, it often swallows entire forests. In the area surrounding Orting, for example, the Electron Mudflow entombed a stand of mature Douglas firs. Excavations during construction of subdivisions and sewers around the town in the past 9 years have exhumed the deeply buried stumps of more than 100 trees. The most impressive stump, Pringle notes, measured more than 7.5 m in circumference about 1 m above the ancient ground level. It would be one of the largest Douglas firs in Washington State if it were still alive today.

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.

Lahars can be triggered by seismic activity or the movement of magma inside a volcano as well as by ash flows sweeping across glaciers. Scientists want to find out how often these debris flows sweep down a mountainside without warning. Sisson says that with one exception--the Electron Mudflow, about 600 years ago--all of Mount Rainier's lahars occurred during periods associated with volcanic activity, when eruptions also laid down tephra deposits.

Maybe an eruption also stimulated the Electron Mudflow but left no evidence, he adds. It's possible that the eruption produced little or no ash, that the tephra fell in winter and then washed away with the spring melt, or that scientists just haven't found the ash deposit yet.

A definite link between lahars and volcanic or seismic activity would be good news for those living in the mountain's shadow. Fortunately, remarks Sisson, "volcanoes usually give warning signs that an eruption's on the way."