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Forget Eruptions, Scientists Say; Rainier's Real Weapon Is Mud : Cascades: Rapidly growing Seattle suburbs are in harm's way, experts warn, and massive lahars cannot be predicted. Huge mudflow in Colombia in 1985 killed 25,000 people.

October 08, 1995|ROBERT COOKE | NEWSDAY

Like duckpins neatly lined up to be bashed by a ball, some of the rapidly growing suburbs surrounding Seattle are standing in harm's way, scientists warn.

Nearby Mt. Rainier--the biggest volcano in the Cascade Range--has a pronounced history of sending dangerous mudflows slithering down its slopes, even when it's not erupting. And the flows--called lahars--are sometimes big enough to wreak havoc across wide areas, even reaching Puget Sound, more than 50 miles away.

Geologic evidence shows, for example, that 55 mudflows--five of them massive--have surged down from the mountain in the last 10,000 years. And more will certainly come, experts say, with millions of people now living in the way.

"These debris flows are a natural part of the life cycle of all the Cascade volcanoes," said hydrologist Carolyn Driedger. "They happen on all the volcanoes, and we expect them to happen again at Mt. Rainier. The conditions exist today for such events." Driedger works at the U.S. Geological Survey's Cascades Volcano Observatory in Vancouver, Wash.

David Zimbelman, a geologist with the U.S. Geological Survey in Denver, added, "It's hard to communicate to people in the Pacific Northwest that the main hazard is lahars." When people do voice concern about the huge volcano, he said, "They always ask when's it going to blow? But that's not its history."

Rainier has had eruptions, but they're usually not as explosive as the eruption of Mt. St. Helens 15 years ago. Instead, geologists have come to realize that the greatest danger is posed by massive mudflows that begin without warning.

According to recent studies, in fact, there is a high statistical probability that homes in the mudflow areas face real danger. Buildings erected in the zones identified as vulnerable are calculated to be 27 times more likely to be destroyed by mud than by fire, researchers said.

"People know the danger of fire, so they buy fire insurance and have smoke alarms," said geologist Kevin Scott. "But most people are not aware of the risks of lahars, and few have applicable flood insurance."

Basically, a lahar is a thick, wet debris flow that begins high on the mountain's flanks. Loaded with ice, rocks and mud, the flowing mass gathers speed as it runs downhill, picking up whole trees, huge boulders, even cars, trucks and bulldozers if they're handy. Mixed to the consistency of wet concrete, the lahar then smashes anything--bridges, houses, highways--in its way.

"The destructive force of these flows, moving between 25 and 50 miles an hour, is enormous," said Scott, who works at the USGS Volcano Observatory, in Vancouver, Wash.

Volcanologist Stanley Williams, at Arizona State University, said there is clear evidence that Mt. Rainier "has giant, very young lahar deposits, on which a couple of million people live right now." Unfortunately, "people tend to live on those deposits because they are nice and flat," making it easy to build homes, roads and other facilities.

The dangerous nature of such lahars was vividly--and tragically--demonstrated in South America in 1985, when a large mudslide poured down from the ice-laden summit of the Nevado del Ruiz volcano in Colombia. In the dark of night, the flow suddenly burst from the mouth of a canyon just above the small agricultural town of Armero, obliterating everything, killing almost 25,000 people. In addition to the smashed buildings, dead livestock and human lives lost, most of what wasn't knocked down got buried under a thick, smothering blanket of light-brown mud.

Ironically, Williams said, in Armero, "the people who survived were those who lived in the rough, hilly parts of town--where nobody wanted to live."

The deadly lahar that rushed down from Nevado del Ruiz was triggered by a relatively small eruption, a volcanic burp that only removed about 5% of the ice that blankets the volcano's summit. If the eruption had continued, or had been larger, damage might have been far greater, and far more deadly.

At Mt. Rainier, the largest known mudflow came off the mountain 5,000 years ago. It traveled rapidly downhill, pouring over what is now the town of Enumclaw, eventually going 60 or 70 miles before dumping debris into Puget Sound.

Mt. Rainier is so dangerous, in terms of lahars, because of its awesome size and weak structure, the geologists said. It stands 14,410 feet tall and its summit carries a huge, permanent load of glacial ice--more ice than all the other Cascade volcanoes in Washington, Oregon and California combined. "There's about 1 cubic mile of ice up there," said Driedger. "The amount of ice on Mt. St. Helens [in 1980] was only about 4% of what's on Rainier." And it is the water locked up in ice that is so important in forming lahars.

In fact, Zimbelman said, "there is a crater lake on Rainier that most people aren't aware of," a potential source of water that could quickly get a lahar going.

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