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.
The closest large city to Mt. Rainier is Tacoma, a metropolitan area that has experienced rapid population growth in the last 20 years. And, according to Scott, land between the base of the volcano and Puget Sound has been filling with population because of expanding suburbs. The known paths taken by past lahars go directly through some of the areas now occupied by suburbanites.
Although eruptions tend to begin slowly, providing ample time to evacuate people, mudflows can be triggered by events other than eruptions. For this reason lahars are difficult--perhaps even impossible--to predict.
"Possible triggers are regional earthquakes, steam explosions or small earthquakes related to crustal loading by the mass of the volcano," Scott said.
It is suspected that the greatest risk comes from the large, gravity-induced flows that seem to occur every 500 to 1,000 years. These landslide-like events may be linked to internal weakening of the mountain's structure.
"Mt. Rainier has been weakened and altered by the constant percolation of hot, chemical-laden waters" through the volcano's interior, Scott explained. As a result, "large landslides, or 'sector collapses,' of this weakened material" begin when the rocks suddenly fail, loosening material that starts streaming downhill.
Zimbelman, who has been studying the chemical changes occurring inside Mt. Rainier, said that chemical alteration "weakens the edifice," the mountain itself. "Volatile gases going up through the rock vertically produce acidic fluids which eat away at the rock." As a result, "you have zones of weakness" that can suddenly collapse.
Alternatively, actual volcanic eruptions can touch off lahars. A summit blast, even a steam explosion, can melt enough ice and snow to set off a large mudflow, the volcano experts said. And, as with other known volcanoes, the sudden release of a small lake's worth of water can be a triggering event.
A major part of the safety problem, of course, is that nobody knows how much danger there really is, or how soon massive mudflows may come cascading down the volcano. But researchers such as Scott are certain the danger is real. The volcano itself has not gone to sleep, and geological studies show that mudflows are a repeating and persistent hazard.