Leif Jones
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Some 5,600 years ago, the body of water we call Puget Sound had an arm that extended 30 miles inland from present-day Elliott Bay in Seattle to a point halfway between Auburn and Sumner. Today, of course, that is the Green River Valley—the narrow, flat suburban land of Kent and Renton and the industrial lowlands of South Seattle. It would be reasonable to think that this change happened gradually, but scientists have determined that most of the long-gone stretch of inland sea was transformed by a single event that created 200 square miles of land in a matter of hours, with waves of mud 20 feet to 600 feet high. Imagine a wall the consistency of wet concrete traveling up to 60 mph. This mudflow destroyed everything in its path, uprooting entire old-growth forests. It hit Puget Sound with such force and with so much material that it flowed underwater for 15 miles, maybe farther. An area of hundreds of square miles was covered with mud and debris up to 350 feet deep.
The source of that enormous mudflow, which geologists call a lahar, was Mount Rainier, about 60 miles south- southeast of Seattle. And it could happen again—maybe even tomorrow. The U.S. Geological Survey estimates there is at least a 1-in-7 chance during an average human lifespan that another catastrophic lahar could pour off the mountain. Little if any eruption would be needed to trigger it. There would be little warning near the mountain, and farther away, the most fortunate residents might have up to an hour to flee.
Mount Rainier is potentially one of the most lethal volcanoes in the world. The United Nations has designated it as one of 14 mountains that could cause catastrophic devastation. The U.S. Geological Survey calls Rainier this country's most dangerous volcano. The explosive potential of a 14,411-foot mountain with a history of 40 ash-producing eruptions since the Ice Age should not be underestimated. But scientists agree that far more menacing, and potentially far more deadly, is the scenario of a lahar burying a large part of suburban Seattle. Under some scenarios, the city itself is vulnerable. Officials have made plans to evacuate areas near the mountain, if there's enough warning, but some emergency planners—and housing development regulations, for that matter—regard Mount Rainier as a threat far away.
In the 1950s, Rocky Crandell was a young USGS geologist just out of graduate school when he was assigned to map the area east of Tacoma. He could find none of the glacial till—the dirt, gravel, and rocks that glaciers leave behind—that he expected in the valleys below Mount Rainier. Instead, he found mud deposits—evidence, he theorized, of a massive flood. But where the flood came from and what caused it were a mystery. "I could see Mount Rainier in the distance, but I didn't make the connection for a long time," Crandell said later.
It was 10,000 feet up on the slopes of Rainier that he found his answer. Samples Crandell took there were the same material he had discovered in the lowlands. The flood below could only have resulted from a massive landslide on Rainier. However, he found no evidence of the huge eruption that he thought would be needed to cause such a landslide. Crandell would go on to forecast the 1980 Mount St. Helens eruption—five years before it happened—and become a legend among geologists. But the mystery on Mount Rainier remained.
As Crandell and others pieced together the Mount Rainier event, what they discovered stunned them: Whatever the cause, the landslide had triggered the greatest flood of mud and debris that man has ever known, and it meant that metropolitan Puget Sound was at risk for another catastrophe, one that would dwarf this year's Hurricane Katrina in human toll and perhaps even surpass the Southeast Asian tsunami of 2004, which took more than 200,000 lives.
Areas that could be affected by an enormous debris avalanche and associated lahars similar in magnitude to the Osceola Mudflow 5,600 years ago.
(Based on research by R.P. Hoblitt, J.S. Walder, C.L. Driedger, K.M. Scott, P.T. Pringle, and J.W. Vallance for the U.S. Geological Survey.) |
It took years to map the lahar that Crandell discovered and to realize its full magnitude. When he and his colleagues finished, the scientist named the monster lahar the Osceola Mudflow, after the tiny valley hamlet near Enumclaw where he had found his best samples. Using radiocarbon dating, scientists were able to put the age of those deposits at 5,600 years, hardly a blink of an eye for 700,000-year-old Rainier. And the cause of this cataclysmic event had not been a massive eruption. It had been a very small eruption, and some scientists thought it had not been an eruption at all but only the movement of magma within the mountain. All agreed, however, about what had happened next. The top 2,000 feet of the mountain slid off and almost instantly was transformed into the Osceola Mudflow.
The 1980 eruption of Mount St. Helens vividly demonstrated not only a volcano's explosive power but also its potential to create lethal lahars. The lahars that swept down from Mount St. Helens were so powerful, they picked up entire logging camps—buildings, cranes, trucks, buses, and even trains with flatbed cars loaded with logs. A catastrophic flood followed the lahar's path and flowed into the Cowlitz River. It cascaded into the mighty Columbia River and unleashed a 20-mile-long logjam, with debris floating to the Pacific Ocean. The Columbia, approximately 60 miles from Mount St. Helens—about the same distance Seattle is from Mount Rainier—was closed to shipping for a long period, at a cost of millions of dollars.
