Heavy Rain, Not Volcano, Triggers Rockslides Mount Adams Saturated Cascades Scientists Say
The biggest rockslides in more than half a century at Mount Adams probably were triggered by rain, and there is no sign the 12,276-foot volcano will erupt anytime soon, scientists say.
“The upper parts of the Cascade volcanoes are as saturated with water as they’ve been for some time,” said Richard Iverson, a landslide specialist for the U.S. Geological Survey’s Cascades Volcano Observatory in Vancouver.
“You can expect slides to happen when you combine the weather pattern with the abundance of very weak rock that forms the summit of Mount Adams,” Iverson said.
Rockslides, a common feature of volcanoes in the Cascade Range, may be triggered by a variety or combination of forces, including earthquakes either related or unrelated to volcanic activity, steam bursts, erosion and weather.
Two slides triggered by rain occurred Sept. 17 on Mount St. Helens. The larger carried about 100,000 cubic meters of rock two miles down the north slope from the crater.
The latest slides on Adams, the third-highest peak in the Cascades and second-highest in Washington state, are estimated at 5 million cubic meters of material Aug. 30-31 and 1 million to 5 million cubic meters early Monday. They were the largest since 1921 on the mountain, 100 miles northeast of Portland.
No damage to structures or injuries were reported.
The Klickitat County Department of Emergency Services had an interagency meeting Wednesday to alert county response teams to the danger.
Klickitat County, population 18,000, shares Mount Adams with Skamania and Yakima counties. Trout Lake, a town 14 miles south of the peak, is built on 6,000-year-old landslide deposits.
“We want to keep all the response agencies apprised of the situation,” said Larry Luloff, county emergency management coordinator.
The last volcanic activity at Adams is believed to have occurred at least 2,500 years ago.
“If something really big was growing, there would be a lot more going on. We’ve seen no increase in seismic (earthquake) activity and have no indication something volcanic is happening,” Iverson said.
A Portland electrical engineer, Charlie Hyman, the only known eyewitness to either of the latest slides, said he was awakened by noise on Aug. 31.
“In the distance, about three-quarters of a mile away … I could see a 50-foot-high river of ice boulders,” Hyman said. “Each one was probably the size of a car and was flowing down the stream bed toward my camp.
“I could hear this rumbling, tumbling, cracking sound, punctuated every few seconds by the pop of a tree succumbing to the inevitable.”
Before he could run to higher ground, the slide stopped a thousand feet from his camp. Taking no chances, he packed up, hiked to his car and drove away.
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