SEATTLE — Some of the biggest rock avalanches in years have been roaring off Mount Rainier the past several days, kicking up billowing clouds of dust and propelling rivers of muddy debris nearly two miles down the volcano's flanks.
No one has been injured, but one group of climbers fled as dust descended on their tent after a rockfall Saturday afternoon.
"From my standpoint of looking at the mountain for 20 years, we've probably had rockfalls like this once every five or 10 years," said Stefan Lofgren, lead climbing ranger for Mount Rainier National Park.
Since Friday, at least three major rockfalls and several smaller ones have sloughed off the rocky ridge called Nisqually Cleaver, at an elevation of about 12,800 feet. The one that let loose Saturday afternoon was the biggest.
University of Washington graduate student Max Stevens and his father were about to head out across the Nisqually Glacier to retrieve a GPS instrument used to measure the glacier's movement.
"I heard it first and looked up and saw a house-sized block of rock falling off Nisqually Cleaver," he said. The pair were on skis, and they skedaddled.
"We got out of there," Stevens said. "This cloud of dust just filled the basin."
When it became clear the avalanche wasn't headed in his direction, Stevens watched the debris flow down the glacier. "There were truck-sized blocks of ice flowing like ice cubes in a stream of water," he said. "I was scared for a moment ... but it quickly turned to pure amazement."
Climbing ranger Chris Kalman was at Camp Muir, a popular day-hike destination and climbers' bivouac at 10,080 feet on the mountain's south side, above Paradise. The day was sunny and warm when a sound like an explosion split the air.
"It was huge," Kalman said. "People were pretty much in awe."
Videos posted on YouTube show a muddy mix of ice, rock and snow surging across the glacier and plunging downhill.
Lofgren flew his private plane over the valley Sunday for a better view of the fingers of debris. "Things like this may happen only a few times in someone's life," he said.
Another avalanche hit Monday.
The events were big enough to register on earthquake sensors, and seismologists at the University of Washington called the park to see what was going on.
Though impressive to bystanders high on the mountain, the rockfalls and the muddy flows they spawned don't pose danger to park visitors or people living downstream by the Nisqually River, said Jim Vallance, a volcanologist at the U.S. Geological Survey's Cascades Volcano Observatory.
Mount Rainier has unleashed massive mudslides, or lahars, in the past. But the current avalanches are tiny by comparison. Nor is there any hint of volcanic activity, which would be required to trigger a lahar from Rainier's south side, Vallance said.
Rangers are advising climbers to avoid the Nisqually Glacier, which is not a common route up the mountain.
It's not clear why the avalanches are so large and frequent this year, Lofgren said. It could be related to heavy snowfall, followed by warmer weather — or something else altogether.
"This is just what happens on mountains," he said.
Information from: The Seattle Times, http://www.seattletimes.com