More than 100 million tons of rock slid down a mountainside in Southeast Alaska on Tuesday morning, sending debris miles across a glacier below and a huge cloud of dust into the air, a scientist says.
It's unclear what exactly caused the 4,000-foot-high mountainside to collapse northwest of Juneau in Glacier Bay National Park, but the mountains in the area are generally young, unstable and eroding quickly, said Colin Stark, a geophysicist at Columbia University's Lamont-Doherty Earth Observatory in New York.
"It rivals anything we've had in several years," Stark said Saturday.
Stark said he plans to spend several days in Alaska during the coming week to study the landslide and collect photographs and samples.
Stark studies the physics of landslides and described the one Tuesday as "exceptionally large." His team at Columbia discovered the landslide through seismic recordings.
According to their preliminary analysis of the seismograms and available imagery, the landslide started at 8:21 a.m. Tuesday when the rock face collapsed on a high, steep slope. For nearly a minute the debris accelerated down the mountain, hitting the ice on Lamplugh Glacier and pushing up snow and ice as it continued across the glacier, Stark said.
He said rough estimates put the size of the slide at about 130 million tons, comparable to roughly 60 million medium-size SUVs tumbling down the mountainside.
"That's a lot of SUVs," Stark said. "It's huge."
Paul Swanstrom, a pilot and owner of Haines-based Mountain Flying Service, saw the aftermath of the landslide from the sky Tuesday about two hours afterward.
Swanstrom said the slide happened roughly 59 miles southwest of Haines. He was on a routine flightseeing tour when he noticed a cloud of dust over Lamplugh Glacier.
"I could tell it was quite big," he said.
Swanstrom said he has flown in Southeast Alaska for more than two decades and has come upon smaller landslides, but this one was massive.
He called the park headquarters and took photographs for the next few days, better understanding the magnitude of the slide as the dust settled.
Michael West, Alaska state seismologist and director of the Alaska Earthquake Center at the University of Alaska Fairbanks, said the landslide was equivalent to about a magnitude-5.5 earthquake.
"That puts it into the range of things we see every several years," he said.
Seismic waves from the landslide were measured as far away as Barrow and Nome.
"It's a longer rumble than an earthquake, but it's a rumble," he said.
West said Tuesday's landslide was particularly huge, but smaller ones are typical for that area of Southeast Alaska. The mountains there are some of the fastest-growing in the world, he said, partly because they are rebounding from the last ice age.
"As that ice has melted away, the mountains have sort of floated, or are buoyed up because they've been unloaded," he said.
He said the area's mountains are young, dynamic and eroding quickly — an unstable combination.
Stark said he worries about the hazards created by the instability.
About 10 miles away, the glacier ends in Johns Hopkins Inlet, a stop for cruise ships. Tuesday's landslide did not reach the inlet, but last fall a similarly sized landslide crashed into remote Taan Fiord, a finger of Icy Bay, and created a huge tsunami wave, Stark said.
No humans were around, he said, but what if a cruise ship had been nearby?
Stark took a team to Taan Fiord in the spring and will return in August. He said he hopes that by studying the landslides, scientists will be able to better assess the hazards in Alaska and elsewhere.