ANCHORAGE, Alaska Scientists say they’ve discovered a massive landslide in an uninhabited area of eastern Alaska that’s the largest detected in North America since the 1980 eruption of Mount St. Helens.
The Oct. 17 slider unleashed 200 million tons of rock down the Taan Fiord valley onto Tyndall Glacier in Icy Bay, according to Columbia University’s Lamont-Doherty Earth Observatory. No one witnessed it, but scientists picked up its “seismic signature” with a method of reading patterns they’ve been honing for six years.
“We’re reading the data, processing the data, in a way that lets us detect the landslides and figure out where they are,” Lamont geomorphologist Colin Stark said Tuesday.
The landslide had a magnitude of 4.9 and was confirmed by satellite images taken a week later, according to Stark. Scientists say the images appear to show tsunami damage along the sides of the fiord.
As large as it was, however, the slide was a distant second from the Mount St. Helens eruption in Washington state that accounted for a 2.5 billion-ton landslide from a flank collapse, he said Tuesday.
Because of the difference in size, the two debris dumps are not really comparable, but the Washington event was included as a frame of reference, Stark said.
“This landslide in Alaska, in Tyndall, was a pretty normal landslide,” he said. “But the Mount St. Helens was pretty freakish.”
The Alaska landslide, whose discovery was announced Friday in San Francisco at the American Geophysical Union meeting, occurred in a section of the state that’s considered one of the world’s most seismically active areas. Lamont scientists say mountains are still developing there and the fragile nature of the rock accounts for a faster erosion rate.
Also contributing to the instability at the landslide site, the Tyndall Glacier has retreated more than 10 miles since the early 1960s, the scientists said. As a result, the weak rock on the valley wall is prone to collapse, they said.
Stark was hesitant to link the landslide to climate change, but considers the glacier retreat a factor, saying it would be like removing scaffolding from a building under construction.
Despite its magnitude, the landslide went unfelt at the closest inhabited location about 20 miles (32 kilometres) to the south at the Icy Bay Lodge, a hunting and fishing business. Todd Robertson, one of the lodge owners, said high tides in the area were noted, but lodge workers still there figured that was result of heavy rains and wind.
That kind of weather is not unusual during that time of year, according to Robertson. But noticeably more debris, including small uprooted trees, has been washing up along the beach, he said.
“That’s unusual,” Robertson said.