Forest fires trigger new type of permafrost landslides in Canada

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There's a new type of landslide affecting northern latitudes, and it could affect more areas as the planet continues to warm.

These landslides, called permafrost detachment slides, occur in areas with relatively thin permafrost and seem to be a long-term impact of wildfire in these environments, according to a paper published Sept. 23 in the journal Geophysical Research Letters . Such slides have not been observed in the region before and may be unprecedented over the past 10,000 years or more, said study leader Joseph Young , who completed the work as part of his doctoral degree at the University of Alberta.

"It has been stable, and then it just flipped a switch, essentially," Young told Live Science.

Young and his colleagues were doing field observations in Canada's Northwest Territories when they noticed large landslide scars in valleys that have long hosted stable permafrost, which is sediment that remains frozen year-round. They began using satellite imagery to map the changes and also conducted fieldwork, which revealed that the slides weren't standard melting permafrost.

"Typically with permafrost, everything just thaws and melts in a slurry and moves like a mud downslope," Young said. "But with these large landslides, we found blocks of frozen material in the debris."

Some of these blocks were 33 feet (10 meters) tall, the height of a two-story house. The researchers ran an electrical current through the ground to determine the depth of the permafrost on the slopes.

They focused on two slides. One, in the Johnson River Valley, took place in 2017 and moved more than 636 million cubic feet (18 million cubic meters) of soil and rock. The other, in the Redstone River Valley, moved almost 159 million cubic feet (4.5 million cubic meters) of debris in 2018. The smaller slide left a scar three-quarters of a mile (1.2 kilometers) long, while the larger one gouged nearly 2 miles (3.2 km) out of the hillside.

The researchers then used the data they'd collected to build a model of how these icy slides occurred. The permafrost in this area has been warming by about 0.54 degrees Fahrenheit (0.3 degrees Celsius) per decade since 1980, when monitoring began, and there were also severe wildfires nearby in 1995.

The researchers found that instead of a top-down melting process by which the permafrost thawed to a muddy slurry from above, the slides were instead caused by a combination of fire-related thaw and warming temperatures, leading to cracks near the top of the slopes. These cracks allowed the thaw to penetrate deeper, eventually melting the bottom of the permafrost layer. This propagation can take 20 years to penetrate 66 feet (20 m), which is exactly what happened in the Johnson River and Redstone River valleys.

"We had a fire in '95 and then 20 years later, we start to get a lot of these widespread disturbances," Young said.

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The two large slides the researchers studied are among more than 150 in the area, Young said. The others are smaller, but they still typically move over 35 million cubic feet (1 million cubic meters) of land apiece. Canada's Northwest Territories are remote, so the permafrost detachment slides have yet to damage any infrastructure. But the two in the study did dam rivers, creating brand-new lakes and rapidly dumping sediment into the waterways downstream, Young said.

There are many other places in Canada and Alaska where the permafrost is 66 to 130 feet (20 to 40 m) deep and thus vulnerable to the same type of slide. This is especially true given the increasing intensity of wildfire at northern latitudes, Young said.

"This is almost a potential glimpse into the future of what we might be able to expect in these areas with changing forest fire dynamics," he said.