Warming and Fire Set Off Canadian Permafrost Slides Carrying House-Size Ice Blocks

Researchers have described a new kind of landslide in Canada's Northwest Territories, in which large blocks of frozen ground break loose and slide downhill. The slides, called permafrost detachment slides, occur where permafrost is relatively thin and appear to be a long-term effect of wildfire, according to a paper published Sept. 23 in Geophysical Research Letters, as reported by Live Science.

Study leader Joseph Young, who did the work as part of his doctoral degree at the University of Alberta, said such slides have not been seen in the region before. He said they may be unprecedented over the past 10,000 years or more. The team warns that the same thing could happen in other warming northern areas.

The key numbers

• 33 feet (10 meters): the height of some of the frozen blocks found in the debris, about the height of a two-story house.
• More than 636 million cubic feet (18 million cubic meters): the amount of soil and rock moved by the slide in the Johnson River Valley in 2017.
• Almost 159 million cubic feet (4.5 million cubic meters): the debris moved by the slide in the Redstone River Valley in 2018.
• 0.54 degrees Fahrenheit (0.3 degrees Celsius) per decade: the rate at which permafrost in the area has warmed since monitoring began in 1980.
• 20 years to penetrate 66 feet (20 m): how long thaw can take to work its way down through the permafrost.
• More than 150: the number of slides in the area, according to Young. The others are smaller but typically still move over 35 million cubic feet (1 million cubic meters) each.

How the team found them

Young and his colleagues were doing field observations when they noticed large landslide scars in valleys that had long held stable permafrost, which is sediment that stays frozen year-round. They mapped the changes with satellite imagery and carried out fieldwork. The researchers also ran an electrical current through the ground to measure how deep the permafrost lay on the slopes.

The fieldwork showed these were not ordinary thaw slides. Young said that permafrost usually thaws into a slurry that flows downslope like mud, but in these slides his team found blocks of frozen material in the debris.

What is behind the slides

The researchers focused on two slides and used their data to build a model. They concluded that the slides did not come from thawing from the top down. Instead, fire-related thaw and warming temperatures combined to open cracks near the tops of the slopes. Those cracks let thaw reach deeper until it melted the bottom of the permafrost layer.

That process matches the timeline in the two valleys, the report said. Severe wildfires burned nearby in 1995, and the large slides came about two decades later. Young described the pattern: a fire in 1995, then widespread disturbances 20 years on. He said the ground had been stable and then, in his words, essentially flipped a switch.

The smaller of the two slides left a scar three-quarters of a mile (1.2 kilometers) long. The larger one gouged nearly 2 miles (3.2 km) out of the hillside.

Why it matters

For now, the effects are limited. The Northwest Territories are remote, and Live Science reported that the slides have not yet damaged any infrastructure. Still, the two slides in the study dammed rivers, creating brand-new lakes and quickly dumping sediment into the waterways downstream, according to Young.

The concern is where else this could happen. Young said there are many places in Canada and Alaska where the permafrost is 66 to 130 feet (20 to 40 m) deep and so could be vulnerable to the same type of slide. He said that risk is especially high given the increasing intensity of wildfire at northern latitudes.

Young called the finding almost a potential glimpse of what these areas could expect as forest fire patterns change. The study describes events in one region, and the report does not say how often similar slides may occur elsewhere.

Around Prague, based on reporting by Live Science