When water penetrates rock crevices in permafrost, it transports heat deep underground, where it causes the frozen rock to thaw. Researchers at the WSL Institute for Snow and Avalanche Research (SLF) have explored which processes destabilize rock to the point of collapse using a high-profile example.

On 13 June 2023, a free-standing rock pillar collapsed on the Hörnligrat, the most prominent access route to the Matterhorn. Around 20 cubic meters of rock fell; fortunately, nobody was injured. For years, water had been seeping into the rock below the pillar during the snowmelt, temporarily thawing and then weakening the rock and so gradually destabilizing it.

“Climate change is accelerating such processes, which are now a common driving force behind the increasing frequency of rockfalls in high alpine permafrost,” says SLF researcher Samuel Weber.

The researchers observed and measured the rock pillar for nine years. Their most important piece of equipment in this work was a GNSS receiver.

With its help, the researchers were able to record every movement of the pillar down to the millimeter. They compared this measurement series with seismic signals, time-lapse images and laser recordings, among other things. Using rock samples taken from the Hörnligrat as a basis, they studied the rock pillar in a laboratory as part of an international project.

“Permafrost thaw significantly reduces the critical angle of friction at which a rock mass starts to move,” explains Weber. He transferred his findings to a computer model. This was a success, as the simulation reproduced the measured movements on the Matterhorn one-to-one.

The research is published in the journal Earth Surface Dynamics.

Chain reaction in the rock

Three effects are exacerbating instability. Due to climate change, the ice in the permafrost that had previously sealed the rock is melting. This allows water to penetrate deeper, putting pressure on the rock.

At the same time, the water brings about warmer temperatures underground. This is a chain reaction, because it causes the permafrost and ice to thaw even faster—which in turn allows the water and thus the heat to penetrate even deeper.

“This also reduces friction at the fracture point by up to 50%, which further weakens the rock,” says Weber.

Ten days before the collapse

The interplay of these effects was made spectacularly apparent on the Hörnligrat. The rock pillar had been slowly tilting for years, with this process speeding up from 2022 onwards.

“Time-lapse photographs document a visible acceleration in the ten days before the collapse in June 2023,” says Weber. At the same time, three seismometers in the vicinity provided evidence of the dynamics of the impending collapse.

“Weather data and temperatures in the permafrost indicate that infiltrating water caused rapid, short-term thawing underground and played a major role in the event,” clarifies Weber.

With a view to better assessing the risk of rockslides in permafrost, Weber wants to learn more about the interaction between temperature, water and ice in frozen rock and its mechanical effects. To do this, he needs more data. “We’re now focusing on the role of water and are combining various measurement methods to this end.”