The same physics behind ice skating may explain why massive ice avalanches can develop so quickly according to new research.
In 2002, a small disturbance on a mountain slope in the Russian Republic of North Ossetia set off a deadly ice avalanche, engulfing two glaciers en route to unsuspecting villagers. The destruction started with the collapse of 100 million cubic meters of ice and rock, which eventually stormed through a river valley toward villages at 175 miles per hour. Over 100 villagers were killed, and similar avalanches in the Alps and in North America have threatened local populations over the past few years.
"Ice avalanches can be particularly devastating," said Barbara Turnbull, an avalanche researcher at the University of Nottingham in the U.K. "They often trigger secondary flows [of rock and snow]."
In an effort to better understand these events, Turnbull simulated avalanche conditions in her lab. Her research has just been published in the journal Physical Review Letters.
Turnbull first dipped water droplets into frigid liquid nitrogen to create ice particles for her experiment. Next, she placed the ice particles in a temperature-controlled rotating drum and filmed the interactions of the particles with a high-speed camera. With data from the camera, Turnbull measured the particle velocities and watched the mini-avalanche evolve over time. Turnbull found that even though the particles were held at temperatures below freezing, they still melted due to friction -- a phenomenon best known in the realms of ice skating and skiing.
When a skater glides across an ice rink, the friction between the skate and the ice causes the top layer to melt. This melting creates a slippery liquid surface, allowing the skater to move gracefully. Similarly, during an avalanche, ice particles collide and start melting one another even at sub-zero air temperatures. In turn, frictional melting creates a slippery surface that speeds up the other particles, causing more collisions.
"I'm more surprised and excited about the fact that [frictional heating] was observed in such a small-scale flow," wrote Demian Schneider, an avalanche researcher at the University of Zurich, in an email to Inside Science.
The positive feedback between frictional heating and particle collisions explains why avalanches develop quickly and unexpectedly. "It's about linking these two aspects together," said Turnbull.
This snowballing cycle can even melt solid rock several meters thick once the avalanche has grown large enough. In 2005, an ice and rock avalanche in Alaska became so massive that seismologists picked up readings on the other side of the globe. Researchers have been detecting more of these large avalanches recently, and evidence suggests that climate change coupled with better detection methods is behind the observed increase.
"We have relatively robust evidence of an increase of high-mountain rockfall and avalanches in the European Alps," Christian Huggel, an avalanche researcher from the University of Zurich told Inside Science in an email. "General theoretical considerations but also field measurements are supporting the view that warming in high-mountain [areas] is likely destabilizing slopes."
The limited field measurements of high-mountain ice temperatures indicate that the problems of climate change are amplified within glaciers. Small rises in air temperature can lead to even greater temperature increases within the ice, suggesting that climate change may have contributed to the recent uptick in observed avalanches in glacial areas.
Although laboratory experiments can never fully recreate avalanche conditions, experts argue that they're an indispensable tool for avalanche preparation. Being able to watch a miniature avalanche evolve over a long period of time can be more useful than studying the aftermath of real avalanches. In particular, climate change may render historical avalanche field data less helpful as avalanches start to affect larger areas, according to Schneider.
Scientists hope to apply their better understanding of avalanche physics toward risk-reduction efforts. Hazard maps of high-mountain environments, for instance, could be improved with a better understanding of how avalanches evolve. But ice avalanches remain stubbornly unpredictable.
"Such large and long reaching avalanches are low-frequency, high-impact events which are difficult to handle in terms of risk, similar to nuclear plants," said Schneider.
Nonetheless, Schneider believes that these most recent results represent an important step toward understanding avalanches: "All in all, it's a great basis for future work."