Avalanches -- triggered from the valley

Dec 02, 2008
The interstices between the ice crystals play a significant role in the formation of cracks in the snow. Credit: Jane Blackford and Chris Jeffree, Universitaet Edinburgh

Everybody knows that skiers swishing down steep slopes can cause extensive slab avalanches. But there is a less well known phenomenon: A person skiing a gentle slope in the valley triggers a slab avalanche on a steeper slope, sometimes several hundred meters further uphill. This scenario doesn't seem to make sense – yet it claims human lives year after year.

But what exactly happens when an avalanche is remotely triggered? "In a slab avalanche, the upper layer of snow slides down into the valley. For that to be able to happen, it first has to become detached from the layer beneath it," says Prof. Dr. Peter Gumbsch, director of the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg.

The view commonly held until now assumes that the layers of snow are separated by shear cracks – the upper layer shifts within a limited area. If the two layers of snow were two hands placed palm to palm, a shear crack would be equivalent to rubbing one hand against the other. The layers of snow can only shift if the slope is steep enough. Shear cracks may be a satisfactory explanation for the breakaway of snow slabs in steep terrain. But how can they be triggered from a distance?

Gumbsch and his colleagues Michael Zaiser and Joachim Heierli at the University of Edinburgh, Scotland, have developed a physical model that explains this phenomenon. "The boundary layer that connects the upper and lower layers of snow is made of ice crystals with fairly large interstices," explains Heierli. The pressure exerted by a skier can cause the ice crystals to break, separate from one another and slip into the interstices – the layer collapses. The layer on top of it subsides. This mass collapse, which can be described as an anti-crack, releases energy that has not previously been taken into account. This energy enables the crack to propagate. To return to our previous analogy, the anti-crack would be like pressing the two hands together.

Experiments carried out by Canadian researchers at the University of Calgary confirm the theory: Whether the slope is gentle or steep, it is equally difficult to trigger the collapse. Once it has started, it propagates as an anti-crack. It can move up or down the mountain and grow to a length of several hundred meters within a few seconds: The layers of snow lose their cohesion. Only the forces of friction can then prevent the snow from slipping. If these are insufficient, the upper layer slides off and a slab avalanche begins.

Source: Fraunhofer-Gesellschaft

Explore further: Better thermal-imaging lens from waste sulfur

add to favorites email to friend print save as pdf

Related Stories

Study shows less snowpack will harm ecosystem

Apr 15, 2014

(Phys.org) —A new study by CAS Professor of Biology Pamela Templer shows that milder winters can have a negative impact both on trees and on the water quality of nearby aquatic ecosystems, far into the warm growing season.

Astronomers looking for clues to water's origins

Mar 27, 2014

A gas and dust cloud collapses to form a star. Amid a whirling disc of debris, little bits of rock coated with liquid water and ice begin to stick together. It is this stage of a star's formation that astronomers ...

The storm before the calm

Feb 06, 2014

In just a week the crew of Concordia research station will be left to survive on one of the remotest places on Earth for nine months. Together, the crew of 13 must operate the base, live and run scientific ...

Recommended for you

Better thermal-imaging lens from waste sulfur

5 hours ago

Sulfur left over from refining fossil fuels can be transformed into cheap, lightweight, plastic lenses for infrared devices, including night-vision goggles, a University of Arizona-led international team ...

How to test the twin paradox without using a spaceship

Apr 16, 2014

Forget about anti-ageing creams and hair treatments. If you want to stay young, get a fast spaceship. That is what Einstein's Theory of Relativity predicted a century ago, and it is commonly known as "twin ...

User comments : 0

More news stories

Better thermal-imaging lens from waste sulfur

Sulfur left over from refining fossil fuels can be transformed into cheap, lightweight, plastic lenses for infrared devices, including night-vision goggles, a University of Arizona-led international team ...

Robotics goes micro-scale

(Phys.org) —The development of light-driven 'micro-robots' that can autonomously investigate and manipulate the nano-scale environment in a microscope comes a step closer, thanks to new research from the ...

Hackathon team's GoogolPlex gives Siri extra powers

(Phys.org) —Four freshmen at the University of Pennsylvania have taken Apple's personal assistant Siri to behave as a graduate-level executive assistant which, when asked, is capable of adjusting the temperature ...

Deadly human pathogen Cryptococcus fully sequenced

Within each strand of DNA lies the blueprint for building an organism, along with the keys to its evolution and survival. These genetic instructions can give valuable insight into why pathogens like Cryptococcus ne ...