The trigger for most avalanches, including the one that claimed seven lives in Sikkim on Tuesday, is fresh precipitation. The physics at play depends on a number of factors, including the nature of precipitation, the composition of the existing snow, and the steepness of the slope. The main forces at play are gravity, the friction that holds the snow together in a pack, and shear that causes a chunk of snow to break off.
How these forces interact also varies from one type of avalanche to another. An avalanche, defined as the rapid movement of a large mass of snow or ice down a mountain slope under the force of gravity, can be of a number of types. Broadly speaking, however, a snow avalanche involves the fall of either loose snow or a slab.
Loose snow avalanche
An avalanche triggered by loose snow usually starts at a point or a small area, and then spreads wider. in the shape of a fan Such avalanches take place when snow has been deposited at a steep angle so that it slides due to gravity. Loose snow avalanches are usually associated with dry snow.
“An avalanche almost always occurs on account of loading because of precipitation, but if it is predominantly dry snow, and it has made the existing mass of snow unstable, then it is from a point that the downward slide of snow takes place,” geologist NC Pant, now retired from Delhi University, said.
Slab avalanche
As the name suggests, it is a slab that gets detached from the existing mass, which inevitably leads to larger destruction. Unlike loose snow, which starts at a point, slab avalanches happen along a broad fracture line.
A slab gets detached on account of the interactions between the layers of snow already present on the surface, and the effect of precipitation on these. “If there is a layer which is hard — snow that has hardened into ice and there is a significant density difference — over which you have snow, then it can get broken along a line,” Pant said.
This happens when a layer of snow of low density lies beneath a denser slab has formed over a significant area. When precipitation increases the weight on the slab, it causes the lower layer to collapse; the frictional forces between the two layers fall. The tipping point comes when the pull of gravity exceeds the strength of the bond holding the crystals in the snow together. The slab breaks off and starts to slide, gathering more snow as it rolls down the slope.
Occasionally, an avalanche can be caused by factors other than fresh precipitation. A disturbance caused by humans can cause a chunk of snow to slide, which could trigger an avalanche if it gathers mass along the way.
Then there are avalanches caused at a cornice, which is an overhanging mass of hardened snow at the edge of a precipice. A cornice forms on account of the wind picking up and depositing the snow. When the mass of the cornice crosses a critical point, a part breaks off due to shear forces, triggering an avalanche.
Other factors: slope angle and avalanche path
There is a range across which a slope can cause an avalanche. This includes an upper limit: after a point, the slope is so steep that it does not allow an avalanche to happen.
The University of Wisconsin, Eau Claire, puts the critical range at 35-40°. “The pull of gravity is usually not strong enough to create an avalanche on slopes less than 35°. On slopes greater than 40°, the snow accumulations do not get deep enough to create an avalanche,” it says on its website.
“The mass of snow comes down, carrying a lot of kinetic energy. The moment it reaches a gentler slope, say around 15°, it tends to stop,” Pant said.
The strength of an avalanche can also depend on whether its path is along a channel or over a wider surface. An avalanche in a smaller, confined zone is likely to cause more damage.
“If it is occurring along a nullah or a channel, it will be confined, and the damage caused will be different for the two. An avalanche that is spread over a surface that is not confined will dissipate the kinetic energy over a wider area,” Pant said.
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