How An Avalanche Forms

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While there are many different ways for avalanches to set up, they are all related in the sense that the snows' frictional hold on the slope has released and gravity is pulling the snow particles down. When the snow is deposited during the storm, the particles are 'bonding' or 'locking' together and creating layers of particles that are relatively similar. Every time the temperature changes during the storm, or the wind shifts directions, it has an effect on how the snow settles and may form a new layer. Some of these layers are denser than others, and some will bond nicely with neighboring layers while others may not. The better the bonding is between the layers, the more stable the snowpack is. When a weak layer is deposited, or created in the snowpack, the chances of that layer collapsing and causing an avalanche are much higher. The layer may fail due to the force of gravity, the weight of new snow on top of it, or forces from a skier or snowmachine on it. An avalanche occurs when the forces due to the previous instances become greater than the mechanical strength of the snowpack. There are two distinct types of avalanches: loose avalanches and slab avalanches. While they are structurally different systems, both can be equally troublesome to those recreating in the mountains.

Loose, or point-release, avalanches occur on slopes where the snow has simply lost its ability to remain on the slope. This is due to cohesionless snow sloughing off the surface, and picking up more snow as it falls down-slope. As the first particles of snow begin to release on the steeper aspects of the slope, they collide with lower particles, and create a fanned, triangular appearance on the slope. This type of avalanche generally occurs on slopes of 35 degrees or more and typically involves only the upper layers of the snow pack.

Slab avalanches happen when a weak layer in the snowpack fails and the cohesive layer above, separate from the rest of the snowpack and flow down the mountain. The layer that separates remains intact as a unit, and resembles a slab of packed snow flowing down the mountain. As it travels downslope, collides with objects and rolls over the terrain, it generally breaks up and is crumpled into smaller, broken pieces of slab by the bottom of its runout.

When either the weak layer fails, or the bond between the slab and the bedsurface releases, the force is drastically increased on all remaining bonds connecting the slab to the slope.
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