College Level Geology

AVALANCHES While anything can be an avalanche, most are associated with falling snow and are also called snow slides. Snow plus ice and debris can rapidly descend any slope under the right conditions. Factors contributing to an avalanche include a lot of new precipitation, weakening of the shear forces on a snowpack, earthquakes, or human activities. Almost all avalanches are a combination of air and snow. Of course, debris can be picked up along the way.

There are two major types of avalanches but many are mixed in their characteristics.

You can have slab avalanches, which are blocks of packed snow that travel across a weaker layer or collapse a weaker layer. Next you have loose or fluffy snow packs. These will gain momentum and accelerate over time as the total mass increases. Air plus snow equals powder, which can also rapidly rush down the mountainside.

You need snowpack that is relatively long-lasting plus a change in conditions. Winter and spring are prime times for this. It is possible to have summer avalanches as well.

Many happen when there are storms that add to the snowpack or erode the top of the snowpack. Heavy melting from the sun are also secondary cause of avalanches.

Earthquakes, rain, rocks and ice falling are both lesser causes of avalanches. Triggers might include skiers, explosives used for a controlled avalanche, and snowmobiles in the area. Sound has no impact on avalanches, contrary to popular belief.

An avalanche tends to start slow but then, like a snowball, it picks up more material. If there is a weak layer, it will fracture and fall as a block. Thousands of cubic meters of frozen snow can be part of this process by the time it is over with. There is shear strength involved that is overcome by the weight of the affected snow. The end result is dependent on the humidity, temperature, and characteristics of the falling snow. The amount of solar radiation also matters because it will change the hardness of the upper layers of the snowpack.

We do not know as much about avalanches as we do about landslides. Computer models and the study of existing and past avalanches will help geologists learn to better predict these and avoid them through the judicious use of explosives to clear the avalanche when it is safest to do this.

204