Avalanches

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.

There are three types of known avalanches:

13. Slab avalanches happen when snow has blown in by wind in a block or slab that fractures off from the rest of the pack. There is a crown fracture like in a landslide, followed by flank fractures, and finally a sauchwall, which is what we call the fracture at the bottom of the slab. Now you can see that the slab can travel downhill with a wide range of depths up to 3 meters thick. About 90 percent of fatalities in hikers and skiers come from these types of avalanches.

14. Powder snow avalanche – these are very large and spectacular avalanches that are mainly a cloud of snow powder. It is sometimes known as a mixed avalanche because there is a denser layer beneath it. The trigger is usually a large recent snowfall of dry powdered snow. These are extremely fast at 300 kilometers per hour, very large, and travel a larger distance than you'd expect – even traveling uphill for a period of time.

15. Wet snow-type avalanches – these are much slower than powdered snow and have more water in them. There is more friction here between the falling snow and the inclined plane. While slower than others, they still reach speed of up to 40 kilometers per hour. Because these are so heavy, they are very destructive to property, vegetation, and the land itself. They start for various reasons but the main causative factor is the very wet snow that is nearly at melting temperature.

These are sometimes called isothermal snow avalanches because they have temperatures near melting but remain frozen except where friction has taken hold. Climate change and springtime warming are the main cause of this.

Avalanches must have some weak layer that is unstable beneath a slab of snow that is more cohesive. This is usually not visible when looking at the avalanche area. There should also be enough slope to overcome the tension holding the snowpack onto the slope. Again, avalanches are much harder to predict than landslides. Also, remember that they only occur on stable snowpacks that persist from year to year in some form. This means you are talking about high elevations and higher latitudes.