Avalanches occur when there is more stress on the toplayer of snow than on the bottom layer on a mountainside.
Avalanches can travel at up to 300km/h and carry thousands of tons of snow. Each year,skiers or even backcountory hikers get covered in a wall of snow in a vally or on a mountainside. If you are burried undernneath several meters of snow, you will run out of oxygen and suffocate. After 35minutes, a person has only a 30percent chance of survival.
Multiple Snow Layers
There are three main causes that contribute to avalanche conditions. On the surface of a mountain is the snowpack. This is the snow that has fallen and accumulated over the past weeks and months. Usually, there are several layers of snow on the mountain. The first layer of snow is called the "bed layer." Some of this snow will melt away, leaving a second thin layer of ice particles on top of the bed layer. It might take several weeks before anothor heavy snowfall. The danger comes when the third layer of snow falls. If it is made of light, loose or fluffy snow (not heavy wet snow), the danger starts. This thid layer is called the weak or slab layer. A weak layer of snow lying on top of ice and bed layers means that an avalanche is more likely to occur.