An avalanche is a sliding mass of snow, or ice, or wet earth and stones. An avalanche of earth and stones is a kind of landslide. It can happen even in regions without snow. A steep mountainside or bluff becomes thoroughly soaked with water, or is weakened in some other way. The earth can no longer stand in such a steep slope without sliding, and an avalanche begins.

This kind of avalanche is most common in the spring, when winter snow and ice melt and soak the ground below. It may also occur where a river wears away its banks, or where man digs a roadway or mine along the base of a steep mountainside. This kind of landslide has often blocked mountain valleys and thereby created large lakes.

A snow avalanche occurs when snow will no longer remain on the mountain slope beneath without sliding. Old, packed, crusted snow will often remain quite firm on even very steep slopes, but fresh, loose, powdery snow may slide very easily on even a gentle slope.

When a small slide of snow begins, it often dislodges larger masses below, and as the avalanche descends, it grows in size and destructiveness. In places where the snow is so situated that it will barely support itself without sliding, even the sound of a voice may disturb it sufficiently to start an avalanche! This is because the sound sets up vibrations, and the vibrations start the snow skidding downwards.

In the Alps, when mountain climbing parties are crossing dangerous slopes, the guides will often insist that everyone remain absolutely silent in order not to set up the vibrations that may cause an avalanche.