Wikipedia: An avalanche is a sudden rapid flow of snow down a slope, occurring when either natural triggers or human activity causes a critical escalating transition from the slow equilibrium evolution of the snow pack. Typically occurring in mountainous terrain, an avalanche can mix air and water with the descending snow. Powerful avalanches have the capability to entrain ice, rocks, trees, and other material on the slope.
Trying to guess at maybe a half mile wide? Probably bigger. This controlled avalanche from Cheget Mountain in Russia is awesome in scope and incredible to watch. They let one fly from the howitzer and a few seconds later the action begins.