An avalanche is  a    natural       disaster that occurs when snow and  ice goes down a hill or mountain. The snow and ice  go at incredible speeds. You probably know about Mt. St. Helens (a volcano in Washington that erupted in 1980), it triggered an avalanche that went 250 mph. It was the fastest avalanche ever.

    Avalanches can also go slowly.  The slowest avalanche was only a couple miles per hour, a quick walker could stay in front of this avalanche.

    An avalanche tumbles like laundry in a dryer and is responsible for many deaths. In World War 1 40,000-80,000 people died because of one.  It was started by gunfire.