An avalanche is any large mass of material that is tumbling downhill. But the word usually refers to snow cascading down a mountainside. A snow avalanche is triggered when snow high on a mountain is disturbed. Falling rocks and earthquakes can destabilize snow. People can also set off avalanches by walking or skiing in the wrong place at the wrong time. Sometimes people will use explosives to cause an avalanche before someone can accidentally set one off and get hurt.
As snow slides downhill, it picks up speed, snow and other debris. Smaller spills of powdery snow are called sluffs. More dangerous avalanches occur when huge slabs of snow break loose from a mountainside. The snow in an avalanche can plunge downward at hundreds of kilometers (miles) per hour. And if someone is buried, it can be nearly impossible to dig out without help. An average of 27 people in the United States alone die in avalanches each year.
In a sentence
The most dangerous avalanches, called dry slab avalanches, occur when a very cold and dry snowpack is disturbed.