A sudden collapse and downward motion of ice and rocks down a mountainside is an avalanche.
Avalanches are rapid mass movements caused when snow, ice, rock, or a mixture of these materials breaks loose and travels downhill under gravity. Snow avalanches are the most familiar type, but ice avalanches and rock avalanches can also occur, especially on steep, unstable slopes. Some events combine snow, ice, soil, and debris into a powerful flow.
Fresh snowfall, heavy rain, strong winds, earthquakes, melting, and human activity can all destabilize a mountainside. The danger is greatest where weak layers form beneath newer snow, allowing a slab to fracture and slide. Avalanches can reach very high speeds and bury roads, buildings, climbers, and skiers within seconds.
An avalanche is not exactly the same as a landslide. Landslides usually involve soil and rock, while avalanches traditionally refer to rapidly moving snow or ice, though rock avalanches are also recognized. Warning systems and controlled explosives are used in some mountain regions to reduce the risk of larger, uncontrolled releases.