Avalanches occur when large amounts of snow fall from steep slopes, either due to a break or fissure in the snow pack or due to an accumulation of too much snow for the slope's angle. In some cases, a snow pack can fracture due to a hiker or skier's actions, or rising temperatures may render the accumulated snow too unstable to stick to the slope.
Under normal circumstances, excess snow on a slope simply falls down to a lower level. In some cases, though, snow can accumulate high up on steep slopes, creating the potential for a dangerous avalanche. Changes in temperature on a mountain can cause sheets of snow to freeze in layers, and gravity or outside action can cause these layers to fracture and come apart, triggering an avalanche.
A minor snowslide may send only powdery snow falling down the mountain, and these are rarely dangerous for those caught in the area of effect. Slab avalanches occur when large snow deposits break loose, and these avalanches can reach speeds of up to 80 miles per hour and spread over a wide area, drawing more snow and debris into the slide. Slab avalanches can prove fatal for any hiker or skier caught on or beneath the snowslide.