Landslides take place when dirt, pebbles, rocks and boulders slide down a slope together. Sometimes these landslides are small, and hardly noticeable. Other times however, they can be substantial, involving the entire side of a mountain.
These destructive slides can be triggered by a number of different causes. Often rain, which adds additional weight to the side of a slope can cause slides. Other times they might be caused by erosion, as the base of a slope is slowly removed by a stream, weakening the entire side of the mountain.
As a slide progresses down a mountain slope, it can pick up tremendous speed, and energy. Some slides have been reported to travel at speeds approaching 200 miles per hour. The resulting winds can be so forceful, that they are known to strip the leaves off of surrounding trees. The momentum of falling material has been known to cause some of the materials to roll several hundred feet back up the other side of a valley.
The amount of material moved in a landslide can be tremendous. In some cases this material is so substantial, that it is measured in cubic miles. This much material falling across a stream, can be the cause for the formation or a new natural lake.