Volcanic structural collapse in the form of avalanches, rock fall, or landslides can be almost any size ranging from a few loose rocks falling from the crater rim of a volcano to large avalanches A debris avalanche is formed when an unstable slope collapses and debris is transported away from the slope. Large scale avalanches normally occur on very steep volcanoes. The larger a debris avalanche is, the faster it moves. The reason for this is due to energy stored in material within the avalanche. Some large avalanches have been known to carry blocks as large as three kilometers in length, several kilometers from their source. Such avalanches can travel close to 300 km/hr.