Movement is characterized by flowage in a dry or wet state, or both.
Debris avalanches commonly originate in massive rockslides which, during their movement, disintegrate into fragments ranging in size from small particles to blocks hundreds of meters across. If the avalanche has a large water content, its matrix may continue to flow downslope as a lahar after its coarser parts have come to rest. Such avalanches form when part of a volcanic edifice fails catastrophically and moves downslope. Steep-sided volcanoes may also fail from other causes, e.g., after gradual weakening by hydrothermal alteration, or after heavy rains which may saturate and weaken parts of the edifice. Debris avalanches typically produce thick hummocky deposits that can extend tens of kilometers from a volcano and cover hundreds of square kilometers.