10.3.6 Debris Avalanches - These are very high velocity flows of large volume mixtures of rock and regolith that result from complete collapse of a mountainous slope. A debris avalanche is a type of slide characterized by the chaotic movement of rocks soil and debris mixed with water or ice (or both) (figure 14). They move down the slope and then travel for considerable distances along relatively gentle slopes. They are often triggered by earthquakes and volcanic eruptions. Debris avalanches differ from debris slides because their movement is much more rapid. This is usually a result of lower cohesion or higher water content and commonly steeper slopes.
Debris avalanches usually occur on large, steep volcanoes and are mainly caused by instability of the volcano's slope. When a slope of a volcano is not stable it can easily collapse (possibly riggered by volcanic earthquakes) causing debris to be transported away from the slope. The bigger the avalanche the bigger its speed and thus its danger. Debris avalanches and landslides can produce numerous dangers. The mixture of debris from a landslide or avalanche with water may produce harmful lahars. They also can dam rivers and cause flooding. Perhaps one of the most important hazards that can be produced by avalanches or landslides is a tsunami.
Figure 14: Example of Debris avalanches in snow mountains