Chapter 5 - Volcanic debris avalanches

Abstract

Volcanoes are growing mountains with hydrothermal and magmatic systems, which have strong controls on volcanic landslides and debris avalanches. Such landslides are conditioned by the nature of volcanic rock, which is highly fractured, usually in granular form, often clay rich and water saturated. In consequence, volcanic landslides are generally more fractured, have more fine material, are more variably saturated than nonvolcanic landslides and they have a tendency to transform into large debris flows. Volcanic landslides vary in size from small failures of a valley side (<million m3) to a large portion of the edifice (tens of km3). The larger landslides are generally more deep seated, because weak hydrothermal and magmatic systems in the volcano core are involved. Volcanoes undergo significant gravitational and tectonic deformation, creating faulting and fracturing in the edifice. The structures form the framework for landslides, and the resulting debris avalanches tend to form hummocky horst and graben topography that reflects the initial structure. As many volcanic landslides descend onto flat plains, this type of topography is often well preserved. Volcanoes of all types in all geological settings suffer landslides, and even extinct volcanoes are landslide prone. About four volcanic landslides occur worldwide per century, meaning they are a significant hazard, especially as they are associated with secondary tsunami, volcanic eruptions, debris flows and lahars.