Large volcanic debris avalanches: Characteristics of source areas, deposits, and associated eruptions
Large volcanic debris avalanches, often exceeding a cubic kilometer in volume, create massive amphitheater-shaped reentrants into the volcanic edifice that differ in morphology and origin from normal collapse calderas. The volume of debris avalanche deposits at the base of these breached craters or calderas often correlates closely with the volumes of the missing sectors of the volcanic edifices, indicating that the dominant process in the formation of these depressions is massive slope failure of a portion of the volcanic cone. Debris avalanche deposits display a hummocky topography with numerous small hills and closed depressions, longitudinal and transverse ridges, and locally homogeneous debris with a jigsaw fit, features that are typical of landslide deposits. The size of the hummocks and the maximum size of breccia blocks within them tends to decrease away from the source. Data on travel distance ( L) of debris avalanches as a function of vertical drop ( H) demonstrates the great mobility (median H/ L = 0.11) of these avalanches, which are emplaced at calculated velocities often exceeding 100 km/hr. Volcanic debris avalanches and associated formation of "avalanche calderas" have occurred at roughly four per century in historic time, several times the historic rate for the formation of Krakatau-type calderas. These depressions often show a preferred orientation normal to the dominant direction of dike emplacement. The differential stress produced by the emplacement of parallel dike swarms is an important factor among the many factors that contribute to large-scale volcanic slope movements. In addition to hazards from rapid emplacement of the avalanches and possible associated directed blasts, a major secondary volcanic hazard from these events is tsunamis produced by the rapid impact of debris avalanches from coastal volcanoes into the sea.