VOLCANIC DEBRIS AVALANCHE PROPAGATION AND EMPLACEMENT MECHANISMS: SEDIMENTOLOGICAL FIELD EVIDENCE AND BIDISPERSE GRANULAR FLOW EXPERIMENTS

Abstract

Volcanic debris avalanches mobilise large volumes and achieve long runouts with high destructive potential. Although many theories have been proposed regarding the mechanisms that enable their long runouts, the processes remain unknown. Theoretical and numerical models are unable to account for their long runout while being consistent with deposit observations. Therefore, evaluation of their dynamics in reference to deposit structure and sedimentology is fundamental for constraining such models.

This thesis explores volcanic debris avalanche propagation dynamics and evaluates potential factors contributing to their long runouts. A systematic comparison of volcanic debris avalanches and lahars allows the evaluation of water as a factor in mass flow propagation. The structure and sedimentology of two deposits from the Canary Islands (Spain) with different properties, and composed of distinct lithologies, are examined and their propagation processes evaluated. A novel technique using structure from motion photogrammetry is developed and proposed for the clast size analysis of indurated/lithified deposits. Finally, analogue granular avalanche experiments are employed to evaluate grain size bidispersity as a factor for enhanced runout. The potential and limitations of analogue experiments are evaluated.

Findings suggest that water does not play a major role in the propagation mechanisms of volcanic debris avalanches and is not a factor contributing to their long runouts. The two studied deposits constitute endmembers consisting of homogenous competent lava lithologies in one case, and less competent pyroclastic products in the other. Consequently, they result in end-member propagation models: (1) brittle fault-accommodated spreading and (2) distributed stress fluidisation respectively. Their comparison suggests that lithological properties are a principal factor for the structural evolution, dynamics and kinematics of volcanic debris avalanches.

Although enhanced runouts are achieved in the analogue experiments with bidisperse grain size distributions, this is the result of scale-dependent processes, dissimilar to real events, generating a collisional flow regime. Consequently, the importance of dynamic scaling for the effective simulation of avalanche dynamics is highlighted.