Distict element numerical modelling of volcanic debris avalanche emplacement geomechanics.

Abstract

Catastrophic collapse of volcanic edifices is a relatively common phenomenon in the geological record, representing the largest subaeriallandsliding events on Earth. Subsequent volcanic debris avalanche (VDA) runout lengths often exceed 50 km and inundated areas may be greater than 1,000 km2. The geomechanical processes that occur during emplacement, however, remain poorly understood as emplacement processes must generally be inferred from deposit analysis. Summarizing the literature, this thesis first introduces the general factors that control edifice collapse, mechanisms thought to control avalanche mobility and commonly observed deposit features. The mechanisms which have led to the formation of characteristic deposit features specifically are then reviewed; commonly discussed themes are then used to develop a general emplacement model which summarizes the geomechanical evolution of VDAs. This model is then tested by analyzing orthophotographic images of VDA deposits; common deposit morphologies are observed in each case, suggesting a common deformation sequence may occur during emplacement. To better understand emplacement processes, a distinct element numerical model is then created. Initial unbonded particulate avalanche simulations allow spatial/temporal variations in avalanche body stress, energy and deformation to be considered in relation to the development of characteristic deposit features. A more sophisticated bonded particle model is then utilized to allow the consideration of emerging brittle behaviour. Resulting simulations display the development of characteristic VDA deposit features from initial block sliding and horst and graben development. Evolution to a fully-flowing granular avalanche occurs through the initiation and propagation of faults generated due to stresses in the avalanche body, reflective of the proposed common deformation sequence. Features commonly observed in VDA deposits, such as toreva blocks and surface hummocks, are created in the bonded avalanche simulations. Use of this innovative numerical model therefore allows for new insight into the geomechanical evolution of rock and debris avalanches to be developed.