Magma fragmentation – the complications of crystals, bubbles and external water

Abstract:

The breaking apart of magma into fragments is intimately related to the style of eruption and thus the nature and footprint of volcanic hazards. Understanding what controls the size distribution of the fragments is particularly important as it affects the efficiency of heating air entrained into pyroclastic plumes and currents, and the settling velocity, and so the residence time, of particles in the atmosphere. The fundamental work three decades ago relating the glass transition to the fragmentation of magmas remains at the heart of conceptual and numerical models of volcanic eruptions. Yet a strain-rate-dependent fragmentation criterion does not predict the sizes of the resulting fragments or take into account the multiphase nature of magmas, or how magma can break in a fluidal manner or by thermal stresses. This talk will address recent advances in these areas from field studies, experiments and models. These will be applied to a variety of topics with an emphasis on 1) how crystallization can affect the nature of fragmentation and the sizes of the resulting fragments, and 2) how and why meteoric water in conduits does (or does not) affect fragmentation, and what pyroclast shapes and internal textures tell us about these processes.