Fragmentation and flow of gas-particle mixtures in volcanic systems

The transport of gas and particle mixtures are a widespread phenomenon, both in industry and in the natural world. In the geosciences these gas-particle mixtures exist during sediment transport by the wind, meteorite impacts, rock and snow avalanches and explosive volcanic eruptions. In this research proposal we focus on gas-particle flows produced during volcanic eruptions, although we expect the physics uncovered will be portable to these other disciplines and plan to host a multidisciplinary workshop to facilitate knowledge transfer.

Ten percent of the world's population (i.e. 100's of millions) live within 100 km of an active volcano. Furthermore, this number is set to rise with the increasing global population. During all explosive volcanic eruptions pyroclastic density currents (PDCs) can form - high temperature multiphase mixtures of rock and gas that rapidly flow away from the volcanic vent. These phenomena are the most lethal of all volcanic hazards and are responsible for more than a third of volcanic related fatalities.

However, despite the lethal nature of pyroclastic density currents we currently lack accurate models to forecast the runout distance achieved by these flows and thus any hazard maps and mitigation strategies are inherently limited. To improve our numerical forecasting models, we need to understand the complex internal flow dynamics within these 'opaque' and hazardous flows. Direct internal observation is not possible, but controlled laboratory experiments offer a way to rigorously study these otherwise hidden phenomena. Specific and targeted experiments aimed at providing key input parameters for numerical flow simulations offer a clear way forward.

During this research programme, we will use two types of scaled laboratory experiments to investigate these phenomena in a controlled and observable manner. Firstly, experiments using a rheometer will measure the flow behaviour (e.g. viscosity; mobility) of natural volcanic gas-particle mixtures. We will quantify how the particle size distribution and gas flow conditions effect the mobility of these flows. Secondly, natural volcanic particles (e.g. bubbly volcanic glass, crystals, rock fragments) will be suspended in air to form a gas-particle mixture at a range of conditions relevant to the volcanic system. The experimental products will be characterised for their size, shape, and abundance as a function of time (i.e., transport distance). This will allow us to quantify the rate at which volcanic particles break, round and change their relative abundance during flow. This will lead to a comprehensive understanding of how the particle textural properties (e.g. size, shape) change during flow and how, in turn, these changes alter the mobility of pyroclastic density currents.

These experimental data, on flow conditions and particle breakage, derived from new state-of-the-art techniques will be incorporated into existing 3D numerical flow models. Numerical simulations will be performed on a case volcano (Tungurahua, Ecuador). This will allow us to illustrate, and quantitatively assess, the impact this 'new physics' has on existing pyroclastic density current models and importantly, their predicted run out distances. In collaboration with the Geophysical Institute of Ecuador these novel results will be used to critically evaluate current hazard maps and risk mitigation strategies. This research programme will therefore provide a platform upon which reliable PDC models and forecasts can be built for eruptions worldwide. Ultimately this will minimise the human and economic cost of these hazardous geophysical flows.