Crystallization and Degassing Processes during Strombolian Eruptions: A Numerical Approach

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Mankind’s interest in volcanic eruptions is probably as old as its existence because, on the one hand, volcanoes provide extremely fertile soil for farming leading to preferred settling grounds at or near volcanoes. Furthermore, they are probably one of the most intriguing natural spectacles making them highly attractive spots for tourists nowadays. On the other hand, they claim human lives, destroy infrastructure, and affect the local as well as the global climate. Understanding and mitigating the dangers of volcanic activity is therefore a major societal task. In this proposal, we address one key aspect of volcanic activity, the transport of magma in a volcanic conduit prior to an eruption, through a detailed modeling study. We specifically focus on strombolian eruptions as they are quite frequent, easy to observe from a safe distance, and a huge amount of different data is available for such eruptions to verify various aspects of numerical models. Models for strombolian explosions, including the ejection of pyroclasts and volcanic gases, call for the rise of large gas slugs (conduit filling gas bubbles, significantly longer than wide) in volcanic conduits that carry a certain overpressure. In between those eruptions, the observed passive degassing is currently explained by two-phase convection in the conduit. Regardless of which of these models is considered, the complex rheology of the magma as a mixture of melt/crystals/bubbles is one of the most fundamental controls on its behavior. In our model, rheological properties of the magma will be derived directly from a thermodynamic model during runtime taking into account cooling/degassing/crystallization of the melt in disequilibrium during transport through the conduit. Knowing the exact composition as well as bubble and crystal content, we employ current models for calculating the viscosity of such a three-phase system. This rheological model will then be implemented into a computational fluid dynamics model to study the gas slug and two-phase convection regimes. Model input parameters will be chosen to best match composition/temperature of magma of Stromboli volcano, Italy. Key questions which will be answered by this project include: 1) How does rheology impact the rise of gas slugs? 2) Are there rheological regimes that promote the development of trains of gas slugs that would explain consecutive eruption pulses? 3) How do transport processes in volcanic conduits change if conduits are not modeled as rigid pipes but rather as hot thermal anomalies in a volcanic edifice? 4) What is the effect of reaction kinetics (non-equilibrium thermodynamics) on transport processes in volcanic conduits? Observations of activity at Stromboli volcano, like ejection velocities, number of pulses during a strombolian eruption, degassing rates, and crystal fraction in ejecta, will be used to test the validity of the model results.