The influence of crystal-size distributions on dynamic processes in magmatic systems and the rheology of magmas
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Abstract
The rheology of a magma is recognized as a one of the crucial parameters for the understanding of processes and features throughout a volcanic system. Despite its key role in governing volcanic processes, magma rheology is extremely difficult to constrain in time and space within a natural volcanic system. It is well known from laboratory studies and observations in natural volcanic systems that the amount of crystals suspended in the melt, and the degree to which they interact, are central parameters governing magma rheology. Accounting for, or neglecting the effect of solid particles in volcanological eruption models can change their results by orders of magnitudes and may be a key parameter controlling whether an eruption is explosive or effusive. However, a key question in suspension rheology remains unanswered: How does the size distribution of suspended particles influence the flow?
Crystal-size distributions (CSD) are a widely used tool to describe aspects of a natural sample’s crystallization history (e.g., nucleation and growth rates, etc.) in order to examine processes within magma chambers and/or the conduit during magma ascent. Because crystals grow during magma rise, and due to the influence of particle concentration, size distribution and shape on suspension rheology, it is expected that the rheology of ascending magmas will change with time and position within a volcanic conduit.
This thesis investigates systematically the influence of CSD's on the rheology of vesicle free magmatic suspensions by using analogue rotational rheometric experiments with glass particles of different shapes in silicone oil acting as a magma equivalent. With the help of multimodal particle suspensions of well-defined size fractions, the analogue experiments of this study reveal that the polydispersity γ of the suspended particles exerts the largest influence on the maximum packing fraction ϕ_m of the suspension and consequently on its rheological behavior. In this thesis, an empirical model is derived in order to estimate the relative effect of crystal content and CSD's on the viscosity of magma directly from textural image analysis of natural rock samples. This model is applied to and tested by the measurement and analyses of the CSDs in experimentally grown crystal populations from high pressure/high temperature (HP/HT) petrological-experiments of Santorini pumice.