The influence of crystal size distributions on dynamic processes in magmatic systems
Final Report Abstract
Flow of magma is an integral part of volcanic activity. Whether a volcano erupts effusively or explosively can often be attributed to different bulk flow properties of the magma brought on by changing geochemical, thermal, and mechanical states of the system. In order to be able to better assess and predict the potential hazards of eruptions, it is therefore of crucial importance to be able to fundamentally and precisely quantify magmatic flow processes – i.e., to know the magma’s rheology. Magma is a complex mixture of a silicate melt, a gaseous phase, and crystals. Particularly the solid crystal phase, and changes therein during ascent to the surface, can have dramatic effects on the rheological response of the magma. During recent years, considerable progress has been made to quantify the effect of the crystals’ volume fraction and shape on rheology. It remained largely unclear, however, in what way and to what extent the crystal size distribution (CSD) affects the flow properties of a magma. It was the primary aim of this study to throw light on this matter. The particular goal was to perform a fundamental and systematic experimental investigation and quantification of the influence of varying particle size distributions on the flow properties of a liquidparticle suspension, to then derive a generalizable model from the findings, and finally to apply the results to crystal-bearing magmatic systems. To this end, we have performed more than 300 shear experiments on suspensions of diverse particles in silicone oil as magma-analog. In these samples, the size distributions of the particles were systematically changed, and the rheological properties of each sample determined using a high-resolution rotational rheometer. Based on the experimental results we developed a semi-empirical model, which relates the polydispersity index and the mean particle aspect ratio of a given suspension to its particle maximum packing fraction, which in turn serves as a proxy for the suspensions’ rheological properties (e.g., viscosity). In other words, with our new model it is possible to estimate the expected flow properties of a magma (or, in fact, any particle bearing liquid) solely on the basis of the size and shape distribution of the crystal load, which can be easily obtained by image analysis of rock thin sections. We finally tested the applicability of the model using real crystal size distributions of rocks of five different volcanic settings. Future refinements of our model (including for example the effects of bubbles) may bring us closer to accurate rheological mapping and behavioural forecasting in active magmatic domains like conduits and lava flows.
Publications
- ‘The influence of crystal size distributions (CSD) on the rheology of magma: new insights from analogue experiments’ Physics of Volcanoes workshop 2016 - Mainz
Klein, J., Mueller, S. P.
- (2017) The influence of crystal size distributions on the rheology of magmas: new insights from analogue experiments. Geochem Geophys Geosyst 18, 4055–4073
Klein J, Mueller SP, Castro JM
(See online at https://doi.org/10.1002/2017GC007114) - (2018) An expanded model and application of the combined effect of crystal-size distribution and crystal shape on the relative viscosity of magmas. J Volcanol Geotherm Res 357, 128-133
Klein J, Mueller SP, Helo C, Schweitzer S, Gurioli L, Castro JM
(See online at https://doi.org/10.1016/j.jvolgeores.2018.04.018)
