Rheology and physical properties of crystal bearing magmas. ETH Zurich (Switzerland)

A series of experiments were performed to determine the rheological and physical properties of crystal-bearing magmas at confining pressures between 250 and 300 MPa and temperatures between 20 and 1000 °C. The data were used to compute flow laws able to describe the rheological behavior of partially crystallized magmas inside volcanic conduits and to quantify the effect of the suspended solid fraction on the propagation velocity of seismic waves. The rheology of crystal-bearing magmas was studied in a system composed of a haplogranitic silicate melt and quartz particles. The high chemical stability of this system enables rigorous control on the parameters relevant for rheology such as the degree of crystallinity and the composition of the melt phase. The experiments provided the possibility to separate and determine the effects of crystal fraction and deformation rate on the viscosity of magma. The results were summarized in a system of equations that account for the complex non-Newtonian behavior of partially crystallized magmas and that were implemented in a numerical code used to simulate the flow dynamics inside volcanic conduits. The numerical modeling revealed that previous codes, which considered magmas as Newtonian fluids, largely overestimated the depth at which the fragmentation of magma takes place. This implies that the recalculated physical properties of magma at the fragmentation depth (e.g. magma vesicularity) were incorrect, inducing significant errors on the reconstruction of eruptive scenarios. The rheological behavior of natural samples from the 1538 AD eruption of Monte Nuovo volcano (Phlegrean fields, Italy) was investigated for two principal reasons: i) The Phlegrean Fields are a highly populated area (about 1 million inhabitants) and the knowledge of the rheological properties of a potential eruptive magma is fundamental for volcanic risk assessment; ii) The material is characterized by the presence of elongated feldspar crystals (about 40-45 vol. %) making it ideal for the study of the effect of elongated crystals on magma rheology. The rheological behavior was characterized as a function of temperature and strain rates. The presence of elongated suspended crystal reduced sensibly the maximum packing fraction (crystal fraction at which the viscosity increases exponentially) of the system. The viscosity resulted 1-1.5 order of magnitude higher with respect to the viscosity of materials with comparable crystal fractions containing sub-spherical particles. Data obtained in this study were combined with literature data to define a mathematical algorithm describing the rheological properties of crystal-bearing magmas over a wide range of crystal fractions and strain rates. Experimental data from each publication were analyzed to extract information on crystallinity, strain rates and crystal shapes. The knowledge of the experimental conditions and the geometrical properties of the particles provided the possibility to assign a physical meaning to the fitting parameters employed in these equations. This, in turn allows describing the variations of viscosity for a variety of different suspensions as a function of crystal fraction, crystal shape and stress conditions. The advancements made on the understanding of the rheological behavior of crystal-bearing materials were supplemented with a study aimed to determine the physical properties of magmatic suspensions. The propagation velocities of shear (Vs) and compressional (Vp) waves were measured on material covering a wide range of crystal fractions (from 0 to 0.7) at 200 MPa confining pressure and temperatures up to 1000 °C. These data, in turn, were utilized to compute elastic moduli. The results reveal a remarkable correspondence between the principal increase of the elastic moduli of the material as function of the crystal fraction, and the onset of an exponential increase of viscosity. Propagation of shear waves, increase of Vp, and a dramatic increase of viscosity are caused by the transition from a liquid-solid suspension to a crystal mush (particles generate a continuous framework) with increasing crystal fraction. The strong differences in elastic properties between solid-liquid suspensions and crystal mushes allow the distinction between a liquid dominated and a relatively highly crystallized magmatic reservoir using the inversion of seismic data. Highly crystallized magmas have very high viscosity that reduces their potential to erupt. Thus, the dataset presented here, combined with seismic inversion tools, could potentially represent a powerful tool to evaluate the probability of a volcanic eruption from a given subvolcanic magma reservoir.