Alain Burgisser

This study presents a three-phase suspension lava flow model with a Herschel–Bulkley rheology. The suspension contains crystals and gas bubbles, and two closures for the evolution of the crystal volume fraction are considered. The first closure minimizes the complexity of the system by treating crystal fraction as a transported quantity subject to relaxation towards an equilibrium state. This closure avoids a parametrization of the many heat transfer mechanisms a lava flow is subjected to. The other closure is on the lava temperature considering four heat transfer mechanisms and a prescribed temperature–crystallinity relationship. We deduce from this system and solve numerically a one-dimensional depth-averaged model. A comparison with a pre-existing model based on real lava flow data suggests that the prediction of flow parameters done with the multi-parametric evolution of temperature yields more accurate results than those obtained with the other closure. The transient nature of our model correctly predicts that confined lava traveling down an irregular steep slope yields a series of cascading fill-then-breakout lumps that causes the overall flow to be pulsatory. These pulses dominate the local dynamics and preclude a strict steady state to be reached. Theoretically, taking gas bubbles into account is best done with a general rheological relationship valid at any capillary number. In the conditions explored herein, bubbles modulate viscosity within a factor 2 with a shear thinning behavior, decelerating slow flows and accelerating fast flows. When a simplified rheology treating bubbles as hard spheres was used, the only dynamic parameter affected was bulk viscosity.

Modeling magma ascent within volcanic conduits is not an easy task since it involves complex physical interactions between melt, crystals and gas. Since the evolution of gas content mainly controls the eruptive dynamics, we have to do our best to understand and quantify how and when this gas phase exsolves from the melt, expands and finds its way toward the surface, possibly independently from the melt. In order to follow these phenomena with time, we developed a new 1.5D two-phase system which takes, among others, effects of pressure, temperature, volatile exchanges (including diffusion and viscous relaxation) between the melt and gas phases into account. First tests with this numerical model have then been conduced in order to see if we manage to reproduce gas content and gas fluxes prior to the July 2013 vulcanian eruption at Tungurahua volcano in Ecuador.

Currently, our ability to interpret the mechanics of magma mingling and mixing is limited by an incomplete understanding of the modes of mixing across all melt fractions and compositions. Here, we present numerical simulations of the emplace- ment of crystal-free magma in crystal-rich reservoirs employing a computational fluid dynamics and discrete element method (CFD–DEM). We performed two runs corresponding to the emplacement of basalt into two end-member types of magmas mush (basaltic and dacitic). We found that the intruded volumes have similar shapes and are surrounded by a halo where the crys- tal volume fraction of the mush is lower. The dynamics of intruded melt are, however, different. Importantly, the mingling of the intruded and host materials starts after emplacement and consists in the incorporation of mush material into the intruded magma. Our findings imply that purely thermo-mechanical processes controlled by grain-scale dynamics are sufficient to explain fundamental aspects of recharge.

Coalescence during bubble nucleation and growth in crystal‐free rhyolitic melt was experimentally investigated, and the percolation threshold, defined as the porosity at which the vesicular melt first becomes permeable, was estimated. Experiments with bubble number densities between 10¹⁴ and 10¹⁵ m⁻³ were compared to four suites of rhyolitic Plinian pumices, which have approximately equal bubble number densities. At the same total porosity, Plinian samples have a higher percentage of coalesced bubbles compared to their experimental counterparts. Percolation modeling of the experimental samples indicates that all of them are impermeable and have percolation thresholds of approximately 80–90%, irrespective of their porosity. Percolation modeling of the Plinian pumices, all of which have been shown to be permeable, gives a percolation threshold of approximately 60%. The experimental samples fall on a distinct trend in terms of connected versus total porosity relative to the Plinian samples, which also have a greater melt‐bubble structural complexity. The same holds true for experimental samples of lower bubble number densities. We interpret the comparatively higher coalescence within the Plinian samples to be a consequence of shear deformation of the erupting magma, together with an inherently greater structural complexity resulting from a more complex nucleation process.