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Numerical modeling of the physical processes causing the reawakening of a magmatic chamber and of the associated geophysical signals

Abstract : The reawakening of volcanoes asleep since hundreds or thousands years (ex. Komagatake, 1640; Pinatubo, 1991) is a mysterious phenomenon. Volcanic eruptions are the surface expressions of processes occurring in the underlying magmatic systems. Long-lived magmatic reservoirs located in the upper crust have been shown to result from the accumulation of intrusions of new magmas, and spend most of their dwelling time as mush. A mush is a magma that is cooled and crystallized, in which the crystals are touching each other to form a rigid framework, that inhibit its ability to flow. The generation of eruptible conditions requires destabilizing the force chains link crystals in contact. Such destabilization is often thought to be triggered by the recharge of the mush with new magma. A better understanding of the physical processes occurring in magmatic reservoirs by the emplacement of new magma is required to identify the conditions that primer in volcanic eruptions. The characterization of the effects of the intrusion on the physical conditions of the magmatic reservoir and on the associated geophysical signals is necessary to best mitigate volcanic hazards.This thesis explores first the physical processes controlling the motions of the crystals in magmas and mush. Previous studies have not considered the presence of lubrication forces. These hydrodynamic forces are opposed to the relative motions between neighboring crystals. By developing scaling relationships of the importance of the forces controlling the motion of the solids, and numerical simulations using a Discrete Element Method coupled with a Computational Fluid Dynamic approach (CFD-DEM), we constrained the effects of lubrication on magmas and defined a dimensionless number able to predict lubrication importance. Results show that lubrication is opposed to the onset or the arrest of motions within the mush. Neglecting lubrication results in underestimating the duration of transient dynamics, but does not affect the quantification of steady-state dynamics.We then used CFD-DEM simulations to explore the emplacement mechanisms of mobile magmas within mush and their effects on the magmatic reservoir. Results show that the behavior of the intrusion is controlled by the injection velocity and by the density contrast between the two melt phases of the resident mush and the intruded material. Under most natural conditions, simulations suggest that the intruded magma is expected to pond at the base of the mush and to be emplaced as a horizontal layer. This scenario does not lead to a high degree of mixing nor to conditions for which an eruption may be triggered rapidly. The recharge in new magmas, however, generates conditions propitious for the extraction of eruptible magmas from the mush. It appears that the unfolding of a recharge event depends on the evolution of the density contrast between the mush interstitial melt and the intruded melt and on the thermal exchanges following the intrusion.The detection of volumes composed by eruptible magmas from seismic signals requires knowledge of their seismic properties, which are yet poorly constrained. We used the coupled phase approach to compute the propagation velocities and attenuation coefficients of seismic waves in magmas. This approach is based on the linearization of the continuum conservation equations that control the motions of the fluid and the solids. Results show that, in crystal-bearing magmas, the propagation velocity of compressional waves is mainly controlled by the crystal volume faction. The measure of the intrinsic attenuation coefficient is required to distinguish the chemical composition of the magmas. The joint monitoring in time of seismic waves velocities and intrinsic attenuation coefficients seems the most suitable method to detect the initiation of an eruption from a magmatic reservoir.
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Alexandre Carrara. Numerical modeling of the physical processes causing the reawakening of a magmatic chamber and of the associated geophysical signals. Volcanology. Université Grenoble Alpes, 2019. English. ⟨NNT : 2019GREAU031⟩. ⟨tel-02497014⟩



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