Convection et dégazage d'un système magmatique : le cas du lac de lave de l'Erebus, Antarctique

Abstract : The observed phenomenon of permanent degassing of the volcano Erebus is accompanied by a cyclic variation of gas composition and level of its crater lake that we proposed to model in the assumption that these fluctuations are caused by the arrival of batch of magma originated at shallow depth, ascending in a conduit through which percolates a continuous flow of gas whose origin is deeper. We first showed that the observed effect of convection on the surface could not be explained solely by thermal convection of a liquid-crystals mixture. If a continuous supply of magma and gas in an open system simulates a pulsating behavior of the surface, this contribution must be large enough for the changes in surface velocity thus generated can be appreciated. The conduit diameter should be sufficient large to ensure the sustainability of convection and maintain the magma above its glass transition temperature. The presence of crystals convection accelerates and improves the efficiency of heat transfer between the upper and lower regions of the magmatic system; these crystals settle at the bottom of the chamber to form a layer of high concentration of about ten meters thickness. The introduction of two batches of magma from different depths of nucleation helped to highlight how their composition and the behavior of the lava lake were sensitive to the temperature at which they are generated. On the other hand, a batch will contribute to the degassing budget in proportions consistent with the observations only if a threshold of sufficient buoyancy was achieved. Otherwise, the migration of the amount of magma required for the degassing will only occur in a conduit of very large diameter. The physical parameters of the crust forming as the surface of the lava lake cools down, such as thickness and permeability, affect its ability to deform under pressure and to allow the trapped pressure to degas under an effusive regime. This pressure in turn influences the porosity of the magma at surface and possibly the degree of degassing of the magma being recirculated toward the magma chamber. This study allowed us to better understand the mechanisms associated with an effusive eruption regime and is a step in understanding the transition to an explosive regime, a major concern for the volcanic monitoring centers.
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Indira Molina. Convection et dégazage d'un système magmatique : le cas du lac de lave de l'Erebus, Antarctique. Volcanologie. Université d'Orléans, 2012. Français. ⟨tel-00767068⟩

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