Magmas in motion: Degassing in volcanic conduits and fabrics of pyroclastic density current.

摘要

Volcanoes are caused by the transport of magma batches from the Earth's crust to the surface. These magmas in motion undergo drastic changes of rheologic properties during their journey to the surface and this work explores how these changes affect volcanic eruptions. The first part of this study is devoted to the dynamic aspects of degassing and permeability in magmas with high pressure, high temperature experiments on natural volcanic rocks. Degassing is measured by the influence of decompression rate on the growth of the bubbles present in the magma while permeability is deduced from the temporal evolution of these bubbles. The parameterization of our results in a numerical model of volcanic conduit flow show that previous models based on equilibrium degassing overestimate the acceleration and the decompression rate of the magma. Assessing permeability effects derived form our results show that the transition between explosive and effusive eruptions is a strong function of the magma initial ascent rate.; The second part of this work is a unification of two end-members of pyroclastic currents (highly concentrated pyroclastic flows and dilute, turbulent pyroclastic surges) using theoretical scaling arguments based on multiphase physics. Starting from the dynamics of the particle interactions with a fundamental eddy, we consider the full spectrum of eddies generated within a turbulent current. We demonstrate that the presence of particles with various sizes induces a density stratification of the current, leading to its segregation into a basal concentrated part overlain by a dilute cloud. To verify our predictions on the interactions of such a segregated pyroclastic current with its surroundings (hills and sea), we studied the products of the 2050 BP caldera-forming eruption of Okmok Volcano (Alaska). This field study allowed us to reconstruct the eruptive sequence and to validate the main aspects of our theoretical model, such as the superposition of a dense and dilute part, their decoupling at sea entrance and the characteristics of the particles they transport.