Controls on magma outgassing and their influence on the effusive-explosive transition of volcanic eruptions

The effusive-explosive transition of silicic volcanic eruptions is explored using a combination of 1D, and 2D multiphase numerical models of conduit flow. The 1D model is steady-state and is used to look into the controls on vertical gas loss through permeable magma and includes turbulence effects (using the Forchheimer equation). Constitutive laws for the permeability and the inertial coefficient were derived from 3D textural analysis of natural pumice that connect the macroscopic equations to the internal geometry of bubble network within the magma (i.e. bubble number density, tortuosity and roughness). Results show that this internal geometry strongly influences the outcome of an eruption. Explosive eruptions are promoted by 1) high bubble number density and high tortuosity because they imply small complex channels through which gas escape is difficult, and 2) high roughness because it leads to an increase in turbulence effects, which keep the gas trapped inside the magma. In nature, such conditions can be reached either when bubble coalescence is inefficient (preserving a high bubble number density), when the strain rates affecting bubble shape are low (deformed bubbles tend to lower tortuosity), or when magma viscosity is high (low viscosity promotes bubble wall smoothing). Spatial and temporal variability of conduit flow dynamics are investigated with the unsteady, 2D model, which is a modified version of the Multiphase Flow with Interphase Exchanges (MFIX) code. Results examine the sensitivity to initial conditions on the evolution of volcanic eruptions.