Formation of a saturation wave front and dynamics of magma state characteristics in its zone

The influence of the diffusion zone width (parameter χ), which bounds the nucleation process in the nucleus neighborhood, on the dynamics of magma melt state at the initial stage of explosive eruption is numerically analyzed in the context of the Iordanskii-Kogarko-van Vijngaarden model of multiphase media. It is shown that in the region of 5 ÷ 50, the value of χ does not affect the time of forming the front of the zone of magma saturation with nuclei, but determines their maximum density, which decreases by three orders of magnitude, from 510¹⁰ to 810⁷ m⁻³, as the parameter χ grows by one order of magnitude. The most considerable losses of gas dissolved in magma are observed for χ = 5 and, consequently, as soon as 100 ms later after the beginning of decompression, there appear zones behind the wavefront, where the melt viscosity grows to hundreds of thousand Pa·s. Under the restriction of diffusion to bubbles, the decompression wave has a classical profile that is slightly perturbed by the front of saturation zone. However, in spite of the fact that the density of the number of nuclei in the cavitating magma upon removing the restriction of gas diffusion into bubbles remains the same as in the case of restriction, the effect of diffusion of additional mass of gas into bubbles changes radically the wavefield structure. Substantially increasing gas pressure in the bubbles due to diffusion with the melt viscosity growing by several orders of magnitude, which prevents bubble growth, changes appreciably the dynamics of the main characteristics of the cavitating magma state.