Numerical study of nucleation and growth of bubbles in viscous magmas

The nucleation and growth processes of bubbles in viscous magmas with a constant decompression rate have been numerically investigated based on a formulation which accounts for effects of viscosity, as well as diffusivity, interfacial tension, and decompression rate. The numerical solutions show two regimes in the nucleation and growth process, a diffusion-controlled regime and a viscosity-controlled regime, mainly depending on the decompression rate, initial saturation pressure and viscosity. The transition between the two regimes dramatically occurs when the value of the above parameter decreases below about 2 × 10³. In the viscosity-controlled regime abundant tiny bubbles (exceeding 10¹⁴ (m^-3)) form with a relatively high internal pressure. In basaltic eruptions the vesiculation is essentially controlled by diffusion, and the viscosity-controlled regime is limited to very high decompression rate and very small water content. When andesitic magma saturated by water at 10 MPa is decompressed through the propagation of rarefaction wave induced by a landslide, the vesiculation is controlled by the viscosity up to 100 m depth. In a rhyolitic magma for the same situation, vesiculation is controlled by the viscosity over the whole depth of the magma column. -from Author