Generation and Evolution of Cavitation in Magma under Decompression Waves

It's considered that presence of the dissolved water (with concentration up to 5-7 % (wt.)) and viscosity of magma play one of the key roles in volcanic processes. According to the geophysical data, both a character and a structure of explosive volcano eruptions are defined by dynamics of cavitation development and the viscosity increasing orders during the diffusion of the dissolved gas from magma in bubbles. This paper represents a full system of the equations, including the kinetics of phase transitions, which allows us to describe dynamics of a magma melt state in a gravity field behind the front of a decompression wave. The problem is solved numerically, using a known expression for the nucleation frequency as a function of current concentration of gas dissolved in magmatic melt. The notion of diffusion zones which arise around cavitation nuclei is introduced. The latter allow us to define real density of cavitation kernels generated in an unit of magmatic melt volume as a result of phase transitions behind a rarefaction-wave front. The dynamics of the size distribution of cavitation bubbles along a magma-melt column (1km height) and also of the magma viscosity changing dynamically vs. a concentration of dissolved water are investigated. (RFBR 06-01-00317a financial support).