How does gas content affect a magmas viscosity?
Only limited data exists concerning the effect of volatiles (in particular, F, Cl, S, H2S, SO2, CO, and CO2) on magma viscosity. No doubt, the effect of dissolved water is to lower viscosity, the effect being greater for silica-rich than silica-poor magmas: The solubility of gases in magmas varies with pressure, temperature and composition of both the gas and the magmatic liquid. Because the volume of a melt with dissolved gas is less than that of a melt and separate gas (vapor) phase, solubility increases as gas pressure increases. At constant gas pressure less than total pressure, any increased load pressure on the melt lowers solubility, because the volume of the melt with dissolved gas is greater than that of melt alone. Dissolved water disrupts the framework of linked Si and Al tetrahedra, but where such polymerization is already minor or absent, there is little effect. F and Cl are though to considerably reduce magma viscosities; in contrast, CO2 increases polymerization, and the
