—New regularities of the viscosity of near-liquid felsic, medium, and mafic hydrous magmas have been established in a wide range of thermodynamic parameters and the Earth’s crust depths (1–30 km). The study was carried out using our new structural and chemical model of reliable predictions and by calculation of the viscosity of magmas of almost any composition. It is shown that these regularities are actually a quantitative physicochemical basis explaining the cause of the relative distribution of intrusive and effusive felsic and mafic rocks in the Earth’s crust. This confirms V.S. Sobolev’s idea of the relationship between the relative distribution of intrusive and effusive felsic, medium, and mafic rocks in the Earth’s crust and the laws of changes in the viscosity of hydrous magmas in a variable P–T field.