In all cases fluids are usually involved. The deformation accompanying metamorphism is often remarkably similar at all size scales. Provides the energy to drive chemical reactions. Expels water from hydrous minerals such as clay. May be very intense along contacts with molten magma.
Chemical reactions are pressure-dependent.
Confining pressure - forces act equally in all directions. Some water in the pore spaces of virtually every rock. Water acts as a catalyst by aiding ion migration. Free waters accompanying metamorphic processes are called hydrothermal solutions. The degree of metamorphism is reflected in the texture and mineralogy of metamorphic rocks.