Low-grade metamorphism refers to changes that occur at low temperatures and pressures, whereas high-grade metamorphism refers to changes that occur at high temperatures and pressures. At progressively higher grades of metamorphism the high temperature drives the water out of the pore spaces and eventually out of the hydrous mineral structures, so that at very high grades of metamorphism, the rocks contain fewer hydrous minerals (e.g., micas). Prograde metamorphism refers to changes that occur while the temperature and pressure are rising and pore fluids are abundant, whereas retrograde meta-morphism refers to changes that occur when temperature and pressure are falling. At this stage, most fluids have already been expelled and the retrograde changes are less pronounced. If this were not so, then all metamorphic rocks would revert back to clays stable at the surface.
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