Any rock that has been modified mineralogically due to geological influences such as temperature, pressure and fluid activity, is said to have been metamorphosed. Modifications may be small but are usually sufficiently extensive to form a new mineralogy that reflects the peak conditions attained during that metamorphic episode. Rocks can undergo several episodes of metamorphism and display complex mineralogical relationships. Deciphering the sequence of metamorphism as reflected in the minerals requires considerable expertise and is the realm of a petrologist.
There are many types of metamorphism depending on the conditions within the crust. The three principal types of metamorphism are burial, thermal and regional. Three other types that tend to occur in more restricted environments include retrogressive metamorphism, dynamic metamorphism and impact metamorphism.
Within each of the principal types different grades are recognized depending on the physical and chemical conditions. Different grades are typically recognized by the formation of one or more new minerals. Original bulk rock compositions also play a major role in the formation of new minerals as does the availability of fluid. Clay-rich rocks (pelites), basaltic rocks and calcareous rocks tend to be more easily modified than quartz-rich rocks.
Some structures and textures of the original rock types can survive even though the minerals change and in the absence of sufficient fluid some minerals may also survive even though the conditions are outside the stability field for that mineral. Mantling by a new mineral and restricted diffusion can lead to overlapping mineralogies.