The parameters that control metamorphism are mainly pressure (P) and temperature (T). Also, metamorphic changes can be plotted on P-T diagrams. It can be noted that metamorphism covers the majority of the PT space. Only the area on the left side of the figure, at HP, very LT is not reached in nature, on Earth. Diagenesis defines the limit at LT, LP. The curve of anatexie (A) indicates the boundary between the solid state transformations of metamorphism and magmatism. This curve indicates the conditions beyond which the rocks begin to melt, producing a magma of granitic composition : the rocks undergo partial melting. This limit is moving, which explains the presence of metamorphic facies at higher HT. These metamorphic facies allow subdivisions to be made in this vast domain. The Steady State Geotherm of the Lithosphère (Géoth.Lithosph.Stable) indicates the increase of T with depth inside a continental lithospheric plate.
It can be pointed out that metamorphic rocks generally record different thermal conditions from those of this geotherm.