1. n. [Geology]
The process by which the characteristics of rocks are altered or the rock is recrystallized. Metamorphism of igneous, sedimentary, or preexisting metamorphic rock can produce new metamorphic rock. Such alteration occurs as rocks respond to changes in temperatures, pressures and fluids, commonly along the edges of colliding lithospheric plates. The pressures and temperatures at which metamorphism occurs are higher than those of diagenesis, but no clear boundary between the two has been established.