Our final stop in looking at the basic rock types defined by the rock cycle are the metamorphic rocks . A metamorphic rock results from extensive changes to texture and mineralogy of the parent rock. This usually occurs with changes in pressure and temperature that occur above the point of diagenesis and below the point of actually melting the rock. One thing to remember, metamorphism occurs while the rock remains solid. If it reaches a point where the rock melts, it then becomes an igneous rock.
Metamorphic rocks are the result of the dynamic nature of our Earth.
Unlike sedimentary and igneous rocks, metamorphic rocks are very complex. Given different conditions of heat and pressure, the same parent rock can form any number of different metamorphic rocks. Because the same parent rock can result in different metamorphic rocks, geologists classify metamorphic rocks in groupings called facies . The different facies reflect the range of pressures and temperatures that the parent rock may be subjected to. Knowing the facies of the metamorphic rock is useful when looking at them in the lab, but in the field, a geologist doesn't always know the specifics of temperature and pressure that might have affected the parent rock.