What does creation of metamorphic rocks have to do with plastic deformation?

Plastic deformation occurs when pressure is applied to a substance and this results in a change of shape of the solid. This occurs to rocks when they undergo metamorphism by pressure. A metamorphic rock is formed by the transformation of preexisting solid rocks under the influence of temperature and pressure. Extreme pressure in metamorphism causes some rocks to be foliated or have a banded appearance. Granite undergoes metamorphism to form gniess.

Metamorphic rocks are rocks formed by modification in the interior of the Earth through metamorphism with heat, pressure and chemical fluids. Plastic deformation is the mode of deformation in which the material does not return to its original norm after being deformed with irreversible thermodynamic changes. Both are related in that the metamorphic rocks were created by means of modification by thermal changes among others and the plastic deformation does not make a creation but a deformation, however, both refer to the change that the rocks and their behavior can have by means of treatments.