Metamorphic rock. Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means change in form. The original rock is subjected to heat and pressure, causing profound physical and/or chemical change.The protolith may be sedimentary rock, igneous rock or another older metamorphic rock.he original rock is subjected to heat and pressure, causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic rock.