During metamorphism, the minerals in the protolith or the original rock will need to change. They need to equilibrate to the new temperature and pressure conditions to form new and stable minerals. This process transforms the protolith, which could be igneous, sedimentary, or metamorphic, into a new metamorphic rock. The resulting rock is denser and more compact. Note that in metamorphism, the protolith does not melt. Even under extreme heat and pressure changes, the protolith was only squished, folded or smashed during the entire process.

Metamorphic rocks may be divided into two, the foliated and nonfoliated. Foliation is when certain minerals align themselves in a parallel arrangement often giving a striped or banded look. The parallel alignment is the response of platy or elongated minerals when the applied pressure squeezes the protolith. Examples of foliated rocks are phyllite, schist, and gneiss. Nonfoliated metamorphic rocks do not contain any platy or elongated minerals so even if great pressure is applied; no foliation will appear. An example of non-foliated metamorphic rock is marble which is metamorphosed limestone.