Introduction

All rocks form at certain temperatures and pressures and are most stable in the conditions under which they form. Therefore, changing the temperature and/or pressure conditions may lead to a different rock that changes to be stable under the new external conditions. If these changes cause the recrystallization of minerals (crystals) under high pressure or high-temperature conditions (but temperatures too low to melt the rock), the new rock is called metamorphic rock. Metamorphism means to change form. A metamorphic change can also occur if the rock’s composition is altered by hot, chemically reactive fluids, causing a change in the mineral content of the rock.

Metamorphic rocks form by the physical and sometimes chemical alteration of a pre-existing rock. To distinguish between the pre-existing rock and the new metamorphic one, the term protolith (proto- meaning “original” and -lith meaning “rock”), or parent rock, is used to describe the pre-existing rock (Figure 13.2). All metamorphic rocks have at least one protolith that was altered during metamorphism. Igneous, sedimentary, and even metamorphic rocks can be altered into completely new or different metamorphic rocks.

A close-up of the sedimentary rock shale (left) and slate (right). — **Figure 13.2:** Heat and pressure alter shale, the protolith, to slate. (*Left:* CC-BY 2.0; James St. John via Flickr; *Right:* CC-BY 4.0; Chloe Branciforte, own work)