High pressures and temperatures within the Earth's crust can change rocks from one type to another. Scientists call this process metamorphism, which comes from the Greek words for "after" and "form." At the junctions where the Earth's crustal plates collide, pushing up mountain ranges, or where one plate slides underneath another, the original rocks, or protoliths (from the Greek words meaning "first" and "rock") undergo reactions that change the chemical or crystal structure of the rocks with little or no actual melting. The grains of minerals within many metamorphic rocks are aligned in parallel due to the forces pushing on them. The pressure-temperature combination causes some sedimentary rocks to lose water molecules or become anhydrous.

Geologists use the phrase metamorphic grade to describe the degree of metamorphism a rock has experienced. The higher the temperature and pressure the rock has experienced over time, the more its metamorphic grade increases. For example, metamorphism can change mudstone, originally a fine-grained sedimentary rock, into smooth-textured slate, which splits easily into thin sheets. Over time, the slate will become phyllite, a metamorphic rock that breaks into thick slabs. If metamorphism continues, phyllite turns into schist, which is considered a medium-grade metamorphic rock, and eventually into gneiss (pronounced "nice"), a high-grade metamorphic rock. Once the rock starts to melt under still higher temperatures, it becomes a migmatite, and when it melts completely, it turns into magma which ultimately hardens into igneous rock.