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.

Thanks to high pressures in a particular direction, the grains in some metamorphic rocks become aligned parallel to each other, giving the rock a layered appearance. Scientists call this property foliation, from the Latin word folium, which means "leaf." In slate, which is a fine-grained metamorphic rock, the foliation occurs at a small scale due to the alignment of tiny flakes of mica and other silicate materials. Geologists say slate has good cleavage because it tends to break into parallel thin sheets. In coarse-grained metamorphic rocks such as gneiss (pronounced "nice"), the foliation may occur as alternating light- and dark-colored bands. These layers may also appear wavy or curved because the forces that changed the rock acted unevenly on the rock. Rocks that metamorphose due to high temperature rather than high pressure may not show any foliation.