Regional metamorphism takes place over large areas of Earth's surface where tectonic plates crash together, pushing up mountain ranges. The metamorphic processes typically happen between temperatures of 350-650 degrees C (660-1,200 degrees F) and at depths of 5-20 km (3-12 miles). High pressures from these enormous collisions cause the rocks to recrystallize and the new mineral grains align into an arrangement like a layer cake, or a sponge when it is squeezed; that texture is called foliation. Low-grade metamorphic rocks, like slate, split easily into sheets. High-grade metamorphic rocks, like gneiss, may be foliated but do not break into sheets.

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.