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.

The elements in metamorphic rocks do not change due to high temperatures and pressures. Iron will remain iron, silicon will still be silicon, and so forth. The basic chemical composition of the rocks remains the same before and after metamorphism. For example, limestone is a sedimentary rock made largely of calcium carbonate; the right conditions of high temperatures and pressures, usually found in mountain-building regions, can recrystallize the minerals in limestone to make marble, which also consists mostly of calcium carbonate. Likewise, sandstone, a sedimentary rock made from sand-sized grains of quartz, can metamorphose into quartzite, a hard rock consisting of interlocking crystals of quartz.