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.

Many igneous and sedimentary protolith rocks that are the precursors to metamorphic rocks contain grains in sizes ranging from pebbles to nearly invisible silt particles. As underground pressures and temperatures increase, grains become compacted together, eliminating the pore spaces between the grains and sometimes making it hard for humans to see the grains at all without magnification. On the other hand, some metamorphic rocks still have grains that are visible to the human eye. Slate and phyllite are two types of fine-grained metamorphic rock. Gneiss (pronounced "nice") has medium to coarse grains and may resemble the igneous rock granite. Geologists use grain sizes and colors to help them identify different types of metamorphic rocks in the field.