Metamorphic rock forms when existing rock is changed by heat, pressure, or chemically active fluids.
The original rock, called the protolith, may be igneous, sedimentary, or older metamorphic rock. During metamorphism, minerals recrystallize or new minerals grow without the rock fully melting.
For example, limestone can become marble, while shale can become slate and then higher-grade rocks such as schist or gneiss. These transformations commonly occur deep underground or near intruding magma.
Metamorphism differs from melting. If rock melts and later solidifies, it becomes igneous rock; if sediments are deposited, compacted, and cemented, they form sedimentary rock.