A protolith is the original rock from which a metamorphic rock forms. It can be any type of rock, including igneous, sedimentary, or another metamorphic rock. When subjected to heat, pressure, or chemically active fluids, the protolith undergoes changes in its mineral composition and texture, resulting in a new rock type. The process of metamorphism transforms the protolith into a metamorphic rock, such as schist or gneiss. Understanding protoliths is essential in geology, as they provide insight into the conditions and processes that shaped the Earth's crust over time.