Metamorphic rocks are formed from existing rocks that undergo changes due to high temperature, pressure, or chemically active fluids. This process, known as metamorphism, alters the mineral composition and structure of the original rock, which can be either igneous or sedimentary. Common examples of metamorphic rocks include schist, gneiss, and marble. These rocks often display unique textures and patterns, such as foliation, which is the alignment of minerals in layers. Metamorphic rocks are important in understanding geological processes and the history of the Earth's crust.