Metamorphic rocks are composed of minerals formed by the alteration of pre-existing rocks, such as igneous, sedimentary, or other metamorphic rocks, due to intense heat, pressure, and chemically active fluids. This transformation, known as metamorphism, does not involve complete melting but rather recrystallization and the development of new mineral assemblages within the parent material. The mechanism of metamorphism dictates the resulting rock structure; for instance, rocks subjected to directed stress often exhibit foliation, where mineral grains align in layers or bands. These rocks are crucial components of the Earth's crust, providing insights into the geologic history and physical conditions under which deep crustal structures formed.