metamorphic rocks
Introduction
Metamorphic rocks (from the Greek meta, change, and morphe, form, “change of form”)[1] are rocks formed by the modification of other pre-existing rocks inside the Earth through a process called metamorphism. Through heat and/or pressure, the transformation of rocks that undergo structural and mineralogical adjustments occurs in chemically active fluids.[2] The agents of metamorphism make it possible for igneous rocks, sedimentary rocks or other metamorphic rocks, when they are subjected to pressures ranging from less than 1000 to up to 16,000 bar "Bar (pressure unit)"), at temperatures ranging from 200 to 1000 °C,[3] and/or an active fluid, cause changes in their composition, providing new substances to them. The rock that is generated will depend on the composition and texture of the original rock, the time it was subjected to the effects of the so-called metamorphic process, as well as the agents of metamorphism itself.[1] The precursor of a metamorphic rock is called protolith.[4].
Metamorphic processes produce many changes in rocks, including an increase in density, growth of larger crystals, reorientation of mineral grains into platy or banded textures, and the transformation of low-temperature minerals into high-temperature minerals.[2] Because of this, there are many ways to conveniently classify metamorphic rocks: For example, they can be grouped into broad lithological types; Other criteria are based on texture (where pressure and temperature conditions intervene) and mineralogy, chemical classes, degree of metamorphism or on the concept of metamorphic facies. A simple and practical method consists of taking into account the type of metamorphism that gave rise to the rocks and dividing them into two main groups according to their texture, that is, foliated and non-foliated.[1].
Origin
Contenido
Las rocas metamórficas son uno de los tres principales tipos de rocas. Se distinguen de las rocas ígneas, que se forman a partir del magma fundido, y las rocas sedimentarias, que se forman a partir de los sedimentos erosionados de la roca existente o precipitados químicamente de los cuerpos de agua.[5].
Las rocas metamórficas se forman cuando la roca existente se transforma física o químicamente a temperatura y/o presión elevadas, sin llegar a fundirse en gran medida. La importancia del calentamiento en la formación de rocas metamórficas fue observada por primera vez por el naturalista escocés pionero, James Hutton, quien a menudo se describe como el padre de la geología moderna. Hutton escribió en 1795 que algunos lechos rocosos de las Tierras Altas de Escocia habían sido originalmente rocas sedimentarias pero que se habían transformado por el gran calor.[6].