Metamorphic rocks

The metamorphic rocks are formed as the result of the modification, in solid state, of pre-existent igneous or sedimentary rocks, the protoliths, that have been subjected to changes in the temperature and pressure conditions or to tectonic forces. The metamorphic processes provoke changes in the mineralogy and in the texture of the rocks and, in some cases, also in the chemical composition.

The metamorphic reactions take place on the terrestrial crust, and it is considered that they encompass an interval of temperatures that go from 200ºC until the fusion of the rock, which can fluctuate between 650ºC and 1 000ºC. In increasing the temperature, a good part of the minerals that constitute the rocks become unstable and the chemical elements are combined in mineral assemblages typical of the new conditions of pressure and temperature. When the fusion starts, the migmatites, mixture of metamorphic and igneous rocks, are formed.

Depending on the extension covered by the effects of the metamorphism, and the cause of the variations in the temperature and the pressure, the metamorphic processes are classified as regional or local.

Regional metamorphism takes place at the edges of convergent plates, where there is high pressure; this is a process that affects very extensive rocky strips. The size of the mineral crystals become bigger as the temperature increases and different mineral assemblages are formed depending on the chemistry of the original rock. As a consequence of the directional pressures, the new minerals grow with a preferential orientation and the rocks acquire a characteristic foliated structure.

For each type of original rock, areas that represent a specific mineralogical association, and which differ from the adjacent areas, can be recognized. This characteristic allows us to establish metamorphic areas, which differentiate by the appearance or the disappearance of a determined identifiable mineral on the terrain. The one most used is that derived from pelites.

Local or contact metamorphism is produced as a result of the temperature increase in the rocks in which an intrusive body is emplaced, a batholith; the heat released by the magma produces the recrystallization of the rocks, which become massive and hard. The mineralogy varies depending on the lithology of the country rock and on the distance to the intrusive body.

The classification of the metamorphic rocks is based on the mineral assemblages that characterize them and on the new textures of the rocks. These depend on the lithology of the original rock, of the pressure conditions and the temperature reached and on the time during which the metaphoric processes have been in action. This being the result of the superimposition of various processes on the pre-existing rocks, the variability is great. Most metamorphic rocks have proper names, but when one wants to refer to them making mention of the original rock, the prefix meta- is used, followed by the name of the protoliths. For example, the rocks affected by regional metamorphism that come from pelites, are slates, phyllites, schists and gneisses; the corneans are meta-pelites formed by contact metamorphism; the marbles derive from limestone that is affected by regional or local metamorphism; the meta-basites derive from basic igneous rocks, some gneisses derive from granitic rocks, etc.