Metamorphic rock or metamorphic rock is that caused by the inherent increase of pressure and / or temperature deep in the earth's crust mineralogical changes in rock. The solid state is retained during this transformation. The transformation process is known as metamorphosis .
Compared to other rock-changing processes, such as chemical weathering or diagenesis , rock metamorphosis takes place under significantly increased pressure and temperature conditions. This is often caused by mountain formation or other processes that are more or less closely related to plate tectonics .
During the transformation, new minerals and mineral aggregates are created , the pressure and temperature-dependent formation of which corresponds to the ambient conditions. However, the general chemical composition of a rock does not change during metamorphosis, otherwise one speaks of metasomatosis . In a pressure-stressed metamorphosis, the mineral grains are often aligned in the rock, which reflects the directions from which the greatest pressure occurred. This changes the rock structure (e.g. texture ), which means that metamorphic rocks differ in terms of their structure from chemically similar plutonic rocks that have also formed in the earth's crust .
In contrast to igneous rock and sedimentary rock, there is as yet no uniform nomenclature for metamorphic rock. In practice, different nomenclature systems are used side by side.
Starting from the protolith
If the unmetamorphic parent rock ( protolith ) of a metamorphic rock is recognizable, the name of the protolith is simply prefixed with the prefix meta- , e.g. B. in Meta basalt or Meta grauwacke (the corresponding umbrella terms are Meta volkanit or Meta sediment ).
Starting from the rock structure
The structure and thus the corresponding names reflect whether the metamorphosis was associated with tectonic movements or not. However, the mineral content of a certain mineral class is also partly decisive for this nomenclature.
- As rock massive metamorphic rocks without preferred orientation of mineral grains are called. Banding is not uncommon, but it goes back to the primary stratification of a sedimentary protolith. Metamorphic rocks known as “rocks” arise from contact and submerged metamorphoses.
- All of the following terms refer to rocks whose name-relevant structure goes back to tectonic deformation:
- The term gneiss characterizes medium to coarse-grained metamorphic rocks with a relatively extensive parallel structure. An additional criterion is a high proportion of feldspars (more than 20%). In the case of gneiss, there is also a rough breakdown according to the parent rock : orthogneiss for a non-sedimentary parent rock and paragneiss for a "glaciated" non-carbonate sedimentary rock.
Slate is a rock with a relatively close, layered parallel structure ( foliation ) in the area of <1 cm. In English, a distinction is made between slate and schist :
- "Slates" are very fine-grained slates that have only undergone a very low-grade metamorphosis or no metamorphosis at all, and therefore in fact still have their original mineral inventory. A typical example are the mostly dark paleozoic slates of many German low mountain ranges. They are not to be confused with "schistig" formed clay stones, which were exclusively exposed to diagenetic processes without significant tectonic deformation, e.g. B. the Posidonia slate of the Black Jurassic or the copper slate of the Zechstein (these are called "shale" in English).
- Medium to coarse-grained slates (“ crystalline slates”) with a relatively high degree of metamorphism and therefore extensive mineral transformations are referred to as “Schist” .
- Phyllite refers to a very fine-grained metamorphic rock with a very close, layered parallel structure and silky, shiny cleavage surfaces that often have wrinkled unevenness.
- Mylonite is a fine-grained metamorphite, the schisty texture of which was created in a ductile shear zone .
The rock names, especially the gneiss and crystalline slate, are further specified by placing the names of minerals in front of them, which the corresponding rock contains in larger proportions. The minerals are sorted in ascending order of frequency. For example, one contains sillimanite - garnet - mica schist more mica as garnet and more than garnet sillimanite.
Migmatite is the name for a rock that is characterized by a flow structure as a result of partial melting ( anatexis ). Since metamorphosis takes place by definition without partial melting, migmatites lead over to the igneous rocks.
Based on the mineral inventory
The most frequently used terms are:
- Amphibolite , a rock consisting mainly of amphiboles and plagioclase
- Eclogite , a high -pressure and high-temperature rock characterized by a high proportion of garnet and clinopyroxene
- Marble , metamorphic carbonate rocks, which consist mainly of calcite or dolomite
- Quartzite , metamorphic quartz sandstones, which consist mainly of quartz
- Serpentinite , a metamorphite predominantly composed of serpentine minerals
In addition, there are other designations that cannot be clearly assigned to either the structure or the mineral inventory classification. The term “ fruit slate ” describes a contact metamorphic rock whose slate structure was not created by tectonic movement, as is the case with “real” slates, but is a relic of the primary stratification of the rock.
- Myron G. Best: Igneous and Metamorphic Petrology . WH Freemann & Company, San Francisco 1982, ISBN 0-7167-1335-7 , pp. 341 ff .
- Douglas Fettes & Jacqueline Desmons: Metamorphic Rocks: A Classification and Glossary of Terms . Cambridge University Press, Cambridge 2007, ISBN 0-521-86810-6 .
- Wolfhard Wimmenauer: Petrography of igneous and metamorphic rocks . Enke Verlag, Stuttgart 1985, ISBN 3-432-94671-6 .
- Geology info: classification of metamorphic rocks
- Metamorphic rocks in the mineral atlas
- Metamorphites. E-learning portal of the FU Berlin
- Download BGS Classification Scheme Classification scheme of the British Geological Survey
- ↑ Kate Brodie et al .: Structural terms including fault rock term - Recommendations by the IUGS Subcommission on the Systematics of Metamorphic Rocks: Web version 01.02.07. (PDF; 304 kB) Use of structural terms as recommended by the IUGS sub-commission for metamorphic rocks. In: bgs.ac.uk. Retrieved April 6, 2009 .