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METAMORPHIC ROCKS

METAMORPHIC ROCKS

SRI Petrography ID: 4

Names

[de] METAMORPHE GESTEINE
[en] METAMORPHIC ROCKS

Description

Metamorphic rocks are rocks formed by the alteration of preexisting rocks (igneous, sedimentary or other metamorphic rocks) due to changing geological conditions: variations in temperature, pressure, mechanical stress.
Structure and mineralogy of a metamorphic rock reflect the type of metamorphism and the composition of the parent rock.
It can be distinguished between regional and contact metamorphism: regional metamorphism covers large areas due to movement of geological plates, contact metamorphism is a local to regional phenomenon: close to intrusive magmatic bodies and at the base of lava flows.
Metamorphic rocks are classified by type of facies depending on their mineral assemblages in context with certain temperature and pressure conditions (high pressure – low pressure, high temperature – low temperature).
Most metamorphic rocks show typical metamorphic texture (banding, foliation, layering, folding) and differ from igneous rocks that usually show no orientation of their grains. Sedimentary rocks, on the other hand, mostly show sedimentary textures (banking, bedding).

Color

variable

Grain Size

Coarse grained, fine grained, depending on type

Texture

variable, often grain orientation, banding, foliation, layering

Minerals

variable, most common:
index minerals for metamorphic facies: sillimanite, kyanite, staurolite, andalusite, garnet.
Other typical minerals in metamorphites (but also in magmatites and partly in sedimentites): feldspar, quartz, mica, olivine, pyroxene, amphibole

Occurences

all over the world

Classification

Narrower Terms

igneous protolith

sedimentary protolith