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Definition: metamorphic rock from The Penguin Dictionary of Science

One of the three main types of rock: metamorphic rock has been changed by a number of processes such as heating or high pressure. Metamorphic rocks are significant because they constitute a large part of the continental crust. Examples of metamorphic rock are marble and slate. Compare ➤igneous rock; sedimentary rock.


Summary Article: metamorphic rocks
from Encyclopedia of Environmental Change

Rocks that have been altered in the solid state (recrystallised) as a result of changes in temperature, pressure and/or chemical environment (e.g. hydrothermal effects). Recrystallisation involves changes to minerals and to their texture (shape or arrangement). Most metamorphic rocks have a crystalline texture and a distinctly anisotropic fabric. Contact metamorphism converts rocks in a metamorphic aureole adjacent to an igneous intrusion into hornfels. Regional metamorphism results from heat and stress during orogenesis. mudstone is converted into phyllite, slate, schist and gneiss with increasing degree of metamorphism. Basic igneous rocks are altered to amphibolite, sandstone to psammite or, if rich in quartz, to metaquartzite and limestone to marble. Dynamic metamorphism results from rock deformation and forms rocks such as mylonite in fault zones. Other categories of metamorphism are summarised in the Table.

Much of the geological record, particularly of the precambrian, is preserved in metamorphic rocks, and their interpretation yields important information about the evolution of environments, the atmosphere, past climate and life on Earth. Orogenic metamorphism in the past may have been associated with the release of carbon dioxide into the atmosphere: although the significance of this process is controversial, it suggests that metamorphism may have a long-term effect on climatic change. Although usually considered in relation to rocks, metamorphic processes also underlie the crystalline transitions between snow, firn and glacier ice.

Metamorphic rocks Types of metamorphism associated with the formation of metamorphic rocks.

Location

Process

Description

Local metamorphism

Contact metamorphism

Metamorphic rocks adjacent to and clearly related to igneous rocks

Dynamic metamorphism

Metamorphic rocks associated with severe deformation along fault or shear zones

Impact metamorphism

Metamorphic rocks associated with high pressure-temperature regimes caused by meteorite impact

Micro-contact metamorphism

Small-scale changes due to high-temperature lightning strikes (creating fulgurites)

Regional metamorphism

Orogenic metamorphism

Metamorphic rocks formed in association with subduction and collision-related zones of orogenesis

Burial metamorphism

Metamorphic rocks buried in sedimentary basins, where higher pressures and temperatures have formed new minerals

Oceanic metamorphism

Metamorphic rocks altered by circulating heated seawater driven by hydrothermal activity at midoceanic ridges (MORs)

JBH

[See also greenstone belt, sedimentary rocks]

  • Best, MG (2003) Igneous and metamorphic petrology. Oxford: Blackwell.
  • Bucher, K; Grapes, R (2011) Petrogenesis of metamorphic rocks, 8th edition. Berlin: Springer.
  • Fettes, D; Desmons, J (eds) (2007) Metamorphic rocks: A classification and glossary of terms. Cambridge: Cambridge University Press.
  • Fry, N (1991) The field description of metamorphic rocks. Bath: Geological Society.
  • Kerrick, DM; Caldeira, K (1998) Metamorphic CO2 degassing from orogenic belts. Chemical Geology 145: 213-232.
John B. Hunt
University of Gloucestershire
© by SAGE Publications Ltd.

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