METAMORPHIC ROCKS

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.

[See alsogreenstone belt, sedimentary rocks]