Metamorphism and metamorphic rocks

Metamorphism, a term derived from the Greek, means change of form or shape. Geologists use the term to describe changes in the minerals, chemistry, and texture within a rock. Metamorphism is typically induced by increases in pressure and temperature from burial, regional tectonics, or nearby igneous intrusions.

Any previously formed rocks may be deeply buried by sedimentary cover, affected by regional plate-boundary processes, or be heated close to an igneous intrusion, changing the temperature and pressure conditions from when and where they were formed. Diagenesis refers to early changes that occur to rocks, generally below 390°F (200°C). When temperatures rise above 390°F (200°C), the changes become more profound and are referred to as metamorphism.

When sedimentary rocks are deposited they contain many open spaces filled with water-rich fluids. When these rocks are deeply buried and subjected to very high temperatures and pressures, these fluids react with the mineral grains in the rock and play a vital role in the metamorphic changes that occur. These fluids act as a hot, reactive juice that transports chemical elements from mineral to fluids to new minerals. This is confirmed by observations of rocks heated to the same temperature and pressure without fluids, which hardly change at all.

When rocks are heated, certain minerals become unstable and others stabilize. Chemical reactions transform one assemblage of minerals into a new assemblage. Most temperature changes are accompanied by pressure changes, and it is the combined pressure-temperature (P-T) fluid composition that determines how the rock will change.

In liquids, pressures are equal in all directions, but in rocks pressures can be greater or lesser in one direction, and they are referred to as stresses. Textures in metamorphic rocks often reflect stresses that are greater in one direction than in another. Sheets of planar minerals become oriented with their flat surfaces perpendicular to the strongest or maximum stress. This planar arrangement of platy minerals is known as foliation.

Time is also an important factor in metamor-phism. In general, the longer the reaction time, the larger the mineral grains and the more complete the metamorphic changes.

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