DEFINITION OF METAMORPHISM

Metamorphism, a term derived from 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. Early changes that occur to rocks, generally

less than 390°F (200°C), are referred to as diagenesis. However,

when temperatures rise above 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, and these

hardly change at all.

When rocks are heated, certain minerals become unstable

and others become stable. Chemical reactions transform

one assemblage of minerals into a new assemblage. Most

temperature changes are accompanied by pressure changes,

and it is the combined P-T fluid composition that determines

how the rock will change.

In liquids pressures are equal in all directions, but in rocks

pressures may be greater or lesser in one direction, and we

refer to them 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 metamorphism. In

general, the longer the reaction time the larger the mineral

grains, and the more complete the metamorphic changes.

Grades of Metamorphism

Low-grade metamorphism refers to changes that occur at low

temperatures and pressures whereas high-grade metamorphism

refers to changes that occur at high temperatures and

pressures. At progressively higher grades of metamorphism

the high temperature drives the water out of the pore spaces

and eventually out of the hydrous mineral structures, so that

at very high grades of metamorphism, the rocks contain fewer

hydrous minerals (e.g., micas). Prograde metamorphism refers

to changes that occur while the temperature and pressure are

rising and pore fluids are abundant, whereas retrograde metamorphism

refers to changes that occur when temperature and

pressure are falling. At this stage, most fluids have already

been expelled and the retrograde changes are less pronounced.

If this were not so, then all metamorphic rocks would revert

back to clays stable at the surface.

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