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Metamorphic Rocks

Metamorphic rocks can form from igneous rocks sedimentary

rocks or even from other metamorphic rocks

The agents of metamorphism are heat, pressure (stress), and

chemically active fluids

Perhaps the most important agent of metamorphism is heat,

because it provides the energy to drive chemical reactions.

Rocks found near the surface of Earth may be subjected to

intense heat when they are intruded by magma, a process

called contact metamorphism.

gneiss

slate

Mica schist

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Here the adjacent host rock is baked by the emplaced

magma.

Rocks may also be subjected to high temperatures if they are

carried deep within Earth.

Pressure, like temperature, also increases with depth.

Buried rocks are subjected to pressure, or stress, exerted by the

load above.

This confining pressure is applied equally in all directions and

causes a reduction in volume, thereby generating a more

compact (dense) rock.

Rocks are also subjected to forces during mountain building that

are unequal in different directions, differential stresses.

Chemically active fluids, most commonly hot water containing

ions in solution, also enhance the metamorphic process.

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For example, along oceanic ridges seawater circulates through

hot basaltic rocks, transforming the existing iron- rich minerals

into metamorphic minerals such as serpentine and talc.

Textural and Mineralogical Changes

The degree of metamorphism is reflected in the rocks texture

and/or mineral composition.

When rocks are subjected to low-grade metamorphism, they

become more compact and thus more dense.

Under more extreme pressures and temperatures, some

minerals recrystallize to form larger crystals.

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During metamorphism the crystals o some minerals, such as

micas which have a sheet structure, and hornblende which has

an elongated structure, will recrystallize with a preferred

orientation.

The new orientation will be essentially perpendicular to the

direction of stress.

The resulting mineral alignment usually gives the rock a layered

or banded appearance termed foliation.

Not all metamorphic rocks have a foliated texture. Metamorphic

rocks composed of only one mineral that forms

equidimensional crystals are generally non-foliated.

In some environments, new minerals are formed during the

metamorphic process. For example, rocks adjacent to a large

magma body would be altered by ion-rich hydrothermal

solutions released during the latter stages of crystallization.