Physical Geology

Metamorphic rocks

Metamorphism (L. meta - to change, morph - form) is the process whereby rock is changed due to heat and/or pressure, while the rock is in a solid state (no melting!). These changes do not involve the removal or addition of materials to the rock so much as they involve the rearrangement of ions and mineral crystals to produce new mineral and new textures in the rock.

Metamorphism is caused by a variety of geological processed related to plate collision, burial, and intrusion. Large regions of rock that are highly metamorphosed usually result from plate collisions and their resulting mountain-building episodes. Thus, geologists look to these regions of metamorphic rock to show us the timing and location of collisions and mountain ranges long since past and eroded flat.

Metamorphic Rock in the Eastern United States

Most of New England (including New York City and the bedrock buried beneath the sands of Long Island) is composed of metamorphic rock. The presence of this rock indicates to geologists that this region was once located within the core of a high mountain range. In fact, different provinces of metamorphic rock in New England and along the eastern margin of North America show that four major episodes of mountain building occurred in these areas over the last billion years.

In NYC, the Fordham Gneiss was metamorphosed in an ancient collision that occurred 1.1 Billion years ago. Above the Fordham Gneiss lie layers of Inwood Marble and Manhattan Schist - themselves metamorphosed during a tectonic collision that occurred some 450 million years ago.

The hows and whys of Metamorphism

There are four main ‘geological situations’ that produce the conditions neccesary for metamorphism to occur.

Contact metamorphism: This is metamorphism induced by heating of country rock in contact with an igneous intrusion. The metamorphosed zone of baked rock surrounding the intrusion is called a metamorphic aureole. There is no pressure involved with contact metamorphism. Temperature is dominant factor.

Regional metamorphism: Metamorphism on a very large scale due to plate collision and subduction. Regionally metamorphosed rocks are often highly deformed structurally.

Shock metamorphism: Metamorphism of rock at the site of a meteorite impact.

Temperature, Pressure, and Metamorphic Grade

Metamorphic changes in rock begin to occur at high temperatures and pressures At these and greater Pand T the minerals normally present in in crustal rocks become unstable and begin to rearrange both shape and composition to achieve forms that are stable (in equilibrium) at the higher P and T.

Metamorphism at relatively low combinations of P and T is called low grade metamorphism. High grade metamorphism occurs at high combinations of P and T.

How does metamorphism change a rock?

Recrystallization: A change in mineral composition as atoms rearrange to form new crystal structures that are stable at higher temperatures and pressures. This can occur in response to both heat and pressure.

Regrowth: Rocks that are being heated, but not being squeezed from any particular direction, tend to recrystallize to form larger, more uniform, interlocking mineral grains. This occurs in contact metamorphism, and in burial metamorphism, where the high water content of the newly deposited sediments tends to even-out the pressure being directed from above.

Strain: Regional metamorphism produces differential stress - pressure oriented in a particular direction. Differential stress will cause mineral grains to regrow and recrystallize perpendicular to the direction of stress.

Movement of rock due to tectonic stress can also cause shear as different layers of mineral slide past one-another. Shear also causes mineral growth and elongation in the direction of shear. Shear also works to pulverize mineral grains and string the fragments along the direction of shear - creating elongated mineral layers called lineations.

Metamorphic Rock Textures

Growth along preffered orientations due to stress and shear is particularly noticable with the sheet silicates such as chlorite and mica, which often grow into large, oriented crystals in metamorphic rocks. The layered appearance that results from the growth and reorientation of mineral grains is called foliation.

In foliated rocks where the grains are too small to see, foliation is expressed as cleavage - the tendency for the rock to break in a particular direction.

Foliation that is obvious because of the growth of large crystals of sheet silicates such as mica is called schistocity.

Foliation expressed as distinct layers of different minerals is called banding.

The different kinds of metamorphic rocks:

If we look to where metamorphism most commonly occurs, at the edges of two colliding tectonic plates, we can identify some very general metamorphic environments or facies.

Metamorphic Rocks Produced in different P-T environments

The kind of metamorphic rock that results from metamorphism depends on two factors:

The parent rock or protolith.

The P-T facies

High P / high T

Slate: clay minerals begin to recrystallize into micas, growing larger and becoming oriented to create a ‘slatey’ cleavage.

Phyllite: micas continue to grow, becoming almost visible to the naked eye. Rock has a shiny appearance along cleavage due to larger mineral surfaces.

Schist: micas become large grains, rock is highly foliated. Other minerals present also attain large size. High grade metamorphic minerals such as garnet and kyanite are often present.

Gneiss: non-mica minerals are numerous enough to form segregated layers that lend a distinctly banded appearance to the rock.