Introduction to Physical Geology
Metamorphic Rocks
There are two major kinds of metamorphism: regional and contact.
Regional metamorphism. Most metamorphic rocks are the result of regional metamorphism (also called dynamothermal metamorphism). These rocks were typically exposed to tectonic forces and associated high pressures and temperatures. They are usually foliated and deformed and thought to be remnants of ancient mountain ranges.
Metamorphic grades. The different groups of minerals, or
assemblages, that crystallize and are stable at the different pressure and temperature ranges during regional metamorphism distinguish distinct
metamorphic grades, or
faces. The grades are usually named for the dominant minerals or colors that identify them (Figure
1 ).
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In general, proceeding from low grade (lower pressure and temperature) to high grade (higher pressure and temperature), the following facies are recognized:
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Zeolite: low temperature, low pressure
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Prehnite-pumpellyite: low temperature, low-medium pressure
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Greenschist: low-medium temperature, low-medium pressure
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Blueschist: low-medium temperature, high pressure
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Amphibolite: medium-high temperature, medium-high pressure
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Granulite: high temperature, high pressure
Contact metamorphism. Contact metamorphism (also called thermal metamorphism) is the process by which the country rock that surrounds a hot magma intrusion is metamorphosed by the high heat flow coming from the intrusion. The zone of metamorphism that surrounds the intrusion is called the halo (or aureole) and rarely extends more than 100 meters into the country rock. Geostatic pressure is usually a minor factor, since contact metamorphism generally takes place less than 10 kilometers from the surface.
