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Metamorphism: a solid-state transformation of pre-existing rock into a new rock that is texturally and/or mineralogically distinct.

Temperature, pressure, and the presence of fluid (primarily water or CO₂) are the variables that determine the nature of the metamorphic rock that develops.

The metamorphic grade is a measure of the intensity of the metamorphism that the rock has experienced, which is primarily related to temperature.

Low-grade metamorphism starts after lithification of sediment (often called diagenesis). These are the least intensely deformed and metamorphosed

High-grade metamorphism can entail partial melting, approaching the upper boundary of metamorphism before transitioning into igneous rocks. These are the most intensely deformed and metamorphosed.

The metamorphic grain size increases with increasing metamorphic grade, BUT beware of relict (inherited) grains.

Regional (Dynanothermal) Metamorphism:

• Occurs over hundreds or thousands of square kilometers.
• Were buried beneath thick accumulations of other rocks at some time, where they experienced higher temperatures and pressures.
• The weight of overlying rocks creates most of the pressure, but convergent plate boundaries creates pressure form the side.

Contact Metamorphism:

• Primarily temperature metamorphism due to intrusion of magma.
• Fluids released from the magma may accentuate the metamorphic changes.
• Zone of country rock around the intrusion that has been metamorphosed is called a metamorphic aureole.

Changes During Metramorphism:

1. Recrystallization of existing minerals, usually into larger minerals. Takes place in both contact and regional metamorphism;
2. Development of new minerals and disappearance of old ones;
3. Deformation and reorientation of existing mineral crystals and growth of new ones with distinctive orientation.

Overall result = a rock with a different texture and usually a different mineral content.

Directed pressure deforms and reorientates minerals, as at convergent boundaries.

Lithostatic pressure = burial pressure: pressure distributed equally in all directions.

Metamorphic Textures

Metamorphic Minerals

Metamorphic Minerals & Metamorphic Grade

Parent Rock: often called the protolith. The higher the grade of metamorphism, the more difficult it is to determine what the protolith was.

Classification & Identification of Metamorphic Rocks

Metamorphic Zones

Distinguishing Among Igneous, Sedimentary, & Metamorphic Rocks