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What are the Controls of Metamorphism?

Published in Metamorphic Geology 2 mins read

The main controls of metamorphism are temperature, pressure (lithostatic and directed stress), fluid activity, and the composition of the parent rock (protolith). These factors work together to transform existing rocks into new metamorphic rocks.

Temperature

Temperature is a crucial factor in metamorphism. As temperature increases, the rate of chemical reactions also increases, leading to the formation of new minerals. The source of heat can be geothermal gradient, magmatic intrusions, or tectonic processes like faulting.

  • Low-Grade Metamorphism: Occurs at relatively low temperatures, typically between 200°C and 400°C.
  • High-Grade Metamorphism: Occurs at higher temperatures, often exceeding 600°C or 800°C.

Pressure

Pressure plays a significant role in metamorphism. There are two main types of pressure:

  • Lithostatic Pressure (Confining Pressure): This is equal in all directions and is due to the weight of the overlying rocks. It causes a reduction in volume and can lead to the formation of denser minerals.
  • Directed Stress (Differential Stress): This is not equal in all directions and is commonly associated with tectonic forces. It causes deformation and foliation (alignment of minerals) in metamorphic rocks.

The depth at which a rock is buried significantly affects the pressure it experiences. Deeper burial results in higher pressures.

Fluid Activity

Fluids, primarily water containing dissolved ions, act as catalysts in metamorphic reactions. They can:

  • Transport ions: Fluids help to transport ions between minerals, facilitating chemical reactions.
  • Promote recrystallization: Fluids can promote the recrystallization of existing minerals into new, more stable forms.
  • Hydrate minerals: Water can be incorporated into the crystal structure of new minerals.

The source of fluids can be from the protolith, magmatic intrusions, or from the breakdown of hydrous minerals during metamorphism.

Composition of the Parent Rock (Protolith)

The chemical composition of the protolith significantly influences the type of metamorphic rock that will form. For example:

  • A shale protolith, rich in clay minerals, will likely transform into slate, phyllite, schist, or gneiss depending on the temperature and pressure conditions.
  • A limestone protolith (composed of calcium carbonate) will metamorphose into marble.

The presence of certain elements in the protolith will limit or promote the formation of specific metamorphic minerals. A rock lacking aluminum, for instance, cannot form aluminosilicate minerals like kyanite, sillimanite, or andalusite, regardless of the temperature and pressure conditions.

In summary, the interplay of temperature, pressure, fluid activity, and protolith composition determines the type and extent of metamorphic changes that occur in a rock.