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Earth & Space Science: Session 5

A Closer Look: Metamorphic Rocks

What are metamorphic rocks?

metamorphic change
Metamorphic rock (left) compared with
sedimentary rock (right).

Metamorphic rock is rock that has physically and chemically changed, or "morphed," into new rock. The word "metamorphic" has its origins in classical Greek and means "to change form." Rock of any type (sedimentary, igneous, or metamorphic) that is subjected to high pressure, high temperatures, and/or reactions with chemical solutions can be converted to metamorphic rock. This transformation can involve changes in a rock’s texture (grain size and shape), fabric (how the grains are oriented relative to one another), chemical composition, and mineral content. The rock remains solid as these changes occur. Through the process of metamorphism, the original rock, or protolith, changes into a new metamorphic rock.

How are these high pressures and temperatures generated?

The answer lies in the processes of plate tectonics. Plates that move against each other produce huge forces that create high pressures and temperatures that can deform rock by bending or breaking it. Rock can also be buried and metamorphosed when plates collide. Temperatures and pressure within the Earth increase with depth, so that rock deep in the crust will experience extreme heat and pressure. Rock can also be subjected to high temperatures in regions of volcanism as well as in places beneath the Earth where magma intrudes into the rock above it.

What types of metamorphism exist?

Regional Metamorphism: Many metamorphic rocks form by regional metamorphism, named for the large areas of the crust that are affected. Regional metamorphism usually results from mountain building processes, which are caused by the collision of tectonic plates. These collisions compress and thicken the crust and cause considerable rock deformation.

High Pressure Metamorphism: Some metamorphic rock forms at high pressures but at temperatures that are relatively low. This type of metamorphism occurs at subduction zones. Here, high pressures result when one plate is submerged under the mantle. Temperatures remain relatively low because the crust that forms the upper part of the subducting plate is cool, having been close to the Earth's surface. As the plate subducts, it actually cools the mantle. The subducting plate reaches high pressures faster than it heats to high temperatures, and this pressure is enough to cause metamorphism.

High Temperature Metamorphism: Some metamorphic rock forms at high temperatures but without high pressures. This occurs near hot intrusions of magma from the mantle into the crust. Rock that is in contact with these intrusions undergoes contact, or thermal, metamorphism. This heat causes minerals to react with each other, which produces new minerals.

Hydrothermal Metamorphism: This process is associated with contact metamorphism. When very large masses of magma — called plutons — intrude from the mantle into the crust, a great amount of heat is generated. This huge body of hot magma creates a heat source that can cause fluids in the crust to circulate. Chemical reactions occur as a result of this circulation. This type of metamorphism is common near mid-oceanic ridges and around large plutonic intrusions in the crust.

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