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How is metamorphic rock formed?

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Quick Answer

Metamorphic rock is formed underground through a process that changes a rock's molecular structure due to pressure, heat and chemical reactions. A metamorphic rock forms from a parent rock called a protolith. Depending on conditions, a protolith can transform into any metamorphic rock. Because protoliths are capable of undergoing vast changes, identifying them is sometimes difficult for geologists.

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Under extreme pressures, such as between two colliding tectonic plates, the minerals of a metamorphic rock group together and align to form foliation, which appears as stripes in the rock. One example of a heavily foliated rock is gneiss.

Alternatively, highly heated areas, such as near magma chambers, produce vastly different metamorphic rocks. One example is hornfels.

Another area for metamorphism is at a subduction zone where oceanic plates collide with and bend under continental plates. Because these high-pressure areas are near the ocean, they are cooler and produce different metamorphic results. One example of this is the creation of a blue mineral called glaucophane. This mineral in the rock foliates from high pressure and creates blueschist, a blue-tinted version of schist.

A protolith may change a number of times before reaching its final metamorphic stage. For example, gneiss may begin as shale that turns into slate, phyllite, schist and finally gneiss.

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