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How Are Metemorphic Rocks Formed?

The word "metamorphic" originates from the Greek words "meta" (after) and "morph" (to change form). Metamorphic rocks are the least common of the three types of rock and were once igneous or sedimentary rocks, or even other metamorphic rock that changed form due to a combination of heat and extreme pressure. In an unstable mineral, pressure causes the elements to recombine, forming new minerals. Stable minerals under pressure remain the same mineral but alter in size and shape. If stable minerals are also subjected to heat, their atoms move to create a more stable form of the same mineral. The types of pressure and amount of heat produce a wide variety of metamorphic rocks.

Lithostatic Pressure
- The weight of overlying rock exerts pressure on rock deep below the earth's surface. This pressure, called lithostatic pressure, overburden pressure or vertical stress, is applied equally in all directions. The extreme pressure causes the bond between the mineral's atoms to break, and some atoms go to bind with other atoms in areas of less pressure. The underlying rock is compressed and becomes denser.
Directed Pressure
- Directed pressure occurs along the edges of tectonic plates where they collide, or where one plate is pushed beneath another. This pressure does not result in new mineralization, but the shape and arrangement of the minerals change as the rock is flattened or lengthened. Partially melted or smeared mineral grains may recrystalize into distinct bands or layers, creating foliated metamorphic rocks, such as slate or gneiss.
Nondirected Pressure
- Rocks that undergo nondirected pressure form nonfoliated rocks. Because they are composed primarily of a single mineral, they don't have the mineral bands typical of foliated rocks. These rocks had contact with hot magma or extreme pressure from being confined deep underground. Examples of rocks that have undergone nondirected pressure are marble, which was originally limestone, and quartzite, which was originally sandstone. A sculptor uses marble to create statues but would never use limestone. Although marble and limestone are both primarily calcite, nondirected pressure made the limestone more compact and hard, producing marble.
Contact Metamorphism
- Hot magma causes metamorphism when it comes in contact with existing rock. The change in the existing rock depends on the heat capacity of the magma and rock, the conductivity of the magma and rock, the amount of pressure and the presence of hydrothermal fluids that transport heat. Rocks created by contact metamorphism are usually fine-grained with no mineral banding.

Rocks & Minerals