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How is metamorphic rocks formed?

Metamorphic rocks are formed by heat, pressure, or a combination of both. These conditions cause the minerals from which the rock is made to become unstable and be rearranged into more stable mineral formations. The heat and pressure comes from different sources.

Sometimes, magma melts and pushes up into the crust. As the magma cools it becomes an igneous rock, called an igneous intrusion, such as the the granite at Idaho's City Of Rocks. The existing rock surrounding the rising magma was heated, but did not completely melt into a liquid. When this occurred, some of the minerals were rearranged or changed into other minerals by the extreme heat. This change in the composition of the rock is called contact metamorphism. The farther from the magma, the less the surrounding rock is heated. The type of metamorphic rock the surrounding rock is changed into depends on how close it was to the magma. Contact metamorphism is caused more by heat than pressure.