Metamorphic rocks are rocks that have changed their structure and chemical composition. Heat, pressure, and chemical activity can each cause the changes.
When a rock comes into contact with extreme heat, the atoms within expand and loosen or break their bonds with other atoms. The loose atoms then circulate within the rock, forming new minerals. The rock is literally transformed into another kind of rock.
Pressure ultimately has the same effect on rock, but through compression rather than expansion. Pressure squeezes the rock until the atoms break apart and make new, more compact, bonds, which makes a harder, denser new rock.
The various minerals in a rock react differently to chemicals, forming new atomic structures to become other minerals. Chemicals that cause these changes can be found deep within Earth and in cooling magma. The presence of water can cause transformation as well, since certain minerals alter their structure when they come into contact with water.
A wonderful result of rock metamorphosis, or change, is marble. Under heat and pressure limestone commonly forms marble, without which many of the world’s sculpture treasures would not exist.