Metamorphic rocks are ones that have undergone a change of form during a process called metamorphism. Metamorphic rocks tend to form deep in the Earth’s crust where immense heat and pressure alter the chemical composition of a rock creating a new type in the process. These rocks are then brought to the surface as tectonic plates collide against each, creating uplift or through processes of erosion.
Magma from deep inside the earth is one source of the change needed to form metamorphic rocks. Rocks that undergo change from this method are known as thermal metamorphic rocks. The immense heat literally melts the old rock creating a new one in its place. This type of change is generally local and the rocks formed will be limited in the area they can be found. Marble is one example of a rock that has undergone this process. The original limestone changes as the crystals reform they produce marble. Sandstone changes under thermal forces and reforms into quartz.
Burial metamorphism refers to changes that occur to sedimentary rocks under extreme pressure from the material on top of them. This process occurs frequently in mountain ranges and can lead to large scale changes in rock referred to as regional metamorphism. Plate convergence continental collisions cause regional metamorphism as well. The primary force under this type of change is pressure. The deeper down the rock the more pressure that it will be exposed to.
Tectonic plates colliding with each other have the power to provide the pressure and heat to change rock into metamorphic rock as well. Rocks that are formed this way will be found along fault lines. Amphibolites are examples of rock that is commonly produced in this fashion. The pressure and heat work together to cause change to old rocks. The plates pushing together create higher heat and pressure than would be seen through other methods in which metamorphic rocks are created. Rocks that are formed along fault lines are referred to as mylonites.
Metamorphic rocks come in a variety of shapes and sizes. They have been used for thousands of years by man in building, sculpting, and jewelry. Today we use them in paving, cutting, roofing, tiling, and as abrasives. They were critical for the foundation of ancient civilizations and they are the cornerstone to civilization today as well. Some of the most valuable metamorphic rocks are sapphires, rubies, and diamonds. They have been valued throughout the history of man. They are much more than a source of beauty. The diversity of these rocks provides mankind with a wide range of uses and life would be much more difficult without them.
Further reading: Hornfels Metamorphic Rocks Created by Heat
