Metamorphic rocks form an important part of the rock cycle. They begin their journey as igneous or sedimentary rocks, then are transformed by heat and pressure, leading to a reorganization of minerals. Metamorphic rocks are usually much harder than their parent material, and can be found everywhere onEarth and in many places on other planets.
Metamorphism occurs when rocks are changed by temperature and pressure. The temperature can increase or decrease during metamorphism, but it is the pressure that is most important for this process. The pressure can be applied by tectonic plates, or even by earthquakes and regional folding of the rocks. When rocks undergo metamorphism, the minerals that are present are reformed in response to the changing chemical environment.
The heat and pressure of metamorphism can cause a variety of reactions within a rock, such as dehydration, recrystallization and melting. During dehydration, the minerals within a rock absorb water and swell, creating more space between them and leading to a rearrangement of their atomic structure. Recrystallization is when grains of the original mineral grow larger and thicker, allowing them to better withstand the changing temperature and pressure. Melting occurs when the temperature is high enough to break chemical bonds and cause the solid rock to liquify.
Metamorphic rocks can be classified based on their texture, which refers to the size, shape, and arrangement of the grains of minerals within them. Foliated rocks have visible bands of minerals that are arranged in parallel layers. Examples include schist, gneiss and slate. Nonfoliated rocks usually do not have visible bands of minerals, and include quartzite and marble.
Metamorphic rocks are classified according to their parent material, or the rock that they started out as. Those that began as sedimentary rocks are known as sedimentary protoliths, and can be further classified according to their composition and texture. Some of the common types of sedimentary protoliths are quartzite, olivine and limestone. Rocks that began as igneous rocks are known as igneous protoliths, and examples include gabbro andphyllite.
Metamorphic rocks are extremely varied in their appearance and composition, and can help to provide insights into the history of any given area. They can often help to reveal the forces that shaped a region and the rocks that came before. Metamorphic rocks are an interesting and important part of the rock cycle, and a reminder of the Earths ever-changing nature.
