Heat, pressure, and chemical processes can alter rocks, changing them into a new type of rock known as a metamorphic rock. Igneous, sedimentary, as well as older metamorphic rocks can be transformed into metamorphic rocks. “Metamorphic” comes from the Greek words for “change” and “form.” When exposed to these extreme conditions the texture and sometimes the mineralogical composition of the rocks changes. The composition only changes when the chemical structure of the protolith, or parent rock, was complex enough to allow shifts in the structure of the minerals or when new minerals are introduced in fluids present at the time of change. There are two types of metamorphic rocks, which describe their texture: foliated and non-foliated (also called granular).
Metamorphic rocks generally form between 12 and 16 kilometers in the earth. Pressure is the most significant factor involved in regional metamorphism, although heat is also involved. Contact metamorphism involves mainly heat. Temperatures of 100 to 800 degrees Celsius are required for significant changes to occur. The heat comes from magma and the pressure from the layers and layers of rocks above and sometimes from shifting of tectonic plates. If the heat becomes too intense the rocks will melt and instead become igneous rocks when they cool. Hydrothermal metamorphism involves temperature, pressure, and the influence of mineral-rich liquids. Regional metamorphism, burial metamorphism, cataclystic metamorphism, and shock metamorphism involve varying degrees of the three influential factors.
Texture changes that take place to form metamorphic rocks are often the result of compaction. When parent rocks are compacted their minerals move closer together making the rock more dense and less porous. Sheets of minerals end up developing perpendicular to the direction of the force, which lead to the creation of foliated metamorphic rocks. Foliation often involves sheets of mica like in schist, or gneiss which forms when light and dark minerals separate into layers. Another variety of foliation is called lineation. Lineation can be seen in tourmaline and kyanite as a rod-like alignment of minerals. Sometimes color-banding of a single mineral can be confused with foliation like in marble. Marble is considered a non-foliated metamorphic rock. The non-foliated or granular metamorphic rocks usually result from parent rocks that were composed all of one mineral like quartz and calcite. Slate is a foliated metamorphic rock formed from shale at temperatures of 200 degrees Celsius and depths of 10 kilometers. Schist is another example and there are many types, named for the minerals present, such as mica schist, garnet schist, and tac schist. Gneiss, as mentioned above, is made of a dark mineral like biotite and a light mineral such as quartz. Marble, quartzite, hornfels, and serpentinite are all granular.
Mineral changes are usually in the form of recrystallization, or the rearranging of the structure of pre-existing minerals to form crystals or make larger crystal from smaller ones. Crystals are more thermodynamically stable than grains and sediments. Micas are produced from the minuscule crystals of clay in shale and calcite in limestone becomes more coarse in marble. One of the few instances where new minerals can be created ins in the formation of metamorphic rock. Garnet, staurolite, talc, graphite, and asbestos are all the results of metamorphic processes.
Every type of rock in the rock cycle can be formed into metamorphic rock, and metamorphic rock can rejoin the cycle if it is exposed to weathering (sedimentary rock) or higher temperatures (igneous rock). Metamorphism can take place in a number of ways and the causes of metamorphism influence the type of rock formed, as well as the composition of the original parent rock. These factors also create changes in texture and sometimes the mineral composition of the resulting metamorphic rock.