Rocks found on the Earth are broadly categorized into three types - sedimentary rocks (which are formed as a result of sedimentation of matter), igneous rocks (which are formed as a result of volcanic activity) and metamorphic rocks (which are formed as a result of some existing rock type undergoing the process of metamorphism.) Metamorphic rocks are the least common of the three types of rocks; and that, perhaps, is the reason why people are not aware of some of the most basic facts about them - like how they are formed, what are the sub-types, what are they used for, etc.
Formation of Metamorphic Rocks
Even though least common of the three types, these rocks are a bit unique as they form from an existing sedimentary rock, igneous rock or an old metamorphic rock. In lithology, i.e. the branch of geology which studies various attributes of rocks, the parent rock of any metamorphic rock is referred to as the 'protolith'. While limestone happens to be the protolith of marble, shale is the protolith for slate and sandstone for quartzite. Metamorphic rocks are formed when their respective protoliths are exposed to immense heat and pressure; to an extent that this exposure brings about physical or chemical changes in them and results in formation of new rock. To a certain extent the tectonic movement and volcanic activity also contribute to the formation of these rocks.
The process by which metamorphic rocks are formed by transformation of an existing rock type is referred to as 'metamorphism'. A technically sound definition of metamorphism would be the recrystallization of an existing rock as a result of the physical and chemical changes in its surroundings without changing its solid state. These changes include heat exceeding 302°F, pressure exceeding 1500 bars and the presence of chemically active fluids in the vicinity. The heat and pressure that these rocks are exposed to increases with depth. At times, the plutonic body can provide heat required for the process of metamorphism even when the rock is not lying at great depths as such.
In course of metamorphic rock formation, the bonds between atoms in a mineral are broken and allowed to rearrange. As the new bonds are subjected to a great deal of heat and pressure, they turn out to be more stable than the previous bonds. Those rocks which lie buried deep beneath are exposed to lithostatic pressure i.e. the pressure exerted from material sitting above these rocks, which is usually equal from all sides. The pressure exerted on rocks from a particular side as a result of collision or subduction of tectonic plates along their boundary is referred to as 'directed pressure'. Even though this sort of pressure is not sufficient to facilitate new mineralization, it does have the ability to affect the shape and arrangement of these minerals. The banding of mineral under directed pressure results in formation of 'foliated metamorphic rocks'. These rocks are totally different from 'non-foliated metamorphic rocks' which are formed in absence of directed pressure.
Of the various types of metamorphic rocks found on Earth, marble and quartzite are perhaps the most popular ones, and don't really need any introduction. Other examples include slate, gneiss, schist, etc. Being quite strong, each of these are used in various fields, right from chalkboards to kitchen counter tops. Other than the numerous uses, the study of metamorphic rocks is also important because it helps us study the interior of the Earth; i.e. the temperature and pressure existing at varying levels of depth within the Earth's crust.