Process of Metamorphism:
Metamorphism may be defined as the process of transformation of igneous or sedimentary rocks to metamorphic rocks ( changed rocks ) through the Earth’s movements ( producing heat and pressure). The metamorphism can be thermal when the intrusion of hot mass of igneous rock raises temperature of surrounding rocks. Similarly these can be a contact metamorphism when flowing when action by magma effects rocks in the zone near the batholiths or dike.
Metamorphism produces changes in the minerals. Mostly the change reshapes the rock particles along parallel planes which are different from the bedding. It is called flow cleavage and its best example is slate. Moreover, through the act of metamorphism fine-grained sediments such as shale are turned into Slates and Schists, coarse-grained or crystalline rocks form Quartzite, Gneisses and Granulites and most of the rocks show foliation.
Types of Metamorphism:
On the basis of the agents which bring about the transformation in the rocks, the metamorphism can be divided into four types:
- Contact Metamorphism (thermal)
- Regional Metamorphism
- Shock Metamorphism
- Hydrothermal Metamorphism
- Dynamic Metamorphism
(1). Contact Metamorphism:
This type of metamorphism takes place due to the movement of hot molten magma (Lava). When the hot lava intrudes into the cavities, hollows or cleavage planes of rocks, it exerts pressure and hears up the surrounding rocks. High temperature brings about many changes. For example, it bakes limestone into marble and coal is turned into graphite.
(2). Regional Metamorphism:
It takes place due to the internal mountains building movements which exert great pressure on the rocks. Thus the entire shape of the rocks is changed. For example, shale becomes slate which then changes into schist. Due to mountain building movements, the submerged rocks are totally changed and become crystalline. Similarly, these submerged also bring back to the surface of Earth the deep rocks. Rocks such as graphite and quartzite are formed in this manner. Similarly, shale or muddy caly is turned into slates which have fine grains, Sandstones are changed into quartzite which are white colored but very hard in the texture.
(3). Hydrothermal Metamorphism:
When the rocks come in contact with high-temperature fluid variated composition, they result in hydrothermal metamorphism in them. A set of metamorphic and metasomatic reactions take place due to the difference in the composition of the existing rocks and invading fluid. The hydrothermal fluid may be magmatic (originate in an intruding magma), circulating groundwater, or ocean water. Convective circulation of hydrothermal fluids in the ocean floor basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and other submarine volcanic areas. The fluids eventually escape through vents on the ocean floor known as black smokers. The patterns of this hydrothermal alteration are used as a guide in the search for deposits of valuable metal ores.
(4). Shock Metamorphism:
Shock metamorphism is the result of the collision of external terrestrial bodies like meteorites with the earth’s surface. The shock metamorphism is also known as the Impact metamorphism. Shock metamorphism is, therefore, characterized by ultrahigh-pressure conditions and low temperature. The resulting minerals (such as SiO2 polymorphs coesite and stishovite) and textures are characteristic of these conditions.
(5). Dynamic Metamorphism:
Dynamic metamorphism is associated with zones of high to moderate strain such as fault zones. Cataclasis, crushing and grinding of rocks into angular fragments, occurs in dynamic metamorphic zones, giving cataclastic texture.
The textures of dynamic metamorphic zones are dependent on the depth at which they were formed, as the temperature and confining pressure determine the deformation mechanisms which predominate. Within depths less than 5 km, dynamic metamorphism is not often produced because the confining pressure is too low to produce frictional heat. Instead, a zone of breccia or cataclastic is formed, with the rock milled and broken into random fragments. This generally forms a mélange. At depth, the angular breccias transit into a ductile shear texture and into mylonite zones.
Within the depth range of 5–10 km, pseudotachylyte is formed because the confining pressure is enough to prevent brecciation and milling and thus energy is focused on discrete fault planes. Frictional heating, in this case, may melt the rock to form pseudotachylyte glass.
Within the depth range of 10–20 km, deformation is governed by ductile deformation conditions and hence frictional heating is dispersed throughout shear zones, resulting in a weaker thermal imprint and distributed deformation. Here, deformation forms mylonite, with dynamo-thermal metamorphism observed rarely as the growth of porphyroblasts in mylonite zones.
Overthrusting may juxtapose hot lower crustal rocks against cooler mid and upper crust blocks, resulting in conductive heat transfer and localized contact metamorphism of the cooler blocks adjacent to the hotter blocks, and often retrograde metamorphism in the hotter blocks. The metamorphic assemblages, in this case, are diagnostic of the depth and temperature and the throw of the fault and can also be dated to give an age of the thrusting.