Metamorphism
Metamorphism takes place at temperatures in excess of 150 °C (300 °F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation.
These differ in the characteristic temperatures, pressures, and rate at which they take place and in the extent to which reactive fluids are involved.
Metamorphism is the set of processes by which existing rock is transformed physically or chemically at elevated temperature, without actually melting to any great degree.
The importance of heating in the formation of metamorphic rock was first recognized by the pioneering Scottish naturalist, James Hutton, who is often described as the father of modern geology.
This hypothesis was tested by his friend, James Hall, who sealed chalk into a makeshift pressure vessel constructed from a cannon barrel and heated it in an iron foundry furnace.
French geologists subsequently added metasomatism, the circulation of fluids through buried rock, to the list of processes that help bring about metamorphism.
[1] The transformation converts the minerals in the protolith into forms that are more stable (closer to chemical equilibrium) under the conditions of pressure and temperature at which metamorphism takes place.
[10] Migmatites are rocks formed at this upper limit, which contains pods and veins of material that has started to melt but has not fully segregated from the refractory residue.
Recrystallization generally begins when temperatures reach above half the melting point of the mineral on the Kelvin scale.
[16] Pressure solution begins during diagenesis (the process of lithification of sediments into sedimentary rock) but is completed during early stages of metamorphism.
During metamorphism of basalt to eclogite in subduction zones, hydrous minerals break down, producing copious quantities of water.
The resulting arc volcanoes tend to produce dangerous eruptions, because their high water content makes them extremely explosive.
The orogenic belt is characterized by thickening of the Earth's crust, during which the deeply buried crustal rock is subjected to high temperatures and pressures and is intensely deformed.
This causes crystals of platy minerals, such as mica and chlorite, to become rotated such that their short axes are parallel to the direction of shortening.
[37] Rocks that were subjected to uniform pressure from all sides, or those that lack minerals with distinctive growth habits, will not be foliated.
A different sequence in the northeast of Scotland defines Buchan metamorphism, which took place at lower pressure than the Barrovian.
[40] Burial metamorphism takes place simply through rock being buried to great depths below the Earth's surface in a subsiding basin.
Rocks formed by contact metamorphism may not present signs of strong deformation and are often fine-grained[46][47] and extremely tough.
[48] The Yule Marble used on the Lincoln Memorial exterior and the Tomb of the Unknown Soldier in Arlington National Cemetery was formed by contact metamorphism.
It takes place around intrusions of a rare type of magma called a carbonatite that is highly enriched in carbonates and low in silica.
The difference in composition between an existing rock and the invading fluid triggers a set of metamorphic and metasomatic reactions.
[67] 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.
At still greater depths, where temperatures exceed 300 °C (572 °F), plastic deformation takes over, and the fault zone is composed of mylonite.
[70] There is considerable evidence that cataclasites form as much through plastic deformation and recrystallization as brittle fracture of grains, and that the rock may never fully lose cohesion during the process.
Eskola drew upon the zonal schemes, based on index minerals, that were pioneered by the British geologist, George Barrow.
Prograde metamorphism involves the change of mineral assemblages (paragenesis) with increasing temperature and (usually) pressure conditions.
[80] Metamorphic processes act to bring the protolith closer to thermodynamic equilibrium, which is its state of maximum stability.
where: In other words, a metamorphic reaction will take place only if it lowers the total Gibbs free energy of the protolith.
)[90][91] A petrogenetic grid is a geologic phase diagram that plots experimentally derived metamorphic reactions at their pressure and temperature conditions for a given rock composition.
[88][89] The Al2SiO5 nesosilicate phase diagram shown is a very simple petrogenetic grid for rocks that only have a composition consisting of aluminum (Al), silicon (Si), and oxygen (O).
