Editing Metamorphism (section)

==Metamorphic processes==
[[File:Metamorphic Pressure alignment-white.png|thumb|upright=1.2|(Left) Randomly-orientated grains in a rock before metamorphism. (Right) Grains align [[orthogonal]] to the applied [[Stress (mechanics)|stress]] if a rock is subjected to stress during metamorphism]]

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. Hutton wrote in 1795 that some rock beds of the Scottish Highlands had originally been [[sedimentary rock]], but had been transformed by great heat.{{sfn|Yardley|1989|pp=1–5}}

Hutton also speculated that pressure was important in metamorphism. This hypothesis was tested by his friend, [[Sir James Hall, 4th Baronet|James Hall]], who sealed [[chalk]] into a makeshift [[pressure vessel]] constructed from a cannon barrel and heated it in an iron foundry furnace. Hall found that this produced a material strongly resembling [[marble]], rather than the usual [[quicklime]] produced by heating of chalk in the open air. French geologists subsequently added [[metasomatism]], the circulation of fluids through buried rock, to the list of processes that help bring about metamorphism. However, metamorphism can take place without metasomatism (isochemical metamorphism) or at depths of just a few hundred meters where pressures are relatively low (for example, in contact metamorphism).{{sfn|Yardley|1989|pp=1–5}}

Rock can be transformed without melting because heat causes atomic bonds to break, freeing the atoms to move and form new bonds with other [[atom]]s. Pore fluid present between mineral grains is an important medium through which atoms are exchanged.{{sfn|Yardley|1989|page=5}} This permits [[recrystallization (geology)|recrystallization]] of existing minerals or crystallization of new minerals with different crystalline structures or chemical compositions (neocrystallization).{{sfn|Marshak|2009|p=177}} 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.{{sfn|Yardley|1989|pp=29–30}}{{sfn|Philpotts|Ague|2009|pp=149, 420–425}}

Metamorphism is generally regarded to begin at temperatures of {{convert|100 to 200|C||sp=us}}. This excludes [[diagenesis|diagenetic]] changes due to [[Compaction (geology)|compaction]] and [[lithification]], which result in the formation of sedimentary rocks.{{sfn|Bucher|2002|p=4}} The upper boundary of metamorphic conditions lies at the [[Solidus (chemistry)|solidus]] of the rock, which is the temperature at which the rock begins to melt. At this point, the process becomes an [[igneous]] process.{{sfn|Nelson|2022}} The solidus temperature depends on the composition of the rock, the pressure, and whether the rock is saturated with water. Typical solidus temperatures range from {{convert|650|C||sp=us}} for wet granite at a few hundred [[megapascal]]s (MPa) of pressure{{sfn|Holland|Powell|2001}} to about {{convert|1080|C||sp=us}} for wet basalt at atmospheric pressure.{{sfn|Philpotts|Ague|2009|p=252}} [[Migmatite]]s 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.{{sfn|Philpotts|Ague|2009|p=44}}

The metamorphic process can occur at almost any pressure, from near surface pressure (for contact metamorphism) to pressures in excess of 16 [[kbar]] (1600 MPa).{{sfn|Yardley|1989|pp=49–51}}

===Recrystallization===
[[File:Basalt-hand-sample.tif|thumb|upright=1.35|Basalt hand sample showing fine texture]]
[[File:Amphibolite (Archean, 3.1-3.2 Ga; Norris South roadcut, Madison County, Montana, USA) 1 (45574881922).jpg|thumb|upright=1.35|Amphibolite formed by metamorphism of basalt showing coarse texture]]
The change in the grain size and orientation in the rock during the process of metamorphism is called [[Recrystallization (geology)|recrystallization]]. For instance, the small [[calcite]] crystals in the sedimentary rocks [[limestone]] and [[chalk]] change into larger crystals in the metamorphic rock [[marble]].{{sfn|Yardley|1989|pp=127, 154}} In metamorphosed [[sandstone]], recrystallization of the original [[quartz]] sand grains results in very compact [[quartzite]], also known as metaquartzite, in which the often larger quartz crystals are interlocked.{{sfn|Jackson|1997|loc="metaquartzite"}} Both high temperatures and pressures contribute to recrystallization. High temperatures allow the [[atom]]s and [[ion]]s in solid crystals to migrate, thus reorganizing the crystals, while high pressures cause solution of the crystals within the rock at their points of contact (''[[pressure solution]]'') and redeposition in pore space.{{sfn|Yardley|1989|pp=154–158}}

During recrystallization, the identity of the mineral does not change, only its texture. Recrystallization generally begins when temperatures reach above half the melting point of the mineral on the [[Kelvin]] scale.{{sfn|Gillen|1982|p=31}}

Pressure solution begins during diagenesis (the process of lithification of sediments into sedimentary rock) but is completed during early stages of metamorphism. For a sandstone protolith, the dividing line between diagenesis and metamorphism can be placed at the point where strained quartz grains begin to be replaced by new, unstrained, small quartz grains, producing a ''mortar texture'' that can be identified in [[thin section]]s under a polarizing microscope. With increasing grade of metamorphism, further recrystallization produces ''foam texture'', characterized by polygonal grains meeting at triple junctions, and then ''porphyroblastic texture'', characterized by coarse, irregular grains, including some larger grains ([[porphyroblasts]].){{sfn|Howard|2005}}

[[File:Mylonite Strona.jpg|thumb|upright=1.35|A mylonite (through a [[petrographic microscope]])]]
Metamorphic rocks are typically more coarsely crystalline than the protolith from which they formed. Atoms in the interior of a crystal are surrounded by a stable arrangement of neighboring atoms. This is partially missing at the surface of the crystal, producing a ''[[surface energy]]'' that makes the surface thermodynamically unstable. Recrystallization to coarser crystals reduces the surface area and so minimizes the surface energy.{{sfn|Yardley|1989|pp=148–158}}

Although grain coarsening is a common result of metamorphism, rock that is intensely deformed may eliminate [[strain energy]] by recrystallizing as a fine-grained rock called ''[[mylonite]]''. Certain kinds of rock, such as those rich in quartz, [[carbonate mineral]]s, or olivine, are particularly prone to form mylonites, while feldspar and garnet are resistant to mylonitization.{{sfn|Yardley|1989|p=158}}
{{clear}}

===Phase change===
{{Annotated image
| image = Al2SiO5 phase diagram.svg
| image-width=210
| height=250
| caption = Phase diagram of Al<sub>2</sub>SiO<sub>5</sub>
 <br />([[nesosilicate]]s)

|annotations = 
{{annotation|040|100|[[Kyanite]]}}
{{annotation|070|170|[[Andalusite]]}}
{{annotation|130|120|[[Sillimanite]]}}
}}

Phase change metamorphism is the creating of a new mineral with the same chemical formula as a mineral of the protolith. This involves a rearrangement of the atoms in the crystals. An example is provided by the [[aluminium silicate]] minerals, [[kyanite]], [[andalusite]], and [[sillimanite]]. All three have the identical composition, {{chem2|Al2SiO5}}. Kyanite is stable at surface conditions. However, at atmospheric pressure, kyanite transforms to [[andalusite]] at a temperature of about {{cvt|190|C||}}. Andalusite, in turn, transforms to [[sillimanite]] when the temperature reaches about {{cvt|800|C||}}. At pressures above about 4 kbar (400 MPa), kyanite transforms directly to sillimanite as the temperature increases.{{sfn|Yardley|1989|pp=32–33, 110, 130–131}} A similar phase change is sometimes seen between [[calcite]] and [[aragonite]], with calcite transforming to aragonite at elevated pressure and relatively low temperature.{{sfn|Yardley|1989|pp=183–183}}

===Neocrystallization===
Neocrystallization involves the creation of new mineral crystals different from the protolith. [[Chemical reaction]]s digest the minerals of the protolith which yields new minerals. This is a very slow process as it can also involve the diffusion of atoms through solid crystals.{{sfn|Vernon|1976|p=149}}

An example of a neocrystallization reaction is the reaction of [[fayalite]] with [[plagioclase]] at elevated pressure and temperature to form [[garnet]]. The reaction is:{{sfn|Yardley|1989|pp=110, 130–131}}

{{NumBlk|:
|{{overset|fayalite|3 {{chem|Fe|2|SiO|4}}}} + {{overset|plagioclase|{{chem|CaAl|2|Si|2|O|8}}}} → {{overset|garnet|2 {{chem|CaFe|2|Al|2|Si|3|O|12}}}}
|{{EquationRef|Reaction 1}}}}

Many complex high-temperature reactions may take place between minerals without them melting, and each mineral assemblage produced provides us with a clue as to the temperatures and pressures at the time of metamorphism. These reactions are possible because of rapid diffusion of atoms at elevated temperature. Pore fluid between mineral grains can be an important medium through which atoms are exchanged.{{sfn|Yardley|1989|page=5}}

A particularly important group of neocrystallization reactions are those that release [[Volatile (astrogeology)|volatiles]] such as water and [[carbon dioxide]]. During metamorphism of [[basalt]] to [[eclogite]] in [[subduction zones]], hydrous minerals break down, producing copious quantities of water.{{sfn|Stern|2002|pp=6–10}} The water rises into the overlying mantle, where it lowers the melting temperature of the mantle rock, generating [[magma]] via [[flux melting]].{{sfn|Schmincke|2003|pp=18, 113–126}} The mantle-derived magmas can ultimately reach the Earth's surface, resulting in volcanic eruptions. The resulting [[Volcanic arc|arc volcanoes]] tend to produce dangerous eruptions, because their high water content makes them extremely explosive.{{sfn|Stern|2002|pp=27–28}}

Examples of ''dehydration reactions'' that release water include:{{sfn|Yardley|1989|pp=75,102}}

{{NumBlk|:
|{{overset|[[hornblende]]|7{{chem2|Ca2Mg3Al4Si6O22(OH)2}}}} + {{overset|quartz|10{{chem2|SiO2}}}} → {{overset|[[cummingtonite]]|3{{chem2|Mg7Si8O22(OH)2}}}} + {{overset|[[anorthite]]|14{{chem2|CaAl2Si2O8}}}} + {{overset|water|4{{chem2|H2O}}}}
|{{EquationRef|Reaction 2}}}}

{{NumBlk|:
|{{overset|[[muscovite]]|2{{chem2|KAl2(AlSi3O10)(OH)2}}}} + {{overset|quartz|2{{chem2|SiO2}}}} → {{overset|sillimanite|2{{chem2|Al2SiO5}}}} + {{overset|[[potassium feldspar]]|2{{chem2|KAlSi3O8}}}} + {{overset|water|2{{chem2|H2O}}}}
|{{EquationRef|Reaction 3}}}}

An example of a decarbonation reaction is:{{sfn|Yardley|1989|p=127}}

{{NumBlk|:
|{{overset|calcite|{{chem2|CaCO3}}}} + {{overset|quartz|{{chem2|SiO2}}}} → {{overset|[[wollastonite]]|{{chem2|CaSiO3}}}} + {{overset|carbon dioxide|{{chem2|CO2}}}}
|{{EquationRef|Reaction 4}}}}

===Plastic deformation===
In plastic deformation pressure is applied to the [[protolith]], which causes it to shear or bend, but not break. In order for this to happen temperatures must be high enough that brittle fractures do not occur, but not so high that diffusion of crystals takes place.{{sfn|Vernon|1976|p=149}}
As with pressure solution, the early stages of plastic deformation begin during diagenesis.{{sfn|Boggs|2006|pp=147–154}}