A metamorphic rock provided to be some other kind of rock, yet it was adjusted inside the Earth to become a new form of rock. Words metamorphism originates from prehistoric Greek words for “change” (meta) and “form” (morph). The form of rock that a metamorphic rock used to be, prior to metamorphism, is dubbed the protolith. Throughout metamorphism the mineral content and also texture of the protolith are adjusted as a result of changes in the physical and also chemical environment of the rock. Metamorphism deserve to be led to by burial, tectonic anxiety, heating by magma, or alteration by fluids. At advanced steras of metamorphism, it is common for a metamorphic rock to build such a different set of minerals and such a thoapproximately readjusted texture that it is difficult to identify what the protolith was.

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A rock undergoing metamorphism stays a solid rock in the time of the process. Rocks perform not melt throughout many problems of metamorphism. At the greatest grade of metamorphism, rocks begin to partially melt, at which allude the boundary of metamorphic problems is surpassed and also the igneous component of the rock cycle is gone into.

Even though rocks reprimary solid during metamorphism, liquid is mainly existing in the microscopic spaces between the minerals. This liquid phase might play a significant function in the chemical reactions that are a crucial component of how metamorphism occurs. The liquid normally is composed largely of water.

Metamorphic rocks carry out a document of the procedures that occurred inside Earth as the rock was based on transforming physical and also chemical conditions. This offers the geologist literally “inside information” on what occurs within the Planet in the time of such processes as the formation of new hill arrays, the collision of continents, the subduction of oceanic plates, and the circulation of sea water into warm oceanic crust. Metamorphic rocks are choose probes that have gone dvery own right into the Planet and come earlier, bringing an document of the conditions they encountered on their journey in the depths of the Planet.

Figure 1. The platy layers in this large outchop of metamorphic rock display the effects of press on rocks throughout metamorphism.

In the big outcrop of metamorphic rocks in figure 1, the rocks’ platy appearance is an outcome of the procedure metamorphism. Metamorphism is the addition of warm and/or push to existing rocks, which reasons them to change physically and/or chemically so that they become a brand-new rock. Metamorphic rocks may adjust so a lot that they may not resemble the original rock.


Any form of rock—igneous, sedimentary, or metamorphic—can end up being a metamorphic rock. All that is necessary is enough heat and/or pressure to alter the existing rock’s physical or chemical makeup without melting the rock totally.

Figure 2. A foliated metamorphic rock.

Rocks change in the time of metamorphism because the minerals have to be stable under the new temperature and also press conditions. The require for stability might cause the structure of minerals to rearvariety and also form new minerals. Ions might relocate between minerals to develop minerals of different chemical complace. Hornfels, through its alternating bands of dark and also light crystals, is an excellent example of just how minerals rearvariety themselves during metamorphism. Hornfels is presented in table 1.

Extreme press may likewise bring about foliation, the level layers that create in rocks as the rocks are squeezed by press (figure 2). Foliation generally develops once push is exerted in only one direction. Metamorphic rocks might also be non-foliated. Quartzite and limestone, shown in table 6, are nonfoliated.

The two primary kinds of metamorphism are both pertained to warmth within Earth:

Regional metamorphism: Changes in enormous amounts of rock over a large location led to by the too much pressure from overlying rock or from compression led to by geologic procedures. Deep burial exposes the rock to high temperatures.Contact metamorphism: Changes in a rock that is in call with magma because of the magma’s too much warm.

Factors that Control Metamorphism

The reason rocks undergo metamorphism is that the minerals in a rock are just secure under a restricted range of push, temperature, and also chemical conditions. When rocks are based on large sufficient transforms in these factors, the minerals will undergo chemical reactions that lead to their replacement by new minerals, minerals that are steady in the new conditions.

Chemical Composition of the Protolith

The type of rock undergoing metamorphism is a major aspect in determining what type of metamorphic rock it becomes. In brief the determine of the protolith plays a huge role in the identification of the metamorphic rock. A fluid phase may present or rerelocate chemical substances right into or out of the rock in the time of metamorphism, yet in most metamorphic rock, most of the atoms in the protolith will certainly be present in the metamorphic rock after metamorphism; the atoms will most likely be rearranged right into new mineral develops within the rock. As such, not just does the protolith recognize the initial chemisattempt of the metamorphic rock, a lot of metamorphic rocks carry out not readjust their mass (overall) chemical compositions extremely much throughout metamorphism. The truth that the majority of metamorphic rocks retain a lot of of their original atoms indicates that even if the rock was so thoabout metamorphosed that it no much longer looks at all choose the protolith, the rock can be analyzed in terms of its mass chemical complace to identify what kind of rock the protolith was.


Temperature is one more significant factor of metamorphism. There are two methods to think around just how the temperature of a rock have the right to be raised as an outcome of geologic processes.

If rocks are buried within the Planet, the deeper they go, the higher the temperatures they suffer. This is because temperature inside the Earth boosts alengthy what is called the geothermal gradient, or geotherm for brief. As such, if rocks are simply buried deep sufficient sufficient sediment, they will endure temperatures high sufficient to cause metamorphism. This temperature is around 200ºC (roughly 400ºF).

Tectonic procedures are one more method rocks deserve to be relocated deeper alengthy the geotherm. Faulting and also folding the rocks of the crust, can relocate rocks to a lot greater depth than basic interment have the right to.

Yet another means a rock in the Earth’s crust have the right to have actually its temperature substantially boosted is by the intrusion of magma adjacent. Magma intrusion topics surrounding rock to better temperature with no increase in depth or press.


Prescertain is a meacertain of the stress and anxiety, the physical pressure, being used to the surface of a product. It is defined as the force per unit location acting on the surchallenge, in a direction perpendicular to the surface.

Lithostatic pressure is the pressure exerted on a rock by all the neighboring rock. The resource of the push is the weight of all the rocks over. Lithostatic push rises as depth within the Earth boosts and also is a unidevelop stress—the press uses equally in all directions on the rock.

If pressure does not apply equally in all directions, differential stress occurs. There are 2 kinds of differential stress.

Regular anxiety compresses (pushes together) rock in one direction, the direction of maximum stress and anxiety. At the same time, in a perpendicular direction, the rock undergoes stress (stretching), in the direction of minimum anxiety.

Shear anxiety pushes one side of the rock in a direction parallel to the side, while at the very same time, the various other side of the rock is being pumelted in the opposite direction.

Differential stress and anxiety has a major influence on the appearance of a metamorphic rock. Differential stress and anxiety deserve to flatten pre-existing grains in the rock, as displayed in the diagram below.


Metamorphic minerals that thrive under differential tension will have actually a wanted orientation if the minerals have actually atomic frameworks that tfinish to make them develop either level or elongate crystals. This will certainly be specifically apparent for micas or various other sheet silicates that flourish during metamorphism, such as biotite, muscovite, chlorite, talc, or serpentine. If any type of of these flat minerals are thriving under normal stress and anxiety, they will prosper through their sheets oriented perpendicular to the direction of maximum compression. This results in a rock that can be easily broken alengthy the parallel mineral sheets. Such a rock is said to be foliated, or to have actually foliation.



Any open area between the mineral grains in a rock, but microscopic, might contain a fluid phase. Many commonly, if tright here is a liquid phase in a rock throughout metamorphism, it will certainly be a hydrous liquid, consisting of water and points dissolved in the water. Less commonly, it might be a carbon dioxide liquid or some various other fluid. The visibility of a liquid phase is a significant element during metamorphism because it helps identify which metamorphic reactions will occur and how quick they will certainly happen. The fluid phase have the right to additionally affect the price at which mineral crystals deform or readjust shape. Most of this influence is because of the dissolved ions that pass in and out of the liquid phase. If throughout metamorphism enough ions are introduced to or removed from the rock through the fluid to change the bulk chemical composition of the rock, the rock is sassist to have undergone metasomatism. However, many metamorphic rocks execute not undergo sufficient readjust in their bulk chemisattempt to be taken into consideration metasomatic rocks.


Many metamorphism of rocks takes location progressively inside the Planet. Regional metamorphism takes area on a timerange of millions of years. Metamorphism usually requires sluggish transforms to rocks in the solid state, as atoms or ions diffusage out of unsteady minerals that are breaking down in the provided push and also temperature conditions and move right into new minerals that are stable in those problems. This form of chemical reaction takes a long time.

Grades of Metamorphism

Metamorphic grade refers to the general temperature and press problems that prevailed in the time of metamorphism. As the push and temperature boost, rocks undergo metamorphism at greater metamorphic grade. Rocks transforming from one kind of metamorphic rock to another as they encounter higher qualities of metamorphism are said to be undergoing prograde metamorphism.

Low-grade metamorphism takes area at around 200–320 ºC and also reasonably low press. This is not much past the conditions in which sediments get lithified right into sedimentary rocks, and also it is widespread for a low-grade metamorphic rock to look somewhat favor its protolith. Low grade metamorphic rocks tend to identified by a wealth of hydrous minerals, minerals that contain water within their crystal framework. Examples of low grade hydrous minerals include clay, serpentine, and also chlorite. Under low grade metamorphism many of the metamorphic minerals will certainly not thrive big enough to be viewed without a microscope.

Medium-grade metamorphism takes place at around at 320–450 ºC and at modeprice pressures. Low grade hydrous minerals are reinserted by micas such as biotite and muscovite, and also non-hydrous minerals such as garnet might flourish. Garnet is an example of a mineral which may form porphyroblasts, metamorphic mineral grains that are larger in size and also even more equant in shape (about the very same diameter in all directions), for this reason standing out among the smaller, flatter, or even more elongate minerals.

High-grade metamorphism takes place at temperatures over around 450 ºC. Micas tfinish to break dvery own. New minerals such as hornblende will form, which is secure at higher temperatures. However, as metamorphic grade boosts to also higher grade, all hydrous minerals, which contains hornblende, might break dvery own and also be replaced by other, higher-temperature, non-hydrous minerals such as pyroxene.

Index Minerals

Index minerals, which are indications of metamorphic grade. In a given rock type, which starts via a particular chemical composition, lower-grade index minerals are replaced by higher-grade index minerals in a sequence of chemical reactions that proceeds as the rock undergoes prograde metamorphism. For example, in rocks made of metamorphosed shale, metamorphism may prograde through the complying with index minerals:

chlorite characterizes the lowest local metamorphic gradebiotite reareas chlorite at the next metamorphic grade, which can be thought about medium-low gradegarnet shows up at the following metamorphic grade, tool gradestaurolite marks the next metamorphic grade, which is medium-high gradesillimanite is a characteristic mineral of high grade metamorphic rocks

Index minerals are supplied by geologists to map metamorphic grade in regions of metamorphic rock. A geologist maps and also collects rock samples across the area and marks the geologic map through the place of each rock sample and the kind of index mineral it has. By illustration lines approximately the areas wbelow each type of index mineral occurs, the geologist delineates the zones of different metamorphic grades in the region. The lines are recognized as isograds.

Types of Metamorphism

Regional Metamorphism

Regional metamorphism occurs where huge locations of rock are subjected to large amounts of differential stress and anxiety for long intervals of time, conditions commonly associated through mountain structure. Mountain structure occurs at subduction zones and also at continental collision areas wbelow two plates each bearing continental crust, converge upon each other.

Most foliated metamorphic rocks—slate, phyllite, schist, and gneiss—are formed in the time of local metamorphism. As the rocks become heated at depth in the Earth during neighborhood metamorphism they become ductile, which means they are fairly soft even though they are still solid. The folding and also deformation of the rock while it is ductile might substantially distort the original shapes and also orientations of the rock, creating folded layers and mineral veins that have extremely deformed or also convoluted shapes. The diagram listed below shows folds forming in the time of a very early phase of regional metamorphism, together with development of foliation, in response to normal stress and anxiety.

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The photograph listed below mirrors high-grade metamorphic rock that has undertaken numerous stperiods of foliation development and also folding during local metamorphism, and might even have actually got to such a high temperature that it started to melt.