A metamorphic rock used to be some other form of rock, yet it was readjusted inside the earth to become a new form of rock. The word metamorphism come from ancient Greek words for “change” (meta) and also “form” (morph). The form of rock the a metamorphic rock offered to be, prior to metamorphism, is dubbed the protolith. Throughout metamorphism the mineral content and also texture the the protolith are readjusted due to alters in the physical and also chemical setting of the rock. Metamorphism deserve to be caused by burial, tectonic stress, heater by magma, or modification by fluids. At progressed stages of metamorphism, the is typical for a metamorphic absent to develop such a different set of minerals and such a thoroughly adjusted texture that it is complicated to recognize what the protolith was.

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A rock undergoing metamorphism stays a hard rock throughout the process. Rocks execute not melt during most conditions of metamorphism. At the highest grade that metamorphism, rocks start to partly melt, in ~ which point the border of metamorphic conditions is surpassed and also the igneous component of the rock cycle is entered.

Even though rocks continue to be solid throughout metamorphism, liquid is generally present in the microscopic spaces between the minerals. This fluid phase may play a significant role in the chemical reactions that are crucial part of just how metamorphism occurs. The liquid usually is composed largely the water.

Metamorphic rocks provide a document of the procedures that arisen inside earth as the rock to be subjected to transforming physical and chemical conditions. This provides the geologist accurate “inside information” ~ above what occurs within the Earth during such processes as the formation of new mountain ranges, the collision that continents, the subduction the oceanic plates, and the circulation of sea water into hot oceanic crust. Metamorphic rocks are choose probes that have actually gone down into the Earth and also come back, bringing an record of the conditions they encountered on your journey in the depths of the Earth.

Figure 1. The platy class in this big outcrop that metamorphic rock display the impacts of pressure on rocks throughout metamorphism.

In the large outcrop that metamorphic rocks in figure 1, the rocks’ platy appearance is a an outcome of the procedure metamorphism. Metamorphism is the enhancement of heat and/or push to existing rocks, which causes them to change physically and/or chemically so that they come to be a new rock. Metamorphic rocks may adjust so much that they may not resemble the original rock.


Any kind 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 physics or chemical makeup without melting the rock entirely.

Figure 2. A foliated metamorphic rock.

Rocks change during metamorphism due to the fact that the minerals need to be steady under the brand-new temperature and also pressure conditions. The require for security may cause the framework of mineral to rearrange and type new minerals. Ions may move in between minerals to create minerals of various chemical composition. Hornfels, with its alternate bands the dark and also light crystals, is a an excellent example of how minerals rearrange themselves during metamorphism. Hornfels is presented in table 1.

Extreme push may additionally lead to foliation, the level layers that type in rocks as the rocks room squeezed by press (figure 2). Foliation normally develops when press is exerted in only one direction. Metamorphic rocks may additionally be non-foliated. Quartzite and also limestone, presented in table 6, space nonfoliated.

The 2 main varieties of metamorphism space both related to warmth within Earth:

Regional metamorphism: changes in massive quantities that rock over a vast area led to by the too much pressure native overlying absent or native compression led to by geologic processes. Deep interment exposes the absent to high temperatures.Contact metamorphism: transforms in a rock the is in call with magma since of the magma’s too much heat.

Factors that manage Metamorphism

The reason rocks experience metamorphism is the the minerals in a absent are only stable under a minimal range the pressure, temperature, and chemical conditions. When rocks space subjected to huge enough alters in this factors, the minerals will undergo chemistry reactions that result in their replacement by new minerals, mineral that room stable in the brand-new conditions.

Chemical composition of the Protolith

The kind of rock undergoing metamorphism is a significant factor in identify what type of metamorphic absent it becomes. In quick the determine of the protolith theatre a large role in the identification of the metamorphic rock. A liquid phase may introduce or eliminate chemical substances into or out of the rock during metamorphism, however in many metamorphic rock, most of the atoms in the protolith will certainly be current in the metamorphic absent after metamorphism; the atoms will likely be rearranged into brand-new mineral forms within the rock. Therefore, not only does the protolith identify the early chemistry the the metamorphic rock, most metamorphic rocks execute not readjust their bulk (overall) chemistry compositions an extremely much during metamorphism. The truth that most metamorphic rocks retain most of their initial atoms method that also if the rock to be so extensively metamorphosed that it no longer looks in ~ all choose the protolith, the rock can be analyzed in terms of its mass chemical composition to identify what type of rock the protolith was.


Temperature is another significant factor the metamorphism. There room two means to think around how the temperature of a rock deserve to be boosted as a result of geologic processes.

If rocks are buried within the Earth, the deeper lock go, the greater the temperature they experience. This is due to the fact that temperature inside the planet increases follow me what is called the geothermal gradient, or geotherm for short. Therefore, if rocks are simply buried deep enough enough sediment, lock will suffer temperatures high enough to cause metamorphism. This temperature is about 200ºC (approximately 400ºF).

Tectonic processes are another method rocks deserve to be relocated deeper follow me the geotherm. Faulting and folding the rocks the the crust, have the right to move rocks to much better depth than straightforward burial can.

Yet another means a absent in the earth crust deserve to have the temperature substantially increased is through the intrusion of magma nearby. Magma intrusion subjects nearby rock to higher temperature v no increase in depth or pressure.


Pressure is a measure of the stress, the physics force, being used to the surface ar of a material. It is characterized as the pressure per unit area acting on the surface, in a direction perpendicular come the surface.

Lithostatic press is the press exerted ~ above a rock by all the surrounding rock. The source of the press is the weight of every the rocks above. Lithostatic pressure increases as depth in ~ the planet increases and is a uniform stress—the pressure applies equally in every directions top top the rock.

If push does not use equally in every directions, differential tension occurs. There are two types of differential stress.

Normal stress and anxiety compresses (pushes together) absent in one direction, the direction of best stress. In ~ the exact same time, in a perpendicular direction, the absent undergoes stress (stretching), in the direction that minimum stress.

Shear stress pushes one side of the rock in a direction parallel to the side, while at the same time, the various other side the the rock is being propelled in opposing direction.

Differential stress has actually a major influence top top the figure of a metamorphic rock. Differential stress have the right to flatten pre-existing grains in the rock, as shown in the diagram below.


Metamorphic mineral that flourish under differential stress and anxiety will have actually a preferred orientation if the minerals have actually atomic structures that often tend to make them type either flat or elongate crystals. This will be especially evident for micas or other sheet silicates that grow throughout metamorphism, such as biotite, muscovite, chlorite, talc, or serpentine. If any of these flat minerals are farming under common stress, castle will grow with your sheets oriented perpendicular come the direction of preferably compression. This results in a absent that have the right to be easily damaged along the parallel mineral sheets. Such a absent is said to be foliated, or to have actually foliation.



Any open space between the mineral seed in a rock, but microscopic, may contain a fluid phase. Most commonly, if over there is a fluid phase in a rock during metamorphism, it will be a hydrous fluid, consist of of water and things liquified 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 factor throughout metamorphism because it helps determine which metamorphic reactions will occur and how rapid they will certainly occur. The liquid phase can additionally influence the rate at i m sorry mineral crystals deform or readjust shape. Many of this influence is because of the liquified ions that pass in and out of the liquid phase. If during metamorphism enough ions are introduced to or removed from the rock via the fluid to readjust the mass chemical composition of the rock, the absent is claimed to have undergone metasomatism. However, most metamorphic rocks carry out not experience sufficient readjust in their bulk chemistry to be considered metasomatic rocks.


Most metamorphism of rocks takes place progressively inside the Earth. Regional metamorphism takes ar on a timescale of countless years. Metamorphism usually entails slow changes to rocks in the solid state, as atoms or ions diffuse out of unstable minerals that room breaking down in the offered pressure and also temperature conditions and also migrate into new minerals that room stable in those conditions. This form of chemistry reaction bring away a lengthy time.

Grades of Metamorphism

Metamorphic grade describes the basic temperature and pressure problems that prevailed during metamorphism. As the pressure and also temperature increase, rocks experience metamorphism at higher metamorphic grade. Rocks changing from one kind of metamorphic rock to one more as lock encounter greater grades that metamorphism are claimed to be experience prograde metamorphism.

Low-grade metamorphism takes ar at roughly 200–320 ºC and fairly low pressure. This is no far beyond the problems in which sediments get lithified right into sedimentary rocks, and also it is typical for a low-grade metamorphic absent to look at somewhat prefer its protolith. Short grade metamorphic rocks often tend to defined by wealth of hydrous minerals, minerals the contain water within their crystal structure. Examples of low grade hydrous minerals encompass clay, serpentine, and also chlorite. Under low grade metamorphism numerous of the metamorphic minerals will not grow huge enough to be seen without a microscope.

Medium-grade metamorphism takes place at about at 320–450 ºC and at middle pressures. Short grade hydrous mineral are replaced by micas such together biotite and also muscovite, and non-hydrous minerals such as garnet may grow. Garnet is an example of a mineral i m sorry may type porphyroblasts, metamorphic mineral grains that are larger in size and much more equant in form (about the exact same diameter in every directions), therefore standing out among the smaller, flatter, or more elongate minerals.

High-grade metamorphism takes place at temperatures above around 450 ºC. Micas often tend to break down. Brand-new minerals such together hornblende will certainly form, which is stable at greater temperatures. However, as metamorphic grade rises to even higher grade, all hydrous minerals, which consists of hornblende, may breakdown and be changed by other, higher-temperature, non-hydrous minerals such as pyroxene.

Index Minerals

Index minerals, i m sorry are indications of metamorphic grade. In a offered rock type, i m sorry starts through a details chemical composition, lower-grade index minerals are changed by higher-grade index minerals in a succession of chemistry reactions that proceeds as the rock undergoes prograde metamorphism. Because that example, in rocks made of metamorphosed shale, metamorphism may prograde with the adhering to index minerals:

chlorite characterizes the lowest regional metamorphic gradebiotite replace instead replace chlorite in ~ the next metamorphic grade, which can be considered medium-low gradegarnet appears at the next metamorphic grade, tool gradestaurolite point out the next metamorphic grade, i m sorry is medium-high gradesillimanite is a properties mineral that high class metamorphic rocks

Index minerals are supplied by geologists come map metamorphic grade in regions of metamorphic rock. A geologist maps and also collects absent samples across the an ar and point out the geologic map through the place of each rock sample and the type of table of contents mineral the contains. By drawing lines around the areas where each type of index mineral occurs, the geologist delineates the zones of different metamorphic grades in the region. The currently are known as isograds.

Types that Metamorphism

Regional Metamorphism

Regional metamorphism wake up where huge areas that rock room subjected to large amounts the differential stress for lengthy intervals of time, problems typically associated with hill building. Mountain building occurs in ~ subduction zones and also at continent collision zones where 2 plates each bearing continent crust, converge upon every other.

Most foliated metamorphic rocks—slate, phyllite, schist, and gneiss—are created during local metamorphism. As the rocks end up being heated in ~ depth in the earth during regional metamorphism they become ductile, which way they are relatively soft also though they room still solid. The folding and also deformation the the absent while it is ductile may considerably distort the original shapes and also orientations of the rock, developing folded layers and also mineral veins that have actually highly deformed or also convoluted shapes. The diagram listed below shows folds forming during an early stage of regional metamorphism, along with development of foliation, in response to typical stress.

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The photograph listed below shows high-grade metamorphic absent that has actually undergone several stages of foliation advance and folding during local metamorphism, and also may even have reached such a high temperature that it began to melt.