Identify metamorphic rocks and also the steps of the rock cycle regarded their formation.

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The last form of absent is metamorphic rocks. Let’s watch what this rocks space like and how they’re formed.

What You’ll discover to Do

Define the characteristics of a metamorphic rock.Discuss the effect of heat, pressure and deformation ~ above rocks.

Characteristics the Metamorphic Rocks

A metamorphic rock supplied to be some other kind of rock, however it was adjusted inside the planet to become a new kind of rock. Words metamorphism come from old Greek words for “change” (meta) and also “form” (morph). The kind of rock the a metamorphic rock used to be, before metamorphism, is referred to as the protolith. During metamorphism the mineral content and also texture of the protolith are adjusted due to transforms in the physical and chemical environment of the rock. Metamorphism have the right to be brought about by burial, tectonic stress, heater by magma, or modification by fluids. At progressed stages of metamorphism, that is typical for a metamorphic rock to construct such a different set of minerals and such a thoroughly changed texture the it is an overwhelming to recognize what the protolith was.

A absent undergoing metamorphism continues to be a heavy rock during the process. Rocks carry out not melt throughout most problems of metamorphism. At the greatest grade the metamorphism, rocks begin to partially melt, in ~ which suggest the border of metamorphic conditions is surpassed and also the igneous part of the rock cycle is entered.

Even despite rocks remain solid during metamorphism, liquid is generally current in the microscope spaces between the minerals. This liquid phase might play a significant role in the chemistry reactions that are critical part of how metamorphism occurs. The fluid usually is composed largely that water.

Metamorphic rocks administer a record of the procedures that occurred inside earth as the rock was subjected to an altering physical and also chemical conditions. This gives the geologist literally “inside information” top top what occurs within the Earth throughout such procedures as the development of brand-new mountain ranges, the collision of continents, the subduction of oceanic plates, and also the circulation of sea water into warm oceanic crust. Metamorphic rocks are choose probes that have gone down into the Earth and also come back, bringing an document of the problems they encountered on your journey in the depths of the Earth.

Figure 1. The platy class in this large outcrop the metamorphic rock show the effects of push on rocks throughout metamorphism.

In the huge outcrop the metamorphic rocks in figure 1, the rocks’ platy illustration is a an outcome of the process metamorphism. Metamorphism is the enhancement of warmth and/or press to present rocks, which causes them to readjust physically and/or chemically so that they end up being a brand-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 the is required is enough heat and/or push to transform the present rock’s physical or chemistry makeup without melting the absent entirely.

Figure 2. A foliated metamorphic rock.

Rocks change during metamorphism due to the fact that the minerals should be stable under the brand-new temperature and also pressure conditions. The need for stability may reason the structure of mineral to rearrange and type new minerals. Ions may move between minerals to produce minerals of various chemical composition. Hornfels, v its alternate bands of dark and light crystals, is a an excellent example of just how minerals rearrange themselves during metamorphism. Hornfels is presented in table 1.

Extreme pressure may likewise lead come foliation, the level layers that form in rocks as the rocks room squeezed by press (figure 2). Foliation normally forms when push is exerted in only one direction. Metamorphic rocks may additionally be non-foliated. Quartzite and limestone, displayed in table 6, are nonfoliated.

The two main species of metamorphism room both related to heat within Earth:

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

Factors that manage Metamorphism

The reason rocks experience metamorphism is that the mineral in a absent are only stable under a restricted range the pressure, temperature, and also chemical conditions. As soon as rocks space subjected to huge enough transforms in these factors, the minerals will undergo chemistry reactions that an outcome in your replacement by new minerals, minerals that space stable in the new conditions.

Chemical composition of the Protolith

The kind of rock undergoes metamorphism is a major factor in determing what form of metamorphic rock it becomes. In brief the recognize of the protolith dram a huge role the identification of the metamorphic rock. A liquid phase may introduce or remove chemical substances right into or the end of the rock during metamorphism, yet in many metamorphic rock, most of the atom in the protolith are be current in the metamorphic absent after metamorphism; the atom will likely be rearranged into brand-new mineral develops within the rock. Therefore, not just does the protolith determine the early stage chemistry that the metamorphic rock, many metamorphic rocks perform not readjust their bulk (overall) chemical compositions very much during metamorphism. The truth that most metamorphic rocks retain most of their initial atoms method that even if the rock to be so thoroughly metamorphosed the it no much longer looks at all like the protolith, the rock can be analyzed in terms of its bulk chemical composition to recognize what kind of rock the protolith was.


Temperature is another significant factor of metamorphism. There room two methods to think about how the temperature of a rock can be increased as a result of geologic processes.

If rocks are hidden within the Earth, the deeper lock go, the higher the temperatures they experience. This is since temperature inside the earth increases along what is called the geothermal gradient, or geotherm for short. Therefore, if rocks room simply buried deep enough enough sediment, lock will endure temperatures high enough to cause metamorphism. This temperature is around 200ºC (approximately 400ºF).

Tectonic processes are another means rocks have the right to be moved deeper along the geotherm. Faulting and also folding the rocks that the crust, deserve to move rocks come much higher depth than an easy burial can.

Yet another way a absent in the earth crust have the right to have that is temperature greatly increased is through the intrusion that magma nearby. Magma intrusion subjects nearby rock to greater temperature with no increase in depth or pressure.


Pressure is a measure of the stress, the physical force, being applied to the surface of a material. The is characterized as the force per unit area acting on the surface, in a direction perpendicular come the surface.

Lithostatic push is the press exerted top top a absent by all the surrounding rock. The resource of the push is the weight of every the rocks above. Lithostatic pressure increases as depth within the earth increases and is a uniform stress—the pressure uses equally in every directions on the rock.

If push does not use equally in every directions, differential stress and anxiety occurs. There space two types of differential stress.

Normal anxiety compresses (pushes together) rock in one direction, the direction of preferably stress. At the very same time, in a perpendicular direction, the rock undergoes stress (stretching), in the direction that minimum stress.

Shear stress and anxiety pushes one next of the absent in a direction parallel to the side, while in ~ the same time, the other side that the rock is being driven in the opposite direction.

Differential stress has actually a significant influence top top the the figure of a metamorphic rock. Differential stress can flatten pre-existing grains in the rock, as presented in the diagram below.


Metamorphic minerals that flourish under differential stress will have a desired orientation if the minerals have actually atomic frameworks that have tendency to do them kind either flat or elongate crystals. This will be especially apparent for micas or various other sheet silicates the grow during metamorphism, such as biotite, muscovite, chlorite, talc, or serpentine. If any of these flat minerals are cultivation under normal stress, lock will grow with your sheets oriented perpendicular come the direction of preferably compression. This outcomes in a rock that deserve to be easily broken along the parallel mineral sheets. Such a rock is said to it is in foliated, or to have foliation.



Any open an are between the mineral seed in a rock, however microscopic, might contain a fluid phase. Many commonly, if over there is a fluid phase in a rock throughout metamorphism, it will be a hydrous fluid, consisting of water and things dissolved in the water. Much less commonly, it may be a carbon dioxide fluid or some various other fluid. The presence of a fluid phase is a major factor during metamorphism because it helps identify which metamorphic reactions will certainly occur and also how fast they will certainly occur. The liquid phase can additionally influence the price at i m sorry mineral crystals deform or readjust shape. Most of this affect is due to the liquified ions the pass in and out the the liquid phase. If during metamorphism enough ions are presented to or removed from the rock via the liquid to adjust the mass chemical composition of the rock, the absent is claimed to have undergone metasomatism. However, most metamorphic rocks perform not experience sufficient change in their bulk chemistry to be considered metasomatic rocks.


Most metamorphism of rocks takes place gradually inside the Earth. Local metamorphism takes location on a timescale of millions of years. Metamorphism usually involves slow changes to rocks in the hard state, as atoms or ions diffuse the end of unstable minerals that are breaking down in the given pressure and temperature conditions and also migrate into brand-new minerals that space stable in those conditions. This form of chemistry reaction bring away a lengthy time.

Grades the Metamorphism

Metamorphic grade refers to the basic temperature and pressure problems that prevailed during metamorphism. As the pressure and temperature increase, rocks undergo metamorphism at higher metamorphic grade. Rocks transforming from one type of metamorphic absent to an additional as they encounter greater grades the metamorphism are said to be experience prograde metamorphism.

Low-grade metamorphism takes location at about 200–320 ºC and relatively low pressure. This is not far beyond the problems in which sediments obtain lithified into sedimentary rocks, and it is common for a low-grade metamorphic rock to look at somewhat choose its protolith. Short grade metamorphic rocks have tendency to characterized by wealth of hydrous minerals, minerals the contain water within their crystal structure. Examples of low grade hydrous minerals incorporate clay, serpentine, and also chlorite. Under short grade metamorphism numerous of the metamorphic minerals will not grow big enough to be watched without a microscope.

Medium-grade metamorphism takes ar at roughly at 320–450 ºC and at center pressures. Low grade hydrous mineral are changed by micas such together biotite and also muscovite, and also non-hydrous minerals such together garnet might grow. Garnet is an example of a mineral i beg your pardon may type porphyroblasts, metamorphic mineral grains the are bigger in dimension and more equant in form (about the exact same diameter in all directions), hence standing out among the smaller, flatter, or much more elongate minerals.

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

Index Minerals

Index minerals, i beg your pardon are indicators of metamorphic grade. In a offered rock type, i m sorry starts through a particular chemical composition, lower-grade index minerals are changed by higher-grade table of contents minerals in a succession of chemistry reactions that proceeds together the absent undergoes prograde metamorphism. Because that example, in rocks do of metamorphosed shale, metamorphism might prograde with the adhering to index minerals:

chlorite characterizes the lowest regional metamorphic gradebiotite replaces chlorite in ~ the following metamorphic grade, which can be taken into consideration medium-low gradegarnet appears at the following metamorphic grade, medium gradestaurolite clues the next metamorphic grade, which is medium-high gradesillimanite is a properties mineral the high grade metamorphic rocks

Index minerals are used by geologists to map metamorphic grade in regions of metamorphic rock. A geologist maps and also collects absent samples throughout the an ar and point out the geologic map with the ar of every rock sample and the form of index mineral it contains. By illustration lines around the areas where each type of index mineral occurs, the geologist delineates the zones of different metamorphic grades in the region. The present are well-known as isograds.

Types of Metamorphism

Regional Metamorphism

Regional metamorphism occurs where huge areas the rock space subjected to large amounts the differential tension for long intervals the time, conditions typically associated with hill building. Mountain structure occurs at subduction zones and also at continent collision area where two plates every bearing continent crust, converge upon every other.

Most foliated metamorphic rocks—slate, phyllite, schist, and gneiss—are formed during local metamorphism. As the rocks end up being heated at depth in the earth during local metamorphism they come to be ductile, which method they are fairly soft even though they room still solid. The folding and deformation of the absent while it is ductile may significantly distort the initial shapes and also orientations of the rock, producing folded layers and also mineral veins that have highly deformed or even convoluted shapes. The diagram below shows folds creating during very early stage of local metamorphism, follow me with breakthrough of foliation, in response to regular stress.

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The photograph below shows high-grade metamorphic rock that has actually undergone number of stages the foliation advance and urgent during regional metamorphism, and may also have got to such a high temperature that it began to melt.