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

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

What You’ll learn to Do

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

Characteristics that Metamorphic Rocks

A metamorphic rock supplied to be part other form of rock, yet it was adjusted inside the earth to come to be a new type of rock. The word metamorphism come from old Greek words for “change” (meta) and also “form” (morph). The kind of rock that a metamorphic rock used to be, prior to metamorphism, is referred to as the protolith. Throughout metamorphism the mineral content and also texture that the protolith are changed due to changes in the physical and also chemical setting of the rock. Metamorphism can be caused by burial, tectonic stress, heating by magma, or modification by fluids. At advanced stages that metamorphism, it is common for a metamorphic rock to build such a different collection of minerals and such a thoroughly adjusted texture the it is challenging to recognize what the protolith was.

A rock undergoing metamorphism remains a heavy rock during the process. Rocks carry out not melt during most problems of metamorphism. In ~ the highest possible grade that metamorphism, rocks begin to partly melt, in ~ which allude the boundary of metamorphic problems is surpassed and also the igneous component of the rock cycle is entered.

Even though 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 chemical reactions the are vital part of just how metamorphism occurs. The liquid usually is composed largely that water.

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

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

In the huge outcrop of metamorphic rocks in figure 1, the rocks’ platy illustration is a an outcome of the procedure metamorphism. Metamorphism is the addition of heat and/or pressure to existing rocks, which reasons them to adjust physically and/or chemically so that they become a new rock. Metamorphic rocks may adjust so much that they might not resemble the original rock.


Any kind of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock. All the is required is sufficient heat and/or press to change the currently rock’s physics or chemical makeup without melting the absent entirely.

Figure 2. A foliated metamorphic rock.

Rocks readjust during metamorphism since the minerals need to be steady under the new temperature and pressure conditions. The need for stability may reason the framework of minerals to rearrange and form new minerals. Ions might move in between minerals to create minerals of different chemical composition. Hornfels, v its alternate bands the dark and light crystals, is a good example of how minerals rearrange themselves during metamorphism. Hornfels is shown in table 1.

Extreme push may also lead come foliation, the flat layers that kind in rocks together the rocks are squeezed by pressure (figure 2). Foliation normally forms when push is exerted in only one direction. Metamorphic rocks may also be non-foliated. Quartzite and also limestone, presented in table 6, room nonfoliated.

The two main species of metamorphism room both associated to warmth within Earth:

Regional metamorphism: changes in huge quantities that rock end a vast area resulted in by the excessive pressure from overlying absent or native compression led to by geologic processes. Deep burial exposes the absent to high temperatures.Contact metamorphism: alters in a rock the is in contact with magma since of the magma’s excessive heat.

Factors that regulate Metamorphism

The factor rocks undergo metamorphism is the the minerals in a absent are just stable under a minimal range that pressure, temperature, and also chemical conditions. When rocks room subjected to big enough changes in this factors, the minerals will certainly undergo chemical reactions that result in their replacement by new minerals, mineral that space stable in the brand-new conditions.

Chemical ingredient of the Protolith

The kind of absent undergoes metamorphism is a significant factor in determing what type of metamorphic absent it becomes. In brief the identify of the protolith theatre a huge role the identification of the metamorphic rock. A liquid phase may introduce or eliminate chemical substances into or the end of the rock throughout metamorphism, but in many metamorphic rock, many of the atoms in the protolith room be existing in the metamorphic rock after metamorphism; the atom will likely be rearranged into new mineral develops within the rock. Therefore, not only does the protolith recognize the early stage chemistry that the metamorphic rock, many metamorphic rocks carry out not change their bulk (overall) chemistry compositions an extremely much during metamorphism. The truth that many metamorphic rocks retain most of their initial atoms means that even if the rock was so completely metamorphosed that it no much longer looks at all choose the protolith, the rock deserve to be analyzed in regards to its mass chemical composition to identify what type of rock the protolith was.


Temperature is another major factor that metamorphism. There room two ways to think around how the temperature that a rock can be boosted as a an outcome of geologic processes.

If rocks are hidden within the Earth, the deeper castle go, the greater the temperatures they experience. This is because temperature inside the earth increases along what is called the geothermal gradient, or geotherm because that short. Therefore, if rocks are simply hidden deep sufficient enough sediment, castle will suffer temperatures high sufficient to reason metamorphism. This temperature is around 200ºC (approximately 400ºF).

Tectonic procedures are another way rocks deserve to be moved deeper along the geotherm. Faulting and folding the rocks that the crust, deserve to move rocks to much greater depth than basic burial can.

Yet another method a absent in the earth’s crust can have the temperature substantially increased is by the intrusion of magma nearby. Magma intrusion subjects nearby rock to higher temperature v no rise in depth or pressure.


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

Lithostatic push is the push exerted ~ above a rock by all the bordering rock. The resource of the pressure is the load of every the rocks above. Lithostatic pressure increases as depth in ~ the earth increases and also is a uniform stress—the pressure applies equally in every directions top top the rock.

If pressure does not apply equally in every directions, differential anxiety occurs. There are two types of differential stress.

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

Shear tension pushes one side of the absent in a direction parallel come the side, while in ~ the same time, the other side that the absent is being pushed in opposing direction.

Differential stress has actually a major influence ~ above the the figure of a metamorphic rock. Differential stress deserve to flatten pre-existing grains in the rock, as presented in the diagram below.


Metamorphic minerals that flourish under differential anxiety will have a preferred orientation if the minerals have atomic frameworks that often tend to do them form either flat or elongate crystals. This will certainly be especially apparent for micas or various other sheet silicates the grow during metamorphism, such as biotite, muscovite, chlorite, talc, or serpentine. If any type of of these flat minerals are growing under normal stress, lock will prosper with their sheets oriented perpendicular come the direction of preferably compression. This results in a absent that deserve to be easily broken along the parallel mineral sheets. Together a rock is claimed to it is in foliated, or to have actually foliation.



Any open room between the mineral grains in a rock, but microscopic, might contain a fluid phase. Most commonly, if over there is a liquid phase in a rock during metamorphism, it will certainly be a hydrous fluid, consisting of water and things dissolved in the water. Less commonly, it may be a carbon dioxide liquid or some other fluid. The existence of a fluid phase is a significant factor throughout metamorphism since it helps identify which metamorphic reactions will certainly occur and how rapid they will certainly occur. The liquid phase can additionally influence the price at i beg your pardon mineral crystals deform or readjust shape. Most of this affect is as result of the liquified ions that pass in and also out of the liquid phase. If during metamorphism sufficient ions are introduced to or removed from the rock via the fluid to adjust the bulk chemical ingredient of the rock, the rock is claimed to have undergone metasomatism. However, most metamorphic rocks carry out not undergo sufficient change in their mass chemistry to be thought about metasomatic rocks.


Most metamorphism the rocks takes place gradually inside the Earth. Local metamorphism takes place on a timescale of countless years. Metamorphism usually entails slow alters to rocks in the solid state, as atoms or ion diffuse out of rough minerals that space breaking under in the given pressure and also temperature conditions and also migrate into new minerals that room stable in those conditions. This kind of chemical reaction bring away a lengthy time.

Grades that Metamorphism

Metamorphic grade describes the general temperature and also pressure problems that prevailed throughout metamorphism. Together the pressure and also temperature increase, rocks experience metamorphism at greater metamorphic grade. Rocks changing from one form of metamorphic absent to one more as castle encounter greater grades the metamorphism are stated to be undergoing prograde metamorphism.

Low-grade metamorphism takes place at around 200–320 ºC and fairly low pressure. This is not far beyond the problems in i beg your pardon sediments acquire lithified into sedimentary rocks, and it is common for a low-grade metamorphic rock to look somewhat prefer its protolith. Short grade metamorphic rocks often tend to defined by an abundance of hydrous minerals, minerals the contain water within their crystal structure. Examples of short grade hydrous minerals include clay, serpentine, and chlorite. Under low grade metamorphism countless of the metamorphic minerals will certainly not grow big enough come be watched without a microscope.

Medium-grade metamorphism takes place at around at 320–450 ºC and also at center pressures. Short grade hydrous mineral are replaced by micas such together biotite and muscovite, and also non-hydrous mineral such together garnet may grow. Garnet is an example of a mineral i beg your pardon may form porphyroblasts, metamorphic mineral grains the are larger in size and more equant in shape (about the exact same diameter in all directions), thus standing out amongst the smaller, flatter, or more elongate minerals.

High-grade metamorphism takes location at temperatures above about 450 ºC. Micas have tendency to rest down. Brand-new minerals such together hornblende will certainly form, i beg your pardon is stable at higher temperatures. However, together metamorphic grade boosts to even higher grade, all hydrous minerals, which includes hornblende, may break down and be replaced by other, higher-temperature, non-hydrous mineral such together pyroxene.

Index Minerals

Index minerals, i m sorry are indicators of metamorphic grade. In a provided rock type, i m sorry starts v a specific chemical composition, lower-grade table of contents minerals are changed by higher-grade index minerals in a sequence of chemical reactions the proceeds as the absent undergoes prograde metamorphism. For example, in rocks make of metamorphosed shale, metamorphism may prograde v the following index minerals:

chlorite characterizes the lowest local metamorphic gradebiotite replace instead replace chlorite at the following metamorphic grade, which might be taken into consideration medium-low gradegarnet appears at the following metamorphic grade, medium gradestaurolite marks the following metamorphic grade, i m sorry is medium-high gradesillimanite is a characteristic mineral of high grade metamorphic rocks

Index mineral are offered by geologists come map metamorphic class in areas of metamorphic rock. A geologist maps and also collects absent samples across the region and clues the geologic map through the location of every rock sample and also the kind of index mineral the contains. By drawing lines roughly the areas where each type of index mineral occurs, the geologist delineates the area of various metamorphic grades in the region. The lines are known as isograds.

Types the Metamorphism

Regional Metamorphism

Regional metamorphism wake up where huge areas that rock space subjected to huge amounts that differential anxiety for lengthy intervals the time, conditions typically linked with mountain building. Mountain building occurs in ~ subduction zones and at continent collision zones where two plates every bearing continental crust, converge upon every other.

Most foliated metamorphic rocks—slate, phyllite, schist, and gneiss—are created during regional metamorphism. As the rocks become heated in ~ depth in the planet during regional metamorphism they become ductile, which method they are fairly soft even though they space still solid. The folding and deformation the the absent while the is ductile may considerably distort the initial shapes and orientations of the rock, creating folded layers and mineral veins that have actually highly deformed or also convoluted shapes. The diagram below shows folds forming during an early stage of local metamorphism, follow me with advancement of foliation, in an answer to normal stress.

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