Identify metamorphic rocks and the steps of the rock cycle related to their formation.

Lastly, metamorphic rocks are rock types.Check out how these rocks are formed and what they're like.

What You’ll Learn to Do

Characteristics of Metamorphic Rocks

Metamorphic rocks originate from another type of rock that was changed inside the Earth into a new type of rock.Originally, metamorphism meant "change" (meta) and "form" (morph).Prior to metamorphism, a metamorphic rock used to be a protolith.A rock's physical and chemical environment causes changes to its mineral content and texture during metamorphism.Burial, tectonic stress, magma heating, or fluid alteration can result in metamorphism.The minerals within a metamorphic rock often develop such a different composition and such a radically different texture at advanced stages of metamorphism that it is hard to tell what the protolith once was.

Rocks undergo metamorphism and remain solid during the process.The rock usually fails to melt in this process.By the time the highest grade of metamorphism takes place, rocks are partially melted, at which point the boundary of metamorphic conditions is crossed and the igneous portion of the rock cycle begins.

Rocks may remain solid during metamorphism, however, fluids are generally present in the microscopic spaces between the minerals.Fluid phases may play a vital role in metamorphosis by facilitating chemical reactions.The fluid is mostly liquid.

As a result of physical and chemical changes that occurred within Earth, metamorphic rocks preserve evidence of what occurred inside.Geologists can thus gain a firsthand understanding of how processes such as the formation of new mountain ranges, the collision of continents, the subduction of oceanic plates, and circulation of sea water within hot oceanic crust tend to take place.A metamorphic rock is like a probe that has gone down into the Earth and then come up, providing a record of the conditions they encountered as they traveled to the depths.

FIGURE ONE.There are a number of platy layers visible in this large rock outcrop that illustrate how pressure shaped rocks during metamorphism.

Metamorphic processes caused the rocks to have a platy appearance in figure 1.A metamorphic rock is formed by adding heat and/or pressure to an existing rock, causing it to change physically and/or chemically.In certain cases, metamorphic rocks may change so much they don't resemble the original rock.


It is possible for any type of rock to become metamorphic, whether it is igneous, sedimentary, or metamorphic.There is no need to melt the existing rock; it only needs enough heat and/or pressure to alter the material's physical or chemical makeup.

See figure 2.Metamorphic rock foliated.

In metamorphic rocks, minerals are stabilised by the changes in temperature and pressure.As minerals rearrange their structure to achieve stability, new minerals are formed.In this scenario, ions may move between minerals to form minerals with different chemical properties.A good example of how minerals rearrange themselves during metamorphism can be found at Hornfels, with its alternating bands of dark and light crystals.Table 1 shows Hornfels.

A high level of pressure may also result in foliation, the upward-facing layers that form in rocks when they are squeezed by pressure (figure 2).If only one direction of pressure is applied, a flolation normally occurs.However, metamorphic rocks may also display no foliation.The table 6 shows quartzite and limestone that are non-foliated.

In terms of metamorphism on Earth, two main types are related to heat:

Metamorphosis regional: Metamorphic contact:

Factors that Control Metamorphism

.Minerals undergo chemical reactions when rocks are subjected to large enough changes in these factors to result in replacement of the minerals in the rock by new minerals that are stable under the new conditions.

Metamorphic types are largely determined by the type of rock that undergoes metamorphism.Shortly, the identity of the protolith plays a huge role in the identity of the metamorphic rock.In some metamorphic rocks, a fluid phase may introduce or remove chemical substances from the rock during metamorphism, but in most metamorphic rocks, the protolithic atoms will be preserved in the metamorphic rock after metamorphism. These atoms will be rearranged into new crystal forms within the rock during metamorphism.Because of this, not only does the protolith dictate the initial chemical composition of the metamorphic rock, but the bulk (overall) composition of metamorphic rocks does not change very much during metamorphism.

Meteorites also undergo metamorphism due to the temperature.Two ways can be used to understand how geologic processes can result in an increase in the temperature of a rock.

Generally, the deeper a rock is buried within the Earth, the higher its temperature will be.As a result, temperature increases in the Earth along the geothermal gradient, or geotherm, as it's called.In other words, rocks may undergo metamorphism by simply being buried in sediment for long enough.200°C (approximately 400°F) is the temperature at which this is happening.

The geotherm also allows rocks to be moved deeper by tectonic processes.When the rocks of the crust are faulted and folded, they can be moved to greater depths than can be achieved by simple burial.

In addition to magma intrusion, another way to increase temperature of Earth's crust rocks is by passing beneath it.With no increase in pressure or depth, magma intrusion causes nearby rock to get warmer.

It is the measure of the physical force or stress applied to the surface of the material.Force per unit area perpendicular to the surface acting on a surface.

All the surrounding rock exerts a lithostatic pressure on a rock.That pressure is created by all the rocks over it.Lithostatic pressure increases with depth within the Earth, and is a uniform stress - it applies equally on all surfaces.

Pressure can be differential in all directions if it is not applied equally.Difference in pressure can be either vector or axial.

Under normal conditions, rock is compressed (pushed together) in only one direction, the direction of maximum stress.Meanwhile, the rock is also under tension (stretching) along a perpendicular plane.

While one side of the rock is pushed parallel to the side, the opposing side is being pushed away from the parallel side.

Metamorphic rocks are affected largely by differential stress.During differential stress in the rock, preexisting grains can become flattened, as shown in the diagram below.


Metamorphic minerals that grow under differential stress have a preferred orientation if their atomic structure tends to cause them to form flat or elongated crystals.Particularly evident will be this phenomenon in micas or other sheet silicates which are produced during metamorphism, such as biotite, muscovite, chlorite, talc, or serpentine.In any of these flat minerals that are subject to normal stress, their sheets will grow perpendicular to the direction of maximum compression.In turn, this results in a rock that can be broken along parallel mineral sheets.A rock of this nature is foliated, or has foliation.


Even the smallest crack between mineral grains in a rock may contain a fluid phase.In metamorphic rocks, fluid phases normally consist of water and dissolved substances.Carbon dioxide or another fluid may be used less frequently.When metamorphism occurs, the presence of a fluid phase determines the types of reactions that will happen and how rapidly.Deformations or changes in the shape of mineral crystals can also be affected by the fluid phase.There is a great deal of influence here due to dissolved ions that pass in and out of the fluid phase.When enough ions are introduced to or removed from a rock by means of the fluid during metamorphism to change the bulk chemical composition of the rock, it is said to have undergone metasomatism.In most cases, metamorphic rocks do not undergo sufficient changes to be considered metasomatic.

Inside the Earth, metamorphism of rocks typically occurs slowly.Metamorphisms take place at a regional scale over several million years.In metamorphic rocks, atoms or ions diffuse out of unstable minerals under pressure and temperature conditions, and then migrate into new minerals that are stable under those pressure and temperature conditions.The chemical reaction takes a long time.

Grades of Metamorphism

During metamorphism, a grade refers to general temperature and pressure conditions that prevailed.Rocks undergo metamorphism at higher metamorphic grades as pressure and temperature increase.Prograde metamorphism occurs when rocks undergo metamorphosis from one metamorphic rock type to another as they encounter higher grades of metamorphism.

.In fact, such conditions are not dissimilar to how sedimentary rocks get lithified, and it is not uncommon for a low-grade metamorphic rock to resemble its protolith.Low grade metamorphic rocks tend to contain a large amount of hydrous minerals, minerals which contain water within their crystal structure.There are several low grade hydrous minerals, including clays, serpentine, and chlorites.In low grade metamorphism, metamorphic minerals won't grow large enough to be seen without a microscope.

320 oC to 450 oC and moderate pressures are optimal for medium-grade metamorphism. .It is one of the minerals which may form porphyroblasts, or metamorphic mineral grains whose size and shape are more equal (equal in all directions), making it distinct from smaller, flatter, or longer minerals.

Metamorphism occurs at temperatures above about 450 oC.High-grade metamorphism takes place at temperatures above about 450 ºC. The highest temperatures for high-grade metamorphism are between 450 oC and 550 oC, which is when mica breaks down.This will result in the formation of new minerals such as hornblende, which is stable at higher temperatures.With increasing metamorphic grade, all hydrous minerals, including hornblende, can dissolve and be replaced by other non-hydrous minerals such as pyroxene at higher temperatures.

Index Minerals

These minerals are indicators of the metamorphic grade.During prograde metamorphism, lower-grade index minerals are substituted for higher-grade index minerals in a specific rock type beginning with a particular chemical composition.In metamorphosed shale, for instance, metamorphism may prograde through the following index minerals:

The index minerals are used by geologists to map metamorphic grade in metamorphic rock formations.Geologists map and collect rock samples within a region and mark the geologic map with their location and the index mineral they contain.Geologists delimit the zones of different metamorphic grades in each region by tracing lines around the locations where index minerals occur.This is known as an isograding.

Types of Metamorphism

.There is mountain building at subduction zones and at continental collision zones when two plates, each carrying continental crust, collide.

Most foliated metamorphic rocks-slate, phyllite, schist, and gneiss-are formed by regional metamorphism.A rock becomes ductile when heated at depth in the Earth during regional metamorphism, which means it is relatively soft as well as solid.It can cause great distortion of the original shapes and orientations of the rock to undergo folding or deformation when it is ductile, resulting in folded layers and mineral veins that are highly deformed or convoluted.Within a naturally stressed environment, folds and foliation form during an early stage of regional metamorphism.


It shows metamorphic rock that is high-grade that has gone through several phases of foliation development and folding during regional metamorphism, and which may have reached a temperature so high that it melted.