Metamorphism is the transformation of existing
rock
Rock most often refers to:
* Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids
* Rock music, a genre of popular music
Rock or Rocks may also refer to:
Places United Kingdom
* Rock, Caerphilly, a location in Wales ...
(the
protolith
A protolith () is the original, unmetamorphosed rock from which a given metamorphic rock is formed.
For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock and th ...
) to rock with a different
mineral composition or
texture
Texture may refer to:
Science and technology
* Surface texture, the texture means smoothness, roughness, or bumpiness of the surface of an object
* Texture (roads), road surface characteristics with waves shorter than road roughness
* Texture ( ...
. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from
weathering or
diagenesis
Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...
, which are changes that take place at or just beneath Earth's surface.
Various forms of metamorphism exist, including
regional
In geography, regions, otherwise referred to as zones, lands or territories, are areas that are broadly divided by physical characteristics (physical geography), human impact characteristics (human geography), and the interaction of humanity and t ...
,
contact, hydrothermal, shock, and dynamic metamorphism. These differ in the characteristic temperatures, pressures, and rate at which they take place and in the extent to which reactive fluids are involved. Metamorphism occurring at increasing pressure and temperature conditions is known as ''prograde metamorphism'', while decreasing temperature and pressure characterize ''retrograde metamorphism''.
Metamorphic petrology
Petrology () is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous, metamorphic, and sedimentary petrology. Igneous and metamorphic petrology are commonly taught together ...
is the study of metamorphism. Metamorphic petrologists rely heavily on
statistical mechanics
In physics, statistical mechanics is a mathematical framework that applies statistical methods and probability theory to large assemblies of microscopic entities. It does not assume or postulate any natural laws, but explains the macroscopic be ...
and
experimental petrology to understand metamorphic processes.
Metamorphic processes
Metamorphism is the set of processes by which existing rock is transformed physically or chemically at elevated temperature, without actually melting to any great degree. The importance of heating in the formation of
metamorphic rock was first recognized by the pioneering Scottish naturalist,
James Hutton
James Hutton (; 3 June O.S.172614 June 1726 New Style. – 26 March 1797) was a Scottish geologist, agriculturalist, chemical manufacturer, naturalist and physician. Often referred to as the father of modern geology, he played a key role ...
, who is often described as the father of modern geology. Hutton wrote in 1795 that some rock beds of the Scottish Highlands had originally been
sedimentary rock, but had been transformed by great heat.
Hutton also speculated that pressure was important in metamorphism. This hypothesis was tested by his friend,
James Hall, who sealed
chalk
Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Chalk ...
into a makeshift
pressure vessel
A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.
Construction methods and materials may be chosen to suit the pressure application, and will depend on the size o ...
constructed from a cannon barrel and heated it in an iron foundry furnace. Hall found that this produced a material strongly resembling
marble, rather than the usual
quicklime
Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term "''lime''" connotes calcium-containing inorganic ma ...
produced by heating of chalk in the open air. French geologists subsequently added
metasomatism, the circulation of fluids through buried rock, to the list of processes that help bring about metamorphism. However, metamorphism can take place without metasomatism (isochemical metamorphism) or at depths of just a few hundred meters where pressures are relatively low (for example, in contact metamorphism).
Rock can be transformed without melting because heat causes atomic bonds to break, freeing the atoms to move and form new bonds with other
atoms. Pore fluid present between mineral grains is an important medium through which atoms are exchanged. This permits
recrystallization of existing minerals or crystallization of new minerals with different crystalline structures or chemical compositions (
neocrystallization). The transformation converts the minerals in the protolith into forms that are more stable (closer to
chemical equilibrium) under the conditions of pressure and temperature at which metamorphism takes place.
Metamorphism is generally regarded to begin at temperatures of . This excludes
diagenetic
Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...
changes due to
compaction and
lithification, which result in the formation of sedimentary rocks. The upper boundary of metamorphic conditions lies at the
solidus
Solidus (Latin for "solid") may refer to:
* Solidus (coin)
The ''solidus'' (Latin 'solid'; ''solidi'') or nomisma ( grc-gre, νόμισμα, ''nómisma'', 'coin') was a highly pure gold coin issued in the Late Roman Empire and By ...
of the rock, which is the temperature at which the rock begins to melt. At this point, the process becomes an
igneous process. The solidus temperature depends on the composition of the rock, the pressure, and whether the rock is saturated with water. Typical solidus temperatures range from for wet granite at a few hundred
megapascals (Mpa) of pressure to about for wet basalt at atmospheric pressure.
Migmatites are rocks formed at this upper limit, which contains pods and veins of material that has started to melt but has not fully segregated from the refractory residue.
The metamorphic process can occur at almost any pressure, from near surface pressure (for contact metamorphism) to pressures in excess of 16
kbar
The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea lev ...
(1500 Mpa).
Recrystallization
The change in the grain size and orientation in the rock during the process of metamorphism is called
recrystallization. For instance, the small
calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...
crystals in the sedimentary rocks
limestone and
chalk
Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Chalk ...
change into larger crystals in the metamorphic rock
marble. In metamorphosed
sandstone, recrystallization of the original
quartz sand grains results in very compact
quartzite, also known as metaquartzite, in which the often larger quartz crystals are interlocked. Both high temperatures and pressures contribute to recrystallization. High temperatures allow the
atoms and
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conv ...
s in solid crystals to migrate, thus reorganizing the crystals, while high pressures cause solution of the crystals within the rock at their points of contact (''
pressure solution
In structural geology and diagenesis, pressure solution or pressure dissolution is a deformation mechanism that involves the dissolution of minerals at grain-to-grain contacts into an aqueous pore fluid in areas of relatively high stress and ...
'') and redeposition in pore space.
During recrystallization, the identity of the mineral does not change, only its texture. Recrystallization generally begins when temperatures reach above half the melting point of the mineral on the
Kelvin scale.
Pressure solution begins during diagenesis (the process of lithification of sediments into sedimentary rock) but is completed during early stages of metamorphism. For a sandstone protolith, the dividing line between diagenesis and metamorphism can be placed at the point where strained quartz grains begin to be replaced by new, unstrained, small quartz grains, producing a ''mortar texture'' that can be identified in
thin section
In optical mineralogy and petrography, a thin section (or petrographic thin section) is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and electron ...
s under a polarizing microscope. With increasing grade of metamorphism, further recrystallization produces ''foam texture'', characterized by polygonal grains meeting at triple junctions, and then ''porphyroblastic texture'', characterized by coarse, irregular grains, including some larger grains (
porphyroblasts
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained matrix. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape.
The most common porphyrobl ...
.)
Metamorphic rocks are typically more coarsely crystalline than the protolith from which they formed. Atoms in the interior of a crystal are surrounded by a stable arrangement of neighboring atoms. This is partially missing at the surface of the crystal, producing a ''
surface energy
In surface science, surface free energy (also interfacial free energy or surface energy) quantifies the disruption of intermolecular bonds that occurs when a surface is created. In solid-state physics, surfaces must be intrinsically less energe ...
'' that makes the surface thermodynamically unstable. Recrystallization to coarser crystals reduces the surface area and so minimizes the surface energy.
Although grain coarsening is a common result of metamorphism, rock that is intensely deformed may eliminate
strain energy
In physics, the elastic potential energy gained by a wire during elongation with a tensile (stretching) force is called strain energy. For linearly elastic materials, strain energy is:
: U = \frac 1 2 V \sigma \epsilon = \frac 1 2 V E \epsilon ...
by recrystallizing as a fine-grained rock called ''
mylonite''. Certain kinds of rock, such as those rich in quartz,
carbonate mineral
Carbonate minerals are those minerals containing the carbonate ion, .
Carbonate divisions Anhydrous carbonates
*Calcite group: trigonal
**Calcite CaCO3
** Gaspéite (Ni,Mg,Fe2+)CO3
**Magnesite MgCO3
** Otavite CdCO3
**Rhodochrosite MnCO3
**Sid ...
s, or olivine, are particularly prone to form mylonites, while feldspar and garnet are resistant to mylonitization.
Phase change
Phase change metamorphism is the creating of a new mineral with the same chemical formula as a mineral of the protolith. This involves a rearrangement of the atoms in the crystals. An example is provided by the
aluminium silicate minerals,
kyanite
Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks generall ...
,
andalusite
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5. This mineral was called andalousite by Delamétehrie, who thought it came from Andalusia, Spain. It soon became clear that it was a locality error, and that the spe ...
, and
sillimanite. All three have the identical composition, . Kyanite is stable at surface conditions. However, at atmospheric pressure, kyanite transforms to
andalusite
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5. This mineral was called andalousite by Delamétehrie, who thought it came from Andalusia, Spain. It soon became clear that it was a locality error, and that the spe ...
at a temperature of about . Andalusite, in turn, transforms to
sillimanite when the temperature reaches about . At pressures above about 4 kbar (400 Mpa), kyanite transforms directly to sillimanite as the temperature increases. A similar phase change is sometimes seen between
calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...
and
aragonite
Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including pr ...
, with calcite transforming to aragonite at elevated pressure and relatively low temperature.
Neocrystallization
Neocrystallization involves the creation of new mineral crystals different from the protolith.
Chemical reaction
A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breakin ...
s digest the minerals of the protolith which yields new minerals. This is a very slow process as it can also involve the diffusion of atoms through solid crystals.
An example of a neocrystallization reaction is the reaction of
fayalite
Fayalite (, commonly abbreviated to Fa) is the iron-rich end-member of the olivine solid-solution series. In common with all minerals in the olivine group, fayalite crystallizes in the orthorhombic system (space group ''Pbnm'') with cell param ...
with
plagioclase at elevated pressure and temperature to form
garnet. The reaction is:
Many complex high-temperature reactions may take place between minerals without them melting, and each mineral assemblage produced provides us with a clue as to the temperatures and pressures at the time of metamorphism. These reactions are possible because of rapid diffusion of atoms at elevated temperature. Pore fluid between mineral grains can be an important medium through which atoms are exchanged.
A particularly important group of neocrystallization reactions are those that release
volatiles such as water and
carbon dioxide. During metamorphism of
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
to
eclogite in
subduction zones, hydrous minerals break down, producing copious quantities of water. The water rises into the overlying mantle, where it lowers the melting temperature of the mantle rock, generating
magma via
flux melting. The mantle-derived magmas can ultimately reach the Earth's surface, resulting in volcanic eruptions. The resulting
arc volcanoes tend to produce dangerous eruptions, because their high water content makes them extremely explosive.
Examples of ''dehydration reactions'' that release water include:
An example of a decarbonation reaction is:
Plastic deformation
In plastic deformation pressure is applied to the
protolith
A protolith () is the original, unmetamorphosed rock from which a given metamorphic rock is formed.
For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock and th ...
, which causes it to shear or bend, but not break. In order for this to happen temperatures must be high enough that brittle fractures do not occur, but not so high that diffusion of crystals takes place.
As with pressure solution, the early stages of plastic deformation begin during diagenesis.
Types
Regional
''Regional metamorphism'' is a general term for metamorphism that affects entire regions of the Earth's crust. It most often refers to ''dynamothermal metamorphism'', which takes place in ''
orogenic belts
Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted ...
'' (regions where
mountain building is taking place), but also includes ''burial metamorphism'', which results simply from rock being buried to great depths below the Earth's surface in a subsiding basin.
Dynamothermal
To many geologists, regional metamorphism is practically synonymous with dynamothermal metamorphism. This form of metamorphism takes place at
convergent plate boundaries, where two
continental plates
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
or a continental plate and an
island arc collide. The collision zone becomes a belt of
mountain formation
Mountain formation refers to the geological processes that underlie the formation of mountains. These processes are associated with large-scale movements of the Earth's crust (tectonic plates). Folding, faulting, volcanic activity, igneous int ...
called an ''
orogeny''. The orogenic belt is characterized by thickening of the Earth's crust, during which the deeply buried crustal rock is subjected to high temperatures and pressures and is intensely deformed. Subsequent
erosion of the mountains exposes the roots of the orogenic belt as extensive outcrops of metamorphic rock, characteristic of mountain chains.
Metamorphic rock formed in these settings tends to shown well-developed
foliation
In mathematics (differential geometry), a foliation is an equivalence relation on an ''n''-manifold, the equivalence classes being connected, injectively immersed submanifolds, all of the same dimension ''p'', modeled on the decomposition of ...
. Foliation develops when a rock is being shortened along one axis during metamorphism. This causes crystals of platy minerals, such as
mica and
chlorite
The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...
, to become rotated such that their short axes are parallel to the direction of shortening. This results in a banded, or foliated, rock, with the bands showing the colors of the minerals that formed them. Foliated rock often develops planes of
cleavage.
Slate is an example of a foliated metamorphic rock, originating from
shale, and it typically shows well-developed cleavage that allows slate to be split into thin plates.
The type of foliation that develops depends on the metamorphic grade. For instance, starting with a
mudstone, the following sequence develops with increasing temperature: The mudstone is first converted to slate, which is a very fine-grained, foliated metamorphic rock, characteristic of very low grade metamorphism. Slate in turn is converted to
phyllite, which is fine-grained and found in areas of low grade metamorphism.
Schist is medium to coarse-grained and found in areas of medium grade metamorphism. High-grade metamorphism transforms the rock to
gneiss, which is coarse to very coarse-grained.
Rocks that were subjected to uniform pressure from all sides, or those that lack minerals with distinctive growth habits, will not be foliated. Marble lacks platy minerals and is generally not foliated, which allows its use as a material for sculpture and architecture.
Collisional orogenies are preceded by
subduction
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...
of oceanic crust. The conditions within the subducting slab as it plunges toward the
mantle
A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that.
Mantle may refer to:
*Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear
**Mantle (vesture), an Eastern Orthodox ve ...
in a subduction zone produce
their own distinctive regional metamorphic effects, characterized by
paired metamorphic belts.
The pioneering work of
George Barrow on regional metamorphism in the Scottish Highlands showed that some regional metamorphism produces well-defined, mappable zones of increasing metamorphic grade. This ''
Barrovian metamorphism'' is the most recognized
metamorphic series in the world. However, Barrovian metamorphism is specific to
pelitic rock, formed from
mudstone or
siltstone
Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, ...
, and it is not unique even in pelitic rock. A different sequence in the northeast of Scotland defines ''
Buchan metamorphism'', which took place at lower pressure than the Barrovian.
Burial
Burial metamorphism takes place simply through rock being buried to great depths below the Earth's surface in a subsiding basin. Here the rock subjected to high temperatures and the great pressure caused by the immense weight of the rock layers above. Burial metamorphism tends to produced low-grade metamorphic rock. This shows none of the effects of deformation and folding so characteristic of dynamothermal metamorphism.
Examples of metamorphic rocks formed by burial metamorphism include some of the rocks of the
Midcontinent Rift System
The Midcontinent Rift System (MRS) or Keweenawan Rift is a long geological rift in the center of the North American continent and south-central part of the North American plate. It formed when the continent's core, the North American craton, ...
of North America, such as the
Sioux Quartzite
The Sioux Quartzite is a Proterozoic quartzite that is found in the region around the intersection of Minnesota, South Dakota, and Iowa, and correlates with other rock units throughout the upper midwestern and southwestern United States. It was ...
, and in the
Hamersley Basin of Australia.
Contact (thermal)
Contact metamorphism occurs typically around
intrusive igneous rocks as a result of the temperature increase caused by the intrusion of
magma into cooler
country rock
Country rock is a genre of music which fuses rock and country. It was developed by rock musicians who began to record country-flavored records in the late 1960s and early 1970s. These musicians recorded rock records using country themes, vocal ...
. The area surrounding the intrusion where the contact metamorphism effects are present is called the metamorphic aureole, the contact aureole, or simply the aureole. Contact metamorphic rocks are usually known as
hornfels. Rocks formed by contact metamorphism may not present signs of strong deformation and are often fine-grained and extremely tough.
Contact metamorphism is greater adjacent to the intrusion and dissipates with distance from the contact. The size of the aureole depends on the heat of the intrusion, its size, and the temperature difference with the wall rocks. Dikes generally have small aureoles with minimal metamorphism, extending not more than one or two dike thicknesses into the surrounding rock, whereas the aureoles around
batholiths
A batholith () is a large mass of intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, such ...
can be up to several kilometers wide.
The metamorphic grade of an aureole is measured by the peak metamorphic mineral which forms in the aureole. This is usually related to the metamorphic temperatures of
pelitic or aluminosilicate rocks and the minerals they form. The metamorphic grades of aureoles at shallow depth are
albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicate. ...
-
epidote hornfels, hornblende hornfels,
pyroxene hornfels, and sillimanite hornfels, in increasing order of temperature of formation. However, the albite-epidote hornfels is often not formed, even though it is the lowest temperature grade.
Magmatic fluids coming from the intrusive rock may also take part in the
metamorphic reaction
A metamorphic reaction is a chemical reaction that takes place during the geological process of metamorphism wherein one assemblage of minerals is transformed into a second assemblage which is stable under the new temperature/pressure conditions r ...
s. An extensive addition of magmatic fluids can significantly modify the chemistry of the affected rocks. In this case the metamorphism grades into
metasomatism. If the intruded rock is rich in
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
the result is a
skarn.
Fluorine
Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reactiv ...
-rich magmatic waters which leave a cooling granite may often form
greisen
Greisen is a highly altered granitic rock or pegmatite, usually composed predominantly of quartz and micas (mostly muscovite). Greisen is formed by self-generated alteration of a granite and is a class of moderate- to high-temperature magmatic- ...
s within and adjacent to the contact of the granite. Metasomatic altered aureoles can localize the deposition of metallic
ore
Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 April ...
minerals and thus are of economic interest.
''Fenitization'', or ''Na-metasomatism'', is a distinctive form of contact metamorphism accompanied by metasomatism. It takes place around intrusions of a rare type of magma called a ''
carbonatite'' that is highly enriched in
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
s and low in
silica. Cooling bodies of carbonatite magma give off highly alkaline fluids rich in sodium as they solidify, and the hot, reactive fluid replaces much of the mineral content in the aureole with sodium-rich minerals.
A special type of contact metamorphism, associated with fossil fuel fires, is known as
pyrometamorphism.
Hydrothermal
Hydrothermal
Hydrothermal circulation in its most general sense is the circulation of hot water ( Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with t ...
metamorphism is the result of the interaction of a rock with a high-temperature fluid of variable composition. The difference in composition between an existing rock and the invading fluid triggers a set of metamorphic and
metasomatic
Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical co ...
reactions. The hydrothermal fluid may be magmatic (originate in an intruding magma), circulating
groundwater, or ocean water. Convective circulation of hydrothermal fluids in the ocean floor
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
s produces extensive hydrothermal metamorphism adjacent to spreading centers and other submarine volcanic areas. The fluids eventually escape through vents on the ocean floor known as
black smokers
A hydrothermal vent is a fissure on the seabed from which geothermally heated water discharges. They are commonly found near volcanically active places, areas where tectonic plates are moving apart at mid-ocean ridges, ocean basins, and hotspot ...
. The patterns of this
hydrothermal alteration are used as a guide in the search for deposits of valuable metal ores.
Shock
Shock metamorphism occurs when an extraterrestrial object (a
meteorite for instance) collides with the Earth's surface. Impact metamorphism is, therefore, characterized by ultrahigh pressure conditions and low temperature. The resulting minerals (such as SiO
2 polymorphs coesite
Coesite is a form ( polymorph) of silicon dioxide Si O2 that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (), are applied to quartz. Coesite was first synthesized by Loring Coes Jr., a chemist at the ...
and
stishovite) and textures are characteristic of these conditions.
Dynamic
Dynamic metamorphism is associated with zones of high strain such as
fault zones. In these environments, mechanical deformation is more important than chemical reactions in transforming the rock. The minerals present in the rock often do not reflect conditions of chemical equilibrium, and the textures produced by dynamic metamorphism are more significant than the mineral makeup.
There are three
deformation mechanisms by which rock is mechanically deformed. These are ''
cataclasis A cataclastic rock is a type of fault rock that has been wholly or partly formed by the progressive fracturing and comminution of existing rocks, a process known as ''cataclasis''. Cataclasis involves the granulation, crushing, or milling of the or ...
'', the deformation of rock via the fracture and rotation of mineral grains; plastic deformation of individual mineral crystals; and movement of individual atoms by diffusive processes. The textures of dynamic metamorphic zones are dependent on the depth at which they were formed, as the temperature and confining pressure determine the deformation mechanisms which predominate.
At the shallowest depths, a fault zone will be filled with various kinds of unconsolidated
cataclastic rock A cataclastic rock is a type of fault rock that has been wholly or partly formed by the progressive fracturing and comminution of existing rocks, a process known as ''cataclasis''. Cataclasis involves the granulation, crushing, or milling of the ori ...
, such as ''
fault gouge
Fault gouge is a type of fault rock best defined by its grain size. It is found as incohesive fault rock (rock which can be broken into its component granules at the present outcrop, only aided with fingers/pen-knife), with less than 30% clasts ...
'' or ''
fault breccia
Fault breccia ( or ; Italian for "breach"), or tectonic breccia, is a breccia (a rock type consisting of angular clasts) that was formed by tectonic forces.
Fault breccia is a tectonite formed by localized zone of brittle deformation (a fault zon ...
''. At greater depths, these are replaced by consolidated cataclastic rock, such as ''crush breccia'', in which the larger rock fragments are cemented together by calcite or quartz. At depths greater than about , ''
cataclasites'' appear; these are quite hard rocks consist of crushed rock fragments in a flinty matrix, which forms only at elevated temperature. At still greater depths, where temperatures exceed , plastic deformation takes over, and the fault zone is composed of mylonite. Mylonite is distinguished by its strong foliation, which is absent in most cataclastic rock. It is distinguished from the surrounding rock by its finer grain size.
There is considerable evidence that cataclasites form as much through plastic deformation and recrystallization as brittle fracture of grains, and that the rock may never fully lose cohesion during the process. Different minerals become ductile at different temperatures, with quartz being among the first to become ductile, and sheared rock composed of different minerals may simultaneously show both plastic deformation and brittle fracture.
The strain rate also affects the way in which rocks deform. Ductile deformation is more likely at low strain rates (less than 10
−14 sec
−1) in the middle and lower crust, but high strain rates can cause brittle deformation. At the highest strain rates, the rock may be so strongly heated that it briefly melts, forming a glassy rock called ''
pseudotachylite''. Pseudotachylites seem to be restricted to dry rock, such as granulite.
Classification of metamorphic rocks
Metamorphic rocks are classified by their protolith, if this can be determined from the properties of the rock itself. For example, if examination of a metamorphic rock shows that its protolith was basalt, it will be described as a metabasalt. When the protolith cannot be determined, the rock is classified by its mineral composition or its degree of foliation.
Metamorphic grades
Metamorphic grade is an informal indication of the amount or degree of metamorphism.
In the Barrovian sequence (described by
George Barrow in zones of progressive metamorphism in Scotland), metamorphic grades are also classified by mineral assemblage based on the appearance of key minerals in rocks of
pelitic (shaly, aluminous) origin:
Low grade ------------------- Intermediate --------------------- High grade
:Greenschist ------------- Amphibolite ----------------------- Granulite
:
Slate ---
Phyllite ----------
Schist ----------------------
Gneiss ---
Migmatite
:
Chlorite
The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...
zone
::::
Biotite
Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumi ...
zone
:::::::
Garnet zone
::::::::::
Staurolite zone
:::::::::::::
Kyanite
Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks generall ...
zone
::::::::::::::::
Sillimanite zone
A more complete indication of this intensity or degree is provided by the concept of
metamorphic facies.
Metamorphic facies
Metamorphic facies are recognizable
terranes
In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its own ...
or zones with an assemblage of key minerals that were in equilibrium under specific range of temperature and pressure during a metamorphic event. The facies are named after the metamorphic rock formed under those facies conditions from
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
.
The particular mineral assemblage is somewhat dependent on the composition of that protolith, so that (for example) the amphibolite facies of a marble will not be identical with the amphibolite facies of a pellite. However, the facies are defined such that metamorphic rock with as broad a range of compositions as is practical can be assigned to a particular facies. The present definition of metamorphic facies is largely based on the work of the Finnish geologist,
Pentti Eskola
Pentti Elias Eskola (8 January 1883 – 6 December 1964) was a Finnish geologist who specialised in the petrology of granites and developed the concept of metamorphic facies. He won the Wollaston Medal in 1958, the Vetlesen Prize in 1964, and ...
in 1921, with refinements based on subsequent experimental work. Eskola drew upon the zonal schemes, based on index minerals, that were pioneered by the British geologist,
George Barrow.
The metamorphic facies is not usually considered when classifying metamorphic rock based on protolith, mineral mode, or texture. However, a few metamorphic facies produce rock of such distinctive character that the facies name is used for the rock when more precise classification is not possible. The chief examples are
amphibolite
Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flak ...
and
eclogite. The British Geological Survey strongly discourages use of ''
granulite'' as a classification for rock metamorphosed to the granulite facies. Instead, such rock will often be classified as a granofels. However, this is not universally accepted.
See diagram for more detail.
Prograde and retrograde
Metamorphism is further divided into prograde and retrograde metamorphism. Prograde metamorphism involves the change of mineral assemblages (
paragenesis Paragenesis is a petrologic concept meaning an ''equilibrium sequence of mineral phases''. It is used in studies of igneous and metamorphic rock genesis and importantly in studies of the hydrothermal deposition of ore minerals and the rock alterat ...
) with increasing temperature and (usually) pressure conditions. These are solid state dehydration reactions, and involve the loss of volatiles such as water or carbon dioxide. Prograde metamorphism results in rock characteristic of the maximum pressure and temperature experienced. Metamorphic rocks usually do not undergo further change when they are brought back to the surface.
Retrograde metamorphism involves the reconstitution of a rock via revolatisation under decreasing temperatures (and usually pressures), allowing the mineral assemblages formed in prograde metamorphism to revert to those more stable at less extreme conditions. This is a relatively uncommon process, because volatiles produced during prograde metamorphism usually migrate out of the rock and are not available to recombine with the rock during cooling. Localized retrograde metamorphism can take place when fractures in the rock provide a pathway for groundwater to enter the cooling rock.
Equilibrium mineral assemblages
Metamorphic processes act to bring the protolith closer to
thermodynamic equilibrium
Thermodynamic equilibrium is an axiomatic concept of thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls. In ther ...
, which is its state of maximum stability. For example, shear stress (nonhydrodynamic stress) is incompatible with thermodynamic equilibrium, so sheared rock will tend to deform in ways that relieve the shear stress. The most stable assemblage of minerals for a rock of a given composition is that which minimizes the
Gibbs free energy
In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and p ...
where:
*''U'' is the
internal energy (SI unit:
joule),
* ''p'' is
pressure (SI unit:
pascal
Pascal, Pascal's or PASCAL may refer to:
People and fictional characters
* Pascal (given name), including a list of people with the name
* Pascal (surname), including a list of people and fictional characters with the name
** Blaise Pascal, Frenc ...
),
* ''V'' is
volume (SI unit: m
3),
* ''T'' is the
temperature (SI unit:
kelvin),
* ''S'' is the
entropy (SI unit: joule per kelvin),
In other words, a metamorphic reaction will take place only if it lowers the total Gibbs free energy of the protolith. Recrystallization to coarser crystals lowers the Gibbs free energy by reducing surface energy, while phase changes and neocrystallization reduce the bulk Gibbs free energy. A reaction will begin at the temperature and pressure where the Gibbs free energy of the reagents becomes greater than that of the products.
A mineral phase will generally be more stable if it has a lower internal energy, reflecting tighter binding between its atoms. Phases with a higher density (expressed as a lower molar volume ''V'') are more stable at higher pressure, while minerals with a less ordered structure (expressed as a higher entropy ''S'') are favored at high temperature. Thus andalusite is stable only at low pressure, since it has the lowest density of any aluminium silicate polymorph, while sillimanite is the stable form at higher temperatures, since it has the least ordered structure.
The Gibbs free energy of a particular mineral at a specified temperature and pressure can be expressed by various analytic formulas. These are calibrated against experimentally measured properties and phase boundaries of mineral assemblages. The equilibrium mineral assemblage for a given bulk composition of rock at a specified temperature and pressure can then be calculated on a computer.
However, it is often very useful to represent equilibrium mineral assemblages using various kinds of diagrams. These include petrogenetic grids and compatibility diagrams (compositional phase diagrams.)
Petrogenetic grids
A
petrogenetic grid
A petrogenetic grid is a geological phase diagram that connects the stability ranges or metastability ranges of metamorphic minerals or mineral assemblages to the conditions of metamorphism. Experimentally determined mineral or mineral-assemb ...
is a geologic
phase diagram
A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions (pressure, temperature, volume, etc.) at which thermodynamically distinct phases (such as solid, liquid or gaseou ...
that plots experimentally derived
metamorphic reactions at their pressure and temperature conditions for a given rock composition. This allows metamorphic petrologists to determine the pressure and temperature conditions under which rocks metamorphose. The Al
2SiO
5 nesosilicate phase diagram shown is a very simple petrogenetic grid for rocks that only have a composition consisting of
aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It h ...
(Al),
silicon (Si), and
oxygen (O). As the rock undergoes different temperatures and pressure, it could be any of the three given
polymorphic minerals. For a rock that contains multiple phases, the boundaries between many phase transformations may be plotted, though the petrogenetic grid quickly becomes complicated. For example, a petrogenetic grid might show both the aluminium silicate phase transitions and the transition from aluminum silicate plus potassium feldspar to muscovite plus quartz.
Compatibility diagrams
Whereas a petrogenetic grid shows phases for a single composition over a range of temperature and pressure, a ''compatibility diagram'' shows how the mineral assemblage varies with composition at a fixed temperature and pressure. Compatibility diagrams provide an excellent way to analyze how variations in the rock's composition affect the mineral
paragenesis Paragenesis is a petrologic concept meaning an ''equilibrium sequence of mineral phases''. It is used in studies of igneous and metamorphic rock genesis and importantly in studies of the hydrothermal deposition of ore minerals and the rock alterat ...
that develops in a rock at particular pressure and temperature conditions. Because of the difficulty of depicting more than three components (as a
ternary diagram
A ternary plot, ternary graph, triangle plot, simplex plot, Gibbs triangle or de Finetti diagram is a barycentric plot on three variables which sum to a constant. It graphically depicts the ratios of the three variables as positions in an equi ...
), usually only the three most important components are plotted, though occasionally a compatibility diagram for four components is plotted as a
projected
Projected is an American rock supergroup consisting of Sevendust members John Connolly and Vinnie Hornsby, Alter Bridge and Creed drummer Scott Phillips, and former Submersed and current Tremonti guitarist Eric Friedman. The band released the ...
tetrahedron.
See also
*
*
Metamorphosis of snow
*
Footnotes
References
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* Eskola P., 1920, ''The Mineral Facies of Rocks'', Norsk. Geol. Tidsskr., 6, 143–194
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Further reading
* Winter J.D., 2001, ''An Introduction to Igneous and Metamorphic Petrology'', Prentice-Hall .
External links
Recommendations by the IUGS Subcommission on the Systematics of Metamorphic Rocks, 1. How to Name a Metamorphic RockRecommendations by the IUGS Subcommission on the Systematics of Metamorphic Rocks, 2. Types, Grade, and Facies of MetamorphismRecommendations by the IUGS Subcommission on the Systematics of Metamorphic Rocks, 3. Structural terms including fault rock termsRecommendations by the IUGS Subcommission on the Systematics of Metamorphic Rocks, 4. High P/T Metamorphic Rocks*
ttps://web.archive.org/web/20110720035915/http://metpetdb.rpi.edu/ Metamorphic Petrology Database(
MetPetDB MetPetDB is a relational database and repository for global geochemical data on and images collected from metamorphic rocks from the earth's crust. MetPetDB is designed and built by a global community of metamorphic petrologists in collaboration ...
) – Department of Earth and Environmental Sciences,
Rensselaer Polytechnic Institute
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Geological processes
Metamorphic petrology