Metamorphic rock
![](https://upload.wikimedia.org/wikipedia/commons/thumb/7/7f/Quartzite.jpg/220px-Quartzite.jpg)
Metamorphic rocksarise from the transformation of existingrockto new types of rock in a process calledmetamorphism.The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C (300 to 400 °F) and, often, elevated pressure of 100megapascals(1,000bar) or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition.[1]The protolith may be anigneous,sedimentary,or existing metamorphic rock.
Metamorphic rocks make up a large part of theEarth's crustand form 12% of the Earth's land surface.[2]They are classified by their protolith, theirchemicalandmineralmakeup, and theirtexture.They may be formed simply by being deeply buried beneath the Earth's surface, where they are subject to high temperatures and the great pressure of the rock layers above. They can also form fromtectonicprocesses such as continental collisions, which cause horizontal pressure, friction, and distortion. Metamorphic rock can be formed locally when rock is heated by theintrusionof hot molten rock calledmagmafrom the Earth's interior. The study of metamorphic rocks (now exposed at the Earth's surface following erosion and uplift) provides information about the temperatures and pressures that occur at great depths within the Earth's crust.
Some examples of metamorphic rocks aregneiss,slate,marble,schist,andquartzite.Slate[3]and quartzite[4]tiles are used in building construction. Marble is also prized for building construction[5]and as a medium for sculpture.[6]On the other hand, schistbedrockcan pose a challenge forcivil engineeringbecause of its pronounced planes of weakness.[7]
Origin
Metamorphic rocks form one of the three great divisions of rock types. They are distinguished fromigneous rocks,which form from moltenmagma,andsedimentary rocks,which form fromsedimentseroded from existing rock or precipitated chemically from bodies of water.[8]
Metamorphic rocks are formed when 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 noted by the pioneering Scottish naturalist,James Hutton,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.[9]
Hutton also speculated that pressure was important in metamorphism. This hypothesis was tested by his friend,James Hall,who sealedchalkinto a makeshiftpressure vesselconstructed from a cannon barrel and heated it in an iron foundry furnace. Hall found that this produced a material strongly resemblingmarble,rather than the usualquicklimeproduced by heating of chalk in the open air. French geologists subsequently addedmetasomatism,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, incontact metamorphism).[9]
Metamorphic processes change the texture or mineral composition of the metamorphosed rock.
Mineralogical changes
![](https://upload.wikimedia.org/wikipedia/commons/thumb/2/22/Basalt-hand-sample.tif/lossy-page1-330px-Basalt-hand-sample.tif.jpg)
![](https://upload.wikimedia.org/wikipedia/commons/thumb/0/08/Amphibolite_%28Archean%2C_3.1-3.2_Ga%3B_Norris_South_roadcut%2C_Madison_County%2C_Montana%2C_USA%29_1_%2845574881922%29.jpg/330px-Amphibolite_%28Archean%2C_3.1-3.2_Ga%3B_Norris_South_roadcut%2C_Madison_County%2C_Montana%2C_USA%29_1_%2845574881922%29.jpg)
Metasomatismcan change the bulk composition of a rock. Hot fluids circulating through pore space in the rock can dissolve existing minerals and precipitate new minerals. Dissolved substances are transported out of the rock by the fluids while new substances are brought in by fresh fluids. This can obviously change the mineral makeup of the rock.[10]
However, changes in the mineral composition can take place even when the bulk composition of the rock does not change. This is possible because all minerals are stable only within certain limits of temperature, pressure, and chemical environment. For example, at atmospheric pressure, the mineralkyanitetransforms toandalusiteat a temperature of about 190 °C (374 °F). Andalusite, in turn, transforms tosillimanitewhen the temperature reaches about 800 °C (1,470 °F). All three have the identical composition,Al2SiO5.Likewise,forsteriteis stable over a broad range of pressure and temperature inmarble,but is converted topyroxeneat elevated pressure and temperature in more silicate-rich rock containingplagioclase,with which the forsterite reacts chemically.[11]
Many complex high-temperature reactions may take place between minerals without them melting, and each mineral assemblage produced indicates 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.[10]
Textural changes
The change in the particle size of the rock during the process of metamorphism is calledrecrystallization.For instance, the smallcalcitecrystals in the sedimentary rocklimestoneandchalkchange into larger crystals in the metamorphic rockmarble.[12]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.[13]Both high temperatures and pressures contribute to recrystallization. High temperatures allow theatomsandionsin solid crystals to migrate, thus reorganizing the crystals, while high pressures cause solution of the crystals within the rock at their point of contact.[14]
Description
![](https://upload.wikimedia.org/wikipedia/commons/thumb/d/d9/Staurolite-Almandine-36948.jpg/220px-Staurolite-Almandine-36948.jpg)
Metamorphic rocks are characterized by their distinctive mineral composition and texture.
Metamorphic minerals
Because every mineral is stable only within certain limits, the presence of certain minerals in metamorphic rocks indicates the approximate temperatures and pressures at which the rock underwent metamorphism. These minerals are known asindex minerals.Examples includesillimanite,kyanite,staurolite,andalusite,and somegarnet.[15]
Other minerals, such asolivines,pyroxenes,hornblende,micas,feldspars,andquartz,may be found in metamorphic rocks but are not necessarily the result of the process of metamorphism. These minerals can also form during thecrystallizationof igneous rocks. They are stable at high temperatures and pressures and may remain chemically unchanged during the metamorphic process.[16]
Texture
![](https://upload.wikimedia.org/wikipedia/commons/thumb/1/1e/Mylonite_Strona.jpg/220px-Mylonite_Strona.jpg)
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 asurface energythat makes the surface thermodynamically unstable. Recrystallization to coarser crystals reduces the surface area and so minimizes the surface energy.[17]
Although grain coarsening is a common result of metamorphism, rock that is intensely deformed may eliminatestrain energyby recrystallizing as a fine-grained rock calledmylonite.Certain kinds of rock, such as those rich in quartz,carbonate minerals,or olivine, are particularly prone to form mylonites, while feldspar and garnet are resistant to mylonitization.[18]
Foliation
![](https://upload.wikimedia.org/wikipedia/commons/thumb/6/6a/Migma_ss_2006.jpg/250px-Migma_ss_2006.jpg)
Many kinds of metamorphic rocks show a distinctive layering calledfoliation(derived from theLatinwordfolia,meaning "leaves" ). Foliation develops when a rock is being shortened along one axis during recrystallization. This causes crystals of platy minerals, such asmicaandchlorite,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 ofcleavage.Slateis an example of a foliated metamorphic rock, originating fromshale,and it typically shows well-developed cleavage that allows slate to be split into thin plates.[19]
The type of foliation that develops depends on the metamorphic grade. For instance, starting with amudstone,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 tophyllite,which is fine-grained and found in areas of low grade metamorphism.Schistis medium to coarse-grained and found in areas of medium grade metamorphism. High-grade metamorphism transforms the rock togneiss,which is coarse to very coarse-grained.[20]
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.
Classification
Metamorphic rocks are one of the three great divisions of all rock types, and so there is a great variety of metamorphic rock types. In general, if the protolith of a metamorphic rock can be determined, the rock is described by adding the prefixmeta-to the protolith rock name. For example, if the protolith is known to bebasalt,the rock will be described as a metabasalt. Likewise, a metamorphic rock whose protolith is known to be aconglomeratewill be described as ametaconglomerate.For a metamorphic rock to be classified in this manner, the protolith should be identifiable from the characteristics of the metamorphic rock itself, and not inferred from other information.[21][22][23]
Under theBritish Geological Survey'sclassification system, if all that can be determined about the protolith is its general type, such as sedimentary or volcanic, the classification is based on the mineral mode (the volume percentages of different minerals in the rock). Metasedimentary rocks are divided into carbonate-rich rock (metacarbonates or calcsilicate-rocks) or carbonate-poor rocks, and the latter are further classified by the relative abundance of mica in their composition. This ranges from low-micapsammitethrough semipelite to high-micapelite.Psammites composed mostly of quartz are classified as quartzite. Metaigneous rocks are classified similarly to igneous rocks, bysilicacontent, from meta-ultramafic-rock (which is very low in silica) to metafelsic-rock (with a high silica content).[22]
Where the mineral mode cannot be determined, as is often the case when rock is first examined in thefield,then classification must be based on texture. The textural types are:
- Schists,which are medium-grained strongly foliated rocks.[22]These show the most well-developed schistosity, defined as the extent to which platy minerals are present and are aligned in a single direction, so that the rock easily splits into plates less than a centimeter (0.4 inches) thick.[23]
- Gneisses,which are more coarse grained and show thicker foliation that schists, with layers over 5mm thick.[22]These show less well-developed schistosity.[23]
- Granofels,which show no obvious foliation[22]or schistosity.[23]
Ahornfelsis a granofels that is known to result from contact metamorphism. Aslateis a fine-grained metamorphic rock that easily splits into thin plates but shows no obvious compositional layering. The term is used only when very little else is known about the rock that would allow a more definite classification. Textural classifications may be prefixed to indicate a sedimentary protolith (para-,such as paraschist) or igneous protolith (ortho-,such as orthogneiss). When nothing is known about the protolith, the textural name is used without a prefix. For example, a schist is a rock with schistose texture whose protolith is uncertain.[22]
Special classifications exist for metamorphic rocks with avolcaniclasticprotolith or formed along afaultor throughhydrothermal circulation.A few special names are used for rocks of unknown protolith but known modal composition, such as marble,eclogite,oramphibolite.[22]Special names may also be applied more generally to rocks dominated by a single mineral, or with a distinctive composition or mode or origin. Special names still in wide use include amphibolite,greenschist,phyllite, marble,serpentinite,eclogite,migmatite,skarn,granulite,mylonite, and slate.[23]
The basic classification can be supplemented by terms describing mineral content or texture. For example, a metabasalt showing weak schistosity might be described as a gneissic metabasalt, and a pelite containing abundant staurolite might be described as a staurolite pelite.[22][23]
Metamorphic facies
Figure 1. Diagram showing metamorphic facies inpressure-temperaturespace. The domain of the graph corresponds to conditions within the Earth's crust andupper mantle. |
A metamorphic facies is a set of distinctive assemblages of minerals that are found in metamorphic rock that formed under a specific combination of pressure and temperature. The particular 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 pelite. 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,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.[24]
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 areamphiboliteandeclogite.The British Geological Survey strongly discourages the use ofgranuliteas a classification for rock metamorphosed to the granulite facies. Instead, such rock will often be classified as a granofels.[22]However, this approach is not universally accepted.[23]
Occurrence
Metamorphic rocks make up a large part of theEarth's crustand form 12% of the Earth's land surface.[2]The lowercontinental crustis mostly metamafic-rock and pelite which have reached thegranulite facies.The middle continental crust is dominated by metamorphic rock that has reached the amphibolite facies.[25]Within the upper crust, which is the only part of theEarth's crustgeologists can directly sample, metamorphic rock forms only from processes that can occur at shallow depth. These arecontact (thermal) metamorphism,dynamic (cataclastic) metamorphism,hydrothermal metamorphism,andimpact metamorphism.These processes are relatively local in occurrence and usually reach only the low-pressure facies, such as thehornfelsandsanidinite facies.Most metamorphic rock is formed byregional metamorphismin the middle and lower crust, where the rock reaches the higher-pressure metamorphic facies. This rock is found at the surface only where extensive uplift and erosion hasexhumedrock that was formerly much deeper in the crust.[26]
Orogenic belts
Metamorphic rock is extensively exposed inorogenic beltsproduced by the collision of tectonic plates atconvergent boundaries.Here formerly deeply buried rock has been brought to the surface by uplift and erosion.[27]The metamorphic rock exposed in orogenic belts may have been metamorphosed simply by being at great depths below the Earth's surface, subjected to high temperatures and the great pressure caused by the immense weight of the rock layers above. This kind of regional metamorphism is known asburial metamorphism.This tends to produce low-grade metamorphic rock.[28]Much more common is metamorphic rock formed during the collision process itself.[29]The collision of plates causes high temperatures, pressures and deformation in the rocks along these belts.[30]Metamorphic rock formed in these settings tends to shown well-developed schistosity.[29]
Metamorphic rock of orogenic belts shows a variety of metamorphic facies. Wheresubductionis taking place, the basalt of the subducting slab is metamorphosed to high-pressure metamorphic facies. It initially undergoes low-grade metamorphism to metabasalt of thezeoliteandprehnite-pumpellyite facies,but as the basalt subducts to greater depths, it is metamorphosed to theblueschist faciesand then theeclogite facies.Metamorphism to the eclogite facies releases a great deal of water vapor from the rock, which drivesvolcanismin the overlyingvolcanic arc.Eclogite is also significantly denser than blueschist, which drives further subduction of the slab deep into theEarth's mantle.Metabasalt and blueschist may be preserved in blueschist metamorphic belts formed by collisions between continents. They may also be preserved byobductiononto the overriding plate as part ofophiolites.[31]Eclogites are occasionally found at sites of continental collision, where the subducted rock is rapidly brought back to the surface, before it can be converted to the granulite facies in the hot upper mantle. Many samples of eclogite arexenolithsbrought to the surface by volcanic activity.[32]
Many orogenic belts contain higher-temperature, lower-pressure metamorphic belts. These may form through heating of the rock by ascending magmas of volcanic arcs, but on a regional scale. Deformation and crustal thickening in an orogenic belt may also produce these kinds of metamorphic rocks. These rocks reach thegreenschist,amphibolite, or granulite facies and are the most common of metamorphic rocks produced by regional metamorphosis. The association of an outer high-pressure, low-temperature metamorphic zone with an inner zone of low-pressure, high-temperature metamorphic rocks is called apaired metamorphic belt.The main islands of Japan show three distinct paired metamorphic belts, corresponding to different episodes of subduction.[33][34]
Metamorphic core complexes
Metamorphic rock is also exposed inmetamorphic core complexes,which form in region of crustal extension. They are characterized by low-angle faulting that exposes domes of middle or lower crust metamorphic rock. These were first recognized and studied in theBasin and Range Provinceof southwestern North America,[35]but are also found in southernAegean Sea,in theD'Entrecasteaux Islands,and in other areas of extension.[36]
Granite-greenstone belts
Continental shieldsare regions of exposed ancient rock that make up the stable cores of continents. The rock exposed in the oldest regions of shields, which is ofArcheanage (over 2500 million years old), mostly belong to granite-greenstone belts. Thegreenstone beltscontain metavolcanic and metasedimentary rock that has undergone a relatively mild grade of metamorphism, at temperatures of 350–500 °C (662–932 °F) and pressures of 200–500 MPa (2,000–5,000 bar). They can be divided into a lower group of metabasalts, including rare metakomatiites;a middle group of meta-intermediate-rock and meta-felsic-rock; and an upper group of metasedimentary rock.[37]
The greenstone belts are surrounded by high-grade gneiss terrains showing highly deformed low-pressure, high-temperature (over 500 °C (932 °F)) metamorphism to the amphibolite or granulite facies. These form most of the exposed rock in Archean cratons.[37]
The granite-greenstone belts are intruded by a distinctive group of granitic rocks called thetonalite-trondhjemite-granodioriteor TTG suite. These are the most voluminous rocks in the craton and may represent an important early phase in the formation of continental crust.[37]
Mid-ocean ridges
Mid-ocean ridgesare where newoceanic crustis formed as tectonic plates move apart. Hydrothermal metamorphism is extensive here. This is characterized by metasomatism by hot fluids circulating through the rock. This produces metamorphic rock of the greenschist facies. The metamorphic rock,serpentinite,is particularly characteristic of these settings, and represents chemical transformation of olivine and pyroxene in ultramafic rock toserpentine groupminerals.[38][29]
Contact aureoles
![](https://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Eozoon01.jpg/220px-Eozoon01.jpg)
![](https://upload.wikimedia.org/wikipedia/commons/thumb/8/8e/Rock_contact_metamorphism_eng_big_text.jpg/220px-Rock_contact_metamorphism_eng_big_text.jpg)
Contact metamorphism takes place when magma is injected into the surrounding solid rock (country rock).[29]The changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it. Around the igneous rock that forms from the cooling magma is a metamorphosed zone called acontact aureole.Aureoles may show all degrees of metamorphism from the contact area to unmetamorphosed (unchanged) country rock some distance away. The formation of importantoreminerals may occur by the process ofmetasomatismat or near the contact zone.[39]Contact aureoles around largeplutonsmay be as much as several kilometers wide.[40]
The termhornfelsis often used by geologists to signify those fine grained, compact, non-foliated products of contact metamorphism.[41]The contact aureole typically shows little deformation, and so hornfels is usually devoid of schistosity and forms a tough, equigranular rock. If the rock was originally banded orfoliated(as, for example, a laminated sandstone or a foliated calc-schist) this character may not be obliterated, and a banded hornfels is the product.[41]Contact metamorphism close to the surface produces distinctive low-pressure metamorphic minerals,[29]such asspinel,andalusite,vesuvianite,orwollastonite.[42]
Similar changes may be induced in shales by the burning ofcoalseams.[41]This produces a rock type namedclinker.[43]
There is also a tendency formetasomatismbetween the igneous magma and sedimentary country rock, whereby the chemicals in each are exchanged or introduced into the other. In that case, hybrid rocks calledskarnarise.[41][44]
Other occurrences
Dynamic (cataclastic) metamorphism takes place locally alongfaults.Here intense shearing of the rock typically forms mylonites.[29]
Impact metamorphism is unlike other forms of metamorphism in that it takes place duringimpact eventsby extraterrestrial bodies. It produces rare ultrahigh pressure metamorphic minerals, such ascoesiteandstishovite.[45]Coesite is rarely found in eclogite brought to the surface inkimberlite pipes,but the presence of stishovite is unique to impact structures.[46]
Uses
Slate tiles are used in construction, particularly as roof shingle.[3]
Quartzite is sufficiently hard and dense that it is difficult to quarry. However, some quartzite is used asdimension stone,often as slabs for flooring, walls, or stairsteps. About 6% of crushed stone, used mostly for road aggregate, is quartzite.[4]
Marble is also prized for building construction[47]and as a medium for sculpture.[6]
Hazards
Schistosebedrockcan pose a challenge forcivil engineeringbecause of its pronounced planes of weakness.[7]A hazard may exist even in undisturbed terrain. On August 17, 1959, amagnitude 7.2 earthquakedestabilized a mountain slope nearHebgen Lake,Montana, composed of schist. This caused a massive landslide that killed 26 people camping in the area.[48]
Metamorphosed ultramafic rock contains serpentine group minerals, which includes varieties ofasbestosthat pose a hazard to human health.[49]
See also
- Blueschist
- List of rock types
- List of rock textures
- Metavolcanic rock
- Neomorphism
- Subduction zone metamorphism
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External links
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- Metamorphic textures – Middle East Technical University
- Contact metamorphism exampleArchived2012-02-16 at theWayback Machine
- Metamorphic Rock Database(MetPetDB) – Department of Earth and Environmental Sciences,Rensselaer Polytechnic Institute
- Metamorphic Rocks Tour, an introduction to metamorphic rocks
- Atlas of Metamorphic Rocks– Detailed field and hand specimen photographs of metamorphic rocks grouped by setting and composition (Department of Earth Sciences,University of Oxford)