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54-658: The Grampian Group is an estimated 9,000 m thick sequence of metamorphosed Neoproterozoic sedimentary rocks that outcrop across the Central Highlands of Scotland , east of the Great Glen . It forms a part of the Dalradian Supergroup . The sequence thickness at any one place is limited to 5km. The Glenshirra Subgroup is the lowermost (oldest) part of the succession. These rocks reflect deposition within shallow marine and fluvial environments. It

108-554: 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 to eclogite in subduction zones , hydrous minerals break down, producing copious quantities of water. The water rises into

162-416: A decarbonation reaction is: In plastic deformation pressure is applied to the protolith , 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. Regional metamorphism

216-517: A metabasalt. When the protolith cannot be determined, the rock is classified by its mineral composition or its degree of foliation. 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

270-411: A metamorphic event. The facies are named after the metamorphic rock formed under those facies conditions from basalt . 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

324-559: 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 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 . Compaction (geology) In sedimentology , compaction

378-457: A rare type of magma called a carbonatite that is highly enriched in carbonates 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 metamorphism

432-415: Is a common result of metamorphism, rock that is intensely deformed may eliminate strain energy by recrystallizing as a fine-grained rock called mylonite . 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. Phase change metamorphism is the creating of a new mineral with

486-546: Is a function of the lithology of the base sediment. Both the thickness and structure of the later sequence will be controlled by the underlying geology in the absence of any active tectonics. Buried tilted fault blocks in a rift basin often produce large anticlinal closures in the post-rift section that may form traps for hydrocarbons e.g. the Daqing Field , the largest oil field in the People's Republic of China , in

540-480: 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 (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. To many geologists, regional metamorphism

594-664: 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. The Yule Marble used on the Lincoln Memorial exterior and the Tomb of the Unknown Soldier in Arlington National Cemetery

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648-697: 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 sections 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 .) Metamorphic rocks are typically more coarsely crystalline than

702-488: 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

756-947: Is overlain by the thick sequence of proximal and distal turbidites of the Corrieyairack Subgroup which is in turn overlain by the Glen Spean Subgroup reflective of the infilling of the Corrieyairack, Strath Tummel and Cromdale depositional basins . The Bowmore Sandstone of the Highland Border Complex is correlated with the Corrieyairack Subgroup. Rock successions in Shetland and Islay may also be correlated with this succession. Metamorphism Various forms of metamorphism exist, including regional , contact , hydrothermal , shock , and dynamic metamorphism. These differ in

810-410: Is practically synonymous with dynamothermal metamorphism. This form of metamorphism takes place at convergent plate boundaries , where two continental plates or a continental plate and an island arc collide. The collision zone becomes a belt of mountain formation called an orogeny . The orogenic belt is characterized by thickening of the Earth's crust, during which the deeply buried crustal rock

864-539: 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 . Foliation develops when a rock is being shortened along one axis during metamorphism. This causes crystals of platy minerals, such as mica and chlorite , to become rotated such that their short axes are parallel to

918-430: Is the most recognized metamorphic series in the world. However, Barrovian metamorphism is specific to pelitic rock, formed from mudstone or siltstone , 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 metamorphism takes place simply through rock being buried to great depths below

972-411: Is the process by which a sediment progressively loses its porosity due to the effects of pressure from loading. This forms part of the process of lithification . When a layer of sediment is originally deposited, it contains an open framework of particles with the pore space being usually filled with water. As more sediment is deposited above the layer, the effect of the increased loading is to increase

1026-401: 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 reactions. The hydrothermal fluid may be magmatic (originate in an intruding magma), circulating groundwater , or ocean water. Convective circulation of hydrothermal fluids in

1080-610: 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 , 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

1134-600: 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 650 °C (1,202 °F) for wet granite at a few hundred megapascals (MPa) of pressure to about 1,080 °C (1,980 °F) 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

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1188-419: The Earth's surface in a subsiding basin. Here the rock is subjected to high temperatures and the great pressure caused by the immense weight of the rock layers above. Burial metamorphism tends to produce 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

1242-452: 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 and stishovite ) and textures are characteristic of these conditions. 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

1296-401: 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 reactions . 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

1350-441: The appearance of key minerals in rocks of pelitic (shaly, aluminous) origin: Low grade ------------------- Intermediate --------------------- High grade A more complete indication of this intensity or degree is provided by the concept of metamorphic facies . Metamorphic facies are recognizable terranes or zones with an assemblage of key minerals that were in equilibrium under specific range of temperature and pressure during

1404-463: 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

1458-515: The aureoles around batholiths 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 - epidote hornfels, hornblende hornfels, pyroxene hornfels, and sillimanite hornfels, in increasing order of temperature of formation. However,

1512-528: 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 is the study of metamorphism. Metamorphic petrologists rely heavily on statistical mechanics and experimental petrology to understand metamorphic processes. Metamorphism

1566-403: 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

1620-400: The conditions of pressure and temperature at which metamorphism takes place. Metamorphism is generally regarded to begin at temperatures of 100 to 200 °C (212 to 392 °F). This excludes diagenetic changes due to compaction and lithification , which result in the formation of sedimentary rocks. The upper boundary of metamorphic conditions lies at the solidus of the rock, which

1674-550: The crystals, while high pressures cause solution of the crystals within the rock at their points of contact ( pressure solution ) 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

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1728-400: 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 , such as fault gouge or fault breccia . At greater depths, these are replaced by consolidated cataclastic rock, such as crush breccia , in which

1782-418: 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

1836-399: 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 sec ) in the middle and lower crust, but high strain rates can cause brittle deformation. At the highest strain rates,

1890-403: The larger rock fragments are cemented together by calcite or quartz. At depths greater than about 5 kilometers (3.1 mi), 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 300 °C (572 °F), plastic deformation takes over, and the fault zone

1944-502: 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

1998-407: 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 ions in solid crystals to migrate, thus reorganizing

2052-449: The ocean floor basalts 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 . The patterns of this hydrothermal alteration are used as a guide in the search for deposits of valuable metal ores. Shock metamorphism occurs when an extraterrestrial object (a meteorite for instance) collides with

2106-438: 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

2160-741: The particle-to-particle stresses resulting in porosity reduction primarily through a more efficient packing of the particles and to a lesser extent through elastic compression and pressure solution . The initial porosity of a sediment depends on its lithology . Mudstones start with porosities of >60%, sandstones typically ~40% and carbonates sometimes as high as 70%. Results from hydrocarbon exploration wells show clear porosity reduction trends with depth. Compaction trend estimation and decompaction process are useful for analyzing numerical basin evolution (e.g., subsidence) and evaluating hydrocarbon reservoirs and geological storages. In sediments compacted under self-weight, especially in sedimentary basins,

2214-507: The porosity profiles often show an exponential decrease, called Athy's law as first shown by Athy in 1930. A mathematical analytical solution was obtained by Fowler and Yang to show the theoretical basis for Athy's law. This process can be easily observed in experiments and used as a good approximation to many real data. If there is a variation in thickness and compactability of a sequence, loading by later deposits will give rise to spatially varying amounts of compaction. This form of compaction

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2268-401: 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 that makes the surface thermodynamically unstable. Recrystallization to coarser crystals reduces the surface area and so minimizes the surface energy. Although grain coarsening

2322-516: The protolith. Chemical reactions 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 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

2376-435: 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 (1600 MPa). The change in the grain size and orientation in the rock during the process of metamorphism is called recrystallization . For instance, the small calcite crystals in the sedimentary rocks limestone and chalk change into larger crystals in

2430-437: The result is a skarn . Fluorine -rich magmatic waters which leave a cooling granite may often form greisens within and adjacent to the contact of the granite. Metasomatic altered aureoles can localize the deposition of metallic ore 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

2484-413: 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. 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

2538-428: 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 of oceanic crust. The conditions within

2592-548: 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 , 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

2646-607: The rocks of the Midcontinent Rift System of North America, such as the Sioux Quartzite , and in the Hamersley Basin of Australia. 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 . The area surrounding the intrusion where the contact metamorphism effects are present

2700-500: 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 , andalusite , and sillimanite . All three have the identical composition, Al 2 SiO 5 . Kyanite is stable at surface conditions. However, at atmospheric pressure, kyanite transforms to andalusite at a temperature of about 190 °C (374 °F). Andalusite, in turn, transforms to sillimanite when

2754-414: The subducting slab as it plunges toward the mantle 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

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2808-426: The temperature reaches about 800 °C (1,470 °F). 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 and aragonite , with calcite transforming to aragonite at elevated pressure and relatively low temperature. Neocrystallization involves the creation of new mineral crystals different from

2862-419: Was formed by contact metamorphism. 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

2916-419: Was important in metamorphism. This hypothesis was tested by his friend, James Hall , who sealed chalk into a makeshift pressure vessel 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 produced by heating of chalk in the open air. French geologists subsequently added metasomatism ,

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