Misplaced Pages

#566433

64-419: Schist typically forms during regional metamorphism accompanying the process of mountain building ( orogeny ) and usually reflects a medium grade of metamorphism. Schist can form from many different kinds of rocks, including sedimentary rocks such as mudstones and igneous rocks such as tuffs . Schist metamorphosed from mudstone is particularly common and is often very rich in mica (a mica schist ). Where

128-428: A discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.) of rock masses in, for example, tunnel , foundation , or slope construction. A hazard may exist even in undisturbed terrain. On August 17, 1959, a magnitude 7.2 earthquake destabilized a mountain slope near Hebgen Lake , Montana, composed of schist. This caused a massive landslide that killed 26 people camping in

192-550: A schistose metasandstone (if the protolith is known to have been a sandstone ). If all that is known is that the protolith was a sedimentary rock, the schist will be described as a paraschist , while if the protolith was an igneous rock, the schist will be described as an orthoschist . Mineral qualifiers are important when naming a schist. For example, a quartz-feldspar-biotite schist is a schist of uncertain protolith that contains biotite mica, feldspar , and quartz in order of apparent decreasing abundance. Lineated schist has

256-554: 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

320-416: 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

384-507: 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

448-411: 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

512-489: A preferred orientation. Schists make up one of the three divisions of metamorphic rock by texture , with the other two divisions being gneiss , which has poorly developed schistosity and thicker layering, and granofels , which has no discernible schistosity. Schists are defined by their texture without reference to their composition, and while most are a result of medium-grade metamorphism, they can vary greatly in mineral makeup. However, schistosity normally develops only when

576-557: 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 . Orogenic belt Too Many Requests If you report this error to

640-457: 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

704-411: A strong linear fabric in a rock which otherwise has well-developed schistosity. Schistosity is developed at elevated temperature when the rock is more strongly compressed in one direction than in other directions ( nonhydrostatic stress ). Nonhydrostatic stress is characteristic of regional metamorphism where mountain building is taking place (an orogenic belt ). The schistosity develops perpendicular to

SECTION 10

#1732859391567

768-408: A very large class of metamorphic rock, geologists will formally describe a rock as a schist only when the original type of the rock prior to metamorphism (the protolith ) is unknown and its mineral content is not yet determined. Otherwise, the modifier schistose will be applied to a more precise type name, such as schistose semipelite (when the rock is known to contain moderate amounts of mica) or

832-415: 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

896-476: 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

960-402: Is a thin layering of the rock produced by metamorphism (a foliation ) that permits the rock to easily be split into flakes or slabs less than 5 to 10 millimeters (0.2 to 0.4 in) thick. The mineral grains in a schist are typically from 0.25 to 2 millimeters (0.01 to 0.08 in) in size and so are easily seen with a 10× hand lens . Typically, over half the mineral grains in a schist show

1024-459: 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

1088-634: 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

1152-697: 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

1216-488: 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

1280-483: Is derived ultimately from the Greek word σχίζειν ( schízein ), meaning "to split", which refers to the ease with which schists can be split along the plane in which the platy minerals lie. Before the mid-19th century, the terms slate , shale and schist were not sharply differentiated by those involved with mining. Geologists define schist as medium-grained metamorphic rock that shows well-developed schistosity. Schistosity

1344-466: Is distinct from weathering or diagenesis , which are changes that take place at or just beneath Earth's surface. Various forms of metamorphism exist, including regional , 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

SECTION 20

#1732859391567

1408-451: 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 is the temperature at which the rock begins to melt. At this point, the process becomes an igneous process. The solidus temperature depends on

1472-507: 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 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

1536-410: 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

1600-539: 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

1664-430: 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

1728-480: Is the orientation of grains in the surrounding medium-grained rock. The composition of the rock must permit formation of abundant platy minerals. For example, the clay minerals in mudstone are metamorphosed to mica, producing a mica schist. Early stages of metamorphism convert mudstone to a very fine-grained metamorphic rock called slate , which with further metamorphism becomes fine-grained phyllite . Further recrystallization produces medium-grained mica schist. If

1792-401: 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

1856-414: Is uncommon but can form from metamorphosis of sedimentary beds containing abundant organic carbon . This may be of algal origin. Graphite schist is known to have experienced greenschist facies metamorphism , for example in the northern Andes . Metamorphosis of felsic volcanic rock , such as tuff, can produce quartz- muscovite schist. In geotechnical engineering a schistosity plane often forms

1920-419: 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

1984-452: 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

Schist - Misplaced Pages Continue

2048-401: 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

2112-441: 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

2176-409: The area. Metamorphism Metamorphism is the transformation of existing rock (the protolith ) to rock with a different mineral composition or texture . Metamorphism takes place at temperatures in excess of 150 °C (300 °F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism

2240-515: 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,

2304-454: 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

2368-546: 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

2432-400: 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

2496-454: The direction of greatest compression, also called the shortening direction, as platy minerals are rotated or recrystallized into parallel layers. While platy or elongated minerals are most obviously reoriented, even quartz or calcite may take up preferred orientations. At the microscopic level, schistosity is divided into internal schistosity , in which inclusions within porphyroblasts take a preferred orientation, and external schistosity , which

2560-418: 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

2624-399: 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,

Schist - Misplaced Pages Continue

2688-462: 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 was important in metamorphism. This hypothesis

2752-403: 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

2816-452: 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

2880-502: 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

2944-407: 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

3008-522: The metamorphism proceeds further, the mica schist experiences dehydration reactions that convert platy minerals to granular minerals such as feldspars, decreasing schistosity and turning the rock into a gneiss. Other platy minerals found in schists include chlorite, talc, and graphite. Chlorite schist is typically formed by metamorphism of ultramafic igneous rocks, as is talc schist. Talc schist also forms from metamorphosis of talc-bearing carbonate rocks formed by hydrothermal alteration . Graphite schist

3072-449: 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

3136-438: 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

3200-401: 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

3264-516: 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

SECTION 50

#1732859391567

3328-435: 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

3392-437: 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

3456-537: The rock contains abundant platy minerals, such as mica or chlorite . Grains of these minerals are strongly oriented in a preferred direction in schist, often also forming very thin parallel layers. The ease with which the rock splits along the aligned grains accounts for the schistosity. Though not a defining characteristic, schists very often contain porphyroblasts (individual crystals of unusual size) of distinctive minerals, such as garnet , staurolite , kyanite , sillimanite , or cordierite . Because schists are

3520-413: 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

3584-428: 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

3648-548: 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

3712-583: 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

3776-500: 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

3840-414: 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

3904-426: 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

SECTION 60

#1732859391567

3968-415: The type of the original rock (the protolith ) is discernible, the schist is usually given a name reflecting its protolith, such as schistose metasandstone . Otherwise, the names of the constituent minerals will be included in the rock name, such as quartz-felspar-biotite schist . Schist bedrock can pose a challenge for civil engineering because of its pronounced planes of weakness . The word schist

4032-419: 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

4096-422: 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 , the circulation of fluids through buried rock, to

#566433