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84-606: The Lower Estuarine Series is an obsolete term for a thin sequence of fine-grained clastic sediments of Jurassic age, which are now called the Grantham Formation . The rocks of this formation form a lower part of the ( Bajocian ) Inferior Oolite Group , of Middle Jurassic age. In central England, it normally lies above the Northampton sand and below the Lower Lincolnshire Limestone Member . The group

168-544: A hydrofracture breccia. Hydrothermal clastic rocks are generally restricted to those formed by hydrofracture , the process by which hydrothermal circulation cracks and brecciates the wall rocks and fills them in with veins. This is particularly prominent in epithermal ore deposits and is associated with alteration zones around many intrusive rocks, especially granites . Many skarn and greisen deposits are associated with hydrothermal breccias. A fairly rare form of clastic rock may form during meteorite impact. This

252-431: A biased view of the original mineralogy of the rock. Porosity can also be affected by this process. For example, clay minerals tend to fill up pore space and thereby reducing porosity. In the process of burial, it is possible that siliciclastic deposits may subsequently be uplifted as a result of a mountain building event or erosion . When uplift occurs, it exposes buried deposits to a radically new environment. Because

336-410: A considerably lesser portion of framework grains and minerals. They only make up about 15 percent of framework grains in sandstones and 5% of minerals in shales. Clay mineral groups are mostly present in mudrocks (comprising more than 60% of the minerals) but can be found in other siliciclastic sedimentary rocks at considerably lower levels. Accessory minerals are associated with those whose presence in

420-408: 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. 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

504-451: A distinctive layering called foliation (derived from the Latin word folia , meaning "leaves"). Foliation develops when a rock is being shortened along one axis during recrystallization. This causes crystals of platy minerals, such as mica and chlorite , to become rotated such that their short axes are parallel to the direction of shortening. This results in a banded, or foliated, rock, with

588-410: A few hundred meters where pressures are relatively low (for example, in contact metamorphism ). Metamorphic processes change the texture or mineral composition of the metamorphosed rock. Metasomatism can 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

672-481: A great resistance to decomposition are categorized as stable, while those that do not are considered less stable. The most common stable mineral in siliciclastic sedimentary rocks is quartz (SiO 2 ). Quartz makes up approximately 65 percent of framework grains present in sandstones and about 30 percent of minerals in the average shale. Less stable minerals present in this type of rocks are feldspars , including both potassium and plagioclase feldspars. Feldspars comprise

756-602: A logarithmic size scale. Siliciclastic rocks are clastic noncarbonate rocks that are composed almost exclusively of silicon, either as forms of quartz or as silicates. The composition of siliciclastic sedimentary rocks includes the chemical and mineralogical components of the framework as well as the cementing material that make up these rocks. Boggs divides them into four categories; major minerals, accessory minerals, rock fragments, and chemical sediments. Major minerals can be categorized into subdivisions based on their resistance to chemical decomposition. Those that possess

840-524: 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 the list of processes that help bring about metamorphism. However, metamorphism can take place without metasomatism ( isochemical metamorphism ) or at depths of just

924-464: 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

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1008-522: 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. Under the British Geological Survey's classification 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

1092-546: A muddy matrix that leaves little space for precipitation to occur. This is often the case for mudrocks as well. As a result of compaction, the clayey sediments comprising mudrocks are relatively impermeable. Dissolution of framework silicate grains and previously formed carbonate cement may occur during deep burial. Conditions that encourage this are essentially opposite of those required for cementation. Rock fragments and silicate minerals of low stability, such as plagioclase feldspar, pyroxenes , and amphiboles , may dissolve as

1176-546: A new texture or mineral composition. The protolith may be an igneous , sedimentary , or existing metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. They are classified by their protolith, their chemical and mineral makeup, and their texture . They may be formed simply by being deeply buried beneath the Earth's surface, where they are subject to high temperatures and

1260-413: A pelite containing abundant staurolite might be described as a staurolite pelite. [REDACTED] 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

1344-519: 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 meta komatiites ; a middle group of meta-intermediate-rock and meta-felsic-rock; and an upper group of metasedimentary rock. The greenstone belts are surrounded by high-grade gneiss terrains showing highly deformed low-pressure, high-temperature (over 500 °C (932 °F)) metamorphism to

1428-437: A result of increasing burial temperatures and the presence of organic acids in pore waters. The dissolution of frame work grains and cements increases porosity particularly in sandstones. This refers to the process whereby one mineral is dissolved and a new mineral fills the space via precipitation. Replacement can be partial or complete. Complete replacement destroys the identity of the original minerals or rock fragments giving

1512-491: A sample's environment of deposition . An example of clastic environment would be a river system in which the full range of grains being transported by the moving water consist of pieces eroded from solid rock upstream. Grain size varies from clay in shales and claystones ; through silt in siltstones ; sand in sandstones ; and gravel , cobble , to boulder sized fragments in conglomerates and breccias . The Krumbein phi (φ) scale numerically orders these terms in

1596-498: A sediment is deposited, it becomes subject to cementation through the various stages of diagenesis discussed below. Eogenesis refers to the early stages of diagenesis. This can take place at very shallow depths, ranging from a few meters to tens of meters below the surface. The changes that occur during this diagenetic phase mainly relate to the reworking of the sediments. Compaction and grain repacking, bioturbation , as well as mineralogical changes all occur at varying degrees. Due to

1680-410: A variety of metamorphic facies. Where subduction is taking place, the basalt of the subducting slab is metamorphosed to high-pressure metamorphic facies. It initially undergoes low-grade metamorphism to metabasalt of the zeolite and prehnite-pumpellyite facies , but as the basalt subducts to greater depths, it is metamorphosed to the blueschist facies and then the eclogite facies . Metamorphism to

1764-417: 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 prefix meta- to the protolith rock name. For example, if the protolith is known to be basalt , the rock will be described as a metabasalt. Likewise, a metamorphic rock whose protolith is known to be a conglomerate will be described as a metaconglomerate . For

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1848-426: Is called a paired metamorphic belt . The main islands of Japan show three distinct paired metamorphic belts, corresponding to different episodes of subduction. Metamorphic rock is also exposed in metamorphic 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

1932-544: Is cemented together and lithified it becomes known as sandstone. Any particle that is larger than two millimeters is considered gravel. This category includes pebbles , cobbles and boulders. Like sandstone, when gravels are lithified they are considered conglomerates. Conglomerates are coarse grained rocks dominantly composed of gravel sized particles that are typically held together by a finer grained matrix. These rocks are often subdivided into conglomerates and breccias. The major characteristic that divides these two categories

2016-412: Is compaction. As sediment transport and deposition continues, new sediments are deposited atop previously deposited beds, burying them. Burial continues and the weight of overlying sediments causes an increase in temperature and pressure. This increase in temperature and pressure causes loose grained sediments become tightly packed, reducing porosity, essentially squeezing water out of the sediment. Porosity

2100-534: Is composed primarily of ejecta; clasts of country rock , melted rock fragments, tektites (glass ejected from the impact crater) and exotic fragments, including fragments derived from the impactor itself. Identifying a clastic rock as an impact breccia requires recognising shatter cones , tektites, spherulites , and the morphology of an impact crater , as well as potentially recognizing particular chemical and trace element signatures, especially osmiridium . Metamorphic rock Metamorphic rocks arise from

2184-464: Is dominated by metamorphic rock that has reached the amphibolite facies. Within the upper crust, which is the only part of the Earth's crust geologists can directly sample, metamorphic rock forms only from processes that can occur at shallow depth. These are contact (thermal) metamorphism , dynamic (cataclastic) metamorphism , hydrothermal metamorphism , and impact metamorphism . These processes are relatively local in occurrence and usually reach only

2268-476: Is 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 to serpentine group minerals. Contact metamorphism takes place when magma

2352-782: Is found in the East Midlands of England and was formed when the London-Brabant Island was drifting through the low northern latitudes , in conditions represented today by the Sahara Desert . The formation is often only around 2 - 5 metres thick. Clastic Clastic sedimentary rocks are rocks composed predominantly of broken pieces or clasts of older weathered and eroded rocks. Clastic sediments or sedimentary rocks are classified based on grain size , clast and cementing material ( matrix ) composition, and texture. The classification factors are often useful in determining

2436-514: Is further reduced by the precipitation of minerals into the remaining pore spaces. The final stage in the process is diagenesis and will be discussed in detail below. Cementation is the diagenetic process by which coarse clastic sediments become lithified or consolidated into hard, compact rocks, usually through the deposition or precipitation of minerals in the spaces between the individual grains of sediment. Cementation can occur simultaneously with deposition or at another time. Furthermore, once

2520-403: Is injected into the surrounding solid rock ( country rock ). 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 a contact aureole . Aureoles may show all degrees of metamorphism from

2604-405: Is known as burial metamorphism . This tends to produce low-grade metamorphic rock. Much more common is metamorphic rock formed during the collision process itself. The collision of plates causes high temperatures, pressures and deformation in the rocks along these belts. Metamorphic rock formed in these settings tends to shown well-developed schistosity. Metamorphic rock of orogenic belts shows

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2688-651: 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 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. Many kinds of metamorphic rocks show

2772-456: Is reserved for mudrocks that are laminated, while mudstone refers those that are not. Siliciclastic rocks initially form as loosely packed sediment deposits including gravels, sands, and muds. The process of turning loose sediment into hard sedimentary rocks is called lithification . During the process of lithification, sediments undergo physical, chemical and mineralogical changes before becoming rock. The primary physical process in lithification

2856-482: 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 as index minerals . Examples include sillimanite , kyanite , staurolite , andalusite , and some garnet . Other minerals, such as olivines , pyroxenes , hornblende , micas , feldspars , and quartz , may be found in metamorphic rocks but are not necessarily

2940-506: Is sufficiently hard and dense that it is difficult to quarry. However, some quartzite is used as dimension stone , often as slabs for flooring, walls, or stairsteps. About 6% of crushed stone, used mostly for road aggregate, is quartzite. Marble is also prized for building construction and as a medium for sculpture. Schistose bedrock can pose a challenge for civil engineering because of its pronounced planes of weakness. A hazard may exist even in undisturbed terrain. On August 17, 1959,

3024-429: Is the amount of rounding. The gravel sized particles that make up conglomerates are well rounded while in breccias they are angular. Conglomerates are common in stratigraphic successions of most, if not all, ages but only make up one percent or less, by weight, of the total sedimentary rock mass. In terms of origin and depositional mechanisms they are very similar to sandstones. As a result, the two categories often contain

3108-400: Is uncertain. Special classifications exist for metamorphic rocks with a volcaniclastic protolith or formed along a fault or through hydrothermal circulation . A few special names are used for rocks of unknown protolith but known modal composition, such as marble, eclogite , or amphibolite . Special names may also be applied more generally to rocks dominated by a single mineral, or with

3192-426: 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

3276-475: Is usually devoid of schistosity and forms a tough, equigranular rock. If the rock was originally banded or foliated (as, for example, a laminated sandstone or a foliated calc- schist ) this character may not be obliterated, and a banded hornfels is the product. Contact metamorphism close to the surface produces distinctive low-pressure metamorphic minerals, such as spinel , andalusite, vesuvianite , or wollastonite . Similar changes may be induced in shales by

3360-470: Is very low in silica) to metafelsic-rock (with a high silica content). Where the mineral mode cannot be determined, as is often the case when rock is first examined in the field , then classification must be based on texture. The textural types are: A hornfels is a granofels that is known to result from contact metamorphism. A slate is a fine-grained metamorphic rock that easily splits into thin plates but shows no obvious compositional layering. The term

3444-566: The Basin and Range Province of southwestern North America, but are also found in southern Aegean Sea , in the D'Entrecasteaux Islands , and in other areas of extension. Continental shields are regions of exposed ancient rock that make up the stable cores of continents. The rock exposed in the oldest regions of shields, which is of Archean age (over 2500 million years old), mostly belong to granite-greenstone belts. The greenstone belts contain metavolcanic and metasedimentary rock that has undergone

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3528-450: The Dott scheme , which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of muddy matrix between these larger grains. Rocks that are classified as mudrocks are very fine grained. Silt and clay represent at least 50% of the material that mudrocks are composed of. Classification schemes for mudrocks tend to vary, but most are based on the grain size of

3612-481: The chemical and mineralogic make-up of the single or varied fragments and the cementing material ( matrix ) holding the clasts together as a rock. These differences are most commonly used in the framework grains of sandstones. Sandstones rich in quartz are called quartz arenites , those rich in feldspar are called arkoses , and those rich in lithics are called lithic sandstones . Siliciclastic sedimentary rocks are composed of mainly silicate particles derived from

3696-547: The activity of organisms. Despite being close to the surface, eogenesis does provide conditions for important mineralogical changes to occur. This mainly involves the precipitation of new minerals. Mineralogical changes that occur during eogenesis are dependent on the environment in which that sediment has been deposited. For example, the formation of pyrite is characteristic of reducing conditions in marine environments. Pyrite can form as cement, or replace organic materials, such as wood fragments. Other important reactions include

3780-477: The amphibolite or granulite facies. These form most of the exposed rock in Archean cratons. The granite-greenstone belts are intruded by a distinctive group of granitic rocks called the tonalite - trondhjemite - granodiorite or TTG suite. These are the most voluminous rocks in the craton and may represent an important early phase in the formation of continental crust. Mid-ocean ridges are where new oceanic crust

3864-404: 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 ,

3948-399: The burning of coal seams. This produces a rock type named clinker . There is also a tendency for metasomatism between the igneous magma and sedimentary country rock, whereby the chemicals in each are exchanged or introduced into the other. In that case, hybrid rocks called skarn arise. Dynamic (cataclastic) metamorphism takes place locally along faults . Here intense shearing of

4032-427: The collision of tectonic plates at convergent boundaries . Here formerly deeply buried rock has been brought to the surface by uplift and erosion. 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

4116-581: The composition of sandstone. They generally make up most of the gravel size particles in conglomerates but contribute only a very small amount to the composition of mudrocks . Though they sometimes are, rock fragments are not always sedimentary in origin. They can also be metamorphic or igneous . Chemical cements vary in abundance but are predominantly found in sandstones. The two major types are silicate based and carbonate based. The majority of silica cements are composed of quartz, but can include chert , opal , feldspars and zeolites . Composition includes

4200-491: The contact area to unmetamorphosed (unchanged) country rock some distance away. The formation of important ore minerals may occur by the process of metasomatism at or near the contact zone. Contact aureoles around large plutons may be as much as several kilometers wide. The term hornfels is often used by geologists to signify those fine grained, compact, non-foliated products of contact metamorphism. The contact aureole typically shows little deformation, and so hornfels

4284-423: The eclogite facies releases a great deal of water vapor from the rock, which drives volcanism in the overlying volcanic arc . Eclogite is also significantly denser than blueschist, which drives further subduction of the slab deep into the Earth's mantle . Metabasalt and blueschist may be preserved in blueschist metamorphic belts formed by collisions between continents. They may also be preserved by obduction onto

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4368-866: The field, it may at times be difficult to distinguish between a debris flow sedimentary breccia and a colluvial breccia, especially if one is working entirely from drilling information. Sedimentary breccias are an integral host rock for many sedimentary exhalative deposits . Clastic igneous rocks include pyroclastic volcanic rocks such as tuff , agglomerate and intrusive breccias , as well as some marginal eutaxitic and taxitic intrusive morphologies. Igneous clastic rocks are broken by flow, injection or explosive disruption of solid or semi-solid igneous rocks or lavas . Igneous clastic rocks can be divided into two classes: Clastic metamorphic rocks include breccias formed in faults , as well as some protomylonite and pseudotachylite . Occasionally, metamorphic rocks can be brecciated via hydrothermal fluids, forming

4452-437: 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

4536-654: The formation of chlorite , glauconite , illite and iron oxide (if oxygenated pore water is present). The precipitation of potassium feldspar, quartz overgrowths, and carbonate cements also occurs under marine conditions. In non marine environments oxidizing conditions are almost always prevalent, meaning iron oxides are commonly produced along with kaolin group clay minerals. The precipitation of quartz and calcite cements may also occur in non marine conditions. As sediments are buried deeper, load pressures become greater resulting in tight grain packing and bed thinning. This causes increased pressure between grains thus increasing

4620-516: 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. Hutton also speculated that pressure was important in metamorphism. This hypothesis was tested by his friend, James Hall , who sealed chalk into

4704-440: The forsterite reacts chemically. 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. The change in

4788-443: The great pressure of the rock layers above. They can also form from tectonic processes such as continental collisions, which cause horizontal pressure, friction, and distortion. Metamorphic rock can be formed locally when rock is heated by the intrusion of hot molten rock called magma from the Earth's interior. The study of metamorphic rocks (now exposed at the Earth's surface following erosion and uplift) provides information about

4872-437: The low-pressure facies, such as the hornfels and sanidinite facies . Most metamorphic rock is formed by regional metamorphism in 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 has exhumed rock that was formerly much deeper in the crust. Metamorphic rock is extensively exposed in orogenic belts produced by

4956-431: The major constituents. In mudrocks, these are generally silt, and clay. According to Blatt, Middleton and Murray mudrocks that are composed mainly of silt particles are classified as siltstones. In turn, rocks that possess clay as the majority particle are called claystones. In geology, a mixture of both silt and clay is called mud. Rocks that possess large amounts of both clay and silt are called mudstones. In some cases

5040-501: The mineral kyanite transforms to andalusite at a temperature of about 190 °C (374 °F). Andalusite, in turn, transforms to sillimanite when the temperature reaches about 800 °C (1,470 °F). All three have the identical composition, Al 2 SiO 5 . Likewise, forsterite is stable over a broad range of pressure and temperature in marble , but is converted to pyroxene at elevated pressure and temperature in more silicate-rich rock containing plagioclase , with which

5124-441: 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 the crystals, while high pressures cause solution of the crystals within the rock at their point of contact. Metamorphic rocks are characterized by their distinctive mineral composition and texture. Because every mineral

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5208-460: The overriding plate as part of ophiolites . 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 are xenoliths brought to the surface by volcanic activity. Many orogenic belts contain higher-temperature, lower-pressure metamorphic belts. These may form through heating of

5292-403: The particle size of the rock during the process of metamorphism is called recrystallization . For instance, the small calcite crystals in the sedimentary rock limestone and 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

5376-405: The pores between grain of sediment. The cement that is produced may or may not have the same chemical composition as the sediment. In sandstones, framework grains are often cemented by silica or carbonate. The extent of cementation is dependent on the composition of the sediment. For example, in lithic sandstones, cementation is less extensive because pore space between framework grains is filled with

5460-424: The process brings material to or closer to the surface, sediments that undergo uplift are subjected to lower temperatures and pressures as well as slightly acidic rain water. Under these conditions, framework grains and cement are again subjected to dissolution and in turn increasing porosity. On the other hand, telogenesis can also change framework grains to clays, thus reducing porosity. These changes are dependent on

5544-442: The result of the process of metamorphism. These minerals can also form during the crystallization of igneous rocks. They are stable at high temperatures and pressures and may remain chemically unchanged during the metamorphic process. 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

5628-410: The rock are not directly important to the classification of the specimen. These generally occur in smaller amounts in comparison to the quartz, and feldspars. Furthermore, those that do occur are generally heavy minerals or coarse grained micas (both muscovite and biotite ). Rock fragments also occur in the composition of siliciclastic sedimentary rocks and are responsible for about 10–15 percent of

5712-475: 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 the greenschist , 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

5796-421: The rock by the fluids while new substances are brought in by fresh fluids. This can obviously change the mineral makeup of the rock. 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,

5880-423: 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. Metamorphic rocks are one of the three great divisions of all rock types, and so there

5964-495: The rock typically forms mylonites. Impact metamorphism is unlike other forms of metamorphism in that it takes place during impact events by extraterrestrial bodies. It produces rare ultrahigh pressure metamorphic minerals, such as coesite and stishovite . Coesite is rarely found in eclogite brought to the surface in kimberlite pipes , but the presence of stishovite is unique to impact structures. Slate tiles are used in construction, particularly as roof shingle. Quartzite

6048-486: 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-mica psammite through semipelite to high-mica pelite . Psammites composed mostly of quartz are classified as quartzite. Metaigneous rocks are classified similarly to igneous rocks, by silica content, from meta-ultramafic-rock (which

6132-433: The same sedimentary structures. Sandstones are medium-grained rocks composed of rounded or angular fragments of sand size, that often but not always have a cement uniting them together. These sand-size particles are often quartz but there are a few common categories and a wide variety of classification schemes that classify sandstones based on composition. Classification schemes vary widely, but most geologists have adopted

6216-399: The shallow depths, sediments undergo only minor compaction and grain rearrangement during this stage. Organisms rework sediment near the depositional interface by burrowing, crawling, and in some cases sediment ingestion. This process can destroy sedimentary structures that were present upon deposition of the sediment. Structures such as lamination will give way to new structures associated with

6300-542: The solubility of grains. As a result, the partial dissolution of silicate grains occurs. This is called pressure solutions. Chemically speaking, increases in temperature can also cause chemical reaction rates to increase. This increases the solubility of most common minerals (aside from evaporites). Furthermore, beds thin and porosity decreases allowing cementation to occur by the precipitation of silica or carbonate cements into remaining pore space. In this process minerals crystallize from watery solutions that percolate through

6384-479: The specific conditions that the rock is exposed as well as the composition of the rock and pore waters. Specific pore waters, can cause the further precipitation of carbonate or silica cements. This process can also encourage the process of oxidation on a variety of iron bearing minerals. Sedimentary breccias are a type of clastic sedimentary rock which are composed of angular to subangular, randomly oriented clasts of other sedimentary rocks. They may form either: In

6468-484: The temperatures and pressures that occur at great depths within the Earth's crust. Some examples of metamorphic rocks are gneiss , slate , marble , schist , and quartzite . Slate and quartzite tiles are used in building construction. Marble is also prized for building construction and as a medium for sculpture. On the other hand, schist bedrock can pose a challenge for civil engineering because of its pronounced planes of weakness. Metamorphic rocks form one of

6552-419: The term can also be used to refer to a family of sheet silicate minerals. Silt refers to particles that have a diameter between .062 and .0039 millimeters. The term mud is used when clay and silt particles are mixed in the sediment; mudrock is the name of the rock created with these sediments. Furthermore, particles that reach diameters between .062 and 2 millimeters fall into the category of sand. When sand

6636-405: The term shale is also used to refer to mudrocks and is still widely accepted by most. However, others have used the term shale to further divide mudrocks based on the percentage of clay constituents. The plate-like shape of clay allows its particles to stack up one on top of another, creating laminae or beds. The more clay present in a given specimen, the more laminated a rock is. Shale, in this case,

6720-436: The three great divisions of rock types. They are distinguished from igneous rocks , which form from molten magma , and sedimentary rocks , which form from sediments eroded from existing rock or precipitated chemically from bodies of water. 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

6804-431: The transformation of existing rock to new types of rock in a process called metamorphism . The original rock ( protolith ) is subjected to temperatures greater than 150 to 200 °C (300 to 400 °F) and, often, elevated pressure of 100 megapascals (1,000  bar ) or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to

6888-451: The 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 approach is not universally accepted. Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. The lower continental crust is mostly metamafic-rock and pelite which have reached the granulite facies . The middle continental crust

6972-421: The weathering of older rocks and pyroclastic volcanism. While grain size, clast and cementing material (matrix) composition, and texture are important factors when regarding composition, siliciclastic sedimentary rocks are classified according to grain size into three major categories: conglomerates , sandstones , and mudrocks . The term clay is used to classify particles smaller than .0039 millimeters. However,

7056-561: 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 and eclogite . The British Geological Survey strongly discourages

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