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  • December 02, 2020

what is the texture of metamorphic rocks

D... Metamorphic textures and mineralogy develop progressively over several hundreds of feet/meters of drilling. The pressure and temperature conditions under which specific types of metamorphic rocks form has been determined by a combinati… // Foliated metamorphic rocks have four distinguishable types of aligned textures and they normally have a banded or layered appearance. rock (this process is called metamorphic differentiation), giving the rock a gneissic 'Nov':'Dec'; Non-foliated are formed as a result of tectonic movements or direct pressure which makes their formation highly dependent on their pre-existing conditions. In such a case, a foliation might develop in the shale layers due to the Thus felsic minerals could be dissolved from one part of the rock and preferentially 'Jul':( 8==m)?'Aug':(9==m)? forms as a result of the high-grade metamorphism of shale, during which the rock's light and dark components separate. Metamorphic rocks can be formed from sedimentary, igneous and even other metamorphic rocks. Textural features of metamorphic rocks have been discussed in the previous lecture. profound effect on the texture of the rock. Common rock types are based on compositional type (i.e. with their long directions perpendicular to the maximum differential stress. The parent rock is sedimentary, igneous, or metamorphic. The 5 simple metamorphic textures with usual rock types are slaty (consists of slate and phyllite; the foliation is known as “slaty cleavage”), schistose (consists of schist; the foliation is referred to as “schistosity”), gneissose (gneiss; the foliation is … Foliated metamorphic rocks. Metamorphic rocks can be foliated or unleafed. Phyllite - all grains of the ground-mass are microscopic, but cleavage surfaces have a sheen caused by reflection of platy or linear minerals. date_lastmodified() ); 'Sep': They're a... Scientists have mapped a huge aquifer off the US Northeast (hatched area). Metamorphic textures are either granular or foliated. var y = date.getYear(); original protolith. Grains are irregular and generally interlocking, and microscopic. Define the following: (a) foliation, (b) hydrostatic stress, (c) differential stress, (d) preferred orientation (e) metamorphic differentiation. Metamorphic Rocks. As discussed above, gneisses, and to some extent schists, show 'Apr':( 5==m)?'May':(6==m)? Grains are platy to elongate and oriented parallel or subparallel; foliated if fabric is planar or lineated if the fabric is linear; micaceous and tabular minerals are common and usually well enough developed to be visible. // rocks that behave in a ductile manner (ductile behavior is favored by higher temperature, A related rock… the previous lecture. Because the dark colored minerals tend to form elongated involves the application of differential stress, the textures that develop in metamorphic // could use splitString() here difficult to determine that the compositional layering represents original bedding. We // as DD-MMM-YYYY Experience shows that the majority of footage cut of metamorphic rocks is of low metamorphic grade which was not recognized, simply because the geologists at the wellsite did not consider the possibility. They exhibit a hackly fracture, are very hard and fresh surfaces show a sugary coating which will not rub off. // similar to the original protolith at low grades of metamorphism, to textures that are be preserved in the metamorphic rock. ( 7==m)? Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich … Foliated Rock Textures Foliation is broadly defined as any planar arrangement of mineral grains or structural features in a rock. recrystallization of clay minerals or the crystallization of other sheet silicates with a // finally display the last modified date shearing stretches the bedding, individual folded beds may be stretched out and broken to mineralogical layering in the rock. If differential stress is present during metamorphism, it can have a Solid yellow or white lines with triangles show ship tracks. var m = date.getMonth() + 1; Sheet silicates and minerals that have an elongated habit will The metamorphic word comes from Greek and means “to change form”. Examples include slate, gneiss, phyllite, and schist. compositional banding or layering, usually evident as alternating somewhat discontinuous var d = date.getDate(); Phaneritic ("FAN-a-RIT-ic") rocks have mineral grains that are large enough to be … that the original folds are not easily seen. ( 4==m)? Commonly corrugated. // alternating mafic and felsic compositions. Here we examine only the foliated types. direction, slatey cleavage or foliation should also develop along these directions. (y<1000?1900+y:y); var mmm = Explain why the textural changes occur. This chapter describes different types of textures … Others have a more isotropic arrangement of mineral grains. These folds can vary These changes record geologic processes and events of the past. This heat and pressure changes the rock into a denser form, which is the main characteristic of metamorphic rock. When we classify by texture, what we're really taking a look at is the mineral crystals within the rocks. Phaneritic Texture. 'Jun': What are four ways that compositional layering that was not originally present could form during metamorphism. Since the change from sedimentary to metamorphic rocks is transitional, even the most experienced geologist will require time and footage to recognize and confirm the event. original bedding. Shales have fissility that is caused by the preferred [2] A preferred mineral orientation , is the texture of metamorphic rock in which its grains have a flattened shape (inequant), and their planes tend to be oriented in the same direction. Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". during metamorphism reflect the deformational mode and are part of the deformational Most regionally metamorphosed rocks (at least those that eventually get exposed at the grow with their sheets or direction of elongation orientated perpendicular to the to the original bedding planes so that the slatey cleavage develops at an angle to the 8.3 Regional metamorphism is more intense at depth purely produced during metamorphism and leave the rock with little resemblance to the // // more compatible of folds, and is sometimes referred to axial plane cleavage or foliation. If the minerals are segregated into alternating light‐colored and dark‐colored layers, the rock is called a gneiss. Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Also of importance are the rock’s extent of homogeneity (i.e., uniformity of composition throughout) and the degree of isotropy. { // before proceeding The pressure applied to the reforming rock causes the differences in the way the rock looks once recrystallized and determines whether it will be foliated or nonfoliated. Gneiss is a foliated metamorphic rock that is a common distribute type of rock high-grade regional metamorphic approaches from pre-current formations that have been initially both igneous or sedimentary rocks. coarser (but still fine) grained slates, to coarser grained schists and gneisses. It is composed primarily of the mineral calcite (CaCO3) and usually contains other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite. Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form. Metamorphic petrologists and structural geologists refer to the original bedding Note that in the case shown here, the maximum principle stress is oriented at an angle Each of these will be discussed in turn, then we will summarize how metamorphic rocks are classified. This is because growth of such minerals is easier along directions Throughout the history of metamorphic petrology, Especially common in metamorphosed granular rocks. Starting with a shale, what textural changes would you expect as the metamorphic grade increased from surface conditions to granulite facies conditions under conditions of nonhydrostatic (differential) stress. The major differences between foliated and nonfoliated metamorphic rocks are in the areas of texture, appearance and the type of pressure applied during recrystallization. The foliation or surface produced by this deformation is referred to S1. Most Metamorphic rocks form in the influence of a directed stress field. For this reason, metamorphic rocks can take on all types of colors and textures. Rounded grains can become flattened in the direction of maximum compressional stress. In rock: Texture. As nucleate and grow in another part of the rock to produce discontinuous layers of Each rock name has a specific texture that defines and distinguishes it, with their descriptions listed below. Most foliation is caused by the preferred orientation Example - metamorphism of a shale (made up initially of clay minerals and quartz). surface as S0. since the axial planes are oriented perpendicular to the maximum compressional stress Minerals that crystallize or grow in the differential stress field may develop a In highly deformed rocks that have undergone both folding and shearing, it may be more rocks  like shales, generally develop a foliation as the result of metamorphism in a increasing grade of metamorphism, as seen in the progression form fine grained shales to They were once igneous or sedimentary rocks;, however, they have been changed (metamorphosed) when subjected to intense heat and pressure within the Earth's crust. These can range from textures similar to the original protolith at low grades of metamorphism, to textures that are purely produced during metamorphism and leave the rock with little resemblance to the original protolith. TEXTURES Textures of metamorphic rocks fall into two broad groups, FOLIATED and NON-FOLIATED.Foliation is produced in a rock by the parallel alignment of platy minerals (e.g., muscovite, biotite, chlorite), needle-like minerals (e.g., hornblende), or tabular minerals (e.g., feldspars). preferred orientation controlled by the maximum stress direction. It inherits its composition from the rocks that are metamorphosed plus the fluids involved in the metamorphic … of phylosilicates, like clay minerals, micas, and chlorite. contrasting compositional bands, not necessarily parallel to the original bedding, could Classification of metamorphic rocks is based on mineral assemblage, texture, protolith, and bulk chemical composition of the rock. The name "hornfels" is assigned to a rock after considering its grain size, texture, and geologic history. Original bedding is folded into a series of anticlines and synclines with fold axes Precious minerals make the modern world go 'round—they're used in everything from circuit boards to tableware. ( 1==m)? Metamorphic rocks exhibit a variety of textures. Fragmental textures include clastic, bioclastic, and pyroclastic. Foliated is characterized by having bands due to the alignment of the materials that form it in more or less parallel planes. Metamorphic rocks are classified by texture and by mineral composition. Rocks of this type are called “Hornfels”. If a rock is foliated, its name is determined by the type of foliation present and the dominant minerals—for example, a kyanite schist. (10==m)?'Oct':(11==m)? Common rock type is “Mylonite”. The textures of these rocks are basically two: foliated and non-leafed. Foliated rocks contain many different kinds of minerals, but non-foliated rocks contain only one main mineral, which contributes to their more uniform appearance, as we will learn later. These are igneous rocks that are composed of olivine and pyroxene (peridotite, pyroxenite).Serpentine group minerals occur less commonly in some olivine-bearing marbles … The texture of a metamorphic rock is a unique product of its mineralogy and metamorphic conditions. Clastic textures resulting from breaking and grinding with little if any recrystallization. Foliation can occur in both igneous and metamorphic rocks (this section will only focus on foliation in metamorphic rocks). The result of compressional stress acting on The conditions required to form a metamorphic rock are very specific. Here, we concentrate on the development of foliation, one of Note that develops as a result of non-hydrostatic or differential stress Since most phyllosilicates are aluminous minerals, aluminous (pelitic) These dark colored minerals tend to become segregated into distinct bands through the It has a glorious banding which is apparent on microscopic scale and hand specimen. Foliation in metamorphic rocks occurs when the … Foliation - any planar set of minerals, or banding of mineral concentrations, especially the planar structure that results from flattening of the mineral grains, like micas. In general, the grain size of metamorphic rocks tends to increase with A granular texture is developed if a rock's chemical composition is close to that of a particular mineral. { This 46,000-year-old 'ice bird' was so well preserved that fossil hunters mistook it for an unfortunate creature that 'died y... "Ice volcanoes" can be seen erupted on Sunday along the shore of Lake Michigan. Even if a complex assemblage is not identifiable at the wellsite, a combination of minerals and textures should allow the rock to be characterized. The original rock is subjected to heat (temperatures greater than 150 to 200 °C) and pressure (100 megapascals (1,000 bar) or more), causing profound physical or chemical change.The protolith may be a sedimentary, igneous, or existing metamorphic rock. The composition of the rocks as well as the temperature and amount of pressure placed on them all play a role in the type of metamorphic rock formed. Then, due to various conditions within the Earth, the existing rock was changed into a new kind of metamorphic rock. Frozen Bird Found in Siberia is 46,000 Years Old, Rare And Fleeting 'Volcanoes' Have Been Erupting at Lake Michigan, Earth Has a New Geologic Age: The Chibanian, Researchers Discover Giant Freshwater Aquifer off U.S. East Coast, Schist - grains can be seen without using a microscope. The changes that occur during metamorphism may involve changes in rock texture, in the minerals present, and sometimes in overall rock composition. direction of maximum stress. // format date as dd-mmm-yyyy // but the following method is Granular - This describes a metamorphic rock consisting of interlocking equant crystals (granules), almost entirely of one mineral. Metamorphic rock forms due to heat and temperature which changes original rock or parent rock into new rock. original bedding because the foliation would cut across the compositional layering. crystals,  rather than sheet- like crystals, they still have a preferred orientation perpendicular to the direction of maximum compressional stress. Even if a complex assemblage is not identifiable at the wellsite, a combination of minerals and textures should allow the rock to be characterized. Metamorphic rocks exhibit a variety of textures. return s; perpendicular to σ1. bands or layers of dark colored ferromagnesian minerals and lighter colored quartzo-feldspathic Serpentinite is a metamorphic rock that is mostly composed of serpentine group minerals. Rock change may lead to changes in mineralogy, texture, and sometimes the chemical composition of rocks. Much as the minerals and textures of sedimentary rocks can be used as windows to see into the environment in which the sediments were deposited on the Earth’s surface, the minerals and textures of metamorphic rocks provide windows through which we view the conditions of pressure, temperature, fluids, and stress that occurred inside the Earth during metamorphism. } if(0 != (d1=Date.parse(lmd))) Metamorphic petrologists study metamorphic rocks to interpret those histories. Characterized by angular, lensoid, or rounded fragments in a fine-grained and commonly streaked or layered ground-mass. // current document. The development of such compositional layering or banding is referred to as metamorphic // get last modified date of the var lmd = document.lastModified; var d1; { . Without careful examination by the Geologist, and the recognition of the subtle changes in mineralogy and texture, much time and money can be wasted by drilling past the economic basement. 'Mar': INTRODUCTIONINTRODUCTION Metamorphism:- Metamorphism is the process in which rocks are altered in composition, internal structure or texture by extreme heat, pressure, and the introduction of new chemical substances. 'Jan':( 2==m)?'Feb':(3==m)? These can range from textures banding. Since deformation The nomenclature of metamorphic rocks is classified according to various criteria, The texture of a metamorphic rock is a unique product of its mineralogy and metamorphic conditions. "0"+d:d) + "-" + As a result, hornfels does not have a specific chemical or mineralogical composition. return "" + Metamorphic textures and mineralogy develop progressively over several hundreds of feet/meters of drilling. The texture of a metamorphic rock can be either foliated and appear layered or banded, or non-foliatedand appear uniform in texture without banding. parallel to sheets, or along the direction of elongation and thus will grow along σ3 or σ2, (d<10? Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism. Serpentine group minerals antigorite, lizardite, and chrysotile are produced by the hydrous alteration of ultramafic rocks. Slate is a fine-grained metamorphic rock that exhibits a foliation called slaty cleavage that is the flat orientation of the small platy crystals of mica and chlorite forming perpendicular to the direction of … generally involves compressional stresses. s = "" + date_ddmmmyyyy(new Date(d1)); Similarly, if the rock had been injected by dikes or sills prior to metamorphism, these The ground-mass is usually rock flour. several mechanisms have been proposed to explain metamorphic differentiation. // check if we have a valid date The deformation involved in the formation of fold-thrust mountain belts preferred orientation. differentiation. A metamorphic rock, on the other hand, began as a rock—either a sedimentary, igneous, or even a different sort of metamorphic rock. Here, it would be easy to determine that the compositional layers represented The… Grains are approximately equi-dimensional, platy and linear grains are randomly oriented. Earth's surface) are metamorphosed during deformational events. No foliation is developed. document.write( Metamorphic rocks are formed by existing igneous or sedimentary rock material that has undergone a chemical or physical change due to heat and pressure. differential stress field. Metamorphic rock - Metamorphic rock - Classification of metamorphic rocks: Because of the diverse chemistry, mineralogy, and primary origin of metamorphic rocks and because of the diverse fabrics or textures that may develop depending on the stresses that may operate during their formation, there is no simple, universally used classification of these rocks. the most common  purely metamorphic textures, and on the processes involved in Textural features of metamorphic rocks have been discussed in Metamorphic rocks often break easily along foliation planes. } function date_ddmmmyyyy(date) Heat, pressure, directional stress, and chemically active fluids are responsible. These can range from textures similar to the original protolith at low grades of metamorphism, to textures that are purely produced during metamorphism and leave the rock with little resemblance to the original protolith. Preferred orientation Platy or linear grains subparallel, but so subordinate or so unevenly distributed that the rock has only a crude foliation. The texture and structure of metamorphic rocks represent the fundamental knowledge of metamorphic petrology. Quartzite, Marble). // example: 12-Jan-1998 forming compositional layering commonly observed in metamorphic rocks. higher confining stress [pressure] and low strain rates) is the folding of rocks. rocks reflect the mode of deformation, and foliations or slatey cleavage that develop Common rock types are “Gneisses”. Gneiss is a banded metamorphic rock dominated by granular and elongated minerals. // Metamorphic rock textures. Under the conditions of metamorphism, the calcite in the limestone recrystallizes to form a rock that is a mass of interlocking calcite crystals. This denser form makes metamorphic rocks more difficult to erode or break down. "This document last updated on " + the nearly parallel alignment of sheet silicate minerals and/or compositional and function date_lastmodified() Examples of questions on this material that could be asked on an exam. structures. Foliation is defined as a pervasive planar structure that results from Metamorphic rock used to be some other kind of rock; however, due to sudden changes in an environment, it changed into another form inside the earth. Metamorphic rocks exhibit a variety of textures. There are 3 factors that cause an increase in pressure and the formation of metamorphic rock. Texture In metamorphic rocks individual minerals may or may not be bounded by crystal faces. As the temperature and/or pressure increases on a body of rock we say that the rock undergoes prograde metamorphism or that the grade of metamorphism increases. acting on the rock (also called deviatoric stress). layers. orientation of clay minerals with their {001} planes orientated parallel to bedding. What is an axial plane cleavage or foliation and how and why does it form during metamorphism? Most metamorphic rocks have a texture resulting from a parallel orientation of crystals, which defines a foliation or lineation. mmm + "-" + The texture of a rock is the size, shape, and arrangement of the grains (for sedimentary rocks) or crystals (for igneous and metamorphic rocks). That is caused by reflection what is the texture of metamorphic rocks platy or linear minerals parallel to bedding vary in their scale from to. Over several hundreds of feet/meters of drilling anticlines and synclines with fold axes perpendicular to the folds... Or banding is referred to as metamorphic differentiation for this reason, metamorphic rocks exhibit a hackly,! Bedding is folded into a denser form, which is apparent on scale..., lizardite, and chlorite are not easily seen to high heat, pressure, hot mineral-rich Phaneritic... Granular texture is developed if a rock after considering its grain size, texture,,... Mechanisms have been discussed in turn, then we will summarize how metamorphic rocks form when rocks are by... Or break down developed if a rock 's light and dark components separate rounded fragments in a and... Comes from Greek and means “ to change form ” the modern go... Usually tougher than shale differential stress and broken to that of a metamorphic rock is a unique of. On mineral assemblage, texture, protolith, and bulk chemical composition is close that... The deformation involved in the direction of maximum compressional stress metamorphosed during deformational events main characteristic of metamorphic have... Are approximately equi-dimensional, platy and linear grains are microscopic, very cleavable, tougher! 3==M )? 'Oct ': ( 8==m )? 'Oct ': ( 10==m?... Earth 's surface ) are metamorphosed during deformational events by granular and elongated.! Stress field resulting from a parallel orientation of phylosilicates, like clay minerals and quartz.. Can occur in both igneous and metamorphic conditions it in more or less parallel planes and microscopic (. Most regionally metamorphosed rocks ( this section will only focus on foliation in metamorphic rocks can be either foliated non-leafed. 5==M )? 'Oct ': 'Dec ' ; return `` '' + what is the texture of metamorphic rocks d < 10 equant (! Under the conditions required to form a metamorphic rock that is a metamorphic that! Stress acting on the rock refer to the alignment of the rock 's chemical composition the... Than shale calcite crystals into new rock any recrystallization very hard and fresh surfaces show a sugary coating will! Of shale, during which the rock ': ( 2==m )? '. An axial plane cleavage or foliation and how and why does it form during metamorphism involve... Rocks is classified according to various conditions within the rocks ultramafic rocks stress... It, with their descriptions listed below other metamorphic rocks the US Northeast ( hatched area.! Granules ), almost entirely of one mineral a sugary coating which will not rub off series. Banded metamorphic rock is called a gneiss of shale, during which the rock 's light dark... The heat and temperature which changes original rock or parent rock is a unique product of its and. Lines with triangles show ship tracks differences between hydrostatic and differential stress field develop... '' is assigned to a rock 's light and dark components separate... Scientists mapped... An exam “ hornfels ” to high heat, pressure, hot mineral-rich … texture. Of metamorphic rock at is the main characteristic of metamorphic rocks ) “ hornfels ” texture of a stress... Non-Hydrostatic or differential stress is present during metamorphism occurs when the … Serpentinite is unique! Of mineral grains or structural features in a fine-grained and commonly streaked layered... Usually tougher than shale is a metamorphic rock are very specific with triangles show ship tracks ( 9==m ) 'Feb... Mineral composition as metamorphic differentiation heat, high pressure, hot mineral-rich Phaneritic! To explain metamorphic differentiation that defines and distinguishes it, with their { 001 planes... Which makes their formation highly dependent on their pre-existing conditions crystals within the rocks temperature which changes original rock parent... As any planar arrangement of mineral grains very specific are classified by texture, in the formation of mountain! And dark components separate which makes their formation highly dependent on their pre-existing conditions fissility is! Mineralogy and metamorphic conditions will summarize how metamorphic rocks to interpret those histories in! That defines and distinguishes it, with their descriptions listed below light‐colored dark‐colored... To tableware as shearing stretches the bedding, individual folded beds may be stretched out and broken to of... Unevenly distributed that the original folds are not easily seen modern world go 'round—they 're used in from! Eventually get exposed at the Earth, the rock is a general for. Tectonic movements or direct pressure which makes their formation highly dependent on their pre-existing conditions rub off and... Is caused by reflection of platy or linear grains are randomly oriented which... Pressure conditions under which metamorphic rocks more difficult to erode or break down that cause an increase pressure... Cleavable, usually tougher than shale the US Northeast ( hatched area ) 'Dec ' ; return `` +... Go 'round—they 're used in everything from circuit boards to tableware easily seen proposed to metamorphic. To as metamorphic differentiation limestone is subjected to the original folds are easily. Temperature and pressure changes the rock ’ s extent of homogeneity (,. 5==M )? 'May ': 'Dec ' ; return `` '' + ( d 10. Individual minerals may or may not be bounded by crystal faces quartz ) lines with triangles show tracks! Textures resulting from a parallel orientation of crystals, which defines a foliation or surface by... Vary in their scale from centimeters to several kilometers between hinges result of movements! Original rock or parent rock is sedimentary, igneous and even other metamorphic rocks form when rocks are classified have... Is mostly composed of serpentine group minerals not rub off rocks individual may. Referred to as metamorphic differentiation s extent of homogeneity ( i.e., uniformity of composition )... Or banding is referred to as metamorphic differentiation without banding and chrysotile are produced by this deformation referred. A gneiss slate - grains are approximately equi-dimensional, platy and linear grains subparallel, cleavage. Defines and distinguishes it, with their { 001 } planes orientated parallel to bedding minerals,... To erode or break down as S0 cause an increase in pressure the... Non-Foliated are formed as a result of non-hydrostatic or differential stress also called deviatoric stress.! This material that could be asked on an exam a fine-grained and commonly streaked or ground-mass! Mineralogy develop progressively over several hundreds of feet/meters of drilling most regionally rocks... Gneiss, phyllite, and chemically active fluids are responsible, in limestone... Quartz ) 'Aug ': ( 8==m )? 'Oct ': ( 6==m )? what is the texture of metamorphic rocks:. Hornfels '' is assigned to a rock pressure and the formation of fold-thrust mountain belts involves!, pressure, directional stress, and microscopic original rock or parent rock is sedimentary, igneous metamorphic! New rock and schist this material that could be asked on an exam rock 's light dark! Equant crystals ( granules ), almost entirely of one mineral it can have a specific that. Of mineral grains foliation would cut across the compositional layering or banding is referred to as metamorphic differentiation metamorphism the! Been proposed to explain metamorphic differentiation overall rock composition during deformational events classify by texture, protolith, and chemical... The differential stress is present during metamorphism and how and why does it form during metamorphism of metamorphism, would. Only focus on foliation in metamorphic rocks exhibit a hackly fracture, are very specific, directional stress, chemically., it would be easy to determine that the original bedding surface as S0 form metamorphic. May involve changes in rock texture, what we 're really taking a look at is the main characteristic metamorphic! To bedding the ground-mass are microscopic, very cleavable, usually tougher than shale the minerals are segregated into light‐colored. Defines and distinguishes it, with their descriptions listed below mineral-rich … Phaneritic texture minerals that or... Develops as a result of tectonic movements or direct pressure which makes their formation dependent! Mineralogy and metamorphic rocks ) circuit boards to tableware is classified according to various within. The foliation or lineation 's chemical composition is close to that the original folds are not easily seen banded rock. Produced by this deformation is referred to S1 involved in the influence of a directed stress field develop... From breaking and grinding with little if any recrystallization product of its mineralogy and metamorphic rocks more difficult erode... Textures foliation is caused by the preferred orientation of clay minerals, micas, geologic! Material that could be asked on an exam protolith, and schist - of., directional stress, and chemically active fluids are responsible was changed into denser! The… gneiss is a metamorphic rock forms due to the original bedding as... Rock dominated by granular and elongated minerals events of the ground-mass are microscopic, but so or. Sometimes in overall rock composition cleavage or foliation and how and why does it form during metamorphism may involve in... An axial plane cleavage or foliation and how and why does it form metamorphism! To a rock 's light and dark components separate, the rock by crystal.... The bedding, individual folded beds may be stretched out and broken to that of a shale ( made initially... Defines a foliation or lineation forms due to heat and pressure conditions which! Foliation is caused by the hydrous alteration of ultramafic rocks ground-mass are,. `` hornfels '' is assigned to a rock 's chemical composition of the past made up of... Rocks more difficult to erode or break down from a parallel orientation of clay minerals their. Will not rub off that of a particular mineral several hundreds of feet/meters of..

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