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. ( 4==m)? here review the differences between hydrostatic and differential stress. When we classify by texture, what we're really taking a look at is the mineral crystals within the rocks. Also of importance are the rock’s extent of homogeneity (i.e., uniformity of composition throughout) and the degree of isotropy. // They're a... Scientists have mapped a huge aquifer off the US Northeast (hatched area). Heat, pressure, directional stress, and chemically active fluids are responsible. bands or layers of dark colored ferromagnesian minerals and lighter colored quartzo-feldspathic Common rock types are “Gneisses”. Metamorphic rock forms due to heat and temperature which changes original rock or parent rock into new rock. var m = date.getMonth() + 1; in their scale from centimeters to several kilometers between hinges. Sheet silicates and minerals that have an elongated habit will Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form. return s; What is an axial plane cleavage or foliation and how and why does it form during metamorphism? Metamorphic rocks are classified by texture and by mineral composition. Foliated metamorphic rocks. // before proceeding the previous lecture. rock (this process is called metamorphic differentiation), giving the rock a gneissic Metamorphic rocks can be formed from sedimentary, igneous and even other metamorphic rocks. For this reason, metamorphic rocks can take on all types of colors and textures. Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". Grains are approximately equi-dimensional, platy and linear grains are randomly oriented. Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. 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 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. There are 3 factors that cause an increase in pressure and the formation of metamorphic rock. original bedding because the foliation would cut across the compositional layering. var d1; This denser form makes metamorphic rocks more difficult to erode or break down. Thus felsic minerals could be dissolved from one part of the rock and preferentially perpendicular to σ1. Note that with their long directions perpendicular to the maximum differential stress. Original bedding is folded into a series of anticlines and synclines with fold axes Metamorphic rocks exhibit a variety of textures. Examples of questions on this material that could be asked on an exam. Precious minerals make the modern world go 'round—they're used in everything from circuit boards to tableware. // more compatible Common rock type is “Mylonite”. 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). mmm + "-" + Slate - grains are microscopic, very cleavable, usually tougher than shale. Rounded grains can become flattened in the direction of maximum compressional stress. 'Jun': 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. It inherits its composition from the rocks that are metamorphosed plus the fluids involved in the metamorphic … // example: 12-Jan-1998 Even if a complex assemblage is not identifiable at the wellsite, a combination of minerals and textures should allow the rock to be characterized. 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. The changes that occur during metamorphism may involve changes in rock texture, in the minerals present, and sometimes in overall rock composition. Each of these will be discussed in turn, then we will summarize how metamorphic rocks are classified. 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. These can range from textures // could use splitString() here Even if a complex assemblage is not identifiable at the wellsite, a combination of minerals and textures should allow the rock to be characterized. Clastic textures resulting from breaking and grinding with little if any recrystallization. Foliated Rock Textures Foliation is broadly defined as any planar arrangement of mineral grains or structural features in a rock. the most common  purely metamorphic textures, and on the processes involved in Explain why the textural changes occur. surface as S0. ( 1==m)? Define the following: (a) foliation, (b) hydrostatic stress, (c) differential stress, (d) preferred orientation (e) metamorphic differentiation. generally involves compressional stresses. function date_lastmodified() s = "" + date_ddmmmyyyy(new Date(d1)); If differential stress is present during metamorphism, it can have a "This document last updated on " + A metamorphic rock, on the other hand, began as a rock—either a sedimentary, igneous, or even a different sort of metamorphic rock. 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 … Examples include slate, gneiss, phyllite, and schist. Rock change may lead to changes in mineralogy, texture, and sometimes the chemical composition of rocks. differential stress field. Earth's surface) are metamorphosed during deformational events. "0"+d:d) + "-" + document.write( Most regionally metamorphosed rocks (at least those that eventually get exposed at the The pressure and temperature conditions under which specific types of metamorphic rocks form has been determined by a combinati… crystals,  rather than sheet- like crystals, they still have a preferred orientation Quartzite, Marble). involves the application of differential stress, the textures that develop in metamorphic Granular - This describes a metamorphic rock consisting of interlocking equant crystals (granules), almost entirely of one mineral. Classification of metamorphic rocks is based on mineral assemblage, texture, protolith, and bulk chemical composition of the rock. { The deformation involved in the formation of fold-thrust mountain belts layers. since the axial planes are oriented perpendicular to the maximum compressional stress These are igneous rocks that are composed of olivine and pyroxene (peridotite, pyroxenite).Serpentine group minerals occur less commonly in some olivine-bearing marbles … difficult to determine that the compositional layering represents original bedding. var s = "Unknown"; // format date as dd-mmm-yyyy nucleate and grow in another part of the rock to produce discontinuous layers of structures. parallel to sheets, or along the direction of elongation and thus will grow along σ3 or σ2, Phyllite - all grains of the ground-mass are microscopic, but cleavage surfaces have a sheen caused by reflection of platy or linear minerals. original protolith. contrasting compositional bands, not necessarily parallel to the original bedding, could Most Metamorphic rocks form in the influence of a directed stress field. perpendicular to the direction of maximum compressional stress. 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 … Especially common in metamorphosed granular rocks. Platy or linear grains subparallel, but so subordinate or so unevenly distributed that the rock has only a crude foliation. 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. compositional banding or layering, usually evident as alternating somewhat discontinuous [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. preferred orientation. 'Jan':( 2==m)?'Feb':(3==m)? 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. Fragmental textures include clastic, bioclastic, and pyroclastic. // but the following method is higher confining stress [pressure] and low strain rates) is the folding of rocks. Minerals that crystallize or grow in the differential stress field may develop a (10==m)?'Oct':(11==m)? Metamorphic petrologists and structural geologists refer to the original bedding Preferred orientation The… Most foliation is caused by the preferred orientation As discussed above, gneisses, and to some extent schists, show We ( 7==m)? profound effect on the texture of the rock. 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. This heat and pressure changes the rock into a denser form, which is the main characteristic of metamorphic rock. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich … Textural features of metamorphic rocks have been discussed in the previous lecture. 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. 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. differentiation. The texture of a metamorphic rock can be either foliated and appear layered or banded, or non-foliatedand appear uniform in texture without banding. Grains are irregular and generally interlocking, and microscopic. // get last modified date of the The development of such compositional layering or banding is referred to as metamorphic Then, due to various conditions within the Earth, the existing rock was changed into a new kind of metamorphic rock. 'Apr':( 5==m)?'May':(6==m)? These folds can vary var d = date.getDate(); It has a glorious banding which is apparent on microscopic scale and hand specimen. The texture of a metamorphic rock is a unique product of its mineralogy and metamorphic conditions. Metamorphic rocks exhibit a variety of textures. Metamorphic rocks are formed by existing igneous or sedimentary rock material that has undergone a chemical or physical change due to heat and pressure. The textures of these rocks are basically two: foliated and non-leafed. Common rock types are based on compositional type (i.e. // { several mechanisms have been proposed to explain metamorphic differentiation. 8.3 Regional metamorphism is more intense at depth A granular texture is developed if a rock's chemical composition is close to that of a particular mineral. preferred orientation controlled by the maximum stress direction. The parent rock is sedimentary, igneous, or metamorphic. mineralogical layering in the rock. Note that in the case shown here, the maximum principle stress is oriented at an angle Metamorphic petrologists study metamorphic rocks to interpret those histories. Here, it would be easy to determine that the compositional layers represented (d<10? 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. 'Mar': Metamorphic rocks can be foliated or unleafed. acting on the rock (also called deviatoric stress). shearing stretches the bedding, individual folded beds may be stretched out and broken to Similarly, if the rock had been injected by dikes or sills prior to metamorphism, these Rocks of this type are called “Hornfels”. Phaneritic ("FAN-a-RIT-ic") rocks have mineral grains that are large enough to be … Thus, slatey cleavage or foliation is often seen to be parallel to the axial planes As a result, hornfels does not have a specific chemical or mineralogical composition. // finally display the last modified date . Foliated is characterized by having bands due to the alignment of the materials that form it in more or less parallel planes. The texture and structure of metamorphic rocks represent the fundamental knowledge of metamorphic petrology. The metamorphic word comes from Greek and means “to change form”. that the original folds are not easily seen. 'Sep': rocks reflect the mode of deformation, and foliations or slatey cleavage that develop Here, we concentrate on the development of foliation, one of As Others have a more isotropic arrangement of mineral grains. var mmm = Metamorphic textures and mineralogy develop progressively over several hundreds of feet/meters of drilling. Textural features of metamorphic rocks have been discussed in These changes record geologic processes and events of the past. Texture In metamorphic rocks individual minerals may or may not be bounded by crystal faces. of phylosilicates, like clay minerals, micas, and chlorite. var y = date.getYear(); date_lastmodified() ); 'Jul':( 8==m)?'Aug':(9==m)? forming compositional layering commonly observed in metamorphic rocks. 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. banding. } Marble is a metamorphic rock that forms when limestone is subjected to the heat and pressure of metamorphism. recrystallization of clay minerals or the crystallization of other sheet silicates with a develops as a result of non-hydrostatic or differential stress 'Nov':'Dec'; { Since most phyllosilicates are aluminous minerals, aluminous (pelitic) 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. function date_ddmmmyyyy(date) purely produced during metamorphism and leave the rock with little resemblance to the original bedding. Foliation can occur in both igneous and metamorphic rocks (this section will only focus on foliation in metamorphic rocks). Most metamorphic rocks have a texture resulting from a parallel orientation of crystals, which defines a foliation or lineation. forms as a result of the high-grade metamorphism of shale, during which the rock's light and dark components separate. These dark colored minerals tend to become segregated into distinct bands through the No foliation is developed. Characterized by angular, lensoid, or rounded fragments in a fine-grained and commonly streaked or layered ground-mass. The name "hornfels" is assigned to a rock after considering its grain size, texture, and geologic history. If a rock is foliated, its name is determined by the type of foliation present and the dominant minerals—for example, a kyanite schist. A related rock… similar to the original protolith at low grades of metamorphism, to textures that are } 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. } 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. What are four ways that compositional layering that was not originally present could form during metamorphism. Gneiss is a banded metamorphic rock dominated by granular and elongated minerals. Since deformation Here we examine only the foliated types. // Shales have fissility that is caused by the preferred Metamorphic rocks exhibit a variety of textures. Non-foliated are formed as a result of tectonic movements or direct pressure which makes their formation highly dependent on their pre-existing conditions. Solid yellow or white lines with triangles show ship tracks. The result of compressional stress acting on to the original bedding planes so that the slatey cleavage develops at an angle to the 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. (y<1000?1900+y:y); This is because growth of such minerals is easier along directions Metamorphic rocks often break easily along foliation planes. coarser (but still fine) grained slates, to coarser grained schists and gneisses. Metamorphic rock textures. Phaneritic Texture. Serpentine group minerals antigorite, lizardite, and chrysotile are produced by the hydrous alteration of ultramafic rocks. direction, slatey cleavage or foliation should also develop along these directions. 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. return "" + The nomenclature of metamorphic rocks is classified according to various criteria, In highly deformed rocks that have undergone both folding and shearing, it may be more 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. 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. The texture of a rock is the size, shape, and arrangement of the grains (for sedimentary rocks) or crystals (for igneous and metamorphic rocks). // current document. Example - metamorphism of a shale (made up initially of clay minerals and quartz).

what is the texture of metamorphic rocks

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