Even though the name of the each metamorphic facies is taken from a type of rock that forms under those conditions, that is not the only type of rock that will form in those conditions. Temperature depends on the heat flow, which varies from location to location. Micas tend to break down. Vapours of water, boron, fluorine and many alkali metals emanating from magmatic bodies take active part in changing the mineralogical composition of rocks surrounding these bodies. Such a rock is said to be foliated, or to have foliation. Did you have an idea for improving this content? Water in the form of steam is considered the single most important agent. Foliation normally forms when pressure is exerted in only one direction. Any type of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock. Plagiarism Prevention 5. The type of rock that a metamorphic rock used to be, prior to metamorphism, is called the protolith. They are also metamorphosed in order to be stable under the new conditions. Metamorphic rocks are like probes that have gone down into the Earth and come back, bringing an record of the conditions they encountered on their journey in the depths of the Earth. Names of different styles of foliation come from the common rocks that exhibit such foliation: Nonfoliated metamorphic rocks lack a planar (oriented) fabric, either because the minerals did not grow under differential stress, or because the minerals that grew during metamorphism are not minerals that have elongate or flat shapes. In short the identify of the protolith plays a big role the identity of the metamorphic rock. Expulsive – when some component gets removed from the original composition of the rock. Medium-grade metamorphism takes place at approximately at 320–450 ºC and at moderate pressures. The presence of a fluid phase is a major factor during metamorphism because it helps determine which metamorphic reactions will occur and how fast they will occur. Any given rock at some depth below the surface is subject to pressure from two sources- first, load of the overlying burden and second, crustal movements during the convergence of the tectonic plates. Regional metamorphism takes place on a timescale of millions of years. Blueschist facies and hornfels facies are associated with unusual geothermal gradients. Burial metamorphism occurs to rocks buried beneath sediments to depths that exceed the conditions in which sedimentary rocks form. Three terms are used to express the grades – low grade, medium grade and high grade. Rock flowage is simply the slippage of the grains or crystals essentially in solid state making them orient themselves in such a way as not to offer any further resistance to the applied forces. Schist is a product of medium grades of metamorphism and is characterized by visibly prominent, parallel sheets of mica or similar sheet silicates, usually either muscovite or biotite, or both. This involves development of large-scale changes in the structural and chemical constitution of the pre-existing rocks under the combined action of pressure, temperature and fluids. Figure 1. Any pre-existing type of rock forming a part of the crust may or may not undergo any physical, chemical or structural change for any time after its formation. Lithostatic pressure is the pressure exerted on a rock by all the surrounding rock. Pneumatolytic – when the fluids are in the form of gases or vapours; iii. The photograph below shows high-grade metamorphic rock that has undergone several stages of foliation development and folding during regional metamorphism, and may even have reached such a high temperature that it began to melt. Copyright 9. Because quartz is stable over a wide range of pressure and temperature, little or no new minerals form in quartzite during metamorphism. where they have to withstand much higher temperature. At the maximum pressures and temperatures the rocks may encounter within the Earth in this range of geotherms, they will enter either the granulite or eclogite facies. Shear stress pushes one side of the rock in a direction parallel to the side, while at the same time, the other side of the rock is being pushed in the opposite direction. Schist often contains more than just micas among its minerals, such as quartz, feldspars, and garnet. Tectonic processes are another way rocks can be moved deeper along the geotherm. These minerals are stable only within the temperature-pressure range considered characteristic of that particular grade. In the dynamic metamorphism, there is no or very little formation of new minerals (compare with the thermal metamorphism). Because contact metamorphism is not caused by changes in pressure or by differential stress, contact metamorphic rocks do not become foliated. This theoretical state of no change may be explained by saying that the rock is in equilibrium with the physical and chemical environment which surrounds it. By drawing lines around the areas where each type of index mineral occurs, the geologist delineates the zones of different metamorphic grades in the region. marble—marble is a metamorphic rock made up almost entirely of either calcite or dolomite, for which the protolith was either limestone or dolostone, respectively. These grades are indicated by the presence of a set of minerals that are called the index minerals. Above 850°C, some components of rock may actually start melting and hence rocks formed from their resolidification shall be included in the category of igneous rocks. Later workers have greatly improved upon the diagnostic characters and even expanded the number of facies. Exposure to intense heat and pressure can result in the metamorphism of previously formed rocks. Quartzite is very hard and is often crushed and used in building railroad tracks (see figure 4). These are named below along with their typical mineral assemblages: It is essential that all the minerals in an assemblage must be thoroughly identified before a metamorphic rock is placed into a proper facies. The first of these is an estimate of the energy released, and the value is referred to as magnitude.This is the number that is typically used by the press when a big earthquake happens. We would like to show you a description here but the site wonât allow us. Earth’s surface conditions are near the top left corner of the graph at about 15ºC which is the average temperature at Earth’s surface and 0.1 MPa (megapascals), which is about the average atmospheric pressure on the Earth’s surface. The type of rock undergoes metamorphism is a major factor in determing what type of metamorphic rock it becomes. some felspar is still left unchanged) or into a schorl rock (when all the felspar of granite is changed into tourmaline). Differential stress has a major influence on the the appearance of a metamorphic rock. Pressure is a measure of the stress, the physical force, being applied to the surface of a material. Roof tiles are also sometimes made of slate. Metamorphic rocks may also be non-foliated. A variety of changes may be caused in pre-existing rocks subjected to metamorphic processes. A fluid phase may introduce or remove chemical substances into or out of the rock during metamorphism, but in most metamorphic rock, most of the atoms in the protolith are be present in the metamorphic rock after metamorphism; the atoms will likely be rearranged into new mineral forms within the rock. slate—slates form at low metamorphic grade by the growth of fine-grained chlorite and clay minerals. A metamorphic rock used to be some other type of rock, but it was changed inside the Earth to become a new type of rock. Graphite, the “lead” in pencils, is a mineral commonly found in metamorphic rocks. Blueschist is the name given to this type of metamorphic rock. The process may involve exchange of, addition to or expulsion of metallic or non-metallic compounds from an original rock. Lithostatic pressure increases as depth within the Earth increases and is a uniform stress—the pressure applies equally in all directions on the rock. Presence or absence of chemically active fluids within the body of the rocks (the pore fluids) or around them plays very important role in the process of all types of metamorphism. 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. Figure 4. The pressure from orogenic activity is generally lateral or horizontal and is commonly termed as directed pressure. This grade prevails up to a temperature range of 650°C and is indicated by the index minerals like staurolite and cordierite. In the diagram below, three different geotherms are marked with dashed lines. This concept has been found very convenient in tracing the progress of metamorphism in the given region. By rock flowage is understood actual movement and reorientation of the mineral constituents of rock under the influence of loads acting during the metamorphic processes. Index minerals, which are indicators of metamorphic grade. This is termed burial metamorphism, and it can result in rocks such as jade. The diagram below shows folds forming during an early stage of regional metamorphism, along with development of foliation, in response to normal stress. The process is then called load metamorphism. New minerals are created that are stable in the changed conditions. METAMORHISM is the term used to express the process responsible for all the changes that take place in an original rock under the influence of changes in the surrounding conditions of temperature, pressure and chemically active fluids. Therefore, not only does the protolith determine the initial chemistry of the metamorphic rock, most metamorphic rocks do not change their bulk (overall) chemical compositions very much during metamorphism. Crushed quartzite is sometimes placed under railroad tracks because it is very hard and durable. Hornfels, with its alternating bands of dark and light crystals, is a good example of how minerals rearrange themselves during metamorphism. Report a Violation 10. In such cases following conditions control the extent of recrystallisation: (a) The size of the igneous intrusion or the lava flow which serves as the source of heat. It usually requires a strong microscope see the small grains of zeolite minerals that form during burial metamorphism. The fluid usually consists largely of water. Metamorphic rock fall into two categories, foliated and unfoliated. KaAlSi3O8) may get partially or completely changed into a new mineral tourmaline (borosilicate of aluminium). Under low grade metamorphism many of the metamorphic minerals will not grow large enough to be seen without a microscope. The granitic rock in migmatite probably originated from partial melting of some of the metamorphic rock, though in some migmatites the granite may have intruded the rock from deeper in the crust. Differential stress can flatten pre-existing grains in the rock, as shown in the diagram below. Depending on the composition of the rock and the temperature reached, minerals indicative of high metamorphic grade such as pyroxene may occur in some hornfels, though many hornfels have minerals indicating medium grade metamorphism. Foliated metamorphic rocks are named for their style of foliation. Rocks changing from one type of metamorphic rock to another as they encounter higher grades of metamorphism are said to be undergoing prograde metamorphism. The heat from the nearby magma “bakes” the sedimentary rocks and recrystallizes the minerals in them into a new texture that no longer breaks easily along the original sedimentary bedding planes. Disclaimer 8. The effects result due to intense localized heating (short of melting). Sometimes fluids present around the rocks also come in contact with them at elevated temperatures and react with the minerals within the rocks producing many changes in their composition and structure. This is also referred as REGIONAL METAMORPHISM and may be considered as the most important as well as prevalent type of metamorphic processes. If the fluid introduces substantal amounts of ions into the rock and removes substantial amounts of ions from it, the fluid has metasomatized the rock—changed its chemical composition. The word metamorphism comes from ancient Greek words for “change” (meta) and “form” (morph). The zone of contact metamorphism surrounding an igneous intrusion is called the metamorphic aureole. It is also common for the differential stresses under which phyllite forms to have produced a set of folds in the rock, making the foliation surfaces wavy or irregular, in contrast to the often perfectly flat surfaces of slaty cleavage. This type of chemical reaction takes a long time. Extreme pressure may also lead to foliation, the flat layers that form in rocks as the rocks are squeezed by pressure (figure 2). Some marble, which is considered better quality stone for carving into statues, lacks color bands. Because they lack foliation, these rocks are named entirely on the basis of their mineralogy. The metasomatic replacement of minerals takes place at atomic level and in solid state. Yet another way a rock in the Earth’s crust can have its temperature greatly increased is by the intrusion of magma nearby. Another term isoreaction grade is sometimes used when similar reactions as indicated by mineral assemblages at different places in a metamorphosed area are clearly understood. Many types of gneiss look somewhat like granite, except that the gneiss has dark and light stripes whereas in granite randomly oriented and distributed minerals with no stripes or layers. In practice, an assemblage of index minerals rather an individual mineral is used for drawing isograds. Marble is used for decorative items and in art. Factors of Metamorphism 3. Metasomatism is quite common in silicate and carbonate rocks. Subduction takes the rocks to great depth in the Earth relatively quickly. in schists. Much of the basalt subjected to this type of metamorphism turns into a type of metamorphic rock known as greenschist. Field evidence is often required to know for sure whether rocks are products of regional metamorphism, contact metamorphism, or some other type of metamorphism. Therefore, if rocks are simply buried deep enough enough sediment, they will experience temperatures high enough to cause metamorphism. This effect is most notable in the rocks adjoining the magmatic intrusions or those coming in contact with lava flows. Main features of these three grades are broadly summarized as follows: It prevails within a temperature range of 200°-400° C and a large pressure range. They bear evidence of formation of new minerals as well as imposition of new textures and structures on an extensive scale. Normal stress compresses (pushes together) rock in one direction, the direction of maximum stress. As the rocks become heated at depth in the Earth during regional metamorphism they become ductile, which means they are relatively soft even though they are still solid. If only looking at rock samples in a laboratory, one can be sure of the type of metamorphism that produced a foliated metamorphic rock such as schist or gneiss, or a hornfels, which is unfoliated, but one cannot be sure of the type of metamorphism that produced an unfoliated marble or quartzite. During subduction, a tectonic plate, consisting of oceanic crust and lithospheric mantle, is recycled back into the deeper mantle. http://commons.wvc.edu/rdawes/G101OCL/Basics/metamorphic.html, http://www.ck12.org/book/CK-12-Earth-Science-For-High-School/section/4.0/, Metamorphism of slate, but under greater heat and pressure than slate, Often derived from metamorphism of claystone or shale; metamorphosed under more heat and pressure than phyllite, Metamorphism of various different rocks, under extreme conditions of heat and pressure, Contact metamorphism of various different rock types, biotite, muscovite, quartz, garnet, plagioclase, plagioclase, orthoclase, quartz, biotite, amphibole, pyroxene, chlorite characterizes the lowest regional metamorphic grade, biotite replaces chlorite at the next metamorphic grade, which could be considered medium-low grade, garnet appears at the next metamorphic grade, medium grade, staurolite marks the next metamorphic grade, which is medium-high grade, sillimanite is a characteristic mineral of high grade metamorphic rocks. This gives the geologist literally “inside information” on what occurs within the Earth during such processes as the formation of new mountain ranges, the collision of continents, the subduction of oceanic plates, and the circulation of sea water into hot oceanic crust. Blueschist is generally interpreted as having been produced within a subduction zone, even if the plate boundaries have subsequently shifted and that location is no longer at a subduction zone. A common fact observed in the case of metasomatism is that the total volume of the rock remains by and large unchanged after the process is completed. Similarly, when a magmatic intrusion like a sill or a dyke invades the country rock from below, the host rocks around the margins of intrusion suffer sudden and enormous changes in their temperature. However, as metamorphic grade increases to even higher grade, all hydrous minerals, which includes hornblende, may break down and be replaced by other, higher-temperature, non-hydrous minerals such as pyroxene. Three major kinds of metamorphism differentiated on the basis of factor most dominant in causing it are: It is a general term including a variety of metamorphic processes in which the heat factor has played an important role. The block may be so much heated up that its original minerals are forced to recrystallise and rearrange themselves in accordance with the conditions imposed by the acute rise in temperature. Silicate and carbonate rocks are easily altered by metasomatic changes. There are two ways to think about how the temperature of a rock can be increased as a result of geologic processes. quartzite—quartzite is a metamorphic rock made almost entirely of quartz, for which the protolith was quartz arenite. In the large outcrop of metamorphic rocks in figure 1, the rocks’ platy appearance is a result of the process metamorphism. Most metamorphism of rocks takes place slowly inside the Earth. These are the original textures and structures of the rocks that are partially or totally obliterated. Metasomatism 5. Burial metamorphism is the lowest grade of metamorphism. Metamorphic rocks may change so much that they may not resemble the original rock. The Precambrian (or Pre-Cambrian, sometimes abbreviated pÐ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. Granulation is favoured by hard and insoluble character of the constituent minerals as well as by higher confining pressures. The net result would be a definite change in the bulk chemical composition of the rock as a whole. Quartz and marble are prime examples of unfoliated that can be produced by either regional or contact metamorphism. It is defined as the force per unit area acting on the surface, in a direction perpendicular to the surface. Dynamic Metamorphism: It is also called clastic metamorphism, mechanical metamorphism or dislocation metamorphism and is brought about by conditions in which pressure factor plays a dominant role. Sometimes the pressure is of the type of hydrostatic type such as load of the overlying rocks. At the highest grade of metamorphism, rocks begin to partially melt, at which point the boundary of metamorphic conditions is surpassed and the igneous part of the rock cycle is entered.
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