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. Low-grade metamorphism takes place at approximately 200–320 ºC and relatively low pressure. Most of this influence is due to the dissolved ions that pass in and out of the fluid phase. The three geotherms represent different geological settings in the Earth. Normal stress compresses (pushes together) rock in one direction, the direction of maximum stress. What kind of change will take place in the rock? Medium-grade metamorphism takes place at approximately at 320–450 ºC and at moderate pressures. Much of the basalt subjected to this type of metamorphism turns into a type of metamorphic rock known as greenschist. P. Eskola originally proposed this idea in 1915 who recognized eight facies in metamorphic rocks. Factors of Metamorphism 3. Hydrothermal metamorphism is the result of extensive interaction of rock with high-temperature fluids. This is termed burial metamorphism, and it can result in rocks such as jade. Metamorphic rocks formed through regional metamorphism occur in the form of extensive mountain belts and also as the core portions of many old eroded mountain systems throughout the world. Dynamothermal metamorphism is the typical process of this zone and high-grade schists like biotite-garnet schists are chief rocks formed. The concept of metamorphic facies simplifies the classification of metamorphic rocks in that it eliminates the necessity of knowing the nature of the parent rocks and their original characters. 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. In schist, the sheets of mica are usually arranged in irregular planes rather than perfectly flat planes, giving the rock a schistose foliation (or simply schistosity). 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. At present ten metamorphic facies as proposed by Myashiro in 1973 are often considered sufficient. 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. The hydrothermal fluid may originate from a magma that intruded nearby and caused fluid to circulate in the nearby crust, from circulating hot groundwater, or from ocean water. The preferred orientation of these sheet silicates causes the rock to easily break along parallel planes, giving the rock a slaty cleavage. Instead, the quartz grains recrystallize into a denser, harder rock than the original sandstone. 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. hornfels—hornfels are very hard rocks formed by contact metamorphism of shale, siltstone, or sandstone. The rocks closest to the contact with the intrusion are heated to the highest temperatures, so the metamorphic grade is highest there and diminishes with increasing distance away from the contact. Plutonic metamorphism is the representative kind and rocks formed in this zone include great variety of Gneisses. Disclaimer 8. However, most metamorphic rocks do not undergo sufficient change in their bulk chemistry to be considered metasomatic rocks. Under low grade metamorphism many of the metamorphic minerals will not grow large enough to be seen without a microscope. Rocks are much denser than air and MPa is the unit most commonly uses to express pressures inside the Earth. If any of these flat minerals are growing under normal stress, they will grow with their sheets oriented perpendicular to the direction of maximum compression. (d) The nature of rock – it is the single most important factor in that the same rock under different conditions may suffer different set of changes during the process of recrystallisation. some felspar is still left unchanged) or into a schorl rock (when all the felspar of granite is changed into tourmaline). Because contact metamorphism is not caused by changes in pressure or by differential stress, contact metamorphic rocks do not become foliated. Mountain building occurs at subduction zones and at continental collision zones where two plates each bearing continental crust, converge upon each other. The lines are known as isograds. Two common sources of heat for such a metamorphism to take place are the internal heat (increase in temperature with depth: thermal gradient:) and the magmatic heat (rise in temperature around magmatic intrusions). The difference in composition between the existing rock and the invading fluid drives the chemical reactions. 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. Get high-quality papers at affordable prices. In other words, zones indicate depth-wise extension of particular grades of metamorphism in a general way. 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. This type of chemical reaction takes a long time. Kaolinisation is another such process where felspars are converted to kaolin under the influence of steam vapours. Additive – when the net result of the process is addition of a new constituent; and. Figure 3. Greenschist contains a set of minerals, some of them green, which may include chlorite, epidote, talc, Na-plagioclase, or actinolite. Blueschist facies and hornfels facies are associated with unusual geothermal gradients. Dolomitization is a metasomatic conversion of limestone into dolomite {CaMg (CO3)2} in the presence of Mg++ rich solutions. Ions may move between minerals to create minerals of different chemical composition. 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. This concept has been found very convenient in tracing the progress of metamorphism in the given region. This results in a rock that can be easily broken along the parallel mineral sheets. Foliated metamorphic rocks are named for their style of foliation. During metamorphism the mineral content and texture of the protolith are changed due to changes in the physical and chemical environment of the rock. The diagram below shows metamorphic facies in terms of pressure and temperature condiditons inside the Earth. Watch BBW HD Porn 1080p HD porn videos for free on Eporner.com. In the large outcrop of metamorphic rocks in figure 1, the rocks’ platy appearance is a result of the process metamorphism. In most subduction zones the subducting plate is relatively cold compared with the high temperature it had when first formed at a mid-ocean spreading ridge. A variety of changes may be caused in pre-existing rocks subjected to metamorphic processes. But once there is a significant and effective change in one or more of these surrounding conditions, the equilibrium is disturbed and the rock is unable to exist in its original form- it has to undergo some changes to establish the equilibrium once again. Low grade metamorphic rocks tend to characterized by an abundance of hydrous minerals, minerals that contain water within their crystal structure. Rocks that have their pressure and temperature conditions increased along such a geotherm will metamorphose in the hornfels facies and, if it gets hot enough, in the granulite facies. The folding and deformation of the rock while it is ductile may greatly distort the original shapes and orientations of the rock, producing folded layers and mineral veins that have highly deformed or even convoluted shapes. Quartz and marble are prime examples of unfoliated that can be produced by either regional or contact metamorphism. Prohibited Content 3. This metasomatic change is sometimes termed as Greisening. The fluids eventually escape through vents in the ocean floor known as black smokers, producing thick deposits of minerals on the ocean floor around the vents. Image Guidelines 4. 11.3 Measuring Earthquakes There are two main ways to measure earthquakes. In short the identify of the protolith plays a big role the identity of the metamorphic rock. Generally speaking, the metamorphic process may result in one or more of the following main categories of effects on the involved rocks: All the changes in the direction of mineralogical reconstitution and textural pattern of rocks during the process of metamorphism are collectively expressed by the term recrystallisation. If pressure does not apply equally in all directions, differential stress occurs. Regionally metamorphosed rocks that contain hydrous fluids will begin to melt before they pass beyond the amphibolite facies. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic eon, which is named after Cambria, the Latinised name for Wales, where rocks from this age were first studied. A metamorphic facies may be characterized by a set of minerals or typical rocks. Metamorphic changes in the rocks are primarily the result of three main factors that are also sometimes called as agents of metamorphism: These agents may act individually or collectively. Metamorphism is the addition of heat and/or pressure to existing rocks, which causes them to change physically and/or chemically so that they become a new rock. Three terms are used to express the grades – low grade, medium grade and high grade. This type of change is termed metasomatism. 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. It is defined as the force per unit area acting on the surface, in a direction perpendicular to the surface. Micas tend to break down. Before uploading and sharing your knowledge on this site, please read the following pages: 1. B. Tectonic processes are another way rocks can be moved deeper along the geotherm. The effects result due to intense localized heating (short of melting). At the same time, in a perpendicular direction, the rock undergoes tension (stretching), in the direction of minimum stress. It is the near surface zone and is characterised by a low temperature (generally less than 300°C) and strong shear stress. Pressure metamorphism occurs when sediments are buried deep under the ground; pressure is dominant, and temperature plays a smaller role. High-temperature, low-pressure geotherms occur in the vicinity of igneous intrusions in the shallow crust, underlying a volcanically active area. Most metamorphism of rocks takes place slowly inside the Earth. 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. Just as atmospheric pressure comes from the weight of all the air above a point on the Earth’s surface, pressure inside the Earth comes from the weight of all the rock above a given depth. This depends primarily on following two major factors: (i) The type of rock involved in the process; (ii) The kind of metamorphic process operating on those rocks. Ocean water that penetrates hot, cracked oceanic crust and circulates as hydrothermal fluid in ocean floor basalts produces extensive hydrothermal metamorphism adjacent to mid-ocean spreading ridges and other ocean-floor volcanic zones. Lithostatic pressure is the pressure exerted on a rock by all the surrounding rock. The water may be present in the minerals as water of crystallisation or simply as pore fluid or it may be supplied externally by magmatic bodies. Pneumatolytic – when the fluids are in the form of gases or vapours; iii. It is the high temperature and great depth type metamorphic zone where hydrostatic stresses are quite dominant. Other things being the same, the effect is most intense in the immediate neighbourhood of the magmatic source and decreases with distance from the contact points. 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. If during metamorphism enough ions are introduced to or removed from the rock via the fluid to change the bulk chemical composition of the rock, the rock is said to have undergone metasomatism. Copyright 9. Hornfels, which is a hard metamorphic rock formed from fine-grained clastic sedimentary rocks, is a common product of contact metamorphism. Plagiarism Prevention 5. Did you have an idea for improving this content? Some unfoliated metamorphic rocks, such as hornfels, originate only by contact metamorphism, but others can originate either by contact metamorphism or by regional metamorphism. The metasomatic replacement of minerals takes place at atomic level and in solid state. The surrounding conditions that play important role in this regard are temperature, pressure and chemically active fluids. Because quartz is stable over a wide range of pressure and temperature, little or no new minerals form in quartzite during metamorphism. Metasomatism 5. The chemically active fluids may be provided: (i) From within the rock, such as pore fluids, in which case the end result of metasomatic change would be a mere replacement of the atoms, the total chemical composition of the rock remaining the same. The approximate extent or degree – qualitatively speaking – to which an original rock has been changed due to metamorphism is expressed by the term metamorphic grade. 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. This gives the surfaces of phyllite a satiny luster, much brighter than the surface of a piece of slate. 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. Important index minerals are – laumonite, prehnite, and lawsonite. There will be, obviously, no change in the physical make up or the chemical constitution of the rock if it has adjusted itself completely to the surrounding environment. schist—the size of mineral crystals tends to grow larger with increasing metamorphic grade. The zone of contact metamorphism surrounding an igneous intrusion is called the metamorphic aureole. Content Filtration 6. 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. Yet another way a rock in the Earth’s crust can have its temperature greatly increased is by the intrusion of magma nearby. 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. 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. 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. (ii) From outside the rock, such as from magmatic emanations or ground water sources in which case many new minerals may be formed in the rock by the interaction of the atoms of the invading fluids with those of the rock. New minerals are created that are stable in the changed conditions. Quartzite is very hard and is often crushed and used in building railroad tracks (see figure 4). For example, if the protolith is basalt, it will turn into greenschist under greenschist facies conditions, and that is what facies is named for. There are two ways to think about how the temperature of a rock can be increased as a result of geologic processes. Meaning of Metamorphism 2. Expulsive – when some component gets removed from the original composition of the rock. (b) The initial temperature of the magma or lava and also its rate of cooling; the higher the initial temperature, the greater is the effect; again, the slower the rate of cooling, greater is degree of recrystallisation. In a given rock type, which starts with a particular chemical composition, lower-grade index minerals are replaced by higher-grade index minerals in a sequence of chemical reactions that proceeds as the rock undergoes prograde metamorphism. Sometimes the pressure is of the type of hydrostatic type such as load of the overlying rocks. Magma intrusion subjects nearby rock to higher temperature with no increase in depth or pressure. Many metamorphic changes are induced solely due to the pressure factor whereas in great majority of cases pressure is the dominant factor and is assisted considerably by the heat factor. Temperature depends on the heat flow, which varies from location to location. It usually requires a strong microscope see the small grains of zeolite minerals that form during burial metamorphism. This will be especially apparent for micas or other sheet silicates that grow during metamorphism, such as biotite, muscovite, chlorite, talc, or serpentine. Metasomatism is quite common in silicate and carbonate rocks. Water in the form of steam is considered the single most important agent. Schist often contains more than just micas among its minerals, such as quartz, feldspars, and garnet. Regional metamorphism occurs where large areas of rock are subjected to large amounts of differential stress for long intervals of time, conditions typically associated with mountain building. For example, in rocks made of metamorphosed shale, metamorphism may prograde through the following index minerals: Index minerals are used by geologists to map metamorphic grade in regions of metamorphic rock.