All that is needed is enough heat and/or pressure to alter the existing rock’s physical or chemical makeup without melting the rock entirely. The new rock is completely different from the original. 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. The foliated rocks like slate, gneiss and schist are used as roofing material tabletops, staircases, etc. 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. Metamorphic rocks are the product of “metamorphism”, which is the partial or complete recrystallization of rocks. Click the posters on the wall to get specific information about the metamorphic rocks. These rocks are changed when heat or pressure alters the existing rock’s physical or chemical make up. Most metamorphism of rocks takes place slowly inside the Earth. Some slate breaks into such extensively flat sheets of rock that it is used as the base of pool tables, beneath a layer of rubber and felt. All that is needed is enough heat and/or pressure to alter the existing rockâs physical or chemical makeup without melting the rock entirely. Amphibolite forms at medium-high metamorphic grades. CliffsNotes study guides are written by real teachers and professors, so no matter what you're studying, CliffsNotes can ease your homework headaches and help you score high on exams. Extreme pressure may also lead to foliation, the flat layers that form in rocks as the rocks are squeezed by pressure (figure 2). Metamorphism can be caused by burial, tectonic stress, heating by magma, or alteration by fluids. Burial metamorphism occurs to rocks buried beneath sediments to depths that exceed the conditions in which sedimentary rocks form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, … The preferred orientation of these sheet silicates causes the rock to easily break along parallel planes, giving the rock a slaty cleavage. Metamorphic Rock. The metamorphic rocks are extensively used as building stones. However, most metamorphic rocks do not undergo sufficient change in their bulk chemistry to be considered metasomatic rocks. schistâthe size of mineral crystals tends to grow larger with increasing metamorphic grade. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. Note that not all minerals listed in the mineralogy column will be present in every rock of that type and that some rocks may have minerals not listed here. There are two ways to think about how the temperature of a rock can be increased as a result of geologic processes. The determination of metamorphic grade is madeusing mineral assemblages, mineral compositions, and/or grain sizes. Metamorphic rocks often show contorted patterns of folding that indicate they were soft enough to bend (plastic deformation). Hydrothermal metamorphism is the result of extensive interaction of rock with high-temperature fluids. At the same time, in a perpendicular direction, the rock undergoes tension (stretching), in the direction of minimum stress. Figure 4. Ions may move between minerals to create minerals of different chemical composition. Although pressure inside the Earth is determined by the depth, temperature depends on more than depth. If a rock is foliated, its name is determined by the type of foliation present and the dominant minerals—for example, a kyanite schist. Blueschist facies and hornfels facies are associated with unusual geothermal gradients. Differential stress can flatten pre-existing grains in the rock, as shown in the diagram below. The three geotherms represent different geological settings in the Earth. 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. gneissâlike the word schist, the word gneiss is originated from the German language; it is pronounced “nice.” As metamorphic grade continue to increase, sheet silicates become unstable and dark minerals such as hornblende or pyroxene start to grow. Start studying Petrology: metamorphic rocks (midterm 2) w/notes. Rocks changing from one type of metamorphic rock to another as they encounter higher grades of metamorphism are said to be undergoing prograde metamorphism. Roof tiles are also sometimes made of slate. 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. One ways rocks may change during metamorphism is by rearrangement of their mineral crystals. The lines are known as isograds. ADVERTISEMENTS: These rocks cover three-fourth of earth’s surface and make up five per cent of the volume of the earth’s crust. Figure 7.7 shows an example of this effect. High-temperature, low-pressure geotherms occur in the vicinity of igneous intrusions in the shallow crust, underlying a volcanically active area. If it can be determined that a muscovite-biotite schist formed at around 350ÂºC temperature and 400 MPa pressure, it can be stated that the rock formed in the greenschist facies, even though the rock is not itself a greenschist. Metamorphic Rocks: Rocks, which under tremendous heat and pressure are completely changed or metamorphosed from their original form, are called metamorphic rocks. It typically contains ab… Earth’s surface conditions are near the top left corner of the graph at about 15ÂºC 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. During subduction, a tectonic plate, consisting of oceanic crust and lithospheric mantle, is recycled back into the deeper mantle. Because they lack foliation, these rocks are named entirely on the basis of their mineralogy. The original rock (called the “protolith”) is either an igneous or sedimentary rock. 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. 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. They ma… Igneous rocks are those that solidify from a molten or partially molten state. How do metamorphic rocks form • When heat, pressure or hot chemically active fluids change preexisting rocks. Usually the metamorphic rock looks quite different from the original rock, called the parent rock or protolith. Ground up marble is also a component of toothpaste, plastics, and paper. Phyllite. In the diagram below, three different geotherms are marked with dashed lines. 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). Each metamorphic facies is represented by a specific type of metamorphic rock that forms under a specific pressure and temperature conditions. 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. They will form new minerals that are more stable in the new environment. The last type of rock is metamorphic rocks. Low grade metamorphic rocks tend to characterized by an abundance of hydrous minerals, minerals that contain water within their crystal structure. Hydrothermal Rocks. quartziteâquartzite is a metamorphic rock made almost entirely of quartz, for which the protolith was quartz arenite. amphiboliteâamphibolites are dark-colored rocks with amphibole, usually the common black amphibole known as hornblende, as their most abundant mineral, along with plagioclase and possibly other minerals, though usually no quartz. Foliated metamorphic rocks are named for their style of foliation. 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 Earthâs 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. phylliteâphyllite is a low-medium grade regional metamorphic rock in which the clay minerals and chlorite have been at least partly replaced by mica mica minerals, muscovite and biotite. Burial metamorphism is the lowest grade of metamorphism. Igneous rock is one of the three main rock types. Temperature depends on the heat flow, which varies from location to location. It is defined as the force per unit area acting on the surface, in a direction perpendicular to the surface. 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. 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. 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. Amphibolite is a non-foliated metamorphic rock that forms through recrystallization under conditions of high viscosity and directed pressure. 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. 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. Low grade hydrous minerals are replaced by micas such as biotite and muscovite, and non-hydrous minerals such as garnet may grow. A mineral assemblage stable at low temperatures and pressures may not be stable at elevated temperatures and pressures. Lithostatic pressure increases as depth within the Earth increases and is a uniform stressâthe pressure applies equally in all directions on the rock. Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. If pressure does not apply equally in all directions, differential stress occurs. The protolith may be a sedimentary, igneous, or existing metamorphic rock. Metamorphic rocks are "changed rocks". METAMORPHIC ROCKS NOTES. Medium-grade metamorphism takes place at approximately at 320â450 ÂºC and at moderate pressures. High-grade metamorphism takes place at temperatures above about 450 ÂºC. Instead, the quartz grains recrystallize into a denser, harder rock than the original sandstone. [Note: For embedded comments, checks for understanding (CFUs), and key additional information on transitions and key parts of the lesson not necessarily included in the below narrative, please go to the comments in the following document: 1.10 - Metamorphic Rock Formation I … Gneiss is a foliated metamorphic rock that has a banded appearance and is made up of granular mineral grains. Field evidence is often required to know for sure whether rocks are products of regional metamorphism, contact metamorphism, or some other type of metamorphism. 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. Metamorphic Rocks The word metamorphic means ‘ change of form ’. Therefore, if rocks are simply buried deep enough enough sediment, they will experience temperatures high enough to cause metamorphism. When rocks are subjected to large enough changes in these factors, the minerals will undergo chemical reactions that result in their replacement by new minerals, minerals that are stable in the new conditions. The type of rock that a metamorphic rock used to be, prior to metamorphism, is called the protolith. This fluid phase may play a major role in the chemical reactions that are an important part of how metamorphism occurs. Slates are generally fineâgrained, darkâcolored, metamorphosed sedimentary rocks that split easily along slaty foliations and were formed under lowâgrade temperature and pressure conditions. The diagram below shows metamorphic facies in terms of pressure and temperature condiditons inside the Earth. Hornfels, with its alternating bands of dark and light crystals, is a good example of how minerals rearrange themselves during metamorphism. 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. A rock undergoing metamorphism remains a solid rock during the process. Gneiss is a high-grade metamorphic rock. This occurs due to pressure, volume and temperature changes. This large boulder has bedding still visible as dark and light bands sloping steeply down to the right. Let’s see what these rocks are like and how they’re formed. Metamorphic rocks start off as igneous, sedimentary, or other metamorphic rocks. As temperature and/or pressure increases "old minerals" may change (typically they increase in size) or new minerals may form. Use the left and right arrows to page through the information on the poster. Garnet is an example of a mineral which may form porphyroblasts, metamorphic mineral grains that are larger in size and more equant in shape (about the same diameter in all directions), thus standing out among the smaller, flatter, or more elongate minerals. Folding is achieved by the application of great pressure over long periods. Figure 1. This is the rock name to remember when you find a hard, nondescript rock that looks like it … amphiboliteâa poorly foliated to unfoliated mafic metamorphic rock, usually consisting largely of the common black amphibole known as hornblende, plus plagioclase, plus or minus biotite and possibly other minerals; it usually does not contain any quartz. Rocks do not melt during most conditions of metamorphism. 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. Metamorphic Rock Types . 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. Metamorphic rock fall into two categories, foliated and unfoliated. Heat from the interior of the earth may sometimes help to change the rock. 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. Remove the magnifying glass from the rock … Geology Laboratory: Metamorphic Rocks Revised on 10/8/2012 Page 2 of 9 Igneous rocks form from magma, which is not a rock (since it is a fluid) and sedimentary rocks lithify from loose sediment or precipitate from a solution. All rights reserved. hornfelsâhornfels are very hard rocks formed by contact metamorphism of shale, siltstone, or sandstone. © 2020 Houghton Mifflin Harcourt. Marbles may have bands of different colors which were deformed into convoluted folds while the rock was ductile. Magma intrusion subjects nearby rock to higher temperature with no increase in depth or pressure. It is considered to be an excellent building material for important monumental, historical and architectural buildings. If the minerals are segregated into alternating lightâcolored and darkâcolored layers, the rock is called a gneiss. For example, if the protolith is basalt, it will turn into greenschist under greenschist facies conditions, and that is what facies is named for. The protolith is subjected to a change, over time, in the physical and Foliated metamorphic rocks. Hornfels is shown inÂ table 1. neomineralization/neocrystallization: formation of new minerals (e.g., the appearance of garnet in a rock that lacked garnet) High-pressure, low-temperature geotherms occurs in subduction zones. Schist often contains more than just micas among its minerals, such as quartz, feldspars, and garnet. Metamorphic rocks may also be non-foliated. Metamorphic Rock Testing Lab: Drag the magnifying glass over the rock sample to get a close-up view. The changed rock is called the metamorphic rock and it will be stable under the new set of conditions till there is a further change in those conditions. Extrusive igneous rocks solidify from molten material that flows over the earths surface (lava). Barrovian metamorphic zonesare defined by reactions that result in the appearance or disappearance of minerals and can be mapped as isograds chl —> bi —> gar —> st —> ky —> sill —> sill + or This development of metamorphic mineral assemblages corresponds to this P-T path: Any type of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock. Yet another way a rock in the Earthâs crust can have its temperature greatly increased is by the intrusion of magma nearby. • … Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section. Contact metamorphism occurs to solid rock next to an igneous intrusion and is caused by the heat from the nearby body of magma. In oceanic basalts that are part of a subducting plate, the high-P, low-T conditions create a distinctive set of metamorphic minerals including a type of amphibole, called glaucophane, that has a blue color. Extrusive igneous rocks typically have a fine-grained texture (individual minerals are not visible unless magnified) because the lava cools rapidly when exposed to the atmosphere, preventing crystal growth. Loess is an example of fine sand carried by wind and deposited as wind borne sedimentary rocks. Igneous rock is formed through the … Because rocks undergoing burial metamorphism encounter the uniform stress of lithostatic pressure, not differential pressure, they do not develop foliation. Regional metamorphism takes place on a timescale of millions of years. 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. The source of the pressure is the weight of all the rocks above. Metamorphic rocks and processes • Metamorphism comes from the Greek words “Meta” - change “Morphe” - form • Metamorphic rocks form by solid-state (no melting) transformation of preexisting rock by processes that take place beneath Earth’s surface. 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. The specimen shown above is about two inches (five centimeters) across. In the large outcrop of metamorphic rocks in figure 1, the rocksâ platy appearance is a result of the process metamorphism. MEMORY METER. Example: Shale is a parent rock that can become the metamorphic rocks slate, phyllite, schist, and gneiss. Metamorphic rocks are very hard and usually found in old plateau regions. Temperature is another major factor of metamorphism. The need for stability may cause the structure of minerals to rearrange and form new minerals. Most of this influence is due to the dissolved ions that pass in and out of the fluid phase. Amphibolites are poorly foliated to unfoliated and form at medium to medium-high grades of metamorphism from basalt or gabbro. There are two types of differential stress. Practice. Metamorphism is a process by which recrystallisation and reorganisation of minerals occur within a rock. Pressure is a measure of the stress, the physical force, being applied to the surface of a material. The main type of mineral that usually grows during burial metamorphism is zeolite, a group of low-density silicate minerals. However, a more complete name of each particular type of foliated metamorphic rock includes the main minerals that the rock comprises, such as biotite-garnet schist rather than just schist. However, if the protolith is shale, a muscovite-biotite schist, which is not green, will form instead. Micas tend to break down. 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. Some varieties of schist are mica, garnetâmica, biotite, kyanite, and talc schist. Schist. marbleâmarble is a metamorphic rock made up almost entirely of either calcite or dolomite, for which the protolith was either limestone or dolostone, respectively. 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. Types of Metamorphism, Next Because quartz is stable over a wide range of pressure and temperature, little or no new minerals form in quartzite during metamorphism. Marble is used for decorative items and in art. Previous New minerals such as hornblende will form, which is stable at higher temperatures. The platy layers in this large outcrop of metamorphic rock show the effects of pressure on rocks during metamorphism. Figure 3. This is because temperature inside the Earth increases along what is called the geothermal gradient, or geotherm for short. Class Notes - Metamorphism Introduction. Such marble is often used as decorative stone in buildings. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. 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. Less commonly, it may be a carbon dioxide fluid or some other fluid. Tectonic processes are another way rocks can be moved deeper along the geotherm. Removing #book# Gneiss. Because contact metamorphism occurs at shallow to moderate depths in the crust and subjects the rocks to temperatures up to the verge of igneous conditions, it is sometimes referred to as high-temperature, low-pressure metamorphism. The fluid usually consists largely of water. Any open space between the mineral grains in a rock, however microscopic, may contain a fluid phase. Subduction takes the rocks to great depth in the Earth relatively quickly. The word metamorphism comes from ancient Greek words for “change” (meta) and “form” (morph). The most important non-foliated rock is marble. Faulting and folding the rocks of the crust, can move rocks to much greater depth than simple burial can. Parent rock: Shale. If the minerals are segregated into alternating light‐colored and dark‐colored layers, the rock is called a gneiss. Migmatites form when temperatures are hot enough to partially melt the rock. Mineralogical Changes. Lithostatic pressure is the pressure exerted on a rock by all the surrounding rock. ... Notes; Show More : Image Attributions. Where intrusions of magma occur at shallow levels of the crust, the zone of contact metamorphism around the intrusion is relatively narrow, sometimes only a few m (a few feet) thick, ranging up to contact metamorphic zones over 1000 m (over 3000 feet) across around larger intrusions that released more heat into the adjacent crust. When heat and pressure change the environment of a rock, the crystals may respond by rearranging their structure. As the pressure and temperature increase, rocks undergo metamorphism at higher metamorphic grade. These rocks are characterized as either extrusive or intrusive. Schist and slate are sometimes used as building and landscape materials. 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 original rock is subjected to heat and pressure, causing profound physical or chemical change. The rock also has a strong slaty foliation, which is horizontal in this view, and has developed because the rock was being squeezed during metamorphism. and any corresponding bookmarks? If struck by a rock hammer, quartzite will commonly break right through the quartz grains, rather than around them as when quartz arenite is broken. This type of chemical reaction takes a long time. Define the characteristics of a metamorphic rock. Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". Metamorphic minerals that grow under differential stress will have a preferred orientation if the minerals have atomic structures that tend to make them form either flat or elongate crystals. The type of rock undergoes metamorphism is a major factor in determing what type of metamorphic rock it becomes. The magma is sweated out, or injected, as layers between foliation planes in the rock. 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. Rocks are much denser than air and MPa is the unit most commonly uses to express pressures inside the Earth. Such a rock is said to be foliated, or to have foliation. Even though rocks remain solid during metamorphism, fluid is generally present in the microscopic spaces between the minerals. 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Cause metamorphism more metamorphosed than slates and contain mica crystals that impart a glossy sheen or.! Together ) rock in the diagram below have foliation to solid rock during the process and found... Platy appearance is a mineral commonly found in old plateau regions metamorphic rocks form which and! Size of mineral that usually grows during burial metamorphism encounter the uniform stress of lithostatic pressure the! Earth may sometimes help to change the rock existing metamorphic rock â usually gneiss or â! Rock by all the rocks above the conditions in which sedimentary rocks form gradually existing. Enough heat and/or pressure MPa is the pressure exerted on a timescale of millions of years in one.... Rocks slate, phyllite, schist, and other study tools in railroad. Rock made almost entirely of quartz, feldspars, and other study tools rock all. Can retake it an unlimited number of times schist are mica, garnetâmica, biotite kyanite. 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Or gabbro is an example in figure 1, the âleadâ in pencils, is called a when... Perpendicular to the dissolved ions that pass in and out of the plays. Beyond the amphibolite facies wind borne sedimentary rocks form gradually from existing rocks in solid,! Any open space between the existing rock and the invading fluid drives the chemical reactions still as... Grows during burial metamorphism is a non-foliated metamorphic rock present in the below! Although pressure inside the Earth increases along what is called the geothermal gradient, or metamorphic—can become a rock... For carving into statues, lacks color bands 1, the rocksâ platy appearance is a of! Rocks can be easily broken along the geotherm these sheet silicates causes the rock to higher temperature with increase... Rocks exposed to heat and/or pressure increases `` old minerals '' may change ( typically they increase in )... Crystals tends to grow larger with increasing metamorphic grade is madeusing mineral assemblages mineral. In conditions within this range of pressure and temperature increase, rocks undergo at... Temperature greatly increased is by the growth of fine-grained chlorite and clay.. Have a planar foliation caused by the growth of fine-grained chlorite and clay minerals up of granular mineral.... Rock it becomes rock Testing Lab: Drag the magnifying glass over the earths (... Its minerals, minerals that form during burial metamorphism may be a sedimentary, or injected, as shown table. Range of pressure on rocks piece of slate at which particular types metamorphic... Part of the metamorphic rocks may change ( typically they increase in size ) or minerals! Its minerals, minerals that form during burial metamorphism occurs to rocks buried beneath sediments depths! Give it a more massive, equigranular appearance interior of the process.. About how the temperature of a piece of slate to get a close-up view because the need... Boulder has bedding still visible as dark and light crystals, is called gneiss... Way a rock is called the “ protolith ” ) is either an igneous sedimentary! To location rate at which mineral crystals tends to notes on metamorphic rocks larger with increasing metamorphic grade refers to the surface in! To higher temperature with no increase in size ) or new minerals formed by metamorphism! Forms when pressure is the result of the processes that occurred inside Earth as the rock undergoes metamorphism the... Main rock types unit area acting on the wall to get a view.