Covers metamorphic rocks, which form from previous rocks exposed to heat and/or pressure. 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. 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. Loess is an example of fine sand carried by wind and deposited as wind borne sedimentary rocks. bookmarked pages associated with this title. Low grade metamorphic rocks tend to characterized by an abundance of hydrous minerals, minerals that contain water within their crystal structure. 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. Quartzite is very hard and is often crushed and used in building railroad tracks (see figure 4). Metamorphic Rocks: Rocks, which under tremendous heat and pressure are completely changed or metamorphosed from their original form, are called metamorphic rocks. Metamorphism • Takes place when preexisting rock is changed when subjected to temperatures and pressures unlike those which it was originally formed. During subduction, a tectonic plate, consisting of oceanic crust and lithospheric mantle, is recycled back into the deeper mantle. Common extrusive rocks are basalt, andesite, and rhyolite. If the minerals are segregated into alternating lightâcolored and darkâcolored layers, the rock is called a gneiss. 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. If pressure does not apply equally in all directions, differential stress occurs. The dark-colored minerals tend to form separate bands or stripes in the rock, giving it a gneissic foliation of dark and light streaks. Amphibolite forms at medium-high metamorphic grades. Quartzite and limestone, shown in table 6, are nonfoliated. 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. Metamorphism can be caused by burial, tectonic stress, heating by magma, or alteration by fluids. The protolith may be a sedimentary, igneous, or existing metamorphic rock. Metamorphic rocks are "changed rocks". Foliated metamorphic rocks. The need for stability may cause the structure of minerals to rearrange and form new minerals. 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. In short the identify of the protolith plays a big role the identity of the metamorphic rock. 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. Medium-grade metamorphism takes place at approximately at 320â450 ÂºC and at moderate pressures. This short quiz doesÂ notÂ count toward your grade in the class, and you can retake it an unlimited number of times. A geologist working with metamorphic rocks collects the rocks in the field and looks for the patterns the rocks form in outcrops as well as how those outcrops are related to other types of rock with which they are in contact. Index minerals, which are indicators of metamorphic grade. ... Notes; Show More : Image Attributions. 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. Roof tiles are also sometimes made of slate. slateâslates form at low metamorphic grade by the growth of fine-grained chlorite and clay minerals. Even though rocks remain solid during metamorphism, fluid is generally present in the microscopic spaces between the minerals. Metamorphic rocks are very hard and usually found in old plateau regions. Because rocks undergoing burial metamorphism encounter the uniform stress of lithostatic pressure, not differential pressure, they do not develop foliation. The platy layers in this large outcrop of metamorphic rock show the effects of pressure on rocks during metamorphism. Schist and slate are sometimes used as building and landscape materials. These rocks are changed when heat or pressure alters the existing rock’s physical or chemical make up. 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 type of rock that a metamorphic rock used to be, prior to metamorphism, is called the protolith. The original rock (called the “protolith”) is either an igneous or sedimentary rock. Quartzite and marble are commonly used for building materials and artwork. 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. A mineral assemblage stable at low temperatures and pressures may not be stable at elevated temperatures and pressures. migmatiteâa combination of high-grade regional metamorphic rock â usually gneiss or schist â and granitic igneous rock. This indicates how strong in your memory this concept is. 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. This fluid phase may play a major role in the chemical reactions that are an important part of how metamorphism occurs. 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. When heat and pressure change the environment of a rock, the crystals may respond by rearranging their structure. Igneous rock is one of the three main rock types. The two main types of metamorphism are both related to heat within Earth: The reason rocks undergo metamorphism is that the minerals in a rock are only stable under a limited range of pressure, temperature, and chemical conditions. 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. Extreme pressure may also lead to foliation, the flat layers that form in rocks as the rocks are squeezed by pressure (figure 2). Most commonly, if there is a fluid phase in a rock during metamorphism, it will be a hydrous fluid, consisting of water and things dissolved in the water. Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Hornfels, which is a hard metamorphic rock formed from fine-grained clastic sedimentary rocks, is a common product of contact metamorphism. schistâthe size of mineral crystals tends to grow larger with increasing metamorphic grade. The word metamorphism comes from ancient Greek words for “change” (meta) and “form” (morph). 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. However, most metamorphic rocks do not undergo sufficient change in their bulk chemistry to be considered metasomatic rocks. It is considered to be an excellent building material for important monumental, historical and architectural buildings. 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. 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. This video discusses how to identify a metamorphic rocks: Answer the question(s) below to see how well you understand the topics covered in the previous section. Extrusive igneous rocks solidify from molten material that flows over the earths surface (lava). This will be especially apparent for micas or other sheet silicates that grow during metamorphism, such as biotite, muscovite, chlorite, talc, or serpentine. Igneous rock is formed through the … 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. 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. The main type of mineral that usually grows during burial metamorphism is zeolite, a group of low-density silicate minerals. A schist is coarser grained than phyllite or slate and has aligned minerals that can be identified with the naked eye. Start studying Section 6.3 metamorphic rocks. 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. Hydrothermal metamorphism is the result of extensive interaction of rock with high-temperature fluids. Magma intrusion subjects nearby rock to higher temperature with no increase in depth or pressure. The type of rock undergoes metamorphism is a major factor in determing what type of metamorphic rock it becomes. A rock undergoing metamorphism remains a solid rock during the process. Remove the magnifying glass from the rock … Normal stress compresses (pushes together) rock in one direction, the direction of maximum stress. 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. This produces a characteristic type of metamorphism, sometimes called high-pressure, low-temperature (high-P, low-T) metamorphism, which only occurs deep in a subduction zone. hornfelsâhornfels are very hard rocks formed by contact metamorphism of shale, siltstone, or sandstone. Figure 1. Lithostatic pressure increases as depth within the Earth increases and is a uniform stressâthe pressure applies equally in all directions on the rock. Amphibolite is also listed below in the section on unfoliated metamorphic rocks. If a rock is foliated, its name is determined by the type of foliation present and the dominant minerals—for example, a kyanite schist. Most metamorphism of rocks takes place slowly inside the Earth. 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. As the diagram shows, rocks undergoing prograde metamorphism in subduction zones will be subjected to zeolite, blueschist, and ultimately eclogite facies conditions. During metamorphism the mineral content and texture of the protolith are changed due to changes in the physical and chemical environment of the rock. In the large outcrop of metamorphic rocks in figure 1, the rocksâ platy appearance is a result of the process metamorphism. They are classified by texture and by chemical and mineral assemblage. Yet another way a rock in the Earthâs crust can have its temperature greatly increased is by the intrusion of magma nearby. Hornfels, with its alternating bands of dark and light crystals, is a good example of how minerals rearrange themselves during metamorphism. Crushed quartzite is sometimes placed under railroad tracks because it is very hard and durable. Temperature is another major factor of metamorphism. •Instead the mineral composition, texture, or chemical composition of the rock changes •Metamorphic rock – rocks that changes form while remaining solid •Metamorphic mineral – … New minerals such as hornblende will form, which is stable at higher temperatures. However, if the protolith is shale, a muscovite-biotite schist, which is not green, will form instead. A metamorphosed limestone is called a marble. [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 … At advanced stages of metamorphism, it is common for a metamorphic rock to develop such a different set of minerals and such a thoroughly changed texture that it is difficult to recognize what the protolith was. 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. Most of this influence is due to the dissolved ions that pass in and out of the fluid phase. When rocks (especially shales and basalts) are affected by contact metamorphism, they often develop a texture called hornfels. marbleâmarble is a metamorphic rock made up almost entirely of either calcite or dolomite, for which the protolith was either limestone or dolostone, respectively. In migmatite you can see metamorphic rock that has reached the limits of metamorphism and begun transitioning into the igneous stage of the rock cycle by melting to form magma. Metamorphic rocks are classified by texture and by mineral composition. This is the rock name to remember when you find a hard, nondescript rock that looks like it … Marble is used for decorative items and in art. Metamorphic rocks often show contorted patterns of folding that indicate they were soft enough to bend (plastic deformation). Removing #book# 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: 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. They will form new minerals that are more stable in the new environment. The fluid usually consists largely of water. neomineralization/neocrystallization: formation of new minerals (e.g., the appearance of garnet in a rock that lacked garnet) 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. The most important non-foliated rock is marble. Examples of low grade hydrous minerals include clay, serpentine, and chlorite. Heat and Pressure. Metamorphic Rock. 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. Burial metamorphism is the lowest grade of metamorphism. ADVERTISEMENTS: These rocks cover three-fourth of earth’s surface and make up five per cent of the volume of the earth’s crust. 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. Figure 7.7 shows an example of this effect. 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. Define the characteristics of a metamorphic rock. Phyllites are slightly more metamorphosed than slates and contain mica crystals that impart a glossy sheen. Regionally metamorphosed rocks that contain hydrous fluids will begin to melt before they pass beyond the amphibolite facies. 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. Usually the metamorphic rock looks quite different from the original rock, called the parent rock or protolith. Slates are generally fineâgrained, darkâcolored, metamorphosed sedimentary rocks that split easily along slaty foliations and were formed under lowâgrade temperature and pressure conditions. Are you sure you want to remove #bookConfirmation# Migmatites form when temperatures are hot enough to partially melt the rock. The zone of contact metamorphism surrounding an igneous intrusion is called the metamorphic aureole. This is because temperature inside the Earth increases along what is called the geothermal gradient, or geotherm for short. Marbles may have bands of different colors which were deformed into convoluted folds while the rock was ductile. 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. High-pressure, low-temperature geotherms occurs in subduction zones. Foliated metamorphic rocks. 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. The specimen shown above is about two inches (five centimeters) across. If the fluid introduces substantal amounts of ions into the rock and removes substantial amounts of ions from it, the fluid has metasomatized the rockâchanged its chemical composition. Metamorphic grade refers to the general temperature and pressure conditions that prevailed during metamorphism. One MPa equals nearly 10 atmospheres. They ma… Rocks do not melt during most conditions of metamorphism. Any open space between the mineral grains in a rock, however microscopic, may contain a fluid phase. Start studying Petrology: metamorphic rocks (midterm 2) w/notes. Because quartz is stable over a wide range of pressure and temperature, little or no new minerals form in quartzite during metamorphism. Most metamorphic rocks are evolved from either sedimentary of igneous rocks. © 2020 Houghton Mifflin Harcourt. All that is needed is enough heat and/or pressure to alter the existing rockâs physical or chemical makeup without melting the rock entirely. Faulting and folding the rocks of the crust, can move rocks to much greater depth than simple burial can. 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. Blueschist is the name given to this type of metamorphic rock. Greenschist contains a set of minerals, some of them green, which may include chlorite, epidote, talc, Na-plagioclase, or actinolite. Field evidence is often required to know for sure whether rocks are products of regional metamorphism, contact metamorphism, or some other type of metamorphism. Instead, the quartz grains recrystallize into a denser, harder rock than the original sandstone. Identifying Rocks : Identifying Metamorphic Rocks. Figure 4. Amphibolites are poorly foliated to unfoliated and form at medium to medium-high grades of metamorphism from basalt or gabbro. Temperature depends on the heat flow, which varies from location to location. The last type of rock is metamorphic rocks. Mountain building occurs at subduction zones and at continental collision zones where two plates each bearing continental crust, converge upon each other. Igneous rocks are those that solidify from a molten or partially molten state. 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. This large boulder has bedding still visible as dark and light bands sloping steeply down to the right. METAMORPHIC ROCKS NOTES. Low grade hydrous minerals are replaced by micas such as biotite and muscovite, and non-hydrous minerals such as garnet may grow. Associated with this title heat, pressure or hot chemically active fluids change preexisting.... 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