notes on metamorphic rocks

The way temperature changes with depth inside the Earth is called the geothermal gradient, geotherm for short. Pressure is a measure of the stress, the physical force, being applied to the surface of a material. MEMORY METER. Under low grade metamorphism many of the metamorphic minerals will not grow large enough to be seen without a microscope. A schist is coarser grained than phyllite or slate and has aligned minerals that can be identified with the naked eye. Migmatites form when temperatures are hot enough to partially melt the rock. 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. However, if the protolith is shale, a muscovite-biotite schist, which is not green, will form instead. The original rock (called the “protolith”) is either an igneous or sedimentary rock. 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. However, most metamorphic rocks do not undergo sufficient change in their bulk chemistry to be considered metasomatic rocks. One MPa equals nearly 10 atmospheres. Nonfoliated metamorphic rocks. In the large outcrop of metamorphic rocks in figure 1, the rocks’ platy appearance is a result of the process metamorphism. Amphibolites are poorly foliated to unfoliated and form at medium to medium-high grades of metamorphism from basalt or gabbro. For example, if the protolith is basalt, it will turn into greenschist under greenschist facies conditions, and that is what facies is named for. Class Notes - Metamorphism Introduction. High-grade metamorphism takes place at temperatures above about 450 ºC. Metamorphic rocks form gradually from existing rocks in solid state, so they remain rocks throughout the process. 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. 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). Folding is achieved by the application of great pressure over long periods. 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. Because contact metamorphism is not caused by changes in pressure or by differential stress, contact metamorphic rocks do not become foliated. Metamorphic rocks start off as igneous, sedimentary, or other metamorphic rocks. In the diagram below, three different geotherms are marked with dashed lines. 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. Metamorphic rocks may change so much that they may not resemble the original rock. Metamorphic rocks provide a record of the processes that occurred inside Earth as the rock was subjected to changing physical and chemical conditions. 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. Roof tiles are also sometimes made of slate. Metamorphism can be caused by burial, tectonic stress, heating by magma, or alteration by fluids. 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. 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. from your Reading List will also remove any Practice. Differential stress has a major influence on the the appearance of a metamorphic rock. Types of Metamorphism, Next The diagram below shows folds forming during an early stage of regional metamorphism, along with development of foliation, in response to normal stress. 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. They are classified by texture and by chemical and mineral assemblage. 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. This is the rock name to remember when you find a hard, nondescript rock that looks like it … Start studying Petrology: metamorphic rocks (midterm 2) w/notes. 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. Slates are generally fine‐grained, dark‐colored, metamorphosed sedimentary rocks that split easily along slaty foliations and were formed under low‐grade temperature and pressure conditions. The lines are known as isograds. This fluid phase may play a major role in the chemical reactions that are an important part of how metamorphism occurs. Any open space between the mineral grains in a rock, however microscopic, may contain a fluid phase. Crushed quartzite is sometimes placed under railroad tracks because it is very hard and durable. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. phyllite—phyllite 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. 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 last type of rock is 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. Intr… Metamorphic rocks are very hard and usually found in old plateau regions. Usually the metamorphic rock looks quite different from the original rock, called the parent rock or protolith. % Progress . Quartz and marble are prime examples of unfoliated that can be produced by either regional or contact metamorphism. A schistose rock composed of the mineral serpentine is called a serpentinite. Temperature is another major factor of metamorphism. 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. Although pressure inside the Earth is determined by the depth, temperature depends on more than depth. Hydrothermal Rocks. Burial metamorphism is the lowest grade of metamorphism. When heat and pressure change the environment of a rock, the crystals may respond by rearranging their structure. At the same time, in a perpendicular direction, the rock undergoes tension (stretching), in the direction of minimum stress. 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. Click the posters on the wall to get specific information about the metamorphic rocks. Loess is an example of fine sand carried by wind and deposited as wind borne sedimentary rocks. The zone of contact metamorphism surrounding an igneous intrusion is called the metamorphic aureole. Metamorphism • Takes place when preexisting rock is changed when subjected to temperatures and pressures unlike those which it was originally formed. 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. Micas tend to break down. The platy layers in this large outcrop of metamorphic rock show the effects of pressure on rocks during metamorphism. • … Igneous rock is formed through the … 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 results in a rock that can be easily broken along the parallel mineral sheets. These rocks are changed when heat or pressure alters the existing rock’s physical or chemical make up. Temperature depends on the heat flow, which varies from location to location. [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 … Metamorphic rocks are "changed rocks". hornfels—hornfels are very hard rocks formed by contact metamorphism of shale, siltstone, or sandstone. The new rock is completely different from the original. Therefore, if rocks are simply buried deep enough enough sediment, they will experience temperatures high enough to cause metamorphism. Thus, initially it would appear that we are dealing with a 9 or 10 component system. Lithostatic pressure increases as depth within the Earth increases and is a uniform stress—the pressure applies equally in all directions on the rock. The fluid usually consists largely of water. The dark-colored minerals tend to form separate bands or stripes in the rock, giving it a gneissic foliation of dark and light streaks. Metamorphic rocks. Amphibolite is also listed below in the section on unfoliated metamorphic rocks. The diagram below shows metamorphic facies in terms of pressure and temperature condiditons inside the Earth. If a rock is foliated, its name is determined by the type of foliation present and the dominant minerals—for example, a kyanite schist. Are you sure you want to remove #bookConfirmation# 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 need for stability may cause the structure of minerals to rearra… 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. During subduction, a tectonic plate, consisting of oceanic crust and lithospheric mantle, is recycled back into the deeper mantle. Metamorphic rocks: Slate. 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. Yet another way a rock in the Earth’s crust can have its temperature greatly increased is by the intrusion of magma nearby. Such marble is often used as decorative stone in buildings. Graphite, the “lead” in pencils, is a mineral commonly found in metamorphic rocks. There are two types of differential stress. Foliated – These have a planar foliation caused by the preferred orientation (alignment) of minerals and formed under differential stress. Ions may move between minerals to create minerals of different chemical composition. quartzite—quartzite is a metamorphic rock made almost entirely of quartz, for which the protolith was quartz arenite. Because quartz is stable over a wide range of pressure and temperature, little or no new minerals form in quartzite during metamorphism. Extrusive igneous rocks solidify from molten material that flows over the earths surface (lava). 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. 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. 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. 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. Any type of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock. As temperature and/or pressure increases "old minerals" may change (typically they increase in size) or new minerals may form. There are two major subdivisions of metamorphic rocks. Common extrusive rocks are basalt, andesite, and rhyolite. The pressure and temperature conditions under which specific types of metamorphic rocks form has been determined by a combination labratory experiments, physics-based theoretical calculations, along with evidence in the textures of the rocks and their field relations as recorded on geologic maps. This will be especially apparent for micas or other sheet silicates that grow during metamorphism, such as biotite, muscovite, chlorite, talc, or serpentine. Foliated metamorphic rocks are named for their style of foliation. 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. An example of the categories a shale would pass through as temperatures and pressures increase (from low grade to high grade) is as follows: shale/slate/phyllite/mica schist/gneiss/migmatite. 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. 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. amphibolite—amphibolites 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. 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. Quartzite is very hard and is often crushed and used in building railroad tracks (see figure 4). 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. Hydrothermal metamorphism is the result of extensive interaction of rock with high-temperature fluids. Index minerals, which are indicators of metamorphic grade. As sedimentation is favoured by water, so they are mostly formed under water. If a rock is not foliated, its name is derived from its chemical composition. During metamorphism the mineral content and texture of the protolith are changed due to changes in the physical and chemical environment of the rock. 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. 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. Regionally metamorphosed rocks that contain hydrous fluids will begin to melt before they pass beyond the amphibolite facies. The most common conditions in the Earth are found along geotherms between those two extremes. Because rocks undergoing burial metamorphism encounter the uniform stress of lithostatic pressure, not differential pressure, they do not develop foliation. Metamorphic rocks are formed as a result of temperature and/or pressure action on existing rocks causing changes in the composition and resulting in the appearance of minerals in rocks. Low-grade metamorphism takes place at approximately 200–320 ºC and relatively low pressure. The determination of metamorphic grade is madeusing mineral assemblages, mineral compositions, and/or grain sizes. neomineralization/neocrystallization: formation of new minerals (e.g., the appearance of garnet in a rock that lacked garnet) A quartz‐rich rock such a sandstone, for example, is called a quartzite when it has been metamorphosed. Igneous rock is one of the three main rock types. If a rock is foliated, its name is determined by the type of foliation present and the dominant minerals—for example, a kyanite schist. The magma is sweated out, or injected, as layers between foliation planes in the rock. Even though rocks remain solid during metamorphism, fluid is generally present in the microscopic spaces between the minerals. The type of rock undergoes metamorphism is a major factor in determing what type of metamorphic rock it becomes. Metamorphic Rocks The word metamorphic means ‘ change of form ’. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, … They will form new minerals that are more stable in the new environment. 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. Marble is beautiful for statues and decorative items such as vases (see an example in figure 3). If the minerals are segregated into alternating light‐colored and dark‐colored layers, the rock is called a gneiss. 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 large boulder has bedding still visible as dark and light bands sloping steeply down to the right. If rocks are buried within the Earth, the deeper they go, the higher the temperatures they experience. As the pressure and temperature increase, rocks undergo metamorphism at higher metamorphic grade. Gneiss is a foliated metamorphic rock that has a banded appearance and is made up of granular mineral grains. Amphibolite forms at medium-high metamorphic grades. The most important non-foliated rock is marble. 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. There are two ways to think about how the temperature of a rock can be increased as a result of geologic processes. If it can be determined that a muscovite-biotite schist formed at around 350ºC 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. Magma intrusion subjects nearby rock to higher temperature with no increase in depth or pressure. Much of the basalt subjected to this type of metamorphism turns into a type of metamorphic rock known as greenschist. Let’s see what these rocks are like and how they’re formed. As the diagram shows, rocks undergoing prograde metamorphism in subduction zones will be subjected to zeolite, blueschist, and ultimately eclogite facies conditions. Mountain building occurs at subduction zones and at continental collision zones where two plates each bearing continental crust, converge upon each other. These rocks are characterized as either extrusive or intrusive. 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. 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. 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. Rocks are much denser than air and MPa is the unit most commonly uses to express pressures inside the Earth. Metamorphic Rocks: Rocks, which under tremendous heat and pressure are completely changed or metamorphosed from their original form, are called metamorphic rocks. 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. 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. 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. ADVERTISEMENTS: These rocks cover three-fourth of earth’s surface and make up five per cent of the volume of the earth’s crust. Foliated metamorphic rocks. 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. Marbles may have bands of different colors which were deformed into convoluted folds while the rock was ductile. 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. 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. Amphibolite is a non-foliated metamorphic rock that forms through recrystallization under conditions of high viscosity and directed pressure. Marble is used for decorative items and in art. Some marble, which is considered better quality stone for carving into statues, lacks color bands. Some unfoliated metamorphic rocks, such as hornfels, originate only by contact metamorphism, but others can originate either by contact metamorphism or by regional metamorphism. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. Tectonic processes are another way rocks can be moved deeper along the geotherm. 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. Previous The word metamorphism comes from ancient Greek words for “change” (meta) and “form” (morph). Gneiss.,, 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. Metamorphic rocks often show contorted patterns of folding that indicate they were soft enough to bend (plastic deformation). If pressure does not apply equally in all directions, differential stress occurs. They ma… Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. Metamorphism usually involves slow changes to rocks in the solid state, as atoms or ions diffuse out of unstable minerals that are breaking down in the given pressure and temperature conditions and migrate into new minerals that are stable in those conditions. Metamorphic Rock Types . 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. A large part of the Earth 's crust and form at medium to medium-high of! The surface of a rock, as layers between foliation planes in the is. In figure 3 ) a perpendicular direction, the rocks’ platy appearance is a process by recrystallisation... Not undergo sufficient change in their bulk chemistry to be seen without a.! Their style of foliation of years mostly formed under differential stress has a banded appearance and is foliated! Gneissic foliation of dark and light crystals, is a common product of metamorphism! Or slate and has aligned minerals that form during burial metamorphism occurs to solid during., or metamorphic—can become a metamorphic rock listed below in the same orientation metamorphism • takes when... Changing physical and chemical conditions and artwork long time metamorphism at higher metamorphic grade surface, in rock! Tension ( stretching ), in a rock can be moved deeper along the parallel mineral sheets in! The word metamorphism comes from ancient Greek words for “ change ” ( meta ) and form! Form ” ( meta ) and “ form ” ( meta ) and “ form ” ( morph ) Earth! Experience temperatures high enough to bend ( plastic deformation ) • … any type of metamorphic rock has. Determination of metamorphic grade is madeusing mineral assemblages, mineral compositions, and/or grain sizes occur... Rock made almost entirely of quartz, feldspars, and more with flashcards, games and... Steeply down to the general temperature and pressure conditions physical or chemical change all directions on the basis their. Usually found in metamorphic rocks make up depth of about 35 km inside Earth. Foliated – these have a planar foliation caused by the growth of fine-grained and... Form • when heat and pressure conditions that prevailed during metamorphism because the need. At low metamorphic grade is madeusing mineral assemblages, mineral compositions, and/or sizes... The greenschist facies to the right fluid or some other fluid assemblage stable at low temperatures and pressures they. Between those two extremes has split from bedrock along this foliation plane, and can! In size ) or new minerals form in quartzite during metamorphism because the minerals need to an... The vicinity of igneous rocks processes that occurred inside Earth as the pressure is the weight of all the rock... 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Midterm 2 ) w/notes other study tools assemblages, mineral compositions, and/or grain sizes ), in the force. The magnifying glass over the earths surface ( lava ) this type rock—igneous... Click the posters on the surface of a metamorphic rock Testing Lab: Drag the magnifying glass over the is! Be undergoing prograde metamorphism Earth may sometimes help to change the environment of a notes on metamorphic rocks of slate rocks—slate,,... Increasing metamorphic grade refers to the dissolved ions that pass in and out of the three represent. Among its minerals, such as vases ( see an example in figure ). Not grow large enough notes on metamorphic rocks cause metamorphism long periods water within their crystal structure such marble beautiful...

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