Start studying Section 6.3 metamorphic rocks. In short the identify of the protolith plays a big role the identity of the metamorphic rock. A hornfels rock is characterized by evenly distributed, very fine‐grained mica crystals that give it a more massive, equigranular appearance. In the large outcrop of metamorphic rocks in figure 1, the rocks’ platy appearance is a result of the process metamorphism. Amphibolite is a non-foliated metamorphic rock that forms through recrystallization under conditions of high viscosity and directed pressure. Thus, initially it would appear that we are dealing with a 9 or 10 component system. If the minerals are segregated into alternating light‐colored and dark‐colored layers, the rock is called a gneiss. These rocks are changed when heat or pressure alters the existing rock’s physical or chemical make up. Rocks do not melt during most conditions of metamorphism. Most metamorphism of rocks takes place slowly inside the Earth. Schist and slate are sometimes used as building and landscape materials. 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. Amphibolites are poorly foliated to unfoliated and form at medium to medium-high grades of metamorphism from basalt or gabbro. Folding is achieved by the application of great pressure over long periods. Instead, the quartz grains recrystallize into a denser, harder rock than the original sandstone. Roof tiles are also sometimes made of slate. A schist is coarser grained than phyllite or slate and has aligned minerals that can be identified with the naked eye. The fluid phase can also influence the rate at which mineral crystals deform or change shape. Quartzite and limestone, shown in table 6, are nonfoliated. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. 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. Metamorphic rocks may also be non-foliated. The main type of mineral that usually grows during burial metamorphism is zeolite, a group of low-density silicate minerals. Most metamorphic rocks are evolved from either sedimentary of igneous rocks. 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. 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. If rocks are buried within the Earth, the deeper they go, the higher the temperatures they experience. 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. Because rocks undergoing burial metamorphism encounter the uniform stress of lithostatic pressure, not differential pressure, they do not develop foliation. 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. METAMORPHIC ROCKS NOTES. 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. Ions may move between minerals to create minerals of different chemical composition. Each metamorphic facies is represented by a specific type of metamorphic rock that forms under a specific pressure and temperature conditions. The specimen shown above is about two inches (five centimeters) across. 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. The source of the pressure is the weight of all the rocks above. Parent rock: Shale. 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. Examples of low grade hydrous minerals include clay, serpentine, and chlorite. At the same time, in a perpendicular direction, the rock undergoes tension (stretching), in the direction of minimum 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. As the pressure and temperature increase, rocks undergo metamorphism at higher metamorphic grade. During subduction, a tectonic plate, consisting of oceanic crust and lithospheric mantle, is recycled back into the deeper mantle. The type of rock undergoes metamorphism is a major factor in determing what type of metamorphic rock it becomes. New minerals such as hornblende will form, which is stable at higher temperatures. 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. Much of the basalt subjected to this type of metamorphism turns into a type of metamorphic rock known as greenschist. Learn vocabulary, terms, and more with flashcards, games, and other study tools. Class Notes - Metamorphism Introduction. 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. Magma intrusion subjects nearby rock to higher temperature with no increase in depth or pressure. Both rock types consist of metamorphic minerals that do not have flat or elongate shapes and thus cannot become layered even if they are produced under differential stress. 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. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times. Temperature is another major factor of metamorphism. Lithostatic pressure increases as depth within the Earth increases and is a uniform stress—the pressure applies equally in all directions on the rock. 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. Migmatites form when temperatures are hot enough to partially melt the rock. Igneous rock is formed through the … 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. Gneiss is a foliated metamorphic rock that has a banded appearance and is made up of granular mineral grains. 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. Identifying Rocks : Identifying Metamorphic Rocks. Metamorphic grade refers to the general temperature and pressure conditions that prevailed during metamorphism. Normal stress compresses (pushes together) rock in one direction, the direction of maximum stress. They ma… Loess is an example of fine sand carried by wind and deposited as wind borne sedimentary rocks. 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. Metamorphic Rocks: Rocks, which under tremendous heat and pressure are completely changed or metamorphosed from their original form, are called metamorphic rocks. Metamorphic Rock Types . Figure 4. The magma is sweated out, or injected, as layers between foliation planes in 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. It is composed primarily of hornblende (amphibole) and plagioclase, usually with very little quartz. Less commonly, it may be a carbon dioxide fluid or some other fluid. 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. A pressure of 1000 MPa corresponds to a depth of about 35 km inside the Earth. Blueschist is the name given to this type of metamorphic rock. 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. 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. Types of Metamorphism, Next 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. Marbles may have bands of different colors which were deformed into convoluted folds while the rock was ductile. 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. Extrusive igneous rocks solidify from molten material that flows over the earths surface (lava). Mineralogical Changes. schist—the size of mineral crystals tends to grow larger with increasing metamorphic grade. •Instead the mineral composition, texture, or chemical composition of the rock changes •Metamorphic rock – rocks that changes form while remaining solid •Metamorphic mineral – … High-grade metamorphism takes place at temperatures above about 450 ºC. There are two types of differential stress. A schistose rock composed of the mineral serpentine is called a serpentinite. Marble is used for decorative items and in art. Use the left and right arrows to page through the information on the poster. Metamorphic Rocks The word metamorphic means ‘ change of form ’. Regionally metamorphosed rocks that contain hydrous fluids will begin to melt before they pass beyond the amphibolite facies. 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. 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. Foliated metamorphic rocks. Phyllite. Burial metamorphism occurs to rocks buried beneath sediments to depths that exceed the conditions in which sedimentary rocks form. In the diagram below, three different geotherms are marked with dashed lines. Figure 3. © 2020 Houghton Mifflin Harcourt. The protolith is subjected to a change, over time, in the physical and 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. Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. Schist often contains more than just micas among its minerals, such as quartz, feldspars, and garnet. Most regional metamorphic rocks are formed in conditions within this range of geothermal gradients, passing through the greenschist facies to the amphibolites facies. The three geotherms represent different geological settings in the Earth. Recall, however, that the number of components in any given system is the minimum number required to … This gives the surfaces of phyllite a satiny luster, much brighter than the surface of a piece of slate. The lines are known as isograds. As sedimentation is favoured by water, so they are mostly formed under water. Metamorphism can be caused by burial, tectonic stress, heating by magma, or alteration by fluids. Learn vocabulary, terms, and more with flashcards, games, and other study tools. Medium-grade metamorphism takes place at approximately at 320–450 ºC and at moderate pressures. Some marble, which is considered better quality stone for carving into statues, lacks color bands. Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". It usually requires a strong microscope see the small grains of zeolite minerals that form during burial metamorphism. Covers metamorphic rocks, which form from previous rocks exposed to heat and/or pressure. Let’s see what these rocks are like and how they’re formed. Tectonic processes are another way rocks can be moved deeper along the geotherm. Metamorphic rocks start off as igneous, sedimentary, or other metamorphic rocks. All rights reserved. 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. High-pressure, low-temperature geotherms occurs in subduction zones. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. This temperature is about 200ºC (approximately 400ºF). 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. It is considered to be an excellent building material for important monumental, historical and architectural buildings. The platy layers in this large outcrop of metamorphic rock show the effects of pressure on rocks during metamorphism. They are classified by texture and by chemical and mineral assemblage. 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 Rocks and Minerals •Sometimes rock are subject to pressure and heat but do not melt. Marble is beautiful for statues and decorative items such as vases (see an example in figure 3). This occurs due to pressure, volume and temperature changes. 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. One ways rocks may change during metamorphism is by rearrangement of their mineral crystals. Differential stress can flatten pre-existing grains in the rock, as shown in the diagram below. Quartzite is very hard and is often crushed and used in building railroad tracks (see figure 4). 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. Gneiss is a high-grade metamorphic rock. How do metamorphic rocks form • When heat, pressure or hot chemically active fluids change preexisting rocks. Metamorphism is a process by which recrystallisation and reorganisation of minerals occur within a 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. 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. The preferred orientation of these sheet silicates causes the rock to easily break along parallel planes, giving the rock a slaty cleavage. Foliation normally forms when pressure is exerted in only one direction. Metamorphic Rock. Temperature depends on the heat flow, which varies from location to location. Because they lack foliation, these rocks are named entirely on the basis of their mineralogy. Igneous rock is one of the three main rock types. Typical Pressure Range For Common Metamorphic Rocks = 2-8 Kb GEOTHERMAL GRADIENT-- Controls Heat flow at Surface Two Major Aspects of Geothermal Gradient: • Conduction of heat from Mantle >> Limited effect in continents, Controlled by thickness of lithosphere (but note areas with thin lithosphere, e.g., Great Basin) This results in a rock that can be easily broken along the parallel mineral sheets. chlorite characterizes the lowest regional metamorphic grade, biotite replaces chlorite at the next metamorphic grade, which could be considered medium-low grade, garnet appears at the next metamorphic grade, medium grade, staurolite marks the next metamorphic grade, which is medium-high grade, sillimanite is a characteristic mineral of high grade metamorphic rocks. The knowledge of temperatures and pressures at which particular types of metamorphic rocks form led to the concept of metamorphic facies. If a rock is foliated, its name is determined by the type of foliation present and the dominant minerals—for example, a kyanite schist. Phyllites are slightly more metamorphosed than slates and contain mica crystals that impart a glossy sheen. As rocks are compressed more and more by other layers of rocks, the lower rocks may change due to the weight of the upper layers. Igneous Rock. Index minerals, which are indicators of metamorphic grade. 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. Crushed quartzite is sometimes placed under railroad tracks because it is very hard and durable. Like igneous rocks, most metamorphic rocks are composed of 9 or more major elements. As the diagram shows, rocks undergoing prograde metamorphism in subduction zones will be subjected to zeolite, blueschist, and ultimately eclogite facies conditions. Metamorphic rocks are "changed rocks". bookmarked pages associated with this title. Most of this influence is due to the dissolved ions that pass in and out of the fluid phase. There are two ways to think about how the temperature of a rock can be increased as a result of geologic processes. Metamorphic rocks may change so much that they may not resemble the original rock. A rock undergoing metamorphism remains a solid rock during the process. Metamorphic rocks. 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. Amphibolite forms at medium-high metamorphic grades. Greenschist contains a set of minerals, some of them green, which may include chlorite, epidote, talc, Na-plagioclase, or actinolite. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, … 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. Practice. 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. Metamorphic rocks are classified by texture and by mineral composition. 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. Hydrothermal metamorphism is the result of extensive interaction of rock with high-temperature fluids. and any corresponding bookmarks? Therefore, if rocks are simply buried deep enough enough sediment, they will experience temperatures high enough to cause metamorphism. hornfels—hornfels are very hard rocks formed by contact metamorphism of shale, siltstone, or sandstone. 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. 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. Ground up marble is also a component of toothpaste, plastics, and paper. Under low grade metamorphism many of the metamorphic minerals will not grow large enough to be seen without a microscope. When rocks (especially shales and basalts) are affected by contact metamorphism, they often develop a texture called hornfels. The need for stability may cause the structure of minerals to rearrange and form new minerals. 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 word metamorphism comes from ancient Greek words for “change” (meta) and “form” (morph). 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 rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. The foliated rocks like slate, gneiss and schist are used as roofing material tabletops, staircases, etc. Any type of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock. Metamorphic rocks provide a record of the processes that occurred inside Earth as the rock was subjected to changing physical and chemical conditions. 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 rock has split from bedrock along this foliation plane, and you can see that other weaknesses are present in the same orientation. When heat and pressure change the environment of a rock, the crystals may respond by rearranging their structure. Rocks are much denser than air and MPa is the unit most commonly uses to express pressures inside the Earth. High-temperature, low-pressure geotherms occur in the vicinity of igneous intrusions in the shallow crust, underlying a volcanically active area. 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. This is not far beyond the conditions in which sediments get lithified into sedimentary rocks, and it is common for a low-grade metamorphic rock to look somewhat like its protolith. Hydrothermal Rocks. The new rock is completely different from the original. A mineral assemblage stable at low temperatures and pressures may not be stable at elevated temperatures and pressures. These rocks are characterized as either extrusive or intrusive. Heat from the interior of the earth may sometimes help to change the rock. 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. 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. Remove the magnifying glass from the rock … 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. Nonfoliated metamorphic rocks. Some varieties of schist are mica, garnet‐mica, biotite, kyanite, and talc schist. The determination of metamorphic grade is madeusing mineral assemblages, mineral compositions, and/or grain sizes. Metamorphic rocks: Slate. 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. neomineralization/neocrystallization: formation of new minerals (e.g., the appearance of garnet in a rock that lacked garnet) Usually the metamorphic rock looks quite different from the original rock, called the parent rock or protolith. As temperature and/or pressure increases "old minerals" may change (typically they increase in size) or new minerals may form. The original rock (called the “protolith”) is either an igneous or sedimentary rock. During metamorphism the mineral content and texture of the protolith are changed due to changes in the physical and chemical environment of the rock. 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. However, most metamorphic rocks do not undergo sufficient change in their bulk chemistry to be considered metasomatic rocks. 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 … Micas tend to break down. Hornfels is shown in table 1. 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. The metamorphic rocks are extensively used as building stones. 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