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. It usually requires a strong microscope see the small grains of zeolite minerals that form during burial metamorphism. Hydrothermal Rocks. High-grade metamorphism takes place at temperatures above about 450 ÂºC. Identifying Rocks : Identifying Metamorphic Rocks. There are two major subdivisions of metamorphic rocks. Migmatites form when temperatures are hot enough to partially melt the rock. Types of Metamorphism, Next One MPa equals nearly 10 atmospheres. Contact metamorphism occurs to solid rock next to an igneous intrusion and is caused by the heat from the nearby body of magma. 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. In short the identify of the protolith plays a big role the identity of the metamorphic rock. Each metamorphic facies is represented by a specific type of metamorphic rock that forms under a specific pressure and temperature conditions. Metamorphic rocks may change so much that they may not resemble the original rock. Schist often contains more than just micas among its minerals, such as quartz, feldspars, and garnet. Any type of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock. A metamorphosed limestone is called a marble. 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. If the minerals are segregated into alternating lightâcolored and darkâcolored layers, the rock is called a gneiss. Amphibolite is a non-foliated metamorphic rock that forms through recrystallization under conditions of high viscosity and directed pressure. This occurs due to pressure, volume and temperature changes. Ions may move between minerals to create minerals of different chemical composition. The original rock is subjected to heat and pressure, causing profound physical or chemical change. 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. They are classified by texture and by chemical and mineral assemblage. Slates are generally fineâgrained, darkâcolored, metamorphosed sedimentary rocks that split easily along slaty foliations and were formed under lowâgrade temperature and pressure conditions. 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. In the diagram below, three different geotherms are marked with dashed lines. 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. The original rock (called the “protolith”) is either an igneous or sedimentary rock. Are you sure you want to remove #bookConfirmation# As the diagram shows, rocks undergoing prograde metamorphism in subduction zones will be subjected to zeolite, blueschist, and ultimately eclogite facies conditions. The rock has split from bedrock along this foliation plane, and you can see that other weaknesses are present in the same orientation. This temperature is about 200ÂºC (approximately 400ÂºF). MEMORY METER. However, if the protolith is shale, a muscovite-biotite schist, which is not green, will form instead. 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. One ways rocks may change during metamorphism is by rearrangement of their mineral crystals. This gives the surfaces of phyllite a satiny luster, much brighter than the surface of a piece of slate. 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. Extreme pressure may also lead to foliation, the flat layers that form in rocks as the rocks are squeezed by pressure (figure 2). hornfelsâhornfels are very hard rocks formed by contact metamorphism of shale, siltstone, or sandstone. The fact that most metamorphic rocks retain most of their original atoms means that even if the rock was so thoroughly metamorphosed that it no longer looks at all like the protolith, the rock can be analyzed in terms of its bulk chemical composition to determine what type of rock the protolith was. Any type of rockâigneous, sedimentary, or metamorphicâcan become a metamorphic rock. However, most metamorphic rocks do not undergo sufficient change in their bulk chemistry to be considered metasomatic rocks. 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. Schist. 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. 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. Normal stress compresses (pushes together) rock in one direction, the direction of maximum stress. Learn vocabulary, terms, and more with flashcards, games, and other study tools. 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. The way temperature changes with depth inside the Earth is called the geothermal gradient, geotherm for short. 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. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. 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. Like igneous rocks, most metamorphic rocks are composed of 9 or more major elements. Figure 4. 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. 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. Example: Shale is a parent rock that can become the metamorphic rocks slate, phyllite, schist, and gneiss. Mountain building occurs at subduction zones and at continental collision zones where two plates each bearing continental crust, converge upon each other. Intr… The metamorphic rocks are extensively used as building stones. 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. The dark-colored minerals tend to form separate bands or stripes in the rock, giving it a gneissic foliation of dark and light streaks. Regionally metamorphosed rocks that contain hydrous fluids will begin to melt before they pass beyond the amphibolite facies. Because quartz is stable over a wide range of pressure and temperature, little or no new minerals form in quartzite during metamorphism. 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 NOTES. Extreme pressure may also lead to the form… Marbles may have bands of different colors which were deformed into convoluted folds while the rock was ductile. Most of this influence is due to the dissolved ions that pass in and out of the fluid phase. 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. A geologist maps and collects rock samples across the region and marks the geologic map with the location of each rock sample and the type of index mineral it contains. Low-grade metamorphism takes place at approximately 200â320 ÂºC and relatively low pressure. ADVERTISEMENTS: These rocks cover three-fourth of earth’s surface and make up five per cent of the volume of the earth’s crust. 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. The determination of metamorphic grade is madeusing mineral assemblages, mineral compositions, and/or grain sizes. Quartzite and limestone, shown in table 6, are nonfoliated. Metamorphic rock fall into two categories, foliated and unfoliated. For example, if the protolith is basalt, it will turn into greenschist under greenschist facies conditions, and that is what facies is named for. Folding is achieved by the application of great pressure over long periods. 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. A fluid phase may introduce or remove chemical substances into or out of the rock during metamorphism, but in most metamorphic rock, most of the atoms in the protolith are be present in the metamorphic rock after metamorphism; the atoms will likely be rearranged into new mineral forms within the rock. 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. Low grade hydrous minerals are replaced by micas such as biotite and muscovite, and non-hydrous minerals such as garnet may grow. Low grade metamorphic rocks tend to characterized by an abundance of hydrous minerals, minerals that contain water within their crystal structure. 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. Class Notes - Metamorphism Introduction. Regional metamorphism takes place on a timescale of millions of years. There are two ways to think about how the temperature of a rock can be increased as a result of geologic processes. 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. Most foliated metamorphic rocks originate from regional metamorphism. Foliated metamorphic rocks are named for their style of foliation. 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) Tectonic processes are another way rocks can be moved deeper along the geotherm. Metamorphic rocks: Slate. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. 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. 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. This is the rock name to remember when you find a hard, nondescript rock that looks like it … 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. % Progress . Metamorphic Rocks: Rocks, which under tremendous heat and pressure are completely changed or metamorphosed from their original form, are called metamorphic rocks. 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. The protolith is subjected to a change, over time, in the physical and Schist and slate are sometimes used as building and landscape materials. A schistose rock composed of the mineral serpentine is called a serpentinite. 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. Metamorphic 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. Thus, initially it would appear that we are dealing with a 9 or 10 component system. New minerals such as hornblende will form, which is stable at higher temperatures. The main type of mineral that usually grows during burial metamorphism is zeolite, a group of low-density silicate minerals. They ma… Usually the metamorphic rock looks quite different from the original rock, called the parent rock or protolith. It typically contains ab… Blueschist facies and hornfels facies are associated with unusual geothermal gradients. This large boulder has bedding still visible as dark and light bands sloping steeply down to the right. Metamorphic rocks are "changed rocks". During metamorphism the mineral content and texture of the protolith are changed due to changes in the physical and chemical environment of the rock. Click the posters on the wall to get specific information about the metamorphic rocks. The need for stability may cause the structure of minerals to rearra… Amphibolite forms at medium-high metamorphic grades. marbleâmarble is a metamorphic rock made up almost entirely of either calcite or dolomite, for which the protolith was either limestone or dolostone, respectively. Medium-grade metamorphism takes place at approximately at 320â450 ÂºC and at moderate pressures. In the large outcrop of metamorphic rocks in figure 1, the rocksâ platy appearance is a result of the process metamorphism. The foliated rocks like slate, gneiss and schist are used as roofing material tabletops, staircases, etc. Metamorphic Rocks The word metamorphic means ‘ change of form ’. 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. Let’s see what these rocks are like and how they’re formed. Temperature depends on the heat flow, which varies from location to location. A mineral assemblage stable at low temperatures and pressures may not be stable at elevated temperatures and pressures. bookmarked pages associated with this title. Igneous rock is formed through the … It is composed primarily of hornblende (amphibole) and plagioclase, usually with very little quartz. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, … 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. The preferred orientation of these sheet silicates causes the rock to easily break along parallel planes, giving the rock a slaty cleavage. Lithostatic pressure increases as depth within the Earth increases and is a uniform stressâthe pressure applies equally in all directions on the rock. 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. Subduction takes the rocks to great depth in the Earth relatively quickly. High-pressure, low-temperature geotherms occurs in subduction zones. 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