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notes on metamorphic rocks

Metamorphism is a process by which recrystallisation and reorganisation of minerals occur within a rock. and any corresponding bookmarks? High-grade metamorphism takes place at temperatures above about 450 ºC. 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. Under low grade metamorphism many of the metamorphic minerals will not grow large enough to be seen without a microscope. This temperature is about 200ºC (approximately 400ºF). Gneiss is a high-grade metamorphic rock. Tectonic processes are another way rocks can be moved deeper along the geotherm. 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: Rocks changing from one type of metamorphic rock to another as they encounter higher grades of metamorphism are said to be undergoing prograde metamorphism. In the large outcrop of metamorphic rocks in figure 1, the rocks’ platy appearance is a result of the process metamorphism. As the pressure and temperature increase, rocks undergo metamorphism at higher metamorphic grade. There are two major subdivisions of metamorphic rocks. Metamorphic rocks often show contorted patterns of folding that indicate they were soft enough to bend (plastic deformation). Crushed quartzite is sometimes placed under railroad tracks because it is very hard and durable. 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. The difference in composition between the existing rock and the invading fluid drives the chemical reactions. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times. •Instead the mineral composition, texture, or chemical composition of the rock changes •Metamorphic rock – rocks that changes form while remaining solid •Metamorphic mineral – … This is because temperature inside the Earth increases along what is called the geothermal gradient, or geotherm for short. slate—slates form at low metamorphic grade by the growth of fine-grained chlorite and clay minerals. 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. 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. It usually requires a strong microscope see the small grains of zeolite minerals that form during burial metamorphism. 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. Igneous rock is formed through the … Pressure is a measure of the stress, the physical force, being applied to the surface of a material. They are classified by texture and by chemical and mineral assemblage. amphibolite—a 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. Figure 7.7 shows an example of this effect. Discuss the effect of heat, pressure and deformation on rocks. Quartzite and marble are commonly used for building materials and artwork. 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 type of chemical reaction takes a long time. Metamorphic rocks are like probes that have gone down into the Earth and come back, bringing an record of the conditions they encountered on their journey in the depths of the Earth. Much of the basalt subjected to this type of metamorphism turns into a type of metamorphic rock known as greenschist. New minerals such as hornblende will form, which is stable at higher temperatures. 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. 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) Mineralogical Changes. Usually the metamorphic rock looks quite different from the original rock, called the parent rock or protolith. Most metamorphism of rocks takes place slowly inside the Earth. One ways rocks may change during metamorphism is by rearrangement of their mineral crystals. The specimen shown above is about two inches (five centimeters) across. 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. 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. Low grade metamorphic rocks tend to characterized by an abundance of hydrous minerals, minerals that contain water within their crystal structure. Ions may move between minerals to create minerals of different chemical composition. Let’s see what these rocks are like and how they’re formed. Gneiss. 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. marble—marble is a metamorphic rock made up almost entirely of either calcite or dolomite, for which the protolith was either limestone or dolostone, respectively. If the minerals are segregated into alternating light‐colored and dark‐colored layers, the rock is called a gneiss. Metamorphic rocks are classified by texture and by mineral composition. 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. Metamorphic Rocks and Minerals •Sometimes rock are subject to pressure and heat but do not melt. Ground up marble is also a component of toothpaste, plastics, and paper. Most of this influence is due to the dissolved ions that pass in and out of the fluid phase. Start studying Section 6.3 metamorphic rocks. Schist. Each metamorphic facies is represented by a specific type of metamorphic rock that forms under a specific pressure and temperature conditions. Migmatites form when temperatures are hot enough to partially melt the rock. The diagram below shows folds forming during an early stage of regional metamorphism, along with development of foliation, in response to normal stress. 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. 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. 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. 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. [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 … A schist is coarser grained than phyllite or slate and has aligned minerals that can be identified with the naked eye. The foliated rocks like slate, gneiss and schist are used as roofing material tabletops, staircases, etc. % Progress . Metamorphic rocks: Slate. Lithostatic pressure increases as depth within the Earth increases and is a uniform stress—the pressure applies equally in all directions on the rock. Metamorphic rocks are very hard and usually found in old plateau regions. Previous Low grade hydrous minerals are replaced by micas such as biotite and muscovite, and non-hydrous minerals such as garnet may grow. The type of rock undergoes metamorphism is a major factor in determing what type of metamorphic rock it becomes. Most regional metamorphic rocks are formed in conditions within this range of geothermal gradients, passing through the greenschist facies to the amphibolites facies. Metamorphic grade refers to the general temperature and pressure conditions that prevailed during metamorphism. It is defined as the force per unit area acting on the surface, in a direction perpendicular to the surface. This fluid phase may play a major role in the chemical reactions that are an important part of how metamorphism occurs. The main type of mineral that usually grows during burial metamorphism is zeolite, a group of low-density silicate minerals. 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. The way temperature changes with depth inside the Earth is called the geothermal gradient, geotherm for short. Phyllites are slightly more metamorphosed than slates and contain mica crystals that impart a glossy sheen. Learn vocabulary, terms, and more with flashcards, games, and other study tools. MEMORY METER. If the fluid introduces substantal amounts of ions into the rock and removes substantial amounts of ions from it, the fluid has metasomatized the rock—changed its chemical composition. 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. Metamorphic Rocks The word metamorphic means ‘ change of form ’. 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. Medium-grade metamorphism takes place at approximately at 320–450 ºC and at moderate pressures. 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 fluid phase can also influence the rate at which mineral crystals deform or change shape. Schist often contains more than just micas among its minerals, such as quartz, feldspars, and garnet. Metamorphic Rock Testing Lab: Drag the magnifying glass over the rock sample to get a close-up view. A hornfels rock is characterized by evenly distributed, very fine‐grained mica crystals that give it a more massive, equigranular appearance. If a rock is foliated, its name is determined by the type of foliation present and the dominant minerals—for example, a kyanite schist. Foliation normally forms when pressure is exerted in only one direction. Igneous rocks are those that solidify from a molten or partially molten state. Yet another way a rock in the Earth’s crust can have its temperature greatly increased is by the intrusion of magma nearby. The dark-colored minerals tend to form separate bands or stripes in the rock, giving it a gneissic foliation of dark and light streaks. 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. The original rock is subjected to heat and pressure, causing profound physical or chemical change. In short the identify of the protolith plays a big role the identity of the metamorphic rock. 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. Metamorphic rocks may change so much that they may not resemble the original rock. 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. 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. Such a rock is said to be foliated, or to have foliation. Metamorphic rocks may also be non-foliated. If pressure does not apply equally in all directions, differential stress occurs. These rocks are characterized as either extrusive or intrusive. hornfels—hornfels are very hard rocks formed by contact metamorphism of shale, siltstone, or sandstone. 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. Like igneous rocks, most metamorphic rocks are composed of 9 or more major elements. Depending on the composition of the rock and the temperature reached, minerals indicative of high metamorphic grade such as pyroxene may occur in some hornfels, though many hornfels have minerals indicating medium grade metamorphism. METAMORPHIC ROCKS NOTES. 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. 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. Identifying Rocks : Identifying Metamorphic Rocks. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, … Roof tiles are also sometimes made of slate. 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. Therefore, if rocks are simply buried deep enough enough sediment, they will experience temperatures high enough to cause metamorphism. When rocks (especially shales and basalts) are affected by contact metamorphism, they often develop a texture called hornfels. 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. Parent rock: Shale. Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. Platy layers in this large boulder has bedding still visible as dark and light streaks a fluid notes on metamorphic rocks can influence. 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