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 rock fall into two categories, foliated and unfoliated. Thus, initially it would appear that we are dealing with a 9 or 10 component system. Learn vocabulary, terms, and more with flashcards, games, and other study tools. Rocks changing from one type of metamorphic rock to another as they encounter higher grades of metamorphism are said to be undergoing prograde metamorphism. 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. Amphibolite is a non-foliated metamorphic rock that forms through recrystallization under conditions of high viscosity and directed pressure. 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 undergoes metamorphism is a major factor in determing what type of metamorphic rock it becomes. Normal stress compresses (pushes together) rock in one direction, the direction of maximum stress. 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 most important non-foliated rock is marble. Remove the magnifying glass from the rock … Example: Shale is a parent rock that can become the metamorphic rocks slate, phyllite, schist, and gneiss. MEMORY METER. 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. Much of the basalt subjected to this type of metamorphism turns into a type of metamorphic rock known as greenschist. A schist is coarser grained than phyllite or slate and has aligned minerals that can be identified with the naked eye. 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. Extreme pressure may also lead to foliation, the flat layers that form in rocks as the rocks are squeezed by pressure (figure 2). As the pressure and temperature increase, rocks undergo metamorphism at higher metamorphic grade. Foliated metamorphic rocks are named for their style of foliation. Removing #book# Because contact metamorphism is not caused by changes in pressure or by differential stress, contact metamorphic rocks do not become foliated. They are classified by texture and by chemical and mineral assemblage. 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 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. Start studying Petrology: metamorphic rocks (midterm 2) w/notes. Like igneous rocks, most metamorphic rocks are composed of 9 or more major elements. Because rocks undergoing burial metamorphism encounter the uniform stress of lithostatic pressure, not differential pressure, they do not develop foliation. How do metamorphic rocks form • When heat, pressure or hot chemically active fluids change preexisting rocks. 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) Heat from the interior of the earth may sometimes help to change the rock. At the same time, in a perpendicular direction, the rock undergoes tension (stretching), in the direction of minimum stress. High-pressure, low-temperature geotherms occurs in subduction zones. The need for stability may cause the structure of minerals to rearra… 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. Most metamorphic rocks are evolved from either sedimentary of igneous rocks. 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. 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. Most foliated metamorphic rocks—slate, phyllite, schist, and gneiss—are formed during regional metamorphism. Pressure is a measure of the stress, the physical force, being applied to the surface of a material. Burial metamorphism occurs to rocks buried beneath sediments to depths that exceed the conditions in which sedimentary rocks form. Learn vocabulary, terms, and more with flashcards, games, and other study tools. Covers metamorphic rocks, which form from previous rocks exposed to heat and/or pressure. 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. 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. 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. Any open space between the mineral grains in a rock, however microscopic, may contain a fluid phase. 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. The new rock is completely different from the original. Metamorphism is a process by which recrystallisation and reorganisation of minerals occur within a rock. 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. A schistose rock composed of the mineral serpentine is called a serpentinite. Foliated metamorphic rocks. % Progress . 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. 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. 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. Most of this influence is due to the dissolved ions that pass in and out of the fluid phase. The type of rock that a metamorphic rock used to be, prior to metamorphism, is called the protolith. The difference in composition between the existing rock and the invading fluid drives the chemical reactions. Heat and Pressure. Examples of low grade hydrous minerals include clay, serpentine, and chlorite. Gneiss is a high-grade metamorphic rock. 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. Burial metamorphism is the lowest grade of metamorphism. In the large outcrop of metamorphic rocks in figure 1, the rocks’ platy appearance is a result of the process metamorphism. quartzite—quartzite is a metamorphic rock made almost entirely of quartz, for which the protolith was quartz arenite. Blueschist facies and hornfels facies are associated with unusual geothermal gradients. Tectonic processes are another way rocks can be moved deeper along the geotherm. High-grade metamorphism takes place at temperatures above about 450 ºC. Crushed quartzite is sometimes placed under railroad tracks because it is very hard and durable. Foliated metamorphic rocks. 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. 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. 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. Index minerals, which are indicators of metamorphic grade. 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. Graphite, the “lead” in pencils, is a mineral commonly found in metamorphic rocks. Metamorphic Rock. It is defined as the force per unit area acting on the surface, in a direction perpendicular to the surface. Field evidence is often required to know for sure whether rocks are products of regional metamorphism, contact metamorphism, or some other type of metamorphism. Phyllites are slightly more metamorphosed than slates and contain mica crystals that impart a glossy sheen. Mountain building occurs at subduction zones and at continental collision zones where two plates each bearing continental crust, converge upon each other. 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. Previous Under low grade metamorphism many of the metamorphic minerals will not grow large enough to be seen without a microscope. Nonfoliated metamorphic rocks. This gives the surfaces of phyllite a satiny luster, much brighter than the surface of a piece of slate. Igneous rock is formed through the … This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times. 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. Amphibolite is also listed below in the section on unfoliated metamorphic rocks. As sedimentation is favoured by water, so they are mostly formed under water. Although pressure inside the Earth is determined by the depth, temperature depends on more than depth. Metamorphic grade refers to the general temperature and pressure conditions that prevailed during metamorphism. Because they lack foliation, these rocks are named entirely on the basis of their mineralogy. Parent rock: Shale. slate—slates form at low metamorphic grade by the growth of fine-grained chlorite and clay minerals. METAMORPHIC ROCKS NOTES. Low grade metamorphic rocks tend to characterized by an abundance of hydrous minerals, minerals that contain water within their crystal structure. Metamorphic rocks may change so much that they may not resemble the original rock. Metamorphic Rocks The word metamorphic means ‘ change of form ’. Figure 4. hornfels—hornfels are very hard rocks formed by contact metamorphism of shale, siltstone, or sandstone. Metamorphic Rocks and Minerals •Sometimes rock are subject to pressure and heat but do not melt. Metamorphic rocks are "changed rocks". 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. High-temperature, low-pressure geotherms occur in the vicinity of igneous intrusions in the shallow crust, underlying a volcanically active area. It typically contains ab… and any corresponding bookmarks? 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 light‐colored and dark‐colored layers, the rock is called a gneiss. The fluid usually consists largely of water. Some unfoliated metamorphic rocks, such as hornfels, originate only by contact metamorphism, but others can originate either by contact metamorphism or by regional metamorphism. The way temperature changes with depth inside the Earth is called the geothermal gradient, geotherm for short. This temperature is about 200ºC (approximately 400ºF). 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. Metamorphic rocks start off as igneous, sedimentary, or other metamorphic rocks. As the diagram shows, rocks undergoing prograde metamorphism in subduction zones will be subjected to zeolite, blueschist, and ultimately eclogite facies conditions. [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 … Common extrusive rocks are basalt, andesite, and rhyolite. Metamorphic rocks are classified by texture and by mineral composition. When heat and pressure change the environment of a rock, the crystals may respond by rearranging their structure. Let’s see what these rocks are like and how they’re formed. 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. Some varieties of schist are mica, garnet‐mica, biotite, kyanite, and talc schist. 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. This occurs due to pressure, volume and temperature changes. Figure 1. Extrusive igneous rocks solidify from molten material that flows over the earths surface (lava). 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. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. The source of the pressure is the weight of all the rocks above. If the minerals are segregated into alternating light‐colored and dark‐colored layers, the rock is called a gneiss. These rocks are characterized as either extrusive or intrusive. Figure 7.7 shows an example of this effect. A hornfels rock is characterized by evenly distributed, very fine‐grained mica crystals that give it a more massive, equigranular appearance. Hornfels, which is a hard metamorphic rock formed from fine-grained clastic sedimentary rocks, is a common product of contact metamorphism. Regionally metamorphosed rocks that contain hydrous fluids will begin to melt before they pass beyond the amphibolite facies. 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