You’ve probably never seen or even heard of blueschist; that’s not surprising. The rate of increase of temperature with depth in the Earth (typically around 30˚ C/km within the crust). a blue-coloured sodium-magnesium bearing amphibole mineral that forms during metamorphism at high pressures and relatively low pressures, typically within a subduction zone, a metamorphic facies characterized by relatively low temperatures and high pressures, such as can exist within a subduction zone, a garnet-pyroxene-glaucophane bearing rock that is the product of high-pressure metamorphism of oceanic crustal rock (e.g., basalt), typically within a subduction zone. The deeper rocks are within the stack, the higher the pre… Because burial to 10 to 20 kilometers is required, the areas affected tend … Figures 6.1.1, 6.1.2, 6.1.4, 6.1.5, 6.1.6: © Steven Earle. For example, quartz is stable from environmental temperatures (whatever the weather can throw at it) all the way up to about 1800°C. Characterized by strong directed pressure and increased temperature due to increased burial. Results in foliated rocks (convergent plate boundary) Metamorphic rocks are classified basesd on their texture and composition. All minerals are stable over a specific range of temperatures. Another way to understand metamorphism is by using a diagram that shows temperature on one axis and depth—which is equivalent to pressure—on the other (Figure 6.1.6). For this reason, it is very difficult to study metamorphic processes in a lab. On modern Earth, regional metamorphism occurs in plate boundary zones. What is surprising is that anyone has seen it! The force of the collision causes rocks to be folded, broken, and stacked on each other, so not only is there the squeezing force from the collision, but from the weight of stacked rocks. At 10 km to 15 km, we are in the greenschist zone (where chlorite would form in mafic volcanic rock) and very fine micas form in mudrock, to produce phyllite. Most metamorphic reactions take place at very slow rates. In most parts of southern Canada, the average surface temperature is about 10°C, so at a 1,000 metre depth, it will be about 40°C. Metamorphic rocks formed there are likely to be foliated because of the strong directional pressure (compression) of converging plates. By way of example, if we look at regional metamorphism in areas with typical geothermal gradients, we can see that burial in the 5 kilometre to 10 kilometre range puts us in the clay mineral zone (see Figure 6.1.6), which is equivalent to the formation of slate. Briefly outline how regional metamorphism is related to plate boundaries? Beyond 25 km depth in this setting, we cross the partial melting line for granite (or gneiss) with water present, and so we can expect migmatite to form. Most other common minerals have upper limits between 150°C and 1000°C. 16. Most feldspars are stable up to between 1000°C and 1200°C. Some minerals will crystallize into different polymorphs (same composition, but different crystalline structure) depending on the temperature and pressure. As temperature increases with depth, both p and T contribute to metamorphism. Secondly, water, especially hot water, can have elevated concentrations of dissolved elements (ions), and therefore it is an important medium for moving certain elements around within the crust. CC BY. Draw and label the … Because burial to 10 km to 20 km is required, the areas affected tend to be large. Which rocks does contact metamorphism create? Also, some areas can be found locally within the C.Z. b. evidence of an … Blueschist facies indicate a. formation at high temperature and high pressure. Two settings, continent-continent collisions and continental volcanic arcs are also shown in more detail in Figure 6.1.5. Chapter 1 Introduction to Geology ics of ancient plate boundaries. In other words, if you go 1,000 m down into a mine, the temperature will be roughly 30°C warmer than the average temperature at the surface. This is commonly associated with convergent plate boundaries and the formation of mountain ranges. Metamorphic rocks formed there are likely to be foliated because of the strong directional pressure of converging plates. In situations where different blocks of the crust are being pushed in different directions, the rocks will likely be subjected to shear stress (Figure 6.1.2c). Another way to understand metamorphism is by using a diagram that shows temperature on one axis and depth (which is equivalent to pressure) on the other (Figure 7.20). A convergent boundary is also known as a destructive plate boundary due to subduction. A Practical Guide to Introductory Geology by Siobhan McGoldrick is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. Regional metamorphism also occurs along plate boundaries where an oceanic plate descends (subducts) back into the mantle as a result of plate convergence (this was discussed in the plate tectonics chapter); oceanic plates that subduct into the mantle will form a deep ocean trench, such as the trench along the western margin of South America. Regional metamorphism takes place over a much wider area. At 10 to 15 kilometres, we are in the greenschist zone (where chlorite would form in mafic volcanic rock) and very fine micas form in mudrock, to produce phyllite. In volcanic areas, the geothermal gradient is more like 40° to 50°C/km, so the temperature at 10 km depth is in the 400° to 500°C range. As described above, regional metamorphism occurs when rocks are buried deep in the crust. The various types of metamorphism described above are represented in Figure 6.1.6 with the same letters (a through e) used in Figures 6.1.4 and 6.1.5. Skip to content. So not only does water facilitate metamorphic reactions on a grain-to-grain basis, it also allows for the transportation of elements from one place to another. In other words, when a rock is subjected to increased temperatures, certain minerals may become unstable and start to recrystallize into new minerals, while remaining in a solid state. The collisions result in the formation of long mountain ranges, like those along the western coast of North America. One of the results of directed pressure and shear stress is that rocks become foliated—meaning that they’ll develop a foliation or directional fabric. 2.1 Electrons, Protons, Neutrons, and Atoms, 4.5 Monitoring Volcanoes and Predicting Eruptions, 5.3 The Products of Weathering and Erosion, Chapter 6 Sediments and Sedimentary Rocks, 6.3 Depositional Environments and Sedimentary Basins, Chapter 7 Metamorphism and Metamorphic Rocks, 7.5 Contact Metamorphism and Hydrothermal Processes, 9.1 Understanding Earth through Seismology, 10.1 Alfred Wegener — the Father of Plate Tectonics, 10.2 Global Geological Models of the Early 20th Century, 10.3 Geological Renaissance of the Mid-20th Century, 10.4 Plates, Plate Motions, and Plate-Boundary Processes, 11.5 Forecasting Earthquakes and Minimizing Damage and Casualties, 15.1 Factors That Control Slope Stability, 15.3 Preventing, Delaying, Monitoring, and Mitigating Mass Wasting, Chapter 21 Geological History of Western Canada, 21.2 Western Canada during the Precambrian, Chapter 22 The Origin of Earth and the Solar System, 22.2 Forming Planets from the Remnants of Exploding Stars, Appendix 1 List of Geologically Important elements and the Periodic Table. Are certain types of metamorphic rocks indicative of particular plate boundaries or tectonic settings? For example, if a mudstone is metamorphosed to slate and then buried deeper where it is metamorphosed to gneiss, the parent rock of the gneiss is mudstone, not slate. While rocks can be metamorphosed at depth in most areas, the potential for metamorphism is greatest in the roots of mountain ranges where there is a strong likelihood for burial of relatively young sedimentary rock to … The rock that forms in this way is known as greenstone if it isn’t foliated, or greenschist if it is. While rocks can be metamorphosed at depth in most areas, the potential for metamorphism is greatest in the roots of mountain ranges where there is a strong likelihood for burial of relatively young sedimentary rock to great depths, as depicted in Figure 6.1.5. In volcanic areas, the geothermal gradient is more like 40° to 50°C per kilometre, so the temperature at a 10 kilometre depth is in the 400° to 500°C range. Although an existing metamorphic rock can be further metamorphosed or re-metamorphosed, metamorphic rock doesn’t normally qualify as a “parent rock”. Practice Exercise 6.2 Metamorphic rocks in areas with higher geothermal gradients. It happens in a much larger area. See Appendix 2 for Practice Exercise 6.2 answers. This typical geothermal gradient is shown by the green dotted line in Figure 6.1.6. Each of these types of metamorphism produces typical metamorphic rocks, but they may … One such place is the area around San Francisco; the rock is known as the Franciscan Complex (Figure 7.18). Zeolites are silicate minerals that typically form during low-grade metamorphism of volcanic rocks. continental-continental convergent boundary. Most blueschist forms in subduction zones, continues to be subducted, turns into eclogite at about 35 km depth, and then eventually sinks deep into the mantle — never to be seen again. Because this metamorphism takes place at temperatures well below the temperature at which the rock originally formed (~1200°C), it is known as retrograde metamorphism. Regional metamorphism during the Cenozoic Era is linked to plate tectonics. Which type of plate boundary is associated with regional metamorphism? Regional or Barrovian metamorphism covers large areas of continental crust typically associated with mountain ranges. This is commonly associated with the boundaries of convergent plate and mountain range formation. Large geological processes such as mountain-building cause regional metamorphism. In other words, if you go 1,000 metres down into a mine, the temperature will be roughly 30°C warmer than the average temperature at the surface. Looking at the geothermal gradient for volcanic regions (dotted yellow line in Figure 6.1.6), estimate the depths at which you would expect to find the same types of rock forming from a mudrock protolith. Metamorphism and Plate Tectonics Metamorphic rocks result from the forces active during plate tectonic processes. Regional metamorphism. All of the important processes of metamorphism that we are familiar with can be directly related to geological processes caused by plate tectonics. In most parts of southern Canada, the average surface temperature is about 10°C, so at 1,000 m depth, it will be about 40°C. So, while the water doesn’t necessarily change the outcome of a metamorphic process, it speeds the process up so metamorphism might take place over a shorter time period, or metamorphic processes that might not otherwise have had time to be completed are completed. Divergent plate boundaries are characterized by ____. Contents. The Euro coin is 23 millimetres in diameter. the amount of time available for metamorphism. Magma is produced at convergent boundaries and rises toward the surface, where it can form magma bodies in the upper part of the crust. Such magma bodies, at temperatures of around 1000°C, heat up the surrounding rock, leading to contact metamorphism (Figure 7.19). Regional metamorphism also takes place within volcanic-arc mountain ranges, and because of the extra heat associated with the volcanism, the geothermal gradient is typically a little steeper in these settings (somewhere between 40° and 50°C/km). In most areas, the rate of increase in temperature with depth is 30°C/km. In most areas, the rate of increase in temperature with depth is 30°C per kilometre. Then, if you are even more pressure to gneiss, of would melt into igneous rocks. Chlorite ((Mg5Al)(AlSi3)O10(OH)8) and serpentine ((Mg, Fe)3Si2O5(OH)4) are both “hydrated minerals” meaning that they have water (as OH) in their chemical formulas. The movement of tectonic plates transports sediment and rocks into different geologic setting—these changes can result in metamorphism, particularly in zones where tectonic plates are converging, as in a subduction zone or where continental plates converge, pushing up high mountain ranges while material below the mountains are pushed down under increasing temperature and pressure condition. A Practical Guide to Introductory Geology, Next: 6.2 Classification of Metamorphic Rocks, Creative Commons Attribution 4.0 International License. If the pressure is higher, that upper limit will be even higher. Because the oceanic crust is typically relatively cool by the time it reaches the subduction zone, especially along its sea-floor upper surface, it does not heat up quickly, and the subducting rock remains several hundreds of degrees cooler than the surrounding mantle (Figure 6.1.5 right). Because this happens at relatively shallow depths, in the absence of directed pressure, the resulting rock does not normally develop foliation. Physical Geology by Steven Earle is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. See Appendix 2 for Practice Exercise 6.1 answers. Whereas denser oceanic crust subducts under more buoyant continental crust, with the collision of continental crust blocks, two landmasses instead collide and deform. Because of plate tectonics, pressures within the crust are typically not applied equally in all directions. Describe the three general classes of metamorphic textures, draw them, and give examples of each. When exposed to the surface, these rocks show the incredible pressure that causes the mountain building process to bend and break the rocks. Figure 6.1.6 shows the types of rock that might form from a mudrock protolith at various points along the curve of the “typical” geothermal gradient (dotted green line). Dynamic metamorphism is associated with zones of high to moderate strain such as … It occurs at: 61. divergent plate boundaries, where newly generated oceanic crust is metamorphosed following . Most regional metamorphism takes place within the continental crust. As a result higher grades of metamorphism can take place closer to surface than is the case in other areas (Figure 7.19). Name the … Exercise 7.3 Metamorphic Rocks in Areas with Higher Geothermal Gradients. Studies linking tectonic environments to types of metamorphic rocks, with key examples from the Pacific Rim and Alpine regions, were published as plate tectonic theory became widely accepted (e.g., Miyashiro, 1967, 1973; Ernst, 1971). A special type of metamorphism takes place under these very high-pressure but relatively low-temperature conditions, producing an amphibole mineral known as glaucophane (Na2(Mg3Al2)Si8O22(OH)2), which is blue in colour, and is an important component of a rock known as blueschist. Most regional metamorphism takes place within continental crust. Contact metamorphism is common at both convergent and divergent plate boundaries, in areas where molten rock is produced. Nevertheless, the cleavage front and the front of regional metamorphism can be found near its western and southern boundaries, in the transition to the more internal parts of the orogen and in relation with the early stages of deformation. The temperature that the rock is subjected to is a key variable in controlling the type of metamorphism that takes place. That’s uncomfortably hot, so deep mines must have effective ventilation systems. quartzite, hornfels, marble . The three heavy dotted lines on this diagram represent Earth’s geothermal gradients under different conditions. Considering that the normal geothermal gradient (the rate of increase in temperature with depth) is around 30°C per kilometre, rock buried to 9 kilometres below sea level in this situation could be close to 18 kilometres below the surface of the ground, and it is reasonable to expect temperatures up to 500°C. Pressure is important in metamorphic processes for two main reasons. Because burial is required from 10 km to 20 km, the affected areas tend to be large. Second, it has implications for the texture of metamorphic rocks. What is a little surprising is that anyone has seen it! This type of metamorphism occurs with rocks that are buried deep down the Earth’s crust. Figure 7.20 shows the types of rock that might form from mudrock at various points along the curve of the “typical” geothermal gradient (dotted green line). REGIONAL METAMORPHISM: Instead of from heat, the key catalyst for regional metamorphism is mostly from pressure. An example would be the Himalayan Range. Toggle Menu. The three heavy dotted lines on this diagram represent Earth’s geothermal gradients under different conditions. regional metamorphism takes place within the continental crust. Contact processes work by raising the local temperature and producing hornfels. Metamorphism occurs along a more-or-less stable geothermal gradient; the resulting metamorphic mineral assemblages are characterized by low recrystallization temperatures and an absence o… Most regional metamorphism takes place within continental crust. At an oceanic spreading ridge, recently formed oceanic crust of gabbro and basalt is slowly moving away from the plate boundary (Figure 7.16). In only a few places in the world, where the subduction process has been interrupted by some other tectonic process, has partially subducted blueschist rock returned to the surface. This is commonly associated with convergent plate boundaries and the formation of mountain ranges. What are the defining features of metamorphic textures? At 15 km to 20 km, larger micas form to produce schist, and at 20 km to 25 km amphibole, feldspar, and quartz form to produce gneiss. How do these factors differ across an area affected by regional metamorphism (e.g., a continent-continent plate boundary) List and describe examples of index minerals for low, medium, and high grade metamorphism. At a subduction zone, oceanic crust is forced down into the hot mantle. zones of regional metamorphism. How do slaty cleavage, schistosity, and gneissic textures differ from each … When metamorphosed ocean crust is later subducted, the chlorite and serpentine are converted into new non-hydrous minerals (e.g., garnet and pyroxene) and the water that is released migrates into the overlying mantle, where it contributes to flux melting (Chapter 3, section 3.2). belts at convergent plate boundaries Hikaru Iwamori Department of Earth and Planetary Sciences, University of Tokyo, Tokyo, Japan Received 2 February 2002; revised 31 December 2002; accepted 25 February 2003; published 28 June 2003. A mountain range takes tens of millions of years to form, and tens of millions of years more to be eroded to the extent that we can see the rocks that were metamorphosed deep beneath it. Regional metamorphism, as its name suggests, works over much larger areas. Blueschists are created in the subduction zone and ultra-high pressure metamorphic (UHPM) rocks are created in collision zones due to deep subduction of continental lithosphere; granulites are created deep under continental and oceanic plateaus and in arcs and collision zones [high-pressure (HP) granulites, ultra … A sheet silicate mineral (e.g., biotite). Most regional metamorphism takes place within the continental crust. The collision of plates, subduction, and the sliding of plates along transform faults create differential stress, friction, shearing, compressive stress, folding, faulting, and increased heat flow. Commonly, they show evidence of having been deformed and metamorphosed at great depth in the crust. The critical feature of the parent rock is its mineral composition because it is the stability of minerals that counts when metamorphism takes place. Regional metamorphism is a type of metamorphism where the formation of a metamorphic rock occurs in a wide area. The passage of this water through the oceanic crust at 200° to 300°C promotes metamorphic reactions that change the original pyroxene in the rock to chlorite and serpentine. Sedimentary or igneous rocks can be considered the parent rocks for metamorphic rocks. the temperature at which metamorphism takes place. Most blueschist forms in subduction zones, continues to be subducted, turns into eclogite at about 35 kilometres depth, and then eventually sinks deep into the mantle—never to be seen again because that rock will eventually melt. Based on the approximate average diameter of the garnets visible, estimate how long this metamorphic process might have taken. Rocks that are subjected to very high confining pressures are typically denser than others because the mineral grains are squeezed together (Figure 6.1.2a), and also because they may contain minerals that have greater density because the atoms are more closely packed. On the other hand, most clay minerals are only stable up to about 150° or 200°C; above that, they transform into micas. At 15 to 20 kilometres, larger micas form to produce schist, and at 20 to 25 kilometres amphibole, feldspar, and quartz form to produce gneiss. In only a few places in the world, where the subduction process has been interrupted by some tectonic process, has partially subducted blueschist rock returned to the surface. Metamorphism affecting a large area or regional metamorphism involves large increases of temperature and pressure. Looking at the geothermal gradient for volcanic regions (dotted yellow line in Figure 7.20), estimate the depths at which you would expect to find the same types of rock forming from a mudrock parent. Regional Metamorphism - no discernible source of heat (no nearby magma chamber, for example) - with increasing depth the temperature and pressure increase. a. hydrothermal alteration and contact metamorphism b. regional and contact metamorphism c. regional and dynamic metamorphism d. dynamic and contact metamorphism e. hydrothermal alteration and dynamic metamorphism. Metamorphism can also take place if cold rock near the surface is intruded and heated by a hot igneous body. Regional metamorphism is associated with the major events of Earth dynamics, and the vast majority of metamorphic rocks are so produced.They are the rocks involved in the cyclic processes of erosion, sedimentation, burial, metamorphism, and mountain building (), events that are all related to major convective processes in Earth’s mantle. There are relatively few terrains for which any investigation of the source of the heat for regional metamorphism has been made (Richardson and Powell, 1976), and, on theoretical and observational grounds, sources internal and ex¬ ternal to the metamorphic pile would both appear possible in appropriate areas. One such place is the area around San Francisco; the rock is known as the Franciscan Complex. At this continent-continent convergent boundary, sedimentary rocks have been both thrust up to great heights (nearly 9,000 metres above sea level) and also buried to great depths. The conditions under which they were metamorphosed are those of regional metamorphism. https://courses.lumenlearning.com/earthscience/chapter/metamorphic-rocks The presence of water is important for two main reasons. For example, one important metamorphic setting is many kilometres deep within the roots of mountain ranges. By way of example, if we look at regional metamorphism in areas with typical geothermal gradients, we can see that burial in the 5 km to 10 km range puts us in the zeolite[1] and clay mineral zone (see Figure 7.20), which is equivalent to the formation of slate. The relationships between plate tectonics and metamorphism are summarized in Figure 6.1.4. This is very important in hydrothermal processes, and in the formation of mineral deposits. A special type of metamorphism takes place under these very high-pressure but relatively low-temperature conditions, producing an amphibole mineral known as glaucophane (Na2(Mg3Al2)Si8O22(OH)2), which is blue in colour, and is a major component of a rock known as blueschist. First, water facilitates the transfer of ions between minerals and within minerals, and therefore increases the rates at which metamorphic reactions take place. Each type of metamorphism generates distinct rock types. Contact metamorphism is a result of the temperature increase caused by the intrusion of magma into cooler country rock. All of the important processes of metamorphism that we are familiar with can be directly related to geological processes caused by plate tectonics. Metamorphism also occurs at subduction zones, where oceanic crust is forced down into the hot mantle. Regional metamorphism occurs over wide areas, affects large volumes of rocks, and is associated with tectonic processes such as plate collision and crustal thickening (orogenic metamorphism) and ocean-floor spreading (ocean-floor metamorphism). Along subduction zones, as described above, the cold oceanic crust keeps temperatures low, so the gradient is typically less than 10°C per kilometre. As we learned in the context of igneous rocks, mineral stability is a function of temperature, pressure, and the presence of fluids (especially water). Considering that the normal geothermal gradient (the rate of increase in temperature with depth) is around 30°C per kilometre, rock buried to 9 km below sea level in this situation could be close to 18 km below the surface of the ground, and it is reasonable to expect temperatures up to 500°C. Metamorphic index minerals are used by geologists to distinguish among different _____. the amount and type of pressure during metamorphism, the types of fluids (mostly water) that are present during metamorphism, and. Water within the crust is forced to rise in the area close to the source of volcanic heat, and this draws more water in from farther out, which eventually creates a convective system where cold seawater is drawn into the crust and then out again onto the sea floor near the ridge. 1.2 Plates, Plate Motions, and Plate Boundaries, Lab 2: Mineral Properties and Non-Silicate Minerals, 5.2 The Products of Weathering and Erosion, 5.5 Depositional Environments and Sedimentary Basins, Lab 6: Metamorphic Rocks and the Rock Cycle, Lab 7: Relative Dating and Geological Time, 9.3 Estimating Dip Direction from a Geological Map, Appendix 1: List of Geologically Important Elements and the Periodic Table, Appendix 2: Answers to Practice Exercises. the transformation of a parent rock into a new rock as a result of heat and pressure that leads to the formation of new minerals, or recrystallization of existing minerals, without melting, the original, un-metamorphosed parent rock from which a given metamorphic rock is formed. Home; Read; Sign in; Search in book: Search Generally, this metamorphism technique is associated with plate boundaries and formation of mountains ranges. Along subduction zones, as described above, the cold oceanic crust keeps temperatures low, so the gradient is typically less than 10°C/km. Typically, a regionally metamorphosed area is situated under a fold/thrust mountain range or along a boundary between tectonic plates. Creative Commons Attribution 4.0 International License. At a 10 kilometre depth, the temperature is about 300°C and at 20 kilometres it’s about 600°C. A. 1. Assume that the diameters of the garnets increased at a rate of 1 millimetre per million years. Metamorphic rocks typically have different mineral assemblages and different textures from their parent rocks, or protoliths, but they may have the same overall chemical composition. In areas of plate convergence, for example, the pressure in one direction (perpendicular to the direction of convergence) is typically greater than in the other directions (Figure 6.1.2b). the mineral composition of the protolith. First, it has implications for mineral stability (Figure 6.1.1). An example would be the Himalayan Range. The minerals kyanite, andalusite, and sillimanite are polymorphs with the composition Al2SiO5. Each pair consists of one belt with a low-temperature, high-pressure metamorphic mineral assemblage, and another characterized by high-temperature, low-pressure metamorphic minerals. But because the oceanic crust is now relatively cool, especially along its sea-floor upper surface, it does not heat up quickly, and the subducting rock remains several hundreds of degrees cooler than the surrounding mantle (Figure 7.17). The zone of contact metamorphism around an intrusion is very small (typically metres to tens of metres) compared with the extent of regional metamorphism in other settings (tens of thousands of square kilometres). The type of plate boundary that regional metamorphism is associated with convergent plate boundaries. Beyond a depth of 25 kilometres in this setting, we cross the partial melting line for granite (or gneiss) with water present, and so we can expect migmatite to form. The relationships between plate tectonics and metamorphism are summarized in Figure 7.14, and in more detail in Figures 7.15, 7.16, 7.17, and 7.19.
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