Because burial is required from 10 km to 20 km, the affected areas tend to be large. Because this happens at relatively shallow depths, in the absence of directed pressure, the resulting rock does not normally develop foliation. 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. Figure – Regional metamorphism is often associated with a continental collision where rocks are squeezed between two converging plates, resulting in mountain building. Commonly, they show evidence of having been deformed and metamorphosed at great depth in 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. This is very important in hydrothermal processes, and in the formation of mineral deposits. 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. 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. 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. First, water facilitates the transfer of ions between minerals and within minerals, and therefore increases the rates at which metamorphic reactions take place. A Practical Guide to Introductory Geology, Next: 6.2 Classification of Metamorphic Rocks, Creative Commons Attribution 4.0 International License. As temperature increases with depth, both p and T contribute to metamorphism. Regional metamorphism is a type of metamorphism where the formation of a metamorphic rock occurs in a wide area. At a 10 kilometre depth, the temperature is about 300°C and at 20 kilometres it’s about 600°C. Metamorphism and Plate Tectonics Metamorphic rocks result from the forces active during plate tectonic processes. 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. What is surprising is that anyone has seen it! A Practical Guide to Introductory Geology by Siobhan McGoldrick is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. The rock that forms in this way is known as greenstone if it isn’t foliated, or greenschist if it is. 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. Typically, a regionally metamorphosed area is situated under a fold/thrust mountain range or along a boundary between tectonic plates. If the pressure is higher, that upper limit will be even higher. the amount of time available for metamorphism. First, it has implications for mineral stability (Figure 6.1.1). This is commonly associated with convergent plate boundaries and the formation of mountain ranges. How do slaty cleavage, schistosity, and gneissic textures differ from each … A sheet silicate mineral (e.g., biotite). Sedimentary or igneous rocks can be considered the parent rocks for metamorphic rocks. zones of regional metamorphism. Second, it has implications for the texture of metamorphic rocks. Foliation is a very important aspect of metamorphic rocks, and is described in more detail later in this chapter. 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). Keywords Orogenic Belt Pression Relativement Marked Contrast Pressure Environment Systematic Increase These keywords were added by machine and not by the authors. 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. 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. Although an existing metamorphic rock can be further metamorphosed or re-metamorphosed, metamorphic rock doesn’t normally qualify as a “parent rock”. 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. CC BY. 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. 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. 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. 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. On the other hand, most clay minerals are only stable up to about 150° or 200°C; above that, they transform into micas. 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. Most metamorphism results from the burial of igneous, sedimentary, or pre-existing metamorphic rocks to the point where they experience different pressures and temperatures than those at which they formed. Dynamic metamorphism is associated with zones of high to moderate strain such as … 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). 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. In most areas, the rate of increase in temperature with depth is 30°C per kilometre. Zeolites are silicate minerals that typically form during low-grade metamorphism of volcanic rocks. While the rate of metamorphism is slow, the tectonic processes that lead to metamorphism are also very slow, so in most cases, the chance for metamorphic reactions to be completed is high. The three heavy dotted lines on this diagram represent Earth’s geothermal gradients under different conditions. Name the … 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. 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. The critical feature of the parent rock is its mineral composition because it is the stability of minerals that counts when metamorphism takes place. 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. Along subduction zones, as described above, the cold oceanic crust keeps temperatures low, so the gradient is typically less than 10°C per kilometre. A convergent boundary is also known as a destructive plate boundary due to subduction. 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. This typical geothermal gradient is shown by the green dotted line in Figure 7.20. 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. 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. Chapter 1 Introduction to Geology Figures 6.1.1, 6.1.2, 6.1.4, 6.1.5, 6.1.6: © Steven Earle. Physical Geology by Steven Earle is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. One such place is the area around San Francisco; the rock is known as the Franciscan Complex (Figure 7.18). Based on the approximate average diameter of the garnets visible, estimate how long this metamorphic process might have taken. Because of plate tectonics, pressures within the crust are typically not applied equally in all directions. Metamorphism can also take place if cold rock near the surface is intruded and heated by a hot igneous body. See Appendix 2 for Practice Exercise 6.2 answers. 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. Most regional metamorphism takes place within the continental crust. You’ve probably never seen or even heard of blueschist; that’s not surprising. Large geological processes such as mountain-building cause regional metamorphism. Regional Metamorphism - no discernible source of heat (no nearby magma chamber, for example) - with increasing depth the temperature and pressure increase. The three heavy dotted lines on this diagram represent Earth’s geothermal gradients under different conditions. 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. 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. For example, one important metamorphic setting is many kilometres deep within the roots of mountain ranges. Because burial to 10 km to 20 km is required, the areas affected tend to be large. The rate of increase of temperature with depth in the Earth (typically around 30˚ C/km within the crust). 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). The main factors that control metamorphic processes are: The protolith, or “parent rock”, is the rock that exists before metamorphism starts. When exposed to the surface, these rocks show the incredible pressure that causes the mountain building process to bend and break the rocks. 4. regional metamorphism:results from mountain building and plate tectonic collisions. At an oceanic spreading ridge, recently formed oceanic crust of gabbro and basalt is slowly moving away from the plate boundary (Figure 7.16). 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). 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. the temperature at which metamorphism takes place. 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 … Metamorphism also occurs at subduction zones, where oceanic crust is forced down into the hot mantle. Preface; Acknowledgments; Acknowledgements: eCampusOntario; I.Main Body. Metamorphic rocks formed there are likely to be foliated because of the strong directional pressure (compression) of converging plates. Contact metamorphism is a result of the temperature increase caused by the intrusion of magma into cooler country rock. 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. The presence of water is important for two main reasons. 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. Practice Exercise 6.2 Metamorphic rocks in areas with higher geothermal gradients. Most regional metamorphism takes place within continental crust. quartzite, hornfels, marble . This type of metamorphism occurs with rocks that are buried deep down the Earth’s crust. regional metamorphism takes place within the continental crust. 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. 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… 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). Most regional metamorphism takes place within continental crust. Results in foliated rocks (convergent plate boundary) Metamorphic rocks are classified basesd on their texture and composition. 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 during the Cenozoic Era is linked to plate tectonics. At a subduction zone, oceanic crust is forced down into the hot mantle. A. Regional metamorphism refers to large-scale metamorphism, such as what happens to continental crust along convergent tectonic margins (where plates collide). This photo shows a sample of garnet-mica schist from the Greek island of Syros. Because burial to 10 to 20 kilometers is required, the areas affected tend … Exercise 7.3 Metamorphic Rocks in Areas with Higher Geothermal Gradients. 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). Characterized by strong directed pressure and increased temperature due to increased burial. This metamorphism creates rocks like gneiss and schist. 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. 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). All of the important processes of metamorphism can be understood in the context of geological processes related to plate tectonics. Are certain types of metamorphic rocks indicative of particular plate boundaries or tectonic settings? Each of these types of metamorphism produces typical metamorphic rocks, but they may … Skip to content. The various types of metamorphism described above are represented in Figure 7.20 with the same letters (a through e) used in Figures 7.14 to 7.17 and 7.19. 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. Blueschist facies indicate a. formation at high temperature and high pressure. 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. The conditions under which they were metamorphosed are those of regional metamorphism. Home; Read; Sign in; Search in book: Search Metamorphism affecting a large area or regional metamorphism involves large increases of temperature and pressure. REGIONAL METAMORPHISM: Instead of from heat, the key catalyst for regional metamorphism is mostly from pressure. The relationships between plate tectonics and metamorphism are summarized in Figure 6.1.4. The deeper rocks are within the stack, the higher the pre… 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. 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. the amount and type of pressure during metamorphism, the types of fluids (mostly water) that are present during metamorphism, and. 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. That’s uncomfortably hot, so deep mines must have effective ventilation systems. If you’ve never seen or even heard of blueschist, it’s not surprising. All of the important processes of metamorphism that we are familiar with can be directly related to geological processes caused by plate tectonics. One such place is the area around San Francisco; the rock is known as the Franciscan Complex. 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. As a result higher grades of metamorphism can take place closer to surface than is the case in other areas (Figure 7.19). It occurs at: 61. divergent plate boundaries, where newly generated oceanic crust is metamorphosed following . 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. As described above, regional metamorphism occurs when rocks are buried deep in the crust. Such magma bodies, at temperatures of around 1000°C, heat up the surrounding rock, leading to contact metamorphism (Figure 7.19). It happens in a much larger area. The large reddish crystals are garnet, and the surrounding light coloured rock is dominated by muscovite mica. 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. Divergent plate boundaries are characterized by ____. two or more minerals with the same chemical formula but different crystal structures, the texture of a metamorphic rock with a foliation, metamorphism caused by burial of the parent rock to depths greater than 5 kilometres (typically takes place beneath mountain ranges, and extends over areas of hundreds of km2). 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. This is commonly associated with convergent plate boundaries and the formation of mountain ranges. 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2020 regional metamorphism and plate boundaries