regional metamorphism and plate boundaries
Metamorphism is the change that takes place within a body of rock as a result of it being subjected to conditions that are different from those in which it formed. In most areas, the rate of increase in temperature with depth is 30°C per kilometre. When exposed to the surface, these rocks show the incredible pressure that causes the mountain building process to bend and break the 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. Contents. 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. 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). 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. Although most metamorphism involves temperatures above 150°C, some metamorphism takes place at temperatures lower than those at which the parent rock formed. Metamorphism also occurs at subduction zones, where oceanic crust is forced down into the hot mantle. Results in foliated rocks (convergent plate boundary) Metamorphic rocks are classified basesd on their texture and composition. Regional metamorphism. Metamorphism and Plate Tectonics Metamorphic rocks result from the forces active during plate tectonic processes. If there is water present, it will be lower. 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. 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. 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. 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). Contact metamorphism is a result of the temperature increase caused by the intrusion of magma into cooler country rock. Which type of plate boundary is associated with regional metamorphism? At an oceanic spreading ridge, recently formed oceanic crust of gabbro and basalt is slowly moving away from the plate boundary (Figure 7.16). 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. Most regional metamorphism takes place within the continental crust. A Practical Guide to Introductory Geology by Siobhan McGoldrick is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. Most feldspars are stable up to between 1000°C and 1200°C. https://courses.lumenlearning.com/earthscience/chapter/metamorphic-rocks b. evidence of an … That’s uncomfortably hot, so deep mines must have effective ventilation systems. Regional metamorphism: We find metamorphic rocks exposed over regions of the Earth's surface, either in the cores of mountain belts or the roots of what were once mountain belts. Contact processes work by raising the local temperature and producing hornfels. What is a little surprising is that anyone has seen it! Then, if you are even more pressure to gneiss, of would melt into igneous rocks. This is commonly associated with convergent plate boundaries and the formation of mountain ranges. Regional metamorphism refers to large-scale metamorphism, such as what happens to continental crust along convergent tectonic margins (where plates collide). 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. Characterized by strong directed pressure and increased temperature due to increased burial. Magma is produced at convergent boundaries and rises toward the surface, where it can form magma bodies in the upper part of the crust. Skip to content. The collisions result in the formation of long mountain ranges, like those along the western coast of North America. A. Dynamic metamorphism is associated with zones of high to moderate strain such as … As we learned in the context of igneous rocks, mineral stability is a function of temperature, pressure, and the presence of fluids (especially water). Along subduction zones, as described above, the cold oceanic crust keeps temperatures low, so the gradient is typically less than 10°C/km. Metamorphism affecting a large area or regional metamorphism involves large increases of temperature and pressure. 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 7.15. 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). Along subduction zones, as described above, the cold oceanic crust keeps temperatures low, so the gradient is typically less than 10°C per kilometre. 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). This is very important in hydrothermal processes, and in the formation of mineral deposits. One such place is the area around San Francisco; the rock is known as the Franciscan Complex (Figure 7.18). 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). You’ve probably never seen or even heard of blueschist; that’s not surprising. Regional Metamorphism - no discernible source of heat (no nearby magma chamber, for example) - with increasing depth the temperature and pressure increase. Paired metamorphic belts are sets of parallel linear rock units that display contrasting metamorphic mineral assemblages.These paired belts develop along convergent plate boundaries where subduction is active. Physical Geology by Steven Earle is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. The minerals kyanite, andalusite, and sillimanite are polymorphs with the composition Al2SiO5. One such place is the area around San Francisco; the rock is known as the Franciscan Complex. Large geological processes such as mountain-building cause regional metamorphism. 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. As temperature increases with depth, both p and T contribute to metamorphism. 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 … Preface; Acknowledgments; Acknowledgements: eCampusOntario; I.Main Body. As described above, regional metamorphism occurs when rocks are buried deep in the crust. Comedians in Cars Getting Coffee: "Just Tell Him You’re The President” (Season 7, Episode 1) - Duration: 19:16. blacktreetv Recommended for you Generally, this metamorphism technique is associated with plate boundaries and formation of mountains ranges. REGIONAL METAMORPHISM: Instead of from heat, the key catalyst for regional metamorphism is mostly from pressure. 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. When rocks are buried deep in the crust, regional metamorphism occurs. For example, the growth of new minerals within a rock during metamorphism has been estimated to be about 1 millimetre per million years. Most regional metamorphism takes place within continental crust. 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. (southern part of the Central Coal Basin and Pisuerga- 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. Regional or Barrovian metamorphism covers large areas of continental crust typically associated with mountain ranges. Give three examples of such rocks and indicate the tectonic environment they represent? zones of regional metamorphism. The presence of water is important for two main reasons. Regional metamorphism is a type of metamorphism where the formation of a metamorphic rock occurs in a wide area. 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. Such magma bodies, at temperatures of around 1000°C, heat up the surrounding rock, leading to contact metamorphism (Figure 7.19). Draw and label the … All of the important processes of metamorphism that we are familiar with can be directly related to geological processes caused by plate tectonics. All of the important processes of metamorphism can be understood in the context of geological processes related to plate tectonics. 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. Commonly, they show evidence of having been deformed and metamorphosed at great depth in the crust. 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). 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). Each of these types of metamorphism produces typical metamorphic rocks, but they may … See Appendix 2 for Practice Exercise 6.2 answers. As a result higher grades of metamorphism can take place closer to surface than is the case in other areas (Figure 7.19). 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… 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). Figure – Regional metamorphism is often associated with a continental collision where rocks are squeezed between two converging plates, resulting in mountain building. Regional metamorphism takes place over a much wider area. Because burial to 10 km to 20 km is required, the areas affected tend to be large. Briefly outline how regional metamorphism is related to plate boundaries? 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). A sheet silicate mineral (e.g., biotite). quartzite, hornfels, marble . Figures 6.1.1, 6.1.2, 6.1.4, 6.1.5, 6.1.6: © Steven Earle. First, it has implications for mineral stability (Figure 6.1.1). First, water facilitates the transfer of ions between minerals and within minerals, and therefore increases the rates at which metamorphic reactions take place. Although an existing metamorphic rock can be further metamorphosed or re-metamorphosed, metamorphic rock doesn’t normally qualify as a “parent rock”. All of the important processes of metamorphism that we are familiar with can be directly related to geological processes caused by plate tectonics. 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. The three heavy dotted lines on this diagram represent Earth’s geothermal gradients under different conditions. 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. This type of metamorphism occurs with rocks that are buried deep down the Earth’s crust. This typical geothermal gradient is shown by the green dotted line in Figure 6.1.6. The deeper rocks are within the stack, the higher the pre… 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. In late Precambrian belts regional metamorphism was more frequently of intermediate pressure type, the paired metamorphic belt regime not being recognised. Sedimentary or igneous rocks can be considered the parent rocks for metamorphic rocks. 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. 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. 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. 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 7.3.2. 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. This is commonly associated with convergent plate boundaries and the formation of mountain ranges. Most regional metamorphism takes place within continental crust. Second, it has implications for the texture of metamorphic rocks. 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. the mineral composition of the protolith. 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. 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. This photo shows a sample of garnet-mica schist from the Greek island of Syros. In most cases—but not all—this involves the rock being deeply buried beneath other rocks, where it is subjected to higher temperatures and pressures than those under which it formed. continental-continental convergent boundary. For example, one important metamorphic setting is many kilometres deep within the roots of mountain ranges. 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. 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 and clay mineral zone (see Figure 7.20), which is equivalent to the formation of slate. At 10 km depth, the temperature is about 300°C and at 20 km it’s about 600°C. 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. 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. For this reason, it is very difficult to study metamorphic processes in a lab. 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. Most metamorphic reactions take place at very slow rates. Name the … Because burial to 10 to 20 kilometers is required, the areas affected tend … 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 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). On the other hand, most clay minerals are only stable up to about 150° or 200°C; above that, they transform into micas. Regional metamorphism occurs when rocks are buried deep in the crust. At a 10 kilometre depth, the temperature is about 300°C and at 20 kilometres it’s about 600°C. Because burial is required from 10 km to 20 km, the affected areas tend to be large. Whereas denser oceanic crust subducts under more buoyant continental crust, with the collision of continental crust blocks, two landmasses instead collide and deform. Each type of metamorphism generates distinct rock types. That’s uncomfortably hot, so deep mines must have effective ventilation systems. The large reddish crystals are garnet, and the surrounding light coloured rock is dominated by muscovite mica. 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. They are stable at different pressures and temperatures, and, as we will see later, they are important indicators of the pressures and temperatures that existed during the formation of metamorphic rocks (Figure 6.1.1). Water is the main fluid present within rocks of the crust, and the only one that we’ll consider here. A convergent boundary is also known as a destructive plate boundary due to subduction. This is commonly associated with the boundaries of convergent plate and mountain range formation. 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 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. The relationships between plate tectonics and metamorphism are summarized in Figure 6.1.4. See Appendix 2 for Practice Exercise 6.1 answers. The main factors that control metamorphic processes are: The protolith, or “parent rock”, is the rock that exists before metamorphism starts. 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. Foliation is a very important aspect of metamorphic rocks, and is described in more detail later in this chapter. 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. 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). On modern Earth, regional metamorphism occurs in plate boundary zones. CC BY. It occurs at: 61. divergent plate boundaries, where newly generated oceanic crust is metamorphosed following . Because this happens at relatively shallow depths, in the absence of directed pressure, the resulting rock does not normally develop foliation. 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. Based on the approximate average diameter of the garnets visible, estimate how long this metamorphic process might have taken. 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 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. Chapter 1 Introduction to Geology 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. 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. 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. 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. Metamorphic rocks formed there are likely to be foliated because of the strong directional pressure (compression) of converging plates. 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. Also, some areas can be found locally within the C.Z. Regional metamorphism during the Cenozoic Era is linked to plate tectonics. Describe the three general classes of metamorphic textures, draw them, and give examples of each. Some minerals will crystallize into different polymorphs (same composition, but different crystalline structure) depending on the temperature and pressure. 4. regional metamorphism:results from mountain building and plate tectonic collisions. Divergent plate boundaries are characterized by ____. Metamorphism through plate tectonics ... dynamic and regional. Keywords Orogenic Belt Pression Relativement Marked Contrast Pressure Environment Systematic Increase These keywords were added by machine and not by the authors. Practice Exercise 6.2 Metamorphic rocks in areas with higher geothermal gradients. Metamorphic index minerals are used by geologists to distinguish among different _____. Burial metamorphism mostly affects sedimentary strata in sedimentary basins as a result of compaction due to burial of sediments by overlying sediments. A Practical Guide to Introductory Geology, Next: 6.2 Classification of Metamorphic Rocks, Creative Commons Attribution 4.0 International License. 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). Blueschist facies indicate a. formation at high temperature and high pressure. 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. Which rocks does contact metamorphism create? 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 … At a subduction zone, oceanic crust is forced down into the hot mantle. the temperature at which metamorphism takes place. At this continent-continent convergent boundary, sedimentary rocks have been both thrust up to great heights (nearly 9,000 m above sea level) and also buried to great depths. The conditions under which they were metamorphosed are those of regional metamorphism. Zeolites are silicate minerals that typically form during low-grade metamorphism of volcanic rocks. Toggle Menu. Most regional metamorphism takes place within the continental crust.
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