4. regional metamorphism:results from mountain building and plate tectonic collisions. 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). This typical geothermal gradient is shown by the green dotted line in Figure 7.20. 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). Practice Exercise 6.2 Metamorphic rocks in areas with higher geothermal gradients. Chapter 1 Introduction to Geology 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. An example would be the Himalayan Range. 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). Most regional metamorphism takes place within 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. Home; Read; Sign in; Search in book: Search All of the important processes of metamorphism that we are familiar with can be directly related to geological processes caused by plate tectonics. This is commonly associated with convergent plate boundaries and the formation of mountain ranges. For this reason, it is very difficult to study metamorphic processes in a lab. 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. Briefly outline how regional metamorphism is related to plate boundaries? Because burial to 10 km to 20 km is required, the areas affected tend to be large. Magma is produced at convergent boundaries and rises toward the surface, where it can form magma bodies in the upper part of the crust. Along subduction zones, as described above, the cold oceanic crust keeps temperatures low, so the gradient is typically less than 10°C per kilometre. 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). Contents. Comedians in Cars Getting Coffee: "Just Tell Him You’re The President” (Season 7, Episode 1) - Duration: 19:16. blacktreetv Recommended for you The minerals kyanite, andalusite, and sillimanite are polymorphs with the composition Al2SiO5. 1. Which type of plate boundary is associated with regional metamorphism? Large geological processes such as mountain-building cause regional metamorphism. Assume that the diameters of the garnets increased at a rate of 1 millimetre per million years. A. Contact metamorphism is a result of the temperature increase caused by the intrusion of magma into cooler country rock. If the pressure is higher, that upper limit will be even higher. All minerals are stable over a specific range of temperatures. 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. Water is the main fluid present within rocks of the crust, and the only one that we’ll consider here. 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). The conditions under which they were metamorphosed are those of regional metamorphism. This metamorphism creates rocks like gneiss and schist. 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. REGIONAL METAMORPHISM: Instead of from heat, the key catalyst for regional metamorphism is mostly from pressure. 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. 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. 16. 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. As a result higher grades of metamorphism can take place closer to surface than is the case in other areas (Figure 7.19). 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. https://courses.lumenlearning.com/earthscience/chapter/metamorphic-rocks Preface; Acknowledgments; Acknowledgements: eCampusOntario; I.Main Body. Two settings, continent-continent collisions and continental volcanic arcs are also shown in more detail in Figure 6.1.5. Because this happens at relatively shallow depths, in the absence of directed pressure, the resulting rock does not normally develop foliation. All of the important processes of metamorphism can be understood in the context of geological processes related to plate tectonics. Describe the three general classes of metamorphic textures, draw them, and give examples of each. Creative Commons Attribution 4.0 International License. One such place is the area around San Francisco; the rock is known as the Franciscan Complex. Foliation is a very important aspect of metamorphic rocks, and is described in more detail later in this chapter. 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. 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. What is surprising is that anyone has seen it! 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. That’s uncomfortably hot, so deep mines must have effective ventilation systems. Contact metamorphism is common at both convergent and divergent plate boundaries, in areas where molten rock is produced. the mineral composition of the protolith. Which rocks does contact metamorphism create? At an oceanic spreading ridge, recently formed oceanic crust of gabbro and basalt is slowly moving away from the plate boundary (Figure 7.16). 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. CC BY. 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. b. evidence of an … The rate of increase of temperature with depth in the Earth (typically around 30˚ C/km within the crust). The relationships between plate tectonics and metamorphism are summarized in Figure 6.1.4. 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 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. 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. On the other hand, most clay minerals are only stable up to about 150° or 200°C; above that, they transform into micas. Metamorphism through plate tectonics ... dynamic and regional. 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). Burial metamorphism mostly affects sedimentary strata in sedimentary basins as a result of compaction due to burial of sediments by overlying sediments. Such magma bodies, at temperatures of around 1000°C, heat up the surrounding rock, leading to contact metamorphism (Figure 7.19). Whereas denser oceanic crust subducts under more buoyant continental crust, with the collision of continental crust blocks, two landmasses instead collide and deform. 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… 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). 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. Metamorphism also occurs at subduction zones, where oceanic crust is forced down into the hot mantle. The type of plate boundary that regional metamorphism is associated with convergent plate boundaries. First, it has implications for mineral stability (Figure 6.1.1). 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 … For example, when there are two convergent plates pushing together, there will be immense pressure at the fault in between. Second, it has implications for the texture of metamorphic rocks. 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 … 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). This is very important in hydrothermal processes, and in the formation of mineral deposits. Characterized by strong directed pressure and increased temperature due to increased burial. The critical feature of the parent rock is its mineral composition because it is the stability of minerals that counts when metamorphism takes place. Name the … As temperature increases with depth, both p and T contribute to metamorphism. Generally, this metamorphism technique is associated with plate boundaries and formation of mountains ranges. 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. For example, one important metamorphic setting is many kilometres deep within the roots of mountain ranges. Contact processes work by raising the local temperature and producing hornfels. Based on the approximate average diameter of the garnets visible, estimate how long this metamorphic process might have taken. The collisions result in the formation of long mountain ranges, like those along the western coast of North America. 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). How do slaty cleavage, schistosity, and gneissic textures differ from each … See Appendix 2 for Practice Exercise 6.2 answers. ics of ancient plate boundaries. Because burial is required from 10 km to 20 km, the affected areas tend to be large. Each of these types of metamorphism produces typical metamorphic rocks, but they may … 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. 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). When rocks are buried deep in the crust, regional metamorphism occurs. It occurs at: 61. divergent plate boundaries, where newly generated oceanic crust is metamorphosed following . Physical Geology by Steven Earle is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. the amount and type of pressure during metamorphism, the types of fluids (mostly water) that are present during metamorphism, and. Give three examples of such rocks and indicate the tectonic environment they represent? In most areas, the rate of increase in temperature with depth is 30°C/km. 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. Blueschist facies indicate a. formation at high temperature and high pressure. 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. 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 large reddish crystals are garnet, and the surrounding light coloured rock is dominated by muscovite mica. Regional metamorphism largely occurs at convergent plate boundaries. First, water facilitates the transfer of ions between minerals and within minerals, and therefore increases the rates at which metamorphic reactions take place. Because burial to 10 to 20 kilometers is required, the areas affected tend … 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. 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.
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