This type of metamorphism occurs with rocks that are buried deep down the Earth’s crust. 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). All of the important processes of metamorphism that we are familiar with can be directly related to geological processes caused by plate tectonics. Which type of plate boundary is associated with regional metamorphism? continental-continental convergent boundary. Although most metamorphism involves temperatures above 150°C, some metamorphism takes place at temperatures lower than those at which the parent rock formed. 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. 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. 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. 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. 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). One such place is the area around San Francisco; the rock is known as the Franciscan Complex (Figure 7.18). At a subduction zone, oceanic crust is forced down into the hot mantle. Two settings, continent-continent collisions and continental volcanic arcs are also shown in more detail in Figure 6.1.5. This photo shows a sample of garnet-mica schist from the Greek island of Syros. 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. 16. In most areas, the rate of increase in temperature with depth is 30°C per kilometre. 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. Regional or Barrovian metamorphism covers large areas of continental crust typically associated with mountain ranges. First, it has implications for mineral stability (Figure 6.1.1). 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. Contents. 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. Figure – Regional metamorphism is often associated with a continental collision where rocks are squeezed between two converging plates, resulting in mountain building. 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. Metamorphism affecting a large area or regional metamorphism involves large increases of temperature and pressure. Because burial is required from 10 km to 20 km, the affected areas tend to be large. Most regional metamorphism takes place within the continental crust. 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. 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. The relationships between plate tectonics and metamorphism are summarized in Figure 6.1.4. Which rocks does contact metamorphism create? 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. Second, it has implications for the texture of metamorphic rocks. Along subduction zones, as described above, the cold oceanic crust keeps temperatures low, so the gradient is typically less than 10°C/km. Based on the approximate average diameter of the garnets visible, estimate how long this metamorphic process might have taken. The Euro coin is 23 millimetres in diameter. 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). Results in foliated rocks (convergent plate boundary) Metamorphic rocks are classified basesd on their texture and composition. 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). (southern part of the Central Coal Basin and Pisuerga- This typical geothermal gradient is shown by the green dotted line in Figure 7.20. Metamorphism also occurs at subduction zones, where oceanic crust is forced down into the hot mantle. 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. 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. Most other common minerals have upper limits between 150°C and 1000°C. All of the important processes of metamorphism can be understood in the context of geological processes related to plate tectonics. A Practical Guide to Introductory Geology, Next: 6.2 Classification of Metamorphic Rocks, Creative Commons Attribution 4.0 International License. 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 mineral composition of the protolith. On modern Earth, regional metamorphism occurs in plate boundary zones. Magma is produced at convergent boundaries and rises toward the surface, where it can form magma bodies in the upper part of the crust. The main factors that control metamorphic processes are: The protolith, or “parent rock”, is the rock that exists before metamorphism starts. Assume that the diameters of the garnets increased at a rate of 1 millimetre per million years. The three heavy dotted lines on this diagram represent Earth’s geothermal gradients under different conditions. In most areas, the rate of increase in temperature with depth is 30°C/km. Because burial to 10 to 20 kilometers is required, the areas affected tend … Although an existing metamorphic rock can be further metamorphosed or re-metamorphosed, metamorphic rock doesn’t normally qualify as a “parent rock”. This is commonly associated with the boundaries of convergent plate and mountain range formation. 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. 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. 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). https://courses.lumenlearning.com/earthscience/chapter/metamorphic-rocks regional metamorphism takes place within the 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). the amount of time available for metamorphism. 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. 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. Divergent plate boundaries are characterized by ____. Exercise 7.3 Metamorphic Rocks in Areas with Higher Geothermal Gradients. Then, if you are even more pressure to gneiss, of would melt into igneous rocks. Regional metamorphism, as its name suggests, works over much larger areas. Most regional metamorphism takes place within continental crust. 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. Also, some areas can be found locally within the C.Z. This is very important in hydrothermal processes, and in the formation of mineral deposits. REGIONAL METAMORPHISM: Instead of from heat, the key catalyst for regional metamorphism is mostly from pressure. Large geological processes such as mountain-building cause regional metamorphism. Metamorphic rocks formed there are likely to be foliated because of the strong directional pressure of converging plates. Keywords Orogenic Belt Pression Relativement Marked Contrast Pressure Environment Systematic Increase These keywords were added by machine and not by the authors. The conditions under which they were metamorphosed are those of regional metamorphism. Dynamic metamorphism is associated with zones of high to moderate strain such as … For example, when there are two convergent plates pushing together, there will be immense pressure at the fault in between. 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. CC BY. Regional metamorphism takes place over a much wider area. zones of regional metamorphism. Describe the three general classes of metamorphic textures, draw them, and give examples of each. Characterized by strong directed pressure and increased temperature due to increased burial. 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). Burial metamorphism mostly affects sedimentary strata in sedimentary basins as a result of compaction due to burial of sediments by overlying sediments. Comedians in Cars Getting Coffee: "Just Tell Him You’re The President” (Season 7, Episode 1) - Duration: 19:16. blacktreetv Recommended for you An example would be the Himalayan Range. 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. It happens in a much larger area. Regional Metamorphism - no discernible source of heat (no nearby magma chamber, for example) - with increasing depth the temperature and pressure increase. 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). 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. Toggle Menu. 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. 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. 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. At 10 km depth, the temperature is about 300°C and at 20 km it’s about 600°C. At a 10 kilometre depth, the temperature is about 300°C and at 20 kilometres it’s about 600°C. 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. Chapter 1 Introduction to Geology Sedimentary or igneous rocks can be considered the parent rocks for metamorphic rocks. Zeolites are silicate minerals that typically form during low-grade metamorphism of volcanic rocks. Are certain types of metamorphic rocks indicative of particular plate boundaries or tectonic settings? quartzite, hornfels, marble . Each of these types of metamorphism produces typical metamorphic rocks, but they may … Metamorphic index minerals are used by geologists to distinguish among different _____. 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). A. Whereas denser oceanic crust subducts under more buoyant continental crust, with the collision of continental crust blocks, two landmasses instead collide and deform. This is commonly associated with convergent plate boundaries and the formation of mountain ranges. 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. That’s uncomfortably hot, so deep mines must have effective ventilation systems. As a result higher grades of metamorphism can take place closer to surface than is the case in other areas (Figure 7.19). 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. Contact metamorphism is a result of the temperature increase caused by the intrusion of magma into cooler country rock. 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. 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. Such magma bodies, at temperatures of around 1000°C, heat up the surrounding rock, leading to contact metamorphism (Figure 7.19). Regional metamorphism largely occurs at convergent plate boundaries. 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. 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). Along subduction zones, as described above, the cold oceanic crust keeps temperatures low, so the gradient is typically less than 10°C per kilometre. This metamorphism creates rocks like gneiss and schist. This typical geothermal gradient is shown by the green dotted line in Figure 6.1.6. 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). Foliation is a very important aspect of metamorphic rocks, and is described in more detail later in this chapter. 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). 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 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. Metamorphic rocks formed there are likely to be foliated because of the strong directional pressure (compression) of converging plates. 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. 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. Blueschist facies indicate a. formation at high temperature and high pressure. The minerals kyanite, andalusite, and sillimanite are polymorphs with the composition Al2SiO5. 4. regional metamorphism:results from mountain building and plate tectonic collisions. 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. 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. If you’ve never seen or even heard of blueschist, it’s not surprising. What is a little surprising is that anyone has seen it! Metamorphism and Plate Tectonics Metamorphic rocks result from the forces active during plate tectonic processes. Name the … See Appendix 2 for Practice Exercise 6.2 answers. Because of plate tectonics, pressures within the crust are typically not applied equally in all directions. 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.
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