This is commonly associated with the boundaries of convergent plate and mountain range formation. Which rocks does contact metamorphism create? 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. See Appendix 2 for Practice Exercise 6.2 answers. CC BY. 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. Most feldspars are stable up to between 1000°C and 1200°C. 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. This typical geothermal gradient is shown by the green dotted line in Figure 6.1.6. 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 … Because burial to 10 to 20 kilometers is required, the areas affected tend … Regional Metamorphism - no discernible source of heat (no nearby magma chamber, for example) - with increasing depth the temperature and pressure increase. When rocks are buried deep in the crust, regional metamorphism occurs. 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. Contact metamorphism is a result of the temperature increase caused by the intrusion of magma into cooler country rock. An example would be the Himalayan Range. A convergent boundary is also known as a destructive plate boundary due to subduction. In most areas, the rate of increase in temperature with depth is 30°C/km. 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). Large geological processes such as mountain-building cause regional metamorphism. 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. Two settings, continent-continent collisions and continental volcanic arcs are also shown in more detail in Figure 6.1.5. If there is water present, it will be lower. If the pressure is higher, that upper limit will be even higher. Describe the three general classes of metamorphic textures, draw them, and give examples of each. 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. Results in foliated rocks (convergent plate boundary) Metamorphic rocks are classified basesd on their texture and composition. 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. All of the important processes of metamorphism that we are familiar with can be directly related to geological processes caused by plate tectonics. For example, quartz is stable from environmental temperatures (whatever the weather can throw at it) all the way up to about 1800°C. 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. As temperature increases with depth, both p and T contribute to metamorphism. 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. 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. 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. continental-continental convergent boundary. All of the important processes of metamorphism that we are familiar with can be directly related to geological processes caused by plate tectonics. Because of plate tectonics, pressures within the crust are typically not applied equally in all directions. Regional metamorphism. 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. In most areas, the rate of increase in temperature with depth is 30°C per kilometre. 4. regional metamorphism:results from mountain building and plate tectonic collisions. 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. 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 an important component of a rock known as blueschist. Preface; Acknowledgments; Acknowledgements: eCampusOntario; I.Main Body. Along subduction zones, as described above, the cold oceanic crust keeps temperatures low, so the gradient is typically less than 10°C per kilometre. Metamorphic rocks formed there are likely to be foliated because of the strong directional pressure of converging plates. 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 regional metamorphism takes place within continental 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). That’s uncomfortably hot, so deep mines must have effective ventilation systems. 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). 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. What is a little surprising is that anyone has seen it! Skip to content. 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. Characterized by strong directed pressure and increased temperature due to increased burial. 1. Metamorphic rocks formed there are likely to be foliated because of the strong directional pressure (compression) of converging plates. Although an existing metamorphic rock can be further metamorphosed or re-metamorphosed, metamorphic rock doesn’t normally qualify as a “parent rock”. Regional metamorphism refers to large-scale metamorphism, such as what happens to continental crust along convergent tectonic margins (where plates collide). This type of metamorphism occurs with rocks that are buried deep down the Earth’s crust. As we learned in the context of igneous rocks, mineral stability is a function of temperature, pressure, and the presence of fluids (especially water). zones of regional metamorphism. 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). Commonly, they show evidence of having been deformed and metamorphosed at great depth in the crust. The minerals kyanite, andalusite, and sillimanite are polymorphs with the composition Al2SiO5. The Euro coin is 23 millimetres in diameter. Such magma bodies, at temperatures of around 1000°C, heat up the surrounding rock, leading to contact metamorphism (Figure 7.19). Metamorphism through plate tectonics ... dynamic and regional. b. evidence of an … 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. This is commonly associated with convergent plate boundaries and the formation of mountain ranges. The rate of increase of temperature with depth in the Earth (typically around 30˚ C/km within the crust). You’ve probably never seen or even heard of blueschist; that’s not surprising. The deeper rocks are within the stack, the higher the pre… 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. All of the important processes of metamorphism can be understood in the context of geological processes related to plate tectonics. the amount of time available for metamorphism. What is surprising is that anyone has seen it! 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. Divergent plate boundaries are characterized by ____. Metamorphic index minerals are used by geologists to distinguish among different _____. When exposed to the surface, these rocks show the incredible pressure that causes the mountain building process to bend and break the rocks. Each of these types of metamorphism produces typical metamorphic rocks, but they may … 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. 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. 16. Generally, this metamorphism technique is associated with plate boundaries and formation of mountains ranges. Foliation is a very important aspect of metamorphic rocks, and is described in more detail later in this chapter. 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. 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. Give three examples of such rocks and indicate the tectonic environment they represent? The collisions result in the formation of long mountain ranges, like those along the western coast of North America. Exercise 7.3 Metamorphic Rocks in Areas with Higher Geothermal Gradients. Burial metamorphism mostly affects sedimentary strata in sedimentary basins as a result of compaction due to burial of sediments by overlying sediments. 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, as its name suggests, works over much larger areas. 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. See Appendix 2 for Practice Exercise 6.1 answers. Although most metamorphism involves temperatures above 150°C, some metamorphism takes place at temperatures lower than those at which the parent rock formed.