Some questions about geology, (metamorphism)


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There have some questions about metamorphism, the questions is in the materials please finish those work by hand.

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Geology 1202 Planet Earth Assignment - plate tectonics and metamorphism We have looked at the three main factors controlling metamorphism; that is temperature, pressure and stress, and some of the metamorphic rocks. In this assignment we will take a closer look at one of the tectonic environments that can provide these necessary conditions. One situation in which a variety of metamorphic conditions can exist is at a subduction zone where one plate is descending beneath another. Based on what we know about the properties of the rocks involved we can make predictions about what goes on in this area. The plate that is descending has been at the Earth's surface and is relatively cold. As it moves down the subduction zone, it is heated by the hot asthenosphere above and below and may begin to undergo partial melting. Hot magma penetrates up through the upper plate from the asthensophere below, bringing heat near the surface. The red contours (called isotherms) on the cross-section at right show these patterns of temperature change in the subduction zone. (these lines are similar to the contours you looked at on topographic maps - instead of marking the change in elevation they mark the change in temperature) Two isotherms are marked on the figure - 300 and 600 degrees celsius. Geothermal gradients O If you could drill downward at the points labelled A, B and C on the diagram above you would encounter very different conditions at each one, even though they would all start an the same temperature and pressure at the surface of the Earth. The diagram at right shows the distribution of metamorphic facies with temperature and pressure. Lines A, B, and C show the geothermal gradients that would be encountered at the three marked positions on the diagram of a subduction zone above. 800 0 0 C A hornfels facies zeolite facies 10 amphibolite facies 4 6 5 volcanic 15 plutonic complex 20 25 plate interior 8 granulite facies 30 upper part of subduction zone 10 35 eclogite facies DEPTH IN KILOMETRES B 2 PRESSURE IN KILOBARS At point A, the rocks would be relatively cold at great depth. At C, an area of active volcanism, the temperature would go up much more rapidly with depth. We would describe the geothermal gradient as high. At point B, conditions would be somewhere in-between. TEMPERATURE IN C 200 400 600 0 What do I have to do? The object of this assignment is to predict the facies of metamorphism that occurs at different points near a converging plate boundary, or subduction zone. To predict the metamorphic facies that will develop, it is necessary to know the temperature and the pressure. A D B E C This is the same diagram of a subduction zone you have seen earlier, with isotherms to indicate the variation in temperature. Use the diagram to estimate the temperature conditions at points A to E (indicated by the black dots). If your point is exactly on an isotherm, this is easy. However, if it is between isotherms, you will have to estimate, based on the distance to nearby isotherms. For example, if your point is halfway between the 300 and 600° isotherms you would estimate the temperature at 450°. For simplicity assume that rocks at the surface are 0°. Enter these values in Table 1 (next page). The next thing you need to know is the pressure. Pressure is due to the weight of overlying rocks, so it increases with depth. Your chart of metamorphic facies includes both pressure (on the left hand side) and depth (on the right). We will use depth measurements to estimate the influence of pressure on the type of metamorphism. For each of the points A to E on the cross section, use the vertical scale to measure the depth below the surface - we will discount the overlying water so measure from the surface of the crust. Enter these values in Table 1. With this information you can use the Pressure-Temperature diagram to determine what metamorphic facies would be produced at each point. For each point draw a vertical line down from your estimated temperature (along the top of the graph). Next draw a horizontal line across from your estimated depth (indicated on the right side of the graph) to the left side Record the pressure (as indicated by the scale on the left side of graph) in Table 1. Where the two lines intersect is the metamorphic facies - label this point with the appropriate letter (A to E). Enter the facies in Table 1. EXAMPLE Below is an example of how it should be done using point X estimate the temperature based on the Isotherms measur e the depth X use the scale to convert the depth to kilometres O 0 TEMPERATURE IN C 200 400 450 600 800 0 hornfels facies 5 4 6 15 20 25 8 X granulite facies 30 10 35 eclogite facies 12 40 DEPTH IN KILOMETRES 10 amphibolite facies PRESSURE IN KILOBARS 2 zeolite facies 0 draw a vertical line at the correct temperature draw a horizontal line at the correct depth STUDENT NUMBER: NAME: O 0 TEMPERATURE C 200 400 600 800 0 0 hornfels facies 2 5 zeolite facies 4 6 15 20 25 granulite facies 8 DEPTH IN KILOMETRES amphibolite facies PRESSURE IN KILOBARS 10 30 10 35 eclogite facies Table 1 Point A Point B Point C Point D Point E Temperature (celsius) Depth (km) Pressure (kb) Metamorphic Facies What do I submit? Submit a photograph of your annotated PT diagram and the competed Table. Do not forget to fill in your name and student number on the sheet. ...
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