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  8/30/2013 1 Chapter 6: METAMORPHISM Modification of Rocks by Temperature and Pressure   1 Lecture Outline   1. Metamorphism and the Earth system 2. Causes of metamorphism 3. Types of metamorphism 4. Metamorphic textures 5. Regional metamorphism and metamorphic grade 6. Plate tectonics and metamorphism 2  8/30/2013 2 Definition of metamorphism: ã “Metamorphism is a subsolidus process leading to changes in mineralogy and/or texture (e.g. grain size) and often in chemical composition in a rock. These changes are due to physical and/or chemical conditions that differ from those normally occurring at the surface of planets and in zones of cementation and diagenesis below this surface. They may coexist with partial melting.”   3   2. Causes of metamorphism ●  heat ●  pressure ●  fluid composition 4  8/30/2013 3 ●  temperature increases with depth (geothermal gradient) ●  shallow (20º C / km) (thick lithosphere) ●  steep (50º C / km) (thin lithosphere)  Average = 30º C / km   2. Causes of metamorphism- Temperature 5 2. Causes of metamorphism- Temperature Figure. Estimated ranges of oceanic and continental steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al  . (1980), Earth. Rev. Geophys. Space Sci., 18, 269-311. Q. Why is continental geothermal gradient higher than the oceanic geotherm? b/c concentration of radioactive (LIL) elements is higher in the continental crust 6  8/30/2013 4 Effect of Temperature Increasing temperature has several effects 1)Promotes recrystallization increased grain size: Larger surface/volume ratio of a mineral The grain size increases with increase in T 2) Drive reactions (that are endothermic) that consume unstable mineral(s) and produces new minerals that are stable under the new conditions (isochemical change) 3) Overcomes kinetic barriers to recrystallization 7   Grades of metamorphism ã Metamorphic grade: a general increase in degree of metamorphism without specifying the exact relationship between temperature and pressure 8


Jul 24, 2017
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