9. MetamorphismandDeformation (1)

9. MetamorphismandDeformation (1) - [email protected]

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Metamorphism takes place at temperatures and pressures that fall between those in which sediments are lithified and those in which rocks begin to melt and form magmas. [email protected]
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There are two types of metamorphism: regional and contact metamorphism. Regional metamorphosed rocks are buried beneath thick accumulations of sediment and rock and can occur over hundreds of thousands of square miles. Contact metamorphic rocks form when intruding magma contacts the surrounding rock and causes changes due to the heat and release of fluids from the magma.
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The important changes that take place during metamorphism include: 1.Recrystallization of existing minerals, especially into larger crystals. 1.Chemical breakdown of unstable original minerals and recrystallization and growth of new minerals, that are stable in the metamorphic environment, such as garnets in the schist rock (shown on the right). 2.Deformation and reorientation of existing mineral crystals and the growth of new ones with distinctive orientations (foliation) . Recrystallized quartz minerals in quartzite Garnet Schist Foliated Schist
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What factors control the changes that take place during metamorphism? 1.Mineralogy of the original rock (parent rock or “protolith.” . Limestone , comprised of the mineral calcite (CaCO 3 ) is metamorphosed to form marble. 1.The depth (pressure) and temperature at which the alteration occurred. 1.Deformation and reorientation of existing mineral crystals and the growth of new ones with distinctive orientations. Limestone Marble The combined effects of these processes generally produce metamorphic rocks that differ from their protoliths by being coarser-grained and/or foliated.
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Metamorphic grade refers to pressure/temperature conditions in regional metamorphic rocks. High grade metamorphic rocks were subjected to high temperatures (500- 700° C) and high pressure conditions (equivalent depth of 15 – 35 km). Low grade metamorphic rocks were subjected to relatively low temperatures (250- 400° C) and pressures (equivalent depth of 6-12 km).
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Metamorphic rocks contain many of the same minerals found in igneous and sedimentary rocks. Certain minerals occur almost exclusively in metamorphic rocks, such as garnet, kyanite or sillimanite . Each mineral found in a metamorphic rock has a specific stability range of pressure and temperature. You will learn to identify 8 common metamorphic minerals in lab.
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Mineralogical changes during metamorphism of the sedimentary rock shale. Note that the elemental composition will not change, but the mineral composition will reflect the pressure/temperature condition.
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Non-foliated metamorphic rocks with granular texture.
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