Chpt8pptteacher - Metamorphismand MetamorphicRocks Chapter...

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Metamorphism and  Metamorphic Rocks Chapter 8
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Metamorphism Transition of one rock into another by temperatures and/or pressures unlike those in which it formed. Metamorphic rocks are produced from Igneous rocks Sedimentary rocks Other metamorphic rocks Metamorphism means to change form . The mineralogy, texture and chemical composition of the rock is different from the parent , or the original rock from which it was derived Agents of metamorphism: Heat , pressure and chemically active fluids The degree of the contribution of each of these varies greatly
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Figure 8.1
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Metamorphism During metamorphism the rock must remain solid Metamorphism may progress from low-grade to high-grade . Transition is gradual, changes subtle. Sedimentary shale becomes metamorphic slate , under increasingly more intense metamorphism the slate becomes phyllite , then schist , then gneiss Intense metamorphism obliterates the parent rock along with any structures, minerals or fossils it may have contained.
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Heat as agent of metamorphism Most important agent because it provides energy to drive chemical reactions that causes recrystallization. Even without melting, heat energy will cause movement of ions Recrystallization results in new minerals that are stable in the new environment and generally have a larger grain size Low-temp, fine grained, minerals like clays or precipitates recrystallize at lower temps into larger grains of the same mineral Heat will raise the temp so that the minerals become chemically unstable, atoms rearrange and form new minerals with the same composition that are stable at this new temperature. If ions migrate into the environment from elsewhere, new minerals with a different composition form As metamorphic intensity due to heat increases the grain size increases and the mineralogy changes
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What is the source of heat? Two sources of heat geothermal gradient - an increase in temperature with depth - caused by radioactivity and residual heat from the planets formation, 20-30 o C/km At about 200°C clays start to become unstable and recrystallize to chlorite and muscovite Quartz, feldspars remain stable at this temp This heat can be found where rocks descend at convergent plate boundaries, at the base of thick depositional basins like the Gulf of Mexico, and where continental collisions cause deformation Contact metamorphism – heat from magma bakes adjacent rock
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Figure 8.3
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What is the source of pressure? Pressure is applied as confining pressure and directional pressure Confining pressure applies forces equally in all directions Increases with depth as the rock thickness increases Pore spaces between the grains close, producing a more compact rock with greater density New minerals may form that have a more compact structure This pressure does not cause folding
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Directed pressure – differential stress rocks may also be subjected to
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This note was uploaded on 11/14/2011 for the course GEOL 1301 taught by Professor Gelbaum during the Fall '11 term at Georgia Perimeter.

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Chpt8pptteacher - Metamorphismand MetamorphicRocks Chapter...

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