105_07-metamorphics_part_1 - Metamorphism and Metamorphic...

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Unformatted text preview: Metamorphism and Metamorphic Rocks Metamorphism of Rocks Chapter 7 Metamorphism Definition: Metamorphism is the transformation of preexisting rock into texturally or mineralogically distinct new rock as a result of high temperature, high pressure, or both.. ..but without the rock melting in the process. Metamorphism Solid Solid state = rock does not melt Between Between 200° C and melting of rock Parent Parent rock – original, pre-metamorphosis prerock New minerals melting Sediments rocks Metamorphism Igneous Metamorphism of Rocks Metamorphism Sediment Metamorphic Igneous Characteristics Characteristics of metamorphic rocks are controlled by: – composition of parent rock – heat – pressure – fluid activity – time 1 What are the Agents of Metamorphism? Composition Composition of parent rock – Generally, no new elements or compounds are added to the parent rock during metamorphism. (except sometimes water) What are the Agents of Metamorphism? Heat Heat – Increasing temperature with depth = geothermal gradient geothermal – Radioactive decay – Deformation – Migrating magmas What What are the Agents of Metamorphism? Heat Heat – increases the rate of chemical reactions that produce different minerals What are the Agents of Metamorphism? Pressure – Confining pressure (lithostatic pressure or hydrostatic pressure) What are the Agents of Metamorphism? Lithostatic Lithostatic Pressure Depth = 750 m Depth = 1500 m Pressure Pressure gradient – results from the weight of overlying rocks – mineral grains are more closely packed – recrystallization may occur, producing smaller and denser minerals 2 What are the Agents of Metamorphism? Differential Pressure Differential Differential pressure acts on an object from different sides unequally. Rock is distorted. Pressure – Confining pressure – Differential stress Forces Forces are stronger in some directions than in others – Compressive stress – Shearing Compressive stress Differential Pressure Shearing Shearing Compressive stress Lithostatic pressure Directed pressure 3 Differential pressure … deforms rocks Foliation in quartzmuscovite schist Differential pressure … creates foliation Differential pressure … creates foliation 4 Folded metamorphic rocks Deformed strata, Death Valley Shear: rotated garnets What are the Agents of Metamorphism? Fluid Fluid Activity – water and carbon dioxide are almost always present in some amount in metamorphic regions – these fluids enhance metamorphism by increasing the rate of chemical reactions Fluid Fluid Sources – water trapped in the pore space of sedimentary rocks – magma – dehydration of water-bearing minerals that are watersubjected to heat and pressure What are the Agents of Metamorphism? Time Time Types of Metamorphism Contact Contact Metamorphism Dynamic Dynamic Metamorphism Regional Regional Metamorphism 5 Types of Metamorphism Contact Contact Metamorphism – produced when a body of magma alters the surrounding country rock Contact Metamorphism Factors Factors in contact metamorphism – initial temperature and size of the intrusion – presence and chemistry of fluids Metamorphic Metamorphic aureoles – zones of mineral assemblages surrounding intrusion Contact Metamorphism Baked contact zone (gray) adjacent to a dike (tan) 6 ...
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This note was uploaded on 05/18/2011 for the course GEOL 105 taught by Professor Jack during the Spring '08 term at University of Louisiana at Lafayette.

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