Midterm 3 Chapter Outlines 6 and 7

Midterm 3 Chapter Outlines 6 and 7 - Geology 101: Midterm 3...

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Geology 101: Midterm 3 Chapters 6, 7 Chapter 6: Metamorphism—Modification of Rocks by Temperature and Pressure Metamorphism - changes in the heat, pressure and the chemical environment of rocks can alter mineral compositions and crystalline textures making them metamorphic. Causes of Metamorphism Heat (internal temperature) steepness of geothermal gradient: Shallow (20 º C/km)…. Steep (50 º C/km) average depths of metamorphism: 10-40 km the role of plate tectonic settings with temperature- as depth increases, so does temperature. Pressure (stress) Confining pressure due to burial: pressure from all directions Directed pressure due to tectonics: pressure from discrete direction the role of plate tectonic settings with pressure rates of pressure increase with increasing depth: rate of increase= 0.3 to 0.4 kbar/km Minerals are Geobarometers(pressure gauges)- unstable minerals recrystallize into new metamorphic minerals. Fluids—chemical alteration (fluid composition inside Earth): fluids allow metamorphic changes to occur faster as atoms and ions are more easily mobilized. Metasomatism-the change of a rock’s bulk composition by fluid transport of chemical substances into or out of a rock. Metamorphic Grades – low, medium, high. Grade increases with inc. of heat, pressure, and depth. Types of Metamorphism Regional –occurs over large areas. Most often occurs along convergent plate boundaries Contact –occurs locally, near igneous intrusions. Occurs along fractures in contact w/ hydrothermal fluids. Seafloor (metasomatism) –magmas mixing with percolating sea water. Metasomatism is very common at seafloor metamorphism. Burial –caused by heat and pressure of the overlaying rocks (typically sedimentary) beyond just diagenesis. Often under relatively low temp and low pressure conditions. High-pressure and ultra-high-pressure –highest pressures (8-12 kbar) Shock –instantaneous heat and pressure. Forms Tektites that look like drops of class. Metamorphic Textures Bedding vs. Foliation –bedding is due to layering and foliation is due to metamorphism and compressive forces. Mineral crystals become elongated perpendicular to the compressive force. Foliation- layering and parallel alignment of platy/flat mineral crystals. Foliated Textures –distinguished by slaty cleavage, schistocity, gneissic foliation, mineral grains show preferred orientation. - From slate to migmatite: increase in M grade, heat, pressure, crystal size, coarseness of foliation Slate –flat foliation; resembles mineral cleavage. Low grade Metamorphic. Phyllite –wavy/wrinkled foliation. Intermediate grade Metamorphic. Schist –scaly/glittery layering. Intermediate-high grade M Gneiss –alternating concentrated layers/lenses of light and dark materials. High grade M. Migmatite –mix of igneous + metamorphic rock; very strong banding. Highest grade M.
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This note was uploaded on 03/30/2008 for the course GEOL 101 taught by Professor Mapes during the Fall '07 term at UNC.

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Midterm 3 Chapter Outlines 6 and 7 - Geology 101: Midterm 3...

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