Lecture 13 Feb 18 2008 Metamorphic rocks

Lecture 13 Feb 18 2008 Metamorphic rocks - Mt Moran(gneiss...

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Mt. Moran (gneiss), Grand Teton National Park
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The important factors in metamorphism are 1) temperature 2) pressure (confining and directed) 3) fluids Tectonic processes at convergent boundaries cause rock to move from shallow to deep levels within Earth’s crust, exposing the rock to different pressures and temperatures.
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Pressure increases with depth at about the same rate everywhere. Temperature increases with depth at varying rates depending upon the location on Earth (primarily related to volcanism or magmatism). That is, geothermal gradients vary.
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There are two styles of metamorphism: 1) Regional occurs where both high T and P are imposed over large regions of Earth’s crust. 2) Contact occurs in a very local area of rock immediately adjacent to magmatic intrusions.
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Metamorphosed sedimentary rocks have become marble, schist, and gneiss in this outcrop in Sequoia National Forest, California. Recrystallization under elevated T and P allows the rock to change its mineralogy and texture (= become a different rock!) while remaining solid. Solid-phase reactions are slow. The presence of hydrothermal fluids accelerates the chemical reactions in metamorphism.
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We characterize the extent of metamorphism by how high the temperatures and pressures were when the metamorphic rock formed.
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Directed pressure causes elongated and platy minerals to line up perpendicular to the direction of applied compressive forces.
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wavy parallel planes were produced under directed pressure.
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Lecture 13 Feb 18 2008 Metamorphic rocks - Mt Moran(gneiss...

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