Chapter 8 - Chapter 8 Metamorphic Rocks Metamorphic rock...

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Chapter 8 Metamorphic Rocks Metamorphic rock forms from protolith . Metamorphism does not involve melting, solid-state process Metamorphic texture, the grains in the rock have grown in place and interlock. Metamorphic rocks form very slowly Many different processes of formation: o Recrystallization: changes the shape and size of grains without changing the identity of the mineral constituting the grains. o Phase Change: transforms a grain of one mineral into a grain of another mineral. o Metamorphic reaction/neocrystallization: growth of new mineral crystals that differ from those of the protolith, one or more chemical reactions takes place. o Pressure Solution: a rock is squeezed more strongly in one direction than in others at relatively low pressures and temperatures in the presence of water. Rocks undergo metamorphism when they are subjected to heat, pressure, differential stress, and/or hydrothermal fluids. Near the earth’s surface, minerals with relatively open crystal structures are stable ---- subjected to extreme pressure, denser minerals form. <phase change/neocrystallization Rocks feel the effects of pressure and temperature that effect metamorphism during mountain building. Differential Stress: when a material is squeezed or stretched unequally from different sides --- the push or pull in one direction differs in magnitude from the push or pull in another direction --- two different kinds: o Normal stress: pushes or pulls perpendicular to a surface. push=compression, pull=tension --- compression flattens a material, tension stretches a material o Shear Stress: moves one part of a material sideways, relative to another.
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When rocks are subjected to differential stress at elevated temperatures and pressures they can change shape without breaking. As a rock changes shape, the internal texture of a rock also changes --- results in preferred mineral orientation --- platy grains lie parallel to each other and elongate grains align in the same direction. o Platy and elongate grains are not the same length in all directions (inequant ----eqaunt is the opposite, grains are the same length) Metamorphic reactions usually take place in the presence of hydrothermal fluids, because water occurs throughout the crust --- hydrothermal fluids consist of hot water, steam, and supercritical fluid. Hydrothermal fluids are chemically active, they are able to chemically react with rock Metasomatism: when rock’s chemical composition changes because of hydrothermal fluids Metamorphic rocks are classified into two different classes: Foliated and Nonfoliated o Foliation: repetition of planar surfaces or layers in a metamorphic rock --- can give a rock a striped or streaked appearance in an outcrop ---can give them ability to split into thin sheets --- contains inequant mineral crystals that are aligned parallel to each other or because the rock has alternation dark and light colored layers o Foliated rocks can be distinguished from each other according
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Chapter 8 - Chapter 8 Metamorphic Rocks Metamorphic rock...

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