Note_-_Chapter_7 - Metamorphism A process of change Introduction Metamorphic rock solid-state alteration of a protolith Meta = change Morphe = form

Note_-_Chapter_7 - Metamorphism A process of change...

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Metamorphism: A process of change Introduction - Metamorphic rock - solid-state alteration of a protolith - Meta = change - Morphe = form - Protoliths are preexisting rocks - Metamorphism can alter any protolith - Protoliths undergo slow solid-state changes in: - Texture - Mineralogy - Metamorphic changes are due to variations in: - Temperature - Pressure - Tectonic stresses - Amount of reactive water What is a metamorphic rock? - Metamorphism changes mineralogy - Red shale -- quartz, clay, and iron oxide - Gneiss -- quartz, feldspar, biotite, and garnet - Metamorphism changes texture - Unique texture -- intergrown and interlocking grains - Metamorphism often creates foliation - A texture defined by: - Alignment of platy minerals - Creation of alternating light/dark bands Metamorphic Processes - Recrystallization - minerals change size and shape - Mineral identity need not change - Example: limestone -> marble - Phase change - new minerals form with: - The same chemical formula - Different crystal structure - Example: andalusite -> kyanite - Neocrystallization - new minerals form from old - Initial minerals become unstable, change to new minerals - Original protolith minerals are digested in reactions - Elements restructure to form a new mineral assemblage - Example: Shale -> garnet mica schist - Pressure solution - mineral grains partially dissolve - Dissolution requires small amounts of water - Minerals dissolve where their surfaces press together
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- Ions from the dissolution migrate in the water film - Plastic deformation - mineral grains soften and deform - Requires elevated temperature and pressure - Rock is squeezed or sheared - Minerals change shape without breaking like a plastic Causes of Metamorphism - The agents of metamorphism are: - Heat - Pressure - Compression and shear - Hot fluids - Not all agents are required; they often do co-occur - Rocks may be overprinted by multiple events Metamorphism Due to Heat - One cause of metamorphism is heat - Most metamorphism occurs between 250 degree celsius and 850 - Between diagenesis and melting (up to 1200 degree celsius) - Heat energy breaks and reforms atomic bonds - Solid-state diffusion: migration of atoms between grains - New minerals form - P increases with depth in the crust - Metamorphism occurs mostly in 2-12 kbar range\ - Increase in P packs atoms more tightly together - Creates denser minerals - Involves phase changes or neocrystallization - Formation and stability of many minerals depend on both P and T Metamorphism via Compression - Compression - stress greater in one orientation - Different from pressure (P), which is equal in all directions - Compression is a common result of tectonic forces Metamorphism via Shear - Shear - moves one part of a material sideways - Causes material to be smeared out -
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  • Fall '09
  • Metamorphic rocks

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