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Metamorphic Rocks

Metamorphic Rocks. Evidence of metamorphism The ingredients of metamorphism Classifying metamorphic rocks Metamorphism of shale (& other parent rocks!) Metamorphic minerals. Geological Materials Transformation Processes. The Rock Cycle. Rock Metamorphism (Increased T & P)

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Metamorphic Rocks

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  1. Metamorphic Rocks Evidence of metamorphism The ingredients of metamorphism Classifying metamorphic rocks Metamorphism of shale (& other parent rocks!) Metamorphic minerals

  2. Geological Materials Transformation Processes The Rock Cycle • Rock Metamorphism (Increased T & P) • Meta- morphic rock Partial Melting • Magma

  3. MetamorphismMetamorphic Rocks Definition • Re-crystallization of minerals • While still solid • Into minerals that are stable at different temperatures and • Different pressures

  4. MetamorphismMetamorphic Rocks WHY??? do the minerals recrystallize? • To reach chemical equilibrium!!! Animation of metamorphic rock formation

  5. A. Evidence of Metamorphism Shale For example… Sedimentary Rocks • Made of minerals derived from weathering of a parent rock • Stable at atmospheric temperatures and pressures (low T & P) • Originally horizontal, continuous and uniform layers • 1.Bent (deformed) layers Gneiss

  6. Differential pressure “squashes” rock and included features 2. Flattened Pebbles Evidence of Metamorphism Conglomerate Metaconglomerate

  7. Evidence of Metamorphism • 3.Crystalline Texture Minerals tightly interlocking due to recrystallization under pressure Quartz Sandstone Quartzite

  8. Evidence of Metamorphism • 4. New mineral assemblages Example: Shale: Clay minerals (some quartz)  Metamorphism (Mid-grade) • Forms Schist: Mica, Feldspar and other silicate minerals 

  9. B. Recipe of Metamorphism • 1.Parent Rock • Even though minerals will change • Most elements are provided by parent rock • Except water and some dissolved ions Shale Schist

  10. B. Recipe of Metamorphism Temp. (ºC) 0 C 600 1200 A B • 2.Heat • Geothermal gradient: more pressure = more heat!! • Minerals stable at lower temperatures converted to • minerals stable at higher temperatures • Solid state chemical reactions are accelerated A B C

  11. Recipe of metamorphism: Heat • Geotherm: Temperature increases with depth beneath Earth’s surface • Typical: 30° C/km • Igneous intrusions: Localized increases in temperature

  12. Recipe of Metamorphism • 3.Increased Pressure & Stress • Increased Confining Pressure as rocks are buried • Compression at convergent plate boundary or • Sheared as plates slide past each other

  13. Compressive Stress Shear Stress Causes Foliation Results of Stress

  14. Ingredients of Metamorphism • 4.Addition or removal of fluids (and elements) • Water (and other fluids) within rocks and minerals • Moving during metamorphism • Accelerates solid-state chemical reactions and • May change rock composition • 5.Time  Millions of years!

  15. Metamorphic Grade • Intensity of Metamorphism: • High grade (High P & T) • Low grade (Low P & T)

  16. Metamorphic Facies • The minerals in a rock are clues to the (pressure and temperature) history of the rock • Facies = a set of metamorphic conditions • Each facies is characteristic of its • Tectonic environment • Source rocks in the environment (like shale)

  17. Metamorphism of Shale not found in nature Fig. Story 9.7b

  18. Metamorphic Rocks of other Parent Rocks • Limestone  Marble • Bioclastic calcite  Crystalline calcite Nonfoliated

  19. Metamorphic Rocks of other Parent Rocks • Quartz Sandstone  Quartzite • Granular quartz  Crystalline quartz Nonfoliated

  20. Metamorphic Rocks of other Parent Rocks • Basalt  Amphibole Schist • Granite  Gneiss • Uniform texture  Foliation (schistosity and gneissic banding)

  21. Characteristic Metamorphic Minerals sillimanite kyanite staurolite garnet andalusite For more images: http://skywalker.cochise.edu/wellerr/mineral/

  22. Distinct Mineral Assemblages Grade

  23. Distinct Mineral Assemblages Grade

  24. garnet - the index mineral Sedimentary parent rock Igneous parent rock

  25. Index Minerals • Define the metamorphic conditions or zones • Give information about the “metamorphic grade” Fig. Story 9.7a

  26. Summary • Evidence of metamorphism • Deformed layers, flattened pebbles, crystalline texture, & new mineral assemblages • The ingredients of metamorphism • Parent rock, heat, pressure, fluids, & time • Classifying metamorphic rocks • Low grade --> high grade & facies • Metamorphism of shale (& other parent rocks!) • Shale --> Gneiss, Sandstone --> Quartzite, Limestone --> Marble • Metamorphic minerals • Muscovite and Garnet = Index minerals

  27. To think about… Metamorphic Rocks & Plate Tectonics • Tracing changes in pressure and temperature conditions that a rock experienced … • … gives insight into plate tectonic settings!!

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