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EASC 116 - Ch. 7 Metamorphic Rx


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EASC 116 - Ch. 7 Metamorphic Rx

  1. 1. Metamorphic Rocks Chapter 7
  2. 2. Metamorphic Rx “meta” = change “morphos” = shape/form Parent rock = pre-existing rock that is subject to metamorphism
  3. 3. Metamorphic Rx Form by alteration of parent rock due to: - increase in pressure - increase in temperature - exposure to chemically active fluid
  4. 4. Metamorphism Occurs within Earth’s crust - minerals change until they are ________ under the new conditions
  5. 5. Metamorphic Conditions 1) Increasing pressure (stress) - occurs for two reasons: a) deep burial b) mountain building
  6. 6. Rx reaction to pressure Behave differently depending upon temperature Shallow depths = cooler temp - rx are brittle and tend to break Ex: fault breccia Called “cataclastic metamorphism”
  7. 7. Rx reaction to pressure Deep burial = higher temp’s - rx behave plastically and fold under pressure (Fig. 7.1)
  8. 8. Types of pressure a) Differential stress - applied unequally from different directions - produces foliated texture
  9. 9. Foliated texture Foliation = parallel alignment of minerals w/in a rock - creates a layered appearance - associated w/mountain building
  10. 10. Types of pressure b) Confining pressure - applied equally from all sides - associated with deep burial - nonfoliated texture
  11. 11. Metamorphic Conditions 2) Increasing temperature NOTE: Temperature cannot exceed the melting point of all minerals in rock Why?
  12. 12. Increasing temperature - occurs for two reasons: a) deep burial - geothermal gradient - increases 30o C per km b) magma intrusion
  13. 13. Increasing temperature Changes to parent rock: a) Compositional change - unstable minerals either melt or rearrange into new minerals - partial melting: remaining rock is metamorphic rock
  14. 14. Compositional Change Ex: Shale (clay minerals) Slate (microscopic micas)
  15. 15. Increasing temperature b) Textural Change - partial melting allows minerals to recrystallize - produces nonfoliated texture
  16. 16. Textural Change Ex: Sandstone (quartz) Quartzite (quartz) Note: The composition is still the same
  17. 17. Metamorphic Conditions 3) Chemically active fluid - superheated water with ions in solution - “hydrothermal solution” - occurs near magma chambers
  18. 18. “Microscopic Water” Water in crystalline structure - necessary for atoms to rearrange themselves
  19. 19. Sources of Water a) Groundwater b) Magma chambers c) Crystalline structure of hydrous minerals Ex: Gypsum = CaSO4 2H2O
  20. 20. Metamorphic Rock Classification Foliated textures - degrees of increasing metamorphic intensity (temperature, pressure)
  21. 21. Foliated textures Parent rock = shale Low degree Slate (microscopic micas) Low degree Phyllite (light reflects from larger micas)
  22. 22. Foliated textures Low degree Medium degree Phyllite Schist (visible micas)
  23. 23. Foliated textures Low degree Slate Medium degree Schist High degree Gneiss (black & white layers)
  24. 24. Transitional change Migmatite - rock w/both igneous & metamorphic characteristics - represents high degree of metamorphism (Fig. 7.23)
  25. 25. Foliated textures Most schists have muscovite & biotite = mica schists Accessory minerals w/micas indicate degree of metamorphism w/in schist range
  26. 26. Accessory minerals Referred to as “index minerals” Ex: chlorite mica schist Low grade mm (~200oC) Ex: garnet mica schist Intermediate grade mm
  27. 27. Nonfoliated textures Composition controlled by parent rock Parent Rock Meta. Rock Sandstone Quartzite Limestone Marble
  28. 28. Regional metamorphism
  29. 29. Index minerals
  30. 30. Regional Metamorphism Foliated rocks are often folded
  31. 31. One rock is the raw material for another