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Metamorphism

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  1. Metamorphism Changes in Rock Composition or Texture Due to Heat, Pressure and Action of Fluids

  2. We Do Not Live at “Normal” Conditions • By the standards of Earth’s interior, we live in a frozen vacuum • Things that look “abnormal” to us are normal behavior for materials • Solids can flow • Solids can react chemically with each other • A given material can have several different atomic structures

  3. Chemical Changes in Rocks Weathering • At Surface Diagenesis • Sedimentary Rocks Metamorphism • Starts about 200 C • Outside range of normal near-surface conditions

  4. Where Does the Heat Come from? Uranium and Thorium  Other Elements + Lead + Radiation (about 90%) Potassium-40  Calcium-40 or Argon-40 + Radiation (about 10%) Possibly some primordial heat (controversial)

  5. Where Does the Pressure Come from? • Air Pressure = 14 P.s.i. (1 Atmosphere or 1 Bar = 100 kPa) • Pressure Beneath 10 Meters (33 Ft.) Of Water = 1 Atm. = 1 Bar • Same Pressure Beneath 3.5 m (10 ft.) Of Rock • Pressure in Deepest Part of Ocean = 1000 Bars • Pressure under One Mile of Rock = 500 Bars • 1000 Bars (2 Mi. or 3.5 km Of Rock) = 1 Kilobar (Kb.) = 100 MPa

  6. Pressure Units • 1 Newton (N) = gravitational attraction on 100 grams • One Newton = 7 Fig Newtons • 1 Pascal (Pa) = 1 N/square meter • 1 Pascal = pressure under thin cardboard or 0.1 mm water • A Thin section exerts about 1 Pa pressure on the slide • One bar = 1 atm = 100 kPa • 1 MPa = 10 bars • 1 GPa = 10 kilobars

  7. Types of Metamorphism Contact • Around Intrusions or Local Heat Sources Regional • Usually Accompanied by Deformation Cataclasis • Shearing and fault zones Shock • Ultra-high pressures and rapid deformation

  8. What Happens During Metamorphism Minerals React to Form New Minerals • 2SiO2 + CaMg(CO3) 2 == CaMgSi2O6 + 2CO2 • Quartz + Dolomite == Pyroxene Minerals Change Form • Al2SiO5 == Al2SiO5 • Andalusite == Kyanite New Materials Are Added (Metasomatism) • CaMg(SiO2)2 + 2CO2 == CaMg(CO3)2 + 2SiO2 • Pyroxene + CO2 == Dolomite + Quartz • Minerals in Solution == Ore Bodies Recrystallization Cataclasis Shock Metamorphism

  9. Polymorphism Al2SiO5 • Andalusite • Kyanite • Sillimanite Ice - 6 high pressure forms Diamond - Graphite Calcite - Aragonite Quartz - • - Tridymite - Cristobalite (increasing temperature) • - Coesite - Stishovite (increasing pressure)

  10. Why Don't Rocks "De-metamorphose"? Reactions Can't Reverse Because Ingredients Lost • 2AlSi2O5(OH) == Al2SiO5 + 3SiO2 + H2O • Clay Mineral == Andalusite + Quartz + Water (Lost) • An example of carbonate metamorphism: • CaMg(CO3) 2 + 2SiO2 == CaMgSi2O6 +2CO2 • Dolomite + Quartz == Pyroxene + CO2 (Lost) Reactions "Freeze" Sometimes it Does Happen if Fluids Present • Retrograde Metamorphism • On the surface we call it weathering

  11. Foliation Foliation is a sheetlike structure that forms when rocks are deformed.

  12. Foliation • Flattening • Grain Rotation • Solution • New Minerals • Partial Melting • Shearing

  13. Foliation In every case, the foliation is: • In the direction of least resistance • at right angles to the direction of greatest compression.

  14. MetamorphicGrade - Degree to Which the Rock Has Changed Composition • Can Often See Original Bedding • Can Sometimes Even See Deformed Fossils • At High Grades, Rocks Can Often Lose All Trace of Their Original Appearance

  15. Major Metamorphic Rock Types

  16. Major Metamorphic Rock Types

  17. Major Metamorphic Rock Types

  18. Major Metamorphic Rock Types

  19. Major Metamorphic Rock Types

  20. What About Other Rocks?

  21. Metamorphic Facies

  22. Metamorphic Map

  23. Mantle Rocks

  24. Metamorphism and Plate Tectonics