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Metamorphism

Metamorphism. Changes in Rock Composition or Texture Due to Heat, Pressure and Action of Fluids. Take-Away Points. Rocks change due to heat, pressure, and fluids Heat in the earth is original heat plus heat from radioactive decay

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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. Take-Away Points • Rocks change due to heat, pressure, and fluids • Heat in the earth is original heat plus heat from radioactive decay • Pressure is simply from the weight of overlying rocks. Pressure = Depth • What happens during metamorphism • Different metamorphic conditions result in different rocks and minerals • Metamorphic rocks reveal temperature and pressure conditions in the earth.

  3. 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

  4. Weathering At Surface Diagenesis Sedimentary Rocks Metamorphism Starts about 200 C Outside range of normal near-surface conditions Chemical Changes in Rocks 1. Rocks change due to heat, pressure, and fluids

  5. Where Does the Heat Come from? • Some Heat is Probably Original • Much heat is from Radioactive Decay • Uranium and Thorium  Other Elements + Lead + Radiation (about 90%) • Potassium-40  Calcium-40 or Argon-40 + Radiation (about 10%) • Typical temperature increase • 25 C per kilometer near surface • 1-2 C per kilometer in deep earth 2. Heat in the earth is original heat plus heat from radioactive decay

  6. Where Does the Pressure Come from? • Air Pressure = 14 P.s.i. (1 Atmosphere or 1 Bar) • 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 km Of Rock) = 1 Kilobar (Kb.) 3. Pressure is simply from the weight of overlying rocks. Pressure = Depth

  7. Contact Around Intrusions Shallow: 0-6 Km Low Pressure Local heat source Regional Wide Areas 5-20 Km, Sometimes 30+ High Pressure Usually Accompanied by Deformation and Mountain Building Types of Metamorphism 4. What happens during metamorphism

  8. 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 What Happens During Metamorphism 4. What happens during metamorphism

  9. 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 4. What happens during metamorphism

  10. Metamorphic Grade - 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 5. Different metamorphic conditions result in different rocks and minerals

  11. Major Metamorphic Rock Types 5. Different metamorphic conditions result in different rocks and minerals

  12. Major Metamorphic Rock Types 5. Different metamorphic conditions result in different rocks and minerals

  13. Major Metamorphic Rock Types 5. Different metamorphic conditions result in different rocks and minerals

  14. Major Metamorphic Rock Types 5. Different metamorphic conditions result in different rocks and minerals

  15. Major Metamorphic Rock Types 5. Different metamorphic conditions result in different rocks and minerals

  16. What About Other Rocks?

  17. Al2SiO5 Andalusite Kyanite Sillimanite Ice - 6 high pressure forms Diamond - Graphite Calcite - Aragonite Quartz - - Tridymite - Cristobalite (increasing temperature) - Coesite - Stishovite (increasing pressure) Polymorphism 6. Metamorphic rocks reveal temperature and pressure conditions in the earth.

  18. Metamorphic Facies 6. Metamorphic rocks reveal temperature and pressure conditions in the earth.

  19. Mantle Rocks

  20. Take-Away Points • Rocks change due to heat, pressure, and fluids • Heat in the earth is original heat plus heat from radioactive decay • Pressure is simply from the weight of overlying rocks. Pressure = Depth • What happens during metamorphism • Different metamorphic conditions result in different rocks and minerals • Metamorphic rocks reveal temperature and pressure conditions in the earth.

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