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Understanding Earth Sixth Edition

Grotzinger • Jordan. Understanding Earth Sixth Edition. Chapter 6: METAMORPHISM Modification of Rocks by Temperature and Pressure. © 2011 by W. H. Freeman and Company. Chapter 6: Metamorphism: Alteration of Rocks by Temperature and Pressure. About Metamorphism

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Understanding Earth Sixth Edition

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  1. Grotzinger • Jordan Understanding Earth Sixth Edition Chapter 6: METAMORPHISM Modification of Rocks by Temperature and Pressure © 2011 by W. H. Freeman and Company

  2. Chapter 6: Metamorphism: Alteration of Rocks by Temperature and Pressure

  3. About Metamorphism • Changes in heat, pressure, and the chemical environment of rocks can alter mineral compositions and crystalline textures, making them metamorphic. • Metamorphic changes occur in the solid state, so there is no melting.

  4. Lecture Outline • Causes of metamorphism • 2. Types of metamorphism • 3. Metamorphic textures • 4. Regional metamorphism and metamorphic grade • 5. Plate tectonics and metamorphism

  5. 1. Causes of Metamorphism ● internal heat of Earth ● internal pressure of Earth ● fluid composition inside Earth

  6. 1. Causes of Metamorphism ● temperature increases with depth ● rate = 20º to 60ºC per km ● at 15 km depth: 450ºC

  7. 1. Causes of Metamorphism ● pressure and temperature increase with depth in all regions

  8. 1. Causes of Metamorphism

  9. 1. Causes of Metamorphism ● the role of temperature ● geothermal gradient ● shallow (20ºC / km) ● steep (50ºC / km)

  10. 1. Causes of Metamorphism ● the role of pressure (stress) ● confining pressure ● directed pressure

  11. 1. Causes of Metamorphism ● the role of pressure (stress) ● rate of increase = 0.3 to 0.4 kbar / km ● minerals are geobarometers

  12. 2. Types of Metamorphism ● the role of fluids ● metasomatism ● accelerated chemical reactions

  13. 2. Types of Metamorphism Depth, km Oceanic crust 0 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere

  14. Shock metamorphism Depth, km Oceanic crust 0 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere

  15. Regional metamorphism Shock metamorphism Depth, km Oceanic crust 0 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere

  16. Regional metamorphism Shock metamorphism High-pressure metamorphism Depth, km Oceanic crust 0 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere

  17. Regional metamorphism Shock metamorphism High-pressure metamorphism Contact metamorphism Depth, km Oceanic crust 0 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere

  18. Regional metamorphism Shock metamorphism High-pressure metamorphism Contact metamorphism Depth, km Oceanic crust 0 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere Burial metamorphism

  19. Regional metamorphism Shock metamorphism High-pressure metamorphism Contact metamorphism Depth, km Oceanic crust 0 35 Continental crust Oceanic lithosphere 75 Continental mantle lithosphere Asthenosphere Water Seafloor metamorphism Burial metamorphism

  20. 3. Metamorphic Textures

  21. 3. Metamorphic Textures Staurolite crystal Mica

  22. Foliated rocks contain platy minerals that are aligned along a preferred orientation. Staurolite crystal Mica

  23. 3. Metamorphic Textures Feldspar Quartz Mica Pyrite Staurolite

  24. Foliation is the result of compressive forces. Feldspar Quartz Mica Pyrite Staurolite

  25. Foliation is the result of compressive forces. Mineral crystals become elongated perpendicular to the compressive force. Feldspar Quartz Mica Pyrite Staurolite

  26. 3. Metamorphic Textures ● preferred orientation of crystals ● slaty cleavage ● foliation

  27. 3. Metamorphic Textures shale sandstone layers 5 cm

  28. The original bedding can be seen in the thin sandy layers. shale sandstone layers 5 cm

  29. The original bedding can be seen in the thin sandy layers. foliation plane shale sandstone layers original bedding 5 cm

  30. The original bedding can be seen in the thin sandy layers. Regional metamorphism causes cleavage planes to develop. foliation plane shale sandstone layers original bedding 5 cm

  31. 3. Metamorphic Textures Low grade Intermediate grade

  32. 3. Metamorphic Textures Diagenesis Low grade High grade Intermediate grade Slate Phyllite Migmatite Schist (abundant micaceous minerals) Gneiss (fewer micaceous minerals) Slaty cleavage Schistosity Banding Banding

  33. Foliated rocks are classified by the degree of cleavage, schistosity, and banding. Diagenesis Low grade High grade Intermediate grade Slate Phyllite Migmatite Schist (abundant micaceous minerals) Gneiss (fewer micaceous minerals) Slaty cleavage Schistosity Banding Banding

  34. 3. Metamorphic Textures ● classification of foliated rocks ● metamorphic grade ● crystal size ● type of foliation ● banding

  35. 3. Metamorphic Textures ● nomenclature of foliated rocks ● slate ● phyllite ● schist ● gneiss ● migmatite

  36. Foliated texture: schist with garnet porphroblasts

  37. 3. Metamorphic Textures ● nomenclature of granoblastic (non-foliated) metamorphic rocks ● hornfels ● quartzites ● marbles ● greenstones ● amphibolites ● granulites (granofels)

  38. Granoblastic Texture

  39. Thought questions for this chapter How is slaty cleavage related to tectonic forces? What forces cause minerals to align with one another? Would you choose to rely on chemical composition or type of foliation to determine metamorphic grade? Why?

  40. 4. Regional Metamorphism and Metamorphic Grade ● grades of metamorphism ● low ● intermediate ● high

  41. 4. Regional Metamorphism and Metamorphic Grade ● mineral isograds (zones of change) ● index minerals reflect pressure and temperature conditions ● groups of 2 to 3 index minerals form an isograd

  42. 4. Regional Metamorphism Canada ME NY Isograds VT NH Key: MA Not metamorphosed CT Chlorite zone Low grade RI Biotite zone Garnet zone Medium grade Staurolite zone Sillimanite zone High grade

  43. Index minerals define metamorphic zones. Canada ME NY Isograds VT NH Key: MA Not metamorphosed CT Chlorite zone Low grade RI Biotite zone Garnet zone Medium grade Staurolite zone Sillimanite zone High grade

  44. Index minerals define metamorphic zones. Isograds can be used to plot the level or degree of metamorphism. Canada ME NY Isograds VT NH Key: MA Not metamorphosed CT Chlorite zone Low grade RI Biotite zone Garnet zone Medium grade Staurolite zone Sillimanite zone High grade

  45. Low Grade Increasing metamorphic grade Slate Intermediate Grade Phyllite High Grade Depth (km) Schist Pressure (kilobars) Gneiss Blueschist Migmatite Temperature (°C)

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