Metamorphism

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Metamorphism

  1. 1. Metamorphism – transformation of one rock type into another<br /><ul><li> Metamorphic rocks – produced from pre-existing sedimentary, igneous and metamorphic rocks
  2. 2. Parent rock – origin of a metamorphic rock</li></ul>See next slides for samples of metamorphic rocks<br />
  3. 3. Norway<br />
  4. 4. China<br />
  5. 5. Lake Huron, Ontario, Canada<br />
  6. 6. Michigan, North America<br />
  7. 7. California, North America<br />
  8. 8. California, North America<br />
  9. 9. Metamorphism leads to changes in:<br /><ul><li> Mineral content
  10. 10. Texture
  11. 11. Chemical composition</li></ul>Metamorphic agents are:<br /><ul><li> Heat
  12. 12. Pressure
  13. 13. Chemically active fluids</li></li></ul><li>Metamorphism occurs from a few km into the crust all the way down to the upper mantle.<br />
  14. 14. Degrees of metamorphism<br /><ul><li> Low-grade metamorphism : little change in the parent rock
  15. 15. High-grade metamorphism : identity of parent rock becomes hard to distinguish
  16. 16. Remember : parent rock should not melt, if it does, then igneous processes (not metamorphic) become involved</li></li></ul><li>Example of low-grade metamorphism (from shale to slate)<br />Shale (sedimentary rock)<br />Slate (metamorphic rock)<br />
  17. 17. Example of high-grade metamorphism: from shale (top) to slate (top right) to gneiss (right)<br />
  18. 18. Degrees of metamorphism<br />Agents of metamorphism<br />Time<br />low-grade<br />LOW<br />SHORT<br />Heat<br />Pressure<br />Chemical activity<br />HIGH<br />high-grade<br />LONG<br />
  19. 19. Heat as an agent of metamorphism<br />Heat provides the energy of activation for the chemical transformation in metamorphic rocks.<br />Chemical reactions result in:<br /><ul><li>recrystallization of existing minerals
  20. 20. formation of new minerals </li></ul>Chemical changes are all based on the increased kinetic energies of the ions.<br />
  21. 21. Heat as an agent of metamorphism<br />Temperatures increase with depth at a rate known as the geothermal gradient.<br />Hence, degree of metamorphism increases as depth increases.<br />
  22. 22. The deepest mine in the world<br />The Western Deep Levels Mine in South Africa at 4 km is the deepest in the world.<br />At these depths, the rock is actually hot enough to burn human skin. Miners work in pairs- one digging, the other operating a large fan to keep cool.<br />
  23. 23. What heats up the Earth?<br /><ul><li> Primordial heat – trapped when Earth’s crust first cooled
  24. 24. Radioactivity – energy released when subatomic particles are spontaneously emitted by radioactive elements
  25. 25. Solar radiation – absorbed and converted to thermal energy by the Earth’s surface</li></li></ul><li>Primordial heat<br />Earth initially formed as a molten ball of rock. When the surface cooled to become the crust, the heat became trapped.<br />This trapped heat now drives the convection cells in the mantle and the movement of the molten outer core round the inner core.<br />
  26. 26. Solar radiation – the Earth’s crust absorbs about 50% of the Sun’s energy<br />
  27. 27. The absorbed solar radiation is distributed unevenly. The greatest concentration occurs in the tropical belt (red zone).<br />
  28. 28. Radioactivity<br />Radioactive elements contribute to Earth’s internal heat. Background radiation (not enough to harm) exists all around us. Shown at right is a geologist measuring background radiation with a portable Geiger counter.<br />
  29. 29. Pressure as an agent of metamorphism<br />Two types of pressure:<br /><ul><li> Confining pressure – experienced by buried rocks; forces are applied equally in all directions
  30. 30. Differential stress – forces are unequal in different directions</li></li></ul><li>Confining pressure is evenly distributed.<br /><ul><li> squeezes out the spaces between mineral grains
  31. 31. produces a more compact, denser rock</li></li></ul><li>Confining pressure produces rock layers that are undeformed.<br />
  32. 32. Directonial stress deforms rock layers as shown in C. (the series shows the deformation of sediments deposited in a river flood plain)<br />
  33. 33. The layers of rock shown below have been deformed by directional stress. Such pressure is most active in convergent plate boundaries.<br />
  34. 34. Rocks in shallow depths are pulverized when subjected to differential stress. At greater depths, on the other hand, rocks are more ductile and are compressed and elongated rather than crushed.<br />A sample of conglomerate becomes metaconglomerate when subjected to differential stress at depth (note the elongated rock fragments).<br />
  35. 35. Heat and pressure<br />Heat increases with increasing pressure. This means that areas of mountain building (continental vs continental convergent plates) are hotter due to greater pressure.<br />
  36. 36. METAMORPHIC ROCKS<br /><ul><li> definition
  37. 37. degrees of metamorphism
  38. 38. agents of metamorphism</li></li></ul><li>Where does metamorphism occur?<br /><ul><li> contact metamorphism
  39. 39. regional metamorphism
  40. 40. dynamic metamorphism</li></li></ul><li>Contact metamorphism<br />
  41. 41. Contact metamorphism<br />
  42. 42. Contact metamorphism<br />
  43. 43. Contact metamorphism<br />
  44. 44. Contact metamorphism<br />Granite<br />Gneiss<br />
  45. 45. From limestone to marble<br />
  46. 46. From igneous rocks to marble<br />Syenite<br />(igneous)<br />marble<br />
  47. 47. From sediments to metamorphic rocks<br />
  48. 48. Regional metamorphism<br />Mountain-building:<br /><ul><li> plate tectonics (convergent boundaries)
  49. 49. directional stresses are involved
  50. 50. greatest volume of metamorphic rocks are produced in this way</li></li></ul><li>Review of plate tectonics<br />
  51. 51.
  52. 52. Regional metamorphism<br />
  53. 53. Regional vs contact metamorphism<br />
  54. 54. Textural changes<br />Texture – size, shape, and distribution of particles that constitute a rock<br /><ul><li> foliated
  55. 55. non-foliated</li></li></ul><li>Textural changes (foliation)<br />
  56. 56. Types of foliation:<br /><ul><li> rock or slaty cleavage – minute crystals
  57. 57. schistosity – larger crystals
  58. 58. gneissic texture – segregation of minerals</li></li></ul><li>Rock or Slaty cleavage<br />
  59. 59. Rock or <br />Slaty cleavage<br />
  60. 60. Rock or Slaty cleavage<br />
  61. 61. Schistocity<br />mica schist – most abundant schist type<br />
  62. 62. Schistocity<br />mica schist<br />
  63. 63. Schistocity<br />
  64. 64. Schistocity<br />
  65. 65. Schistocity<br />
  66. 66. Schistocity<br />
  67. 67. Schistocity<br />talc schist<br />
  68. 68. Schistocity<br />talc schist<br />
  69. 69. Schistocity<br />talc schist<br />
  70. 70. Gneissic texture<br />
  71. 71. Gneissic texture<br />
  72. 72. Gneissic texture<br />
  73. 73. Gneissic texture<br />
  74. 74. Nonfoliated texture<br />marble<br />
  75. 75. Nonfoliated texture<br />
  76. 76. Nonfoliated texture<br />
  77. 77. Nonfoliated texture<br />
  78. 78. Nonfoliated texture<br />quartzite<br />
  79. 79. Nonfoliated texture<br />quartzite<br />

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