1. Subduction Zones

3. Geosyncline

4. What We Actually See

5. What Does This Look Like?

8. Andes – True Scale

9. Continent-Continent Collision

10. Continent-Terrane Collision

11. Where Does Ocean Crust Go?
Hugo Benioff, 1954
1. How we know plate tectonics happens

12. Benioff’s Interpretation
1. How we know plate tectonics happens

13. Benioff’s Interpretation Updated
1. How we know plate tectonics happens

14. How Plates Move

15. A Subduction Zone

16. Subduction and Metamorphism

17. Why Mountains are High

18. Where the Plates Meet

19. Terrane Accretion

20. Terranes in Western North America

21. Subduction Zone Rocks

22. Steinmann Trinity (1905) Serpentinite Pillow Basalt Radiolarian Cherts
Characteristic of ophiolite settings
Other Distinctive Rocks
Graywacke and Flysch
Blueschist

23. Bedded Chert

24. Serpentinite

25. Serpentinite

26. Magnesite Mines in Serpentinite

27. Pillow Lava

28. Pillow Lava

29. Pillow Lava

30. Blueschist

31. Graywacke

32. Flysch

33. Melange

34. Melange

35. Eclogite

36. Molasse, Switzerland

37. Molasse and the High Alps

38. Wisconsin 2 Billion Years Ago

39. 1900 MY Penokean Orogeny

40. Before the Penokean Orogeny

41. First Collision

42. Second Phase

43. Marshfield Terrane Collides

44. Assembling North America

45. Assembling North America

46. 1700 MY Granite and Rhyolite

47. Mazatzal Orogeny

48. Cactus Rock

49. Baraboo Interval Quartzites

50. “Baraboo Interval”

51. Yavapai Orogeny

52. Baraboo Quartzite

53. Mazatzal-Yavapai Events

54. 1450 MY Wolf River Batholith

55. Wolf River Batholith

56. 1100 MY Mid-Continent Rift

57. The Grenville Orogeny Begins

58. The Grenville Orogeny

59. A Subduction Zone
The story begins where two oceanic plates converge and one sinks into the earth's mantle, a process called subduction. In these diagrams light green indicates basaltic oceanic crust, dark green is a layer of the earth's mantle about 100 kilometers thick that forms the base of the plate. These two units collectively are called the lithosphere. Brown represents deeper mantle and red represents young igneous rocks. An ocean trench marks the place where the descending plate sinks into the mantle.

61. When the descending plate reaches about 100 kilometers deep it begins to be heated.

62. The descending plate has been soaking in sea water for many millions of years and is wet. Heating drives water into the surrounding mantle.

63. Water doesn't affect just the earth's surface but its deep interior as well. Water lowers the melting point of rocks and causes the adjacent mantle to begin melting. Magma rises upward.

64. How Arcs Grow

68. Exotic Terranes
Continued subduction builds a larger volcanic chain and the weight of the volcanoes causes the crust to sag. Erosion off the volcanic arc sheds sediment onto the flanks. Eventually, convergence of the plates may bring a submarine volcano, or seamount, into the subduction zone.

69. When the seamount enters the subduction zone, something has to give.

70. Often the seamount is thrust onto or beneath the arc.

71. When the seamount reaches the subduction zone, it may be shoved under the other plate or may break off and be thrust onto the other plate. Sediment accumulating in the trench also may get shoved onto the arc as well. This process is called obduction.

72. At times, a sliver of oceanic crust may break off and ride onto the other plate as well to form an ophiolite.

73. Submarine volcanic plateaus may also collide with the volcanic arc.

74. The submarine volcanic plateau begins to be thrust onto the arc
The submarine volcanic plateau begins to be thrust onto the arc. It is too thick to be subducted so it will either be thrust onto the arc, or it will cause the subduction zone to relocate somewhere else.

75. Here a submarine volcanic plateau has been added to the arc
Here a submarine volcanic plateau has been added to the arc. Seamounts, ophiolites and volcanic plateaus are all made of igneous rock but ophiolites, in particular, have a distinctive structure that sets them apart.

76. Eventually so much material can be added to the arc that the subduction zone clogs and a new subduction zone forms. Repeating this process over 100 million years can build up a very sizable land mass like Cuba, the other Greater Antilles, or Costa Rica and Panama.