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PLATETECTONICS Or Are we dancing on an eggshell? Is there a magma ocean below? …and much more…

PLATETECTONICS Or Are we dancing on an eggshell? Is there a magma ocean below? …and much more…. PLATETECTONICS. What causes plate movement? Heat transfer by mantle convection!. geotherm. Earth’s mantle: peridotite !!!. olivine. Olivine (cut gems). Olivine: thin section.

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PLATETECTONICS Or Are we dancing on an eggshell? Is there a magma ocean below? …and much more…

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  1. PLATETECTONICS Or Are we dancing on an eggshell? Is there a magma ocean below? …and much more…

  2. PLATETECTONICS

  3. What causes plate movement? Heat transfer by mantle convection!

  4. geotherm Earth’s mantle: peridotite!!!

  5. olivine

  6. Olivine (cut gems)

  7. Olivine: thin section

  8. Olivine with rims altered to iddingsite

  9. Pyroxenes: Orthopyroxenes: (Mg,Fe2+)2[SiO6] Clinopyroxenes: (Ca,Na,Li)(Mg,Fe2+,Fe3+,Mn,Al,Ti)[(Si,Al)2O6]

  10. Clinopyroxene: thin section

  11. Plagioclase: albite

  12. Plagioclase: thin section

  13. Olivine (coloured) in plagioclase (grey)

  14. Garnet: (Ca,Mg,Fe,Mn)3(Al,Fe,Cr)3[SiO4]3

  15. Garnet (cut gem)

  16. Garnet: thin section (polarized light)

  17. geotherm Earth’s mantle: peridotite!!!

  18. Earth’s geotherm is well-below the solidus! So why does magma form? i.e., why does the mantle melt?

  19. geotherm

  20. …now back to the surface… moving oceanic and continental plates

  21. plate movement in cm/yr

  22. earthquakes: at plateboundaries (in particular along subduction zones)

  23. …creation of oceanic crust… SPREADING ZONES

  24. mantle convection drives plate tectonics!

  25. heat causes convection - heat transport by convection

  26. mid-oceanic ridges / spreading zones: “constructive” and “divergent” plate boundaries

  27. Structure of the oceanic crust

  28. Recent (modern) pillow basalts at the seafloor off the US west-coast

  29. Ancient pillow basalt

  30. Ancient pillow basalt in a road cut black pillow rims are basaltic glass formed by quenching of hot lava during contact with cold seawater

  31. hydrothermal vent: black smoker: hot, acidic, reducing (colour due to formation of black sulphide particles)

  32. hydrothermal vent: white smoker: <200oC (colour due to formation of silica and sulphate particles)

  33. hydrothermal vents: black and white smokers

  34. most recently discovered (!!!): new type of (cold) hydrothermal vent (heat produced by alteration of oceanic crust, white minerals are Ca carbonate)

  35. Structure of the oceanic crust

  36. Ophiolites: • -pieces of oceanic crust thrusted (obducted) onto the edge of continental plates; • - assemblage of mafic (basalt) and ultramafic (perodotite) lavas found in association with sedimentary rocks; • -found in Cyprus, New Guinea, Newfoundland, California, Turkey, and Oman. • Semail ophiolite in southeastern Oman probably formed in the Cretaceous not far from what is now the Persian Gulf. The rocks were later thrust (pushed uphill at a low angle) westward onto the Arabian shield.

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