1. Chronology of geomagnetic field reversals magnetic anomaly “number” Ocean floor age, millions of years (Ma), determined largely from deep sea drilling

3. Age, Ma moho Seafloor spreading is a tape recorder of the geomagnetic field! crust upper mantle The recording head of the tape recorder The tape drive The recorded reversal chronology

4. Ocean Ridge system East Pacific Rise Mid-Atlantic Ridge Modern view of ocean bathymetry derived from satellite altimetry. see EXPLORING THE OCEAN BASINS WITH SATELLITE ALTIMETER DATA Global Bathymetric Prediction for Ocean Modelling and Marine Geophysics

5. Global bathymetry Map shown in next slide

6. Ship tracks across the East Pacific Rise which obtained the magnetic anomalies shown in the next slide. The measurements were made in the 1960 ’ s by the Columbia University research vessel Eltanin. 21 20 19

7. The Eltanin 19 profile is among the most influential geophysical profiles ever published. It provided the “ smoking gun ” evidence for seafloor spreading, evidence that turned a majority of skeptics into a majority of believers. The profile was published together with three others in 1966 by Pitman and Heirtzler in “ Magnetic Anomalies over the Pacific-Antarctic Ridge ” (Science, 154, 1164-1171). The figures above comes from that paper. The track lines (ELT 19-21) of the research vessel Eltanin are shown together with the correlated magnetic anomalies (numbered dashed lines) and the 2000 fathom bathymetric contour. The crosses are earthquake epicenters. The inferred active spreading center would be between anomalies 1 and 1 ’. The voyage occurred in 1965. The famous Eltanin 19 profile

8. The four profiles show total intensity anomalies and bathymetry (ocean depth in km) along the four tracks shown on the previous map. Note that track 20 crosses the ridge system twice. The vertical scale for total intensity anomaly,  F, is shown in “gammas”. This is the same as nanoTeslas or nT. The horizontal lines are at zero anomaly; the scale is thus minus 500 to plus 500 nT. Eltanin profiles of magnetic anomalies

9. The incredible symmetry of the Eltanin 19 profile ESE WNW total intensity anomaly calculated from model WNW ESE measured profile of total intensity anomalies mirror image of measured profile to show symmetry +500 -500 +500 -500 0 0

10. magnetic field intensity,F obs distance along ship track intensiy anomaly,  F 0 0 Smoothly varying global field plus small, short wavelength effects of crustal magnitizations distance along ship track subtract global field to yield total intensity anomaly

11. Magnetized rectangular prisms ( “ stripes ” ) parallel to spreading axis direction of cross section (perpendicular to axis of spreading) thickness of magnetized prisms Strike of spreading ridge

12. Typical model for seafloor spreading type of anomaly: very long rectangular prism (2-D model) longitudinal or strike direction, assumed infinite Vertical, downwards perpendicular to strike

13. Typical model for seafloor spreading type of anomaly: very long rectangular prism (2-D model) longitudinal or strike direction, assumed infinite thickness, t width, w Vertical direction of cross section, z axis horizontal direction of cross section, x axis

14. vertical downwards magnetization equivalent to two parallel strips of magnetic poles as shown: top strip of negative poles and bottom strip of positive poles Vertical, downwards perpendicular to strike strips extend to “ infinity ”

15. + ++++++++++++++ ---------------- J Magnetic field lines for vertically downwards magnetization in cross-sectional view

16. + ++++++++++++++ ---------------- J Magnetic field lines for vertically upwards magnetization

17. + ++++++++++++++ ---------------- J Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Earth ’ s field, He Vertically downwards magnetization parallel to vertical earth ’ s field ocean surface

18. + ++++++++++++++ ---------------- J Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Earth ’ s field, He Magnetized prism field adds to Earth ’ s field,  F positive

19. + ++++++++++++++ ---------------- J Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Earth ’ s field, He Magnetized prism field adds to Earth ’ s field,  F positive

20. + ++++++++++++++ ---------------- J Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Earth ’ s field, He Magnetized prism field perpendicular to He,  F = 0

21. + ++++++++++++++ ---------------- J Magnetic field due to magnetized prism taken along the surface above the prism (directions only) Earth ’ s field, He Magnetized prism field subtracts from He,  F negative

22.  F/2J Magnetic total intensity anomaly from single vertically magnetized prism distance, x, along cross section perpendicular to the trend of the rectangular prism width = 10 km

23.  F/2J Magnetic total intensity anomaly from single vertically magnetized prism width = 80 km distance, x, along cross section perpendicular to the trend of the rectangular prism

24.  F/2J Five rectangular prisms: one central one plus two pairs symmetrically on either side (seafloor spreading model in the region near the spreading axis); magnetic anomaly of each prism is plotted separately Earth ’ s field, He Combination of rectangular prisms WoWd t axis of spreading

25.  F/2J Multiple prisms vertically magnetized in alternate directions: combined effect

26. Map of magnetic anomaly numbers Deep Sea Drilling sites

27. magnetic anomaly number Age (Ma) from geomagnetic reversal chronology extrapolated in South Atlantic assuming constant rate of spreading paleontological age, Ma Seafloor ages from deep sea drilling versus geomagnetic reversal chronology Deep sea drilling in the South Atlantic Ocean

28. Chronology of geomagnetic field reversals magnetic anomaly “number” Ocean floor age, millions of years (Ma), determined largely from deep sea drilling

29. Geologic time scale http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html 4600 My 600 My Age range of modern ocean floor

30. Transform faults

31. 7000 3500 1500 -2500 -1500 -500 0 500 -5500-4500-3500 -6500 South Atlantic Ocean

32. 7000 3500 1500 -2500 -1500 -500 0 500 -5500-4500-3500 -6500 South Atlantic Ocean inactive fracture zone active transform fault active ridge crest

33. inactive fracture zone active transform fault active ridge crest inactive fracture zone

35. Transform faults and fracture zones

36. Map pattern of magnetic anomaly number Deep sea drilling calibration of chronology of geomagnetic field reversals back to 180 Ma. Map pattern of ocean floor age

37. 56 10.9 120.4 anomaly no. 181325313421 M16 M10N M4M0M21M25 154.3180 Age of ocean floor From Muller, et al., 1997

38. Pangea Tethys Ocean Pacific Ocean

43. Creation of Atlantic & Indian oceans and associated “ passive margins ”

46. closing of Tethys Ocean: collisional tectonics of Mediterranean-central and southeast Asia

48. Interaction of East Pacific Rise with western North America

49. Continued subduction beneath western South America as East Pacific Rise comes closer