1. magnetic anomaly number age (Ma) from geomagnetic reversal chronology extrapolated in South Atlantic assuming constant rate of spreading paleontological age Seafloor ages from deep sea drilling versus geomagnetic reversal chronology data for Atlantic ocean; similar data from older oceans permit reversal chronology to be calibrated back to 180 Ma

2. Chronology of geomagnetic field reversals recorded on ocean floor Ocean floor age, millions of years (Ma), determined largely from deep sea drilling (ODP program)

3. Chronology of geomagnetic field reversals recorded on ocean floor magnetic anomaly “number” is a convenient identifier of specific features of the magnetic anomaly profiles that have proven useful for correlation between different profiles. Ocean floor age, millions of years (Ma), determined largely from deep sea drilling (ODP program)

4. Age of the ocean floor Spatial correlation of magnetic anomalies produced by the seafloor spreading “tape recorder” of reversals of the geomagnetic field” Chronology of reversals of geomagnetic field Map pattern of magnetic anomalies and age of ocean floor

5. ODP drilling site Map pattern of magnetic anomaly number

6. Map pattern of magnetic anomaly number: North Atlantic

8. 56 10.9 120.4 anomaly no. 181325313421 M16 M10N M4M0M21M25 154.3180 From Muller, et al., 1997 Map pattern of ocean floor age

11. Continental rifting continental shelf

12. Mid-ocean ridge: rift valley

13. Fracture zones: transform faults

14. 7000 3500 1500 -2500 -1500 -500 0 500 -5500-4500-3500 -6500 South Atlantic Mid-Ocean Ridge

15. 7000 3500 1500 -2500 -1500 -500 0 500 -5500-4500-3500 -6500 South Atlantic Mid-Ocean Ridge inactive fracture zone active transform fault active ridge crest

16. Global topography and bathymetry Bathymetry of world’s oceans: close correlation with ocean floor age shown in next slide

17. seafloor age, Ma Ocean floor age from seafloor spreading tape recorder

18. seafloor age, Ma Seafloor bathymetry Close correlation with ocean floor age and bathymetry

19. spreading ridge heat flow by conduction Heat flow by convection heat flow by conduction Simple thermal model for spreading ridge showing conductive thermal “boundary layer” (= lithosphere) convecting mantle beneath. (oceanic crust is not shown) depth sea water

20. temperature spreading ridge heat flow by conduction Heat flow by convection depth heat flow by conduction profile A depth The temperature profile B (temperature versus depth) at the ridge crest has a very rapid increase from sea bottom temperatures near 0 C to temperatures of 1300 C near the top of the hot, convecting mantle sea water

21. spreading ridge heat flow by conduction Heat flow by convection temperature depth heat flow by conduction profile B At some distance, x, away from the ridge crest, the seafloor has an age = x/v (where v is half the spreading rate). During the time since the plate formed at the spreading ridge, the plate has cooled to form a conductive “boundary layer”. The thickness of the conductive boundary layer increases with time. depth sea water v v x

22. temperature spreading ridge heat flow by conduction Heat flow by convection depth temperature depth heat flow by conduction profile A profile B profile A profile B depth The decrease in temperature due to cooling of the plate as it moves away from the ridge crest leads to thermal contraction and consequent deepening of the ocean bottom. Thus the depth of the ocean increases with the age of the sea floor. sea water

23. ocean depth vrs age of ocean floor See thermal.pdf for derivation of these curves thermal.pdf

24. Thickness of conductive boundary layer (lithosphere) vrs age of ocean floor

25. Subducted oceanic lithosphere Oceanic crust “island arc” subduction zone What happened to the sub-oceanic plates older than 200 Ma? Subduction! Continental Margin subduction zone trench volcanic arc

26. seafloor age, Ma ocean age from seafloor spreading tape recorder

27. 0 Ma NAM SAM AFR EUR

28. 20 Ma

29. 40 Ma

30. 60 Ma

31. 80 Ma

32. 100 Ma

33. 120 Ma

34. 140 Ma

35. 160 Ma

36. 180 Ma The reconstructions are from C.R. Scotese’s PALEOMAP Project

37. Plate motions: the movie Click on the above to play the the “movie”, a file called SFS.MOV. This is a large file (38+ mb); at first it will be slow as the file is downloading, but then you can run it back and forth quite rapidly. It will play faster if you first right-click on the link above, download the file, and then run it from your own machine. Alternatively, you can find the same file (SFS.MOV) on our class folder in the computer lab. The CD that comes with the class text (Stanley) has a lower resolution version of the same reconstruction. The file shows accurate reconstructions of plate positions based on fitting the map patterns of ocean floor ages decoded from the map patterns of magnetic reversals recorded by the seafloor spreading “tape recorder”. The reconstructions are from the PALEOMAP Project (C.R. Scotese) at the University of Texas in Austin.PALEOMAP Project