magnetic field intensity,F obs

1. Smoothly varying global field plus small, short wavelength effects of crustal magnitizations magnetic field intensity,Fobs 0 distance along ship track subtract global field to yield total intensity anomaly intensiy anomaly, DF 0 distance along ship track

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

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

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

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

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

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

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

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

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

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

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

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

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

15. W d t Combination of rectangular prisms 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 DF/2J Earth’s field, He Wo axis of spreading

16. Multiple prisms vertically magnetized in alternate directions: combined effect DF/2J

17. Age of ocean floor M10N anomaly no. 5 6 13 18 21 25 31 34 M0 M4 M16 M21 M25 120.4 154.3 180 10.9 From Muller, et al., 1997

18. Tethys Ocean Pangea Pacific Ocean

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

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

28. Interaction of East Pacific Rise with western North America

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

31. Age of ocean floor M10N anomaly no. 5 6 13 18 21 25 31 34 M0 M4 M16 M21 M25 120.4 154.3 180 10.9 From Muller, et al., 1997