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

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

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

Global bathymetry Map shown in next slide

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

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, ). 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 The famous Eltanin 19 profile

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

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

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

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

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

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

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 ”

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

J Magnetic field lines for vertically upwards magnetization

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

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

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

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

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

 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

 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

 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

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

Map of magnetic anomaly numbers Deep Sea Drilling sites

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

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

Geologic time scale My 600 My Age range of modern ocean floor

Transform faults

South Atlantic Ocean

South Atlantic Ocean inactive fracture zone active transform fault active ridge crest

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

Transform faults and fracture zones

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

anomaly no M16 M10N M4M0M21M Age of ocean floor From Muller, et al., 1997

Pangea Tethys Ocean Pacific Ocean

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

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

Interaction of East Pacific Rise with western North America

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