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

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

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)

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

ODP drilling site Map pattern of magnetic anomaly number

Map pattern of magnetic anomaly number: North Atlantic

anomaly no M16 M10N M4M0M21M From Muller, et al., 1997 Map pattern of ocean floor age

Continental rifting continental shelf

Mid-ocean ridge: rift valley

Fracture zones: transform faults

South Atlantic Mid-Ocean Ridge

South Atlantic Mid-Ocean Ridge inactive fracture zone active transform fault active ridge crest

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

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

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

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

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

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

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

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

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

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

seafloor age, Ma ocean age from seafloor spreading tape recorder

0 Ma NAM SAM AFR EUR

20 Ma

40 Ma

60 Ma

80 Ma

100 Ma

120 Ma

140 Ma

160 Ma

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

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