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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
Eas
t Pac
ific
Ris
e
Mid-A
tlantic 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.
21
20
19
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
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
ESEWNW
total intensity anomaly calculated from model
WNWESE
measured profile of total intensity anomalies
mirror image of measured profile to show symmetry
+500
-500
+500
-500
0
0
mag
net
ic f
ield
inte
nsit
y,F
obs
distance along ship track
inte
nsi
y an
omal
y,
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
Mag
netiz
ed re
ctang
ular p
risms
(“stri
pes”)
paral
lel to
sprea
ding a
xis
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)
long
itudi
nal o
r stri
ke di
recti
on, a
ssum
ed in
finite
Vertical, downwards
perpendicular to strike
Typical model for seafloor spreading type of anomaly: very long rectangular prism (2-D model)
long
itudi
nal o
r stri
ke di
recti
on, a
ssum
ed in
finite
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”
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, HeVertically 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
Wo Wd
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
pale
onto
logi
cal a
ge,
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
http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html
4600 My 600 My
Age range of modern ocean floor
Transform faults
7000
3500
1500
-250
0
-150
0
-500
0 500
-550
0
-450
0
-350
0
-650
0
South Atlantic Ocean
7000
3500
1500
-250
0
-150
0
-500
0 500
-550
0
-450
0
-350
0
-650
0
South Atlantic Ocean
inactive fracture zone
active transform faultactive 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
5 6
10.9 120.4
anomaly no. 1813 25 31 3421 M16M10N
M4M0 M21 M25
154.3 180
Age of ocean floor
From Muller, et al., 1997
Pangea
Tethys OceanPacific 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
