Reversals of the Geomagnetic Field
• Secular variations- historic to modern changes in the field
• Archaeomagnetism: changes during the Holocene
• Reversals of the dipole polarity• Reversal chronology for past 5 million
years: the terrestrial record• reversal chronology for past 200 million
years: the seafloor spreading “tape recorder
Locations of the north pole of the dipole component of the geomagnetic field from 1945-2000.
-30 to 800 BP
800 to 1940 BP
1940 to 3690 BP
Calibrated radiocarbon years before present, (B.P, AD1950=0)
Average pole position for all data(94 poles):88.4 N23.8 W1.6 degrees from geographic North Pole
The north magnetic pole during the past 3700 years.
-30 to 3690 BP
units: nT/yrcontour interval: 5 nT/yrMain field: 30,000 to 60,000 nT
units: minutes/yrcontour interval: 2 min/yr
units: minutes/yrcontour interval: 1 min/yr
Years before present (BP)
Dipole moment determined from the strength of magnetization of archaeological material (archaeomagnetic results from TRM in ancient hearths and pottery)
dipole component
non-dipole component
Schematic plot of magnetic field variations in time
A snapshot of the 3D magnetic field structure simulated with the Glatzmaier-Roberts geodynamo model. Magnetic field lines are blue where the field is directed inward and yellow where directed outward. The rotation axis of the model Earth is vertical and through the center. A transition occurs at the core-mantle boundary from the intense, complicated field structure in the fluid core, where the field is generated, to the smooth, potential field structure outside the core. The field lines are drawn out to two Earth radii. Magnetic field is wrapped around the "tangent cylinder" due to the shear of the zonal fluid flow.
About “36,000 years” into the simulation the magnetic field underwent a reversal of its dipole moment (Figure 3), over a period of a little more than a thousand years. The intensity of the magnetic dipole moment decreased by about a factor of ten during the reversal and recovered immediately after, similar to what is seen in the Earth's paleomagnetic reversal record. Our solution shows how convection in the fluid outer core is continually trying to reverse the field but that the solid inner core inhibits magnetic reversals because the field in the inner core can only change on the much longer time scale of diffusion [2]. Only once in many attempts is a reversal successful, which is probably the reason why the times between reversals of the Earth's field are long and randomly distributed.
500yrs before middle of reversal 500yrs after
The key to determining the chronology of the geomagnetic field reversals is to be able to date the time during which robust magnetizations were attained in a given rock sample. The classical work was done in the latter half of the last century on basaltic rocks, which cool rapidly and acquire a strong thermo-remanent magnetization (TRM). These rocks can be dated effectively with the Potassium-Argon method, which uses the decay of K-40 into the chemically inert Ar-40.
Ar-40 is trapped and accumulates in the rock only since the last time the rock was melted – the time when the basalt was extruded and solidified. While liquid, the prior Ar-40, a gas, leaves the magma. Since the basalt is extruded on the surface, cooling is rapid and the acquisition of TRM occurs soon after the trap is set for accumulation of Ar-40.
Reversal captured in Columbia River basalt flows ( Steens Mtn., Oregon: Miocene, 15.5 Ma)
Steens Mtn: Kiger Gorge from the Steens Mountain Loop Road
Steens Mtn: View northwest from the short trail/road to the summit.
High resolution record of
geomagnetic field reversal
Mankinen, et al., 1985, J. Geophys. Res., v. 90, p, 10400
3500 yrs
3600 yrs
5000 yrs
Reversal captured in Columbia River basalt flows ( Steens Mtn., Oregon: Miocene, 15.5 Ma)
Extrusions at rate of about 43 m/1000 yrs
Steens Mtn results: VGP’s in time
Magnetizations (DRM) recovered from deep ocean sediments
Note minimum intensities during reversals
Magnetizations (DRM) recovered from deep ocean sediments
Geomagnetic field reversal chronology for past 5 million years based mainly on K-Ar dating of terrestrial volcanic rocks
Why only to 5 Ma?
Geomagnetic field reversal chronology for past 5 million years based mainly on K-Ar dating of terrestrial volcanic rocks
Why only to 5 Ma?Errors in K-Ar dates become too large compared to reversal periods
The chronology of geomagnetic field reversals earlier than 5 Ma is well preserved in the magnetization of basalts extruded on the ocean floor in the process of sea-floor spreading.
Age, Ma
moho
Seafloor spreading model
lith
osp
her
e
crust
upper mantle
Schematic representation of upper crustal magnetized layer
1200 deg C
convecting mantle
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 reversal chronology recorded on land
Marine magnetic anomalies
• Ships tow magnetometers which measure the “total intensity” of the geomagnetic field, the magnitude of the geomagnetic field vector, often symbolized by F, or Fobs , to denote that it is the observed total intensity. These measurements lead to a plot of Fobs versus distance along the track.
mag
net
ic f
ield
inte
nsit
y,F
obs
0
Smoothly varying global field plus small, short wavelength effects due to crustal magnetizations
distance along ship track
Marine Magnetic anomalies
• Ships tow magnetometers which measure the “total intensity” of the geomagnetic field, the magnitude of the geomagnetic field vector, often symbolized by F, or Fobs , to denote that it is the observed total intensity. These measurements lead to a plot of Fobs versus distance along the track.
• The main internal geomagnetic field (produced in the outer core), Fg, is determined for the earth as a function of time as the International Geomagnetic Reference Field (IGRF).
• The IGRF field can then be subtracted from the observed value to produce a total intensity anomaly, F = Fobs - Fg
• F results only from effects of rocks magnetized near the surface, and can thus be compared with models of the magnetization of the ocean bottom rocks.
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
Total intensity anomaly, F
Marine Magnetic anomalies
The rocks with the strongest magnetizations by far are the basalts extruded and rapidly cooled, acquiring thermo-remanent magnetization (TRM) via the process of seafloor spreading.
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J
Magnetic field lines for vertically downwards magnetization in cross-sectional view
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- - - - - - - - - - - - - - - -J
Magnetic field lines for vertically upwards magnetization
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- - - - - - - - - - - - - - - -
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 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
sea surface
ocean bottom
Basalt magnetized upon solidification along axis of spreading ridge
axis
of
seaf
loor
sp
read
ing
Direction of modern geomagnetic field
distance along track
Inte
nsi
ty a
nom
aly,
F
reve
rsal
reve
rsal
reve
rsal
reve
rsal
+
-
sea surface
ocean bottom
Basalt magnetized upon solidification along axis of spreading ridge
axis
of
seaf
loor
sp
read
ing
Direction of modern geomagnetic field
distance along track
Inte
nsi
ty a
nom
aly,
F
reve
rsal
reve
rsal
reve
rsal
reve
rsal
Magnetization increases main
field
Magnetization decreases main
field
Magnetization decreases main
field
Global bathymetry, showing ocean ridge system
Eas
t Pac
ific
Ris
e
Mid-A
tlantic Ridge
Global bathymetry, showing ocean ridge system
Eas
t Pac
ific
Ris
e
Mid-A
tlantic Ridge
Global bathymetry, showing ocean ridge system
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 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
Ocean depth, km
Magnetic anomaly, gamma
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
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.
Eltanin profiles of magnetic anomalies
Also note that peaks and troughs in the curves can be correlated from track to track, indicating that the magnetized material on the ocean floor with a positive or negative magnetization can be traced along the strike of the ocean ridge system. These correlations are shown by the numbers, which identify correlatable features in the wiggly lines.
Modeling the magnetic anomaly pattern
mirror image of measured profile to show symmetry
ESEWNW
WNWESE
reversal chronology from paleomagnetic studies on land
cross section through model of normal (black) and reversed (white) magnetized upper crust
total intensity anomaly calculated from model
Observed profile of total intensity anomalies
Age, Ma
moho
Seafloor spreading model
lith
osp
her
e
crust
upper mantle
Schematic representation of upper crustal magnetized layer
1200 deg C
convecting mantle
The seafloor spreading tape recorder extends the record of geomagnetic field reversals out as far as we have ocean basins- this turns out to be about 200 million years worth of recording.
All that is needed is to determine the timing of the recording system back beyond 5 million years.
How? Drilling to the bottom of the sediments that cover the basalts
The Ocean Drilling Program, which started in 1968, and is still working, did just this throughout the world’s oceans.
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
pal
eon
tolo
gic
al a
ge
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
Deep sea drilling in the South Atlantic Ocean
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)