Download ppt - Reversals of the Geomagnetic Field Secular variations- historic to modern changes in the field Archaeomagnetism: changes during the Holocene Reversals

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


inte

nsi

y an

omal

y,

F

0

0

Smoothly varying global field plus small, short wavelength effects of crustal magnitizations


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.

+++ + + + + + + + + + + + +

- - - - - - - - - - - - - - - -

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


Earth’s field, He Magnetized prism field adds to Earth’s field, F positive

+++ + + + + + + + + + + + +

- - - - - - - - - - - - - - - -

J


Earth’s field, He Magnetized prism field perpendicular to He, F = 0

+++ + + + + + + + + + + + +

- - - - - - - - - - - - - - - -

J


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

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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

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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


Eas

t Pac

ific

Ris

e

Mid-A

tlantic Ridge


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.

http://www.oceandrilling.org/

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)