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The Geodynamo and PaleomagnetismBrown and Mussett (1993) ch. 6; Fowler p. 32-50

• In this lecture:The CoreThe Core

• Problems • Outer core

– Physical state– Composition

• Inner core– Physical state– Composition

Paleomagnetism• paramagnetism• ferrimagnetism• remanent magnetizations

The Geodynamo• How it works• The Earth’s magnetic field

Reversals of the Earth’s magnetic fieldWhat can we learn about the deep earth

from them?

LithophileLayered Earth

ChalcophileSiderophile

The Core

Seismology:Outer liquid part (r = 3485 km)

ca. 10,000 kg/m3

29.3% Earth’s mass16% Earth’s volumeLargest magma chamber!

Inner solid part (r = 1225 km)ca. 13,000 kg/m3

1.7% Earth’s mass0.7% volume

CompositionIron + ?

Problems1. Why is the inner core solid?2. How is the Earth’s magnetic field

generated?3. The outer core is made of Fe + what?

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The Core

Composition ?High pressure (diamond-anvil cell) experimentsShock velocities suggest Fe + diluant

Must be capable of alloying with FeMust be capable of partitioning into coreSufficiently abundantProduce alloy that matches seismic properties and density of outer core

Sulfur is the most likely Sulfur is the most likely diluentdiluentIt can reduce the melting point of Fe and aid in separation of the core

from the mantleThe inner core may contain a small amount of siderophile Nickel

LithophileLayered Earth

ChalcophileSiderophile

LithophileLayered Earth

ChalcophileSiderophile

The Earth’s Magnetic Field

Approximately a dipole Approximately a dipole tilted 12tilted 12oo to spin axisto spin axis10% of field is non dipolar10% of field is non dipolarNot a solid bar magnet! Not a solid bar magnet! Why?Why?Two observations important:Two observations important:

1. Above the Curie temperatureMagnetic materials lose permanent magnetismTCurie well below melting point (ca. 550 oC)

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The Earth’s Magnetic Field

2.2. Progressive changes with timeProgressive changes with timeSecular variationSecular variationDirection & strength of field drifts over few Direction & strength of field drifts over few kyrskyrsWestward driftWestward driftInclination (vertical component: I)Inclination (vertical component: I)Declination (horizontal component: D)Declination (horizontal component: D)Magnetic vector (total direction and field strength: H)Magnetic vector (total direction and field strength: H)

Thus, field produced dynamically, not staticallyThus, field produced dynamically, not staticallyThe The GeodynamoGeodynamo modelmodel

The Earth’s Magnetic Field

Approximately a dipole Approximately a dipole tilted 12tilted 12oo to spin axisto spin axis10% of field is non dipolar

• non-dipolar components explain non uniform distribution of total field, inclination, declination over the Earth’s surface

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Magnetization of Rocks

Paramagnetic mineralsRetain information about Earth’s fieldRetain information about Earth’s field1. Contain atoms with odd # of electrons2. Spins & orbits of unpaired electrons generate weak magnetic fields3. When placed into weak external field of the Earth, atomic dipoles rotate into parallelism with the

direction of the external fieldFerrimagnetic minerals (magnetite)

Contain large numbers of unpaired electrons 1. Atoms coupled by interactions of these multiple magnetic fields2. Interaction only below Curie Temperature (550 - 600 oC)3. Alignment of dipoles within intra-crystalline domains gives mineral a magnetic direction (and

intensity) that can be retained as a Permanent or Permanent or RemanentRemanent magnetizationmagnetizationRemanent Magnetization

Can be “hard” or stable for long periods (myrs)1. Thermoremanent Magnetization (TRM)

• Robust I, D, intensity2. Detrital Remanent Magnetization (DRM)

• I, D, commonly lost through coring, low intensities, but still very useful3. Chemical Remanent Magnetization (CRM)

• Forms during precipitation of new minerals at low temperature4. Viscous Remanent Magnetization (VRM)

• Overprinting of original magnetization in weak field

These “hard” components isolated by magnetic cleaning• Spinner or cryogenic magnetometer• Increase external field (AF) or Temperature to progressively remove components to isolate primary

magnetic character of rock

Paleomagnetism

Fossil magnetism retained in certain rocksLava flowsLava flows: Thermoremanent magnetizationSedimentsSediments: Detrital Remanent magnetization

– Measure Inclination, Declination, (vector H including strength, if possible)• Note that D can be obscured by rotation & translation of plates

– Determine the age of the rock using geochronology (e.g., 40Ar/39Ar dating)

– Half of rocks measured give primary remanent magnetization directions 180o

from the others• Polarity of the Earth’s magnetic field has reversed many times• Exploited to construct the Geomagnetic Polarity Time Scale (GPTS)Geomagnetic Polarity Time Scale (GPTS)

– Lengths of polarity Chrons (ca. 1 myr)– Duration of reversals short (kyrs)

– Can determine inclinations for rocks with a range of ages•• Can calculateCan calculate paleolatitudepaleolatitude at which theat which the rock formed• If different than predicted from today’s inclination, either:

– The pole has moved (True Polar Wander)– The continent (plate) has moved

• Calculation to construct Apparent Polar Wander curves

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Paleolatitude from Paleomagnetism

Calculation of paleolatitudetan I = Z/Htan I = 2cosθ/sinθ

= 2cotθ= 2tanλ

λ = magnetic latitude= 90 – θ

Example:Basalt flow at 47oN 20oEMeasured I = 30o

tan 30o = 2tan λλ = tan-1(tan30o/2)

= tan-1(0.2887)= 16.1o

Thus plate has moved 31o

northward since basalt cooled through Tcurie

θ

Z

H

Reversals of Earth’s Magnetic Field

PMAG measurements + K/Ar dating of lava flowsNormal and Reversed polarity intervals first identified on land

Geomagnetic Polarity Time Scale (GPTS)Normal= blackReversed = white

Later verified in ocean crust and sedimentsThe ocean crust as a tape recorder:

Evolution of the GPTS in the 1960’s

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Reversals of Earth’s Magnetic Field

Geomagnetic Polarity Time Scale (GPTS)• most complete calendar for last 100 myr• based on distance vs. age fit for marine magnetic

anomalies corresponding to polarity chronboundaries, and a small number of radioisitopic dates

Cande and Kent(1995) Journal of Geophysical Research

Reversals of Earth’s Magnetic Field

Why does the field reverse?

The Geodynamo modelDynamics of field suggest source in fluid outer core

Magnetic field generated by electric current loops in outer core, powered by convection of molten material

Disk dynamo analogya. Conducting disk rotates in magnetic field

– Generates emf potential, but no current flowsb. Current flows if circuit completed between

rim of disk and axle– Requires external field and energy source– Generates secondary magnetic field

c. If current passed through coil to axle, secondary magnetic field powers the current flow — this is a self-exciting dynamo

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The Geodynamo

Models are in their infancy

• Rotation influences flow within outer core• Magnetic field lines are generated by liquid iron

flowing in helical paths– Tanget cylinder– Influence of the inner core

• Energy source to drive convective flow– Cooling and heat loss by convection

The GeodynamoGlatzmaeir and Roberts Numerical Simulations

– Magnetohydrodynamic equations solved in finite element model using linked supercomputers– Models have been run to simulate several kyrs– Spontaneous reversals have occurred:

1 intial state2. 500 yr prior to reversal

3. Middle of reversal

4. 500 yr after reversal

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The Geodynamo

Glatzmaier and Roberts Model

• Predicts inner core rotation faster than outer core• This has been verified by seismically

– Measure seismic velocities along same path through inner core several years apart

– Seismic velocities decreased between 1996 and 1990– Best explanation is that inner core and its

anisotropic structure has rotated faster than the recording stations.

The Geodynamo

Validating Glatzmaier and Roberts Model

• Of the several models run, one used seismic-tomographically constrained heat flux at the core-mantle boundary as a boundary condition

– This model also produced a 22 kyrlong reversal

• Lava flow recordings of the Matuyama-Brunhes reversal:

– Reversal takes ca. 16 kyr to complete

– Recorded pole positions over regions of reduced heat flux across the core-mantle boundary

• Our lava flow data suggest that this model is the best match to observations of the last reversal

Observed Matuyama-Brunhes

lava VGPs

mean ages of

Maui 777.6 kaTahiti 791.7 kaChile 792.3 kaLa Palma 798.5 ka

lava sequences

-60o-60o

-30o-30o

0o0o

30o 30o

60o 60o

780.3

782.4

778.9 778.9

803.3

776.2

791.2

796.2

789.3794.0

797.4

797.3

785.1

778.7756.2

784.3

787.3

792.2

797.7791.3

791.6

a

Other M-B Lava VGPs

Tongjing, China Iceland La Guadeloupe

Glatzmaier and Roberts numerical simulation tomographic heat flow at CMB

VGP density during 64.2 kyr of model,including 22 kyrreversal

(From Coe et al., 2000)

-60o-60o

-30o

-30o

0o

30o 30o

60o 60ob

Density of the VGP's

T=64,220 yr

c