The Geodynamo and Paleomagnetism Brown and Mussett (1993) ch. 6; Fowler p

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1 In this lecture: The Core The Geodynamo and Paleomagnetism Brown and Mussett (1993) ch. 6; Fowler p Problems Outer core Physical state Composition Inner core Physical state Composition Paleomagnetism paramagnetism ferrimagnetism remanent magnetizations Layered Earth Lithophile The Geodynamo How it works The Earth s magnetic field Chalcophile Siderophile Reversals of the Earth s magnetic field What can we learn about the deep earth from them? The Core Seismology: Outer liquid part (r = 3485 km) ca. 10,000 kg/m % Earth s mass 16% Earth s volume Largest magma chamber! Inner solid part (r = 1225 km) ca. 13,000 kg/m 3 1.7% Earth s mass 0.7% volume Composition Iron +? Problems 1. Why is the inner core solid? 2. How is the Earth s magnetic field generated? 3. The outer core is made of Fe + what?

2 The Core Layered Earth Lithophile Composition? High pressure (diamond-anvil cell) experiments Shock velocities suggest Fe + diluant Must be capable of alloying with Fe Must be capable of partitioning into core Sufficiently abundant Produce alloy that matches seismic properties and density of outer core Sulfur is the most likely diluent It can reduce the melting point of Fe and aid in separation of the core from the mantle The inner core may contain a small amount of siderophile Nickel Chalcophile Siderophile The Earth s Magnetic Field Approximately a dipole tilted 12 o to spin axis 10% of field is non dipolar Not a solid bar magnet! Why? Two observations important: 1. Above the Curie temperature Magnetic materials lose permanent magnetism T Curie well below melting point (ca. 550 o C)

3 The Earth s Magnetic Field 2. Progressive changes with time Secular variation Direction & strength of field drifts over few kyrs Westward drift Inclination (vertical component: I) Declination (horizontal component: D) Magnetic vector (total direction and field strength: H) Thus, field produced dynamically, not statically The Geodynamo model The Earth s Magnetic Field Approximately a dipole tilted 12 o to spin axis 10% of field is non dipolar non-dipolar components explain non uniform distribution of total field, inclination, declination over the Earth s surface

4 Magnetization of Rocks Paramagnetic minerals Retain information about Earth s field 1. Contain atoms with odd # of electrons 2. Spins & orbits of unpaired electrons generate weak magnetic fields 3. When placed into weak external field of the Earth, atomic dipoles rotate into parallelism with the direction of the external field Ferrimagnetic minerals (magnetite) Contain large numbers of unpaired electrons 1. Atoms coupled by interactions of these multiple magnetic fields 2. Interaction only below Curie Temperature ( o C) 3. Alignment of dipoles within intra-crystalline domains gives mineral a magnetic direction (and intensity) that can be retained as a Permanent or Remanent magnetization Remanent Magnetization Can be hard or stable for long periods (myrs) 1. Thermoremanent Magnetization (TRM) Robust I, D, intensity 2. Detrital Remanent Magnetization (DRM) I, D, commonly lost through coring, low intensities, but still very useful 3. Chemical Remanent Magnetization (CRM) Forms during precipitation of new minerals at low temperature 4. 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 rocks Lava flows: Thermoremanent magnetization Sediments: 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., 40 Ar/ 39 Ar dating) Half of rocks measured give primary remanent magnetization directions 180 o from the others Polarity of the Earth s magnetic field has reversed many times Exploited to construct the 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 calculate paleolatitude at 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

5 Paleolatitude from Paleomagnetism Calculation of paleolatitude tan I = Z/H tan I = 2cosθ/sinθ = 2cotθ = 2tanλ λ = magnetic latitude = 90 θ Example: Basalt flow at 47 o N 20 o E Measured I = 30 o H Z tan 30 o = 2tan λ λ = tan -1 (tan30 o /2) = tan -1 (0.2887) = 16.1 o Thus plate has moved 31 o northward since basalt cooled through T curie θ Reversals of Earth s Magnetic Field PMAG measurements + K/Ar dating of lava flows Normal and Reversed polarity intervals first identified on land Geomagnetic Polarity Time Scale (GPTS) Normal= black Reversed = white Later verified in ocean crust and sediments The ocean crust as a tape recorder: Evolution of the GPTS in the 1960 s

6 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 chron boundaries, 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 model Dynamics 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 analogy a. Conducting disk rotates in magnetic field Generates emf potential, but no current flows b. 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

7 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 Geodynamo Glatzmaeir 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 state yr prior to reversal yr after reversal 3. Middle of reversal

8 -60o 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 0 Model o 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 kyr b long reversal Lava flow recordings of the Matuyama- Brunhes reversal: 0 Reversal takes ca. 16 kyr to o complete Recorded pole positions over regions of reduced heat flux across the core-mantle boundary c Our lava flow data suggest that this model is the best match to observations of the last reversal a 30 o 30 o -30 o -30 o 60 o 60 o o -60 o o 30 o -60 o T=64,220 yr 60 o -60 o -30 o -30 o 0 o Observed Matuyama-Brunhes lava VGPs mean ages of lava sequences Maui ka Tahiti ka Chile ka La Palma ka 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 kyr reversal (From Coe et al., 2000) Density of the VGP's