The Geodynamo and Paleomagnetism
Brown and Mussett (1993) ch. 6; Fowler p. 32-50
• In this lecture:
The Core
• Problems
• Outer core
– Physical state
– Composition
Layered Earth
Lithophile
• Inner core
– Physical state
– Composition
Paleomagnetism
• paramagnetism
• ferrimagnetism
• remanent magnetizations
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/m3
29.3% Earth’s mass
16% Earth’s volume
Largest magma chamber!
Inner solid part (r = 1225 km)
ca. 13,000 kg/m3
1.7% Earth’s mass
0.7% volume
Composition
Iron + ?
Problems
1.
2.
3.
Why is the inner core solid?
How is the Earth’s magnetic field
generated?
The outer core is made of Fe + what?
1
The Core
Layered Earth
Lithophile
Composition ?
High pressure (diamond-anvil cell) experiments
Shock velocities suggest Fe + diluant
Must be capable of alloying with Fe
Chalcophile
Must be capable of partitioning into core
Siderophile
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
The Earth’s Magnetic Field
Approximately a dipole
tilted 12o 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
TCurie well below melting point (ca. 550 oC)
2
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 12o 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
3
Magnetization of Rocks
Paramagnetic minerals
Retain information about Earth’s field
1.
2.
3.
Contain atoms with odd # of electrons
Spins & orbits of unpaired electrons generate weak magnetic fields
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.
2.
3.
Atoms coupled by interactions of these multiple magnetic fields
Interaction only below Curie Temperature (550 - 600 oC)
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)
2.
Detrital Remanent Magnetization (DRM)
3.
Chemical Remanent Magnetization (CRM)
•
•
•
4.
Robust I, D, intensity
I, D, commonly lost through coring, low intensities, but still very useful
Forms during precipitation of new minerals at low temperature
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:
flows Thermoremanent magnetization
Sediments:
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., 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)
–
–
–
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
4
Paleolatitude from Paleomagnetism
Calculation of paleolatitude
tan I = Z/H
tan I = 2cosθ/sinθ
= 2cotθ
= 2tanλ
λ = magnetic latitude
= 90 – θ
Example:
H
Basalt flow at 47oN 20oE
Measured I = 30o
Z
θ
tan 30o = 2tan λ
λ = tan-1(tan30o/2)
= tan-1(0.2887)
= 16.1o
Thus plate has moved 31o
northward since basalt
cooled through Tcurie
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
5
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
–
b.
–
–
c.
Generates emf potential, but no current flows
Current flows if circuit completed between
rim of disk and axle
Requires external field and energy source
Generates secondary magnetic field
If current passed through coil to axle,
secondary magnetic field powers the current
flow — this is a self-exciting dynamo
6
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
4. 500 yr after
reversal
2. 500 yr prior to
reversal
3. Middle of
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
756.2
784.3
778.7
a
60o
60o
Observed
Matuyama-Brunhes
lava VGPs
785.1
30
30o
o
Validating Glatzmaier and Roberts
Model
•
•
797.3
o
0
791.6
797.7
791.3
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 MatuyamaBrunhes reversal:
0
–
Reversal takes ca. 16 kyr to
complete
–
Recorded pole positions over
regions of reduced heat flux across
the core-mantle boundary
-30o
787.3
792.2
794.0
789.3
797.4
796.2
o
0
780.3
782.4
803.3
791.2
-30o
778.9 778.9
o
-60
mean ages of
lava sequences
Maui
777.6 ka
Tahiti
791.7 ka
Chile
792.3 ka
La Palma 798.5 ka
-60
o
776.2
o
60
60o
30o
30o
o
-30
o
•
o
-30
o
-60o
Other M-B Lava VGPs
Tongjing, China
Iceland
La Guadeloupe
-60
c
Our lava flow data suggest that this
model is the best match to
observations of the last reversal
Glatzmaier and Roberts
numerical simulation
tomographic heat
flow at CMB
VGP density during
64.2 kyr of model,
including 22 kyr
reversal
T=64,220 yr
(From Coe et al., 2000)
Density of the VGP's
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