Earth s major plates
Note: Boundaries rarely correspond to the
contact between oceans and continents!
Implied rates of plate divergence, convergence and strike-slip motion
Rayleigh number =
Buoyancy
Viscous drag
acceleration
Momentum diffusivity
X Thermal diffusivity
Thermal expansion
Temperature contrast
Length scale
Kinematic viscosity
Thermal diffusivity
A numerical model of whole-mantle convection in a
2-D earth
Types of plate boundaries
Relation between igneous activity and plate boundaries
Starting point: Continents and oceans are the pieces of an Eden
broken up by the great flood:
Alfred Wegener proposes continental drift ca. 1912
1: The earth is divided into continents
and oceans
Alfred Wegener, ca. 1920,
in a weird hat.
2: The continents look like pieces of a puzzle, and share elements
of their geology.
3: The distribution of fossil animals and plants re-enforces the
tie-points between the continents
4: The edges of the continents are locations of faulting
and seismicity
5: when continents rift
apart, you find an ocean
between them
Horst
Rift
Graben
Put it all together, and see
that the continents were
once connected but have
drifted apart by sliding over
or plowing through the ocean floor
Driving force: mystery, or
centrifugal force, or both
Counter-arguments
This is just stupid
Any force strong enough to push a continent over a bed of
ocean floor would internally deform the continent instead
Counter-arguments
Not all pieces of the continental puzzle really match
Counter-arguments
Many continental margins don t even fit geometrically
Counter-arguments
Even if they fit, so what! Lots of fits are possible just by chance
Breakthrough:
Harry Hess and the exploration of the ocean floor
A ridge
• Led to discovery that the ocean floor is the active part of the plate
system, not the passive medium through which continents move.
A ridge and its transforms seen in plan view
Ahh, trenches
The real story with seismicity
Wagener was only partially correct. The ring of fire is a locus
of faulting, but there are other loci not expected if continents drift
Benioff-Wadati zones
And, the seismicity at the edges of the ring of fire don t represent
continents sliding over oceans; they are places where ocean floor plunges
into the deep earth interior.
Active Volcanism
Centered on most great belts of seismicity, and
is rare elsewhere
1600: Recognition that earth has a magnetic field; supposed
to be like a ‘lodestone’ (magnetite with a permanent
magnetic field)
1919: Larmor proposes it is actually a dynamo
1946: Elsasser explains field as consequence of convection
of induced electric currents in liquid outer core
Geodynamo
• Ohmic decay would dissipate earth’s field, if static, in ~20,000 years
Therefore, must be sustained by some dissipative process (convection)
• Fundamental cause of magnetism is induction in a convecting, conducting medium
– electrically conductive medium
– convection in response to density gradients in that medium
– maintenance of density gradients by heat generation/exchange and/or
chemical differentiation
• ~80% dipole; rest other modes
• Varies in intensity and orientation and relative strength of
various harmonic components with a wide range of t constants
Numerical model of ‘tangled’ magnetic field lines in core
Terrestrial Magnetism
Inclination…
…and declination
Map of
inclination
Intensity
(in mysterious units
— 10,000 γ s)
Preservation in rocks of the
orientation of the magnetic field
Ferrimagnetism in magnetite
Fe+3 only
Fe+3 and Fe+2
spin of unpaired electrons in the Fe+2 and Fe+3 sub-lattices
can order at low temperature. These oppose each other,
but are unequal, leading to a net magnetic moment
Variation through time of the apparent location of the
north magnetic pole, based on records from North American rocks
Phanerozoic records of magnetic polar wander from Europe and North
America disagree…unless they have moved relative to each other (or, the
shape of the Earth s magnetic field has varied)
Interesting, but all it really does is support and flesh out
Wegener s view of continental drift .
Much more important is an incredibly subtle detail to the fine
structure of the modern magnetic field…
A closer look…
Measured
Anomaly
(I.e., relative to long-wavelength field)
The anomalies represent positive and negative interference from
magnetic rocks in the crust
A magnetic reversal occurs between
these two times
How do we know? We ve seen them appear after volcanic
eruptions on the sea floor.
Some real examples
Calibrating the ocean floor s strip-chart recorder
1: Collecting samples with the Glomar Challenger
K-Ar dating
e- emission; λβ = 4.982x10-10 yr-1
40Ca
88.8 %
40K
0.01167 % of natural K
e- capture; λe = 0.581x10-10 yr-11
40Ar
11.2 %
λ = λe + λβ = 5.543x10-10 yr-1
40Ar
= λe/λ40K(eλt-1) + 40Ar0
Some closure temperatures w/r to K/Ar dating:
Amphibole: 500 to 700 ˚C
Biotite: 300 to 400 ˚C
K-feldspar: 200-250 ˚C
Use in dating magnetic reversals
Maps of magnetic stripes
Sometimes it s fun to pretend that our record of the seafloor s
magnetic stripes is complete: