Download 1: The earth is divided into continents and oceans

Earth’s major plates
Note: Boundaries rarely correspond to the
contact between oceans and continents!
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
Terrestrial Magnetism
Dip
Intensity
(in mysterious units
— 10,000 ’s)
Preservation in rocks of the
orientation of the magnetic field
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…
Anomaly
Measured
(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
0.01167 % of natural K
e- emission;  = 4.982x10-10 yr-1
40Ca
88.8 %
40K
e- capture; e = 0.581x10-10 yr-11
40Ar
11.2 %
 = e +  = 5.543x10-10 yr-1
40Ar
= e/40K(et-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:
Implied rates of plate divergence, convergence and strike-slip motion