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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:.

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slide1

Earth’s major plates

Note: Boundaries rarely correspond to the contact between oceans and continents!

slide5

Alfred Wegener proposes continental drift ca. 1912

1: The earth is divided into continents and oceans

Alfred Wegener, ca. 1920,

in a weird hat.

slide10

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

slide11

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

slide12

Counter-arguments

Not all pieces of the continental puzzle really match

slide13

Counter-arguments

Many continental margins don’t even fit geometrically

slide14

Counter-arguments

Even if they fit, so what! Lots of ‘fits’ are possible just by chance

slide15

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.

slide18

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

slide19

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.

slide20

Active Volcanism

Centered on most great belts of seismicity, and

is rare elsewhere

slide22

Dip

Intensity

(in mysterious units

— 10,000 ’s)

slide24

Variation through time of the apparent location of the

north magnetic pole, based on records from North American rocks

slide25

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’.

slide26

Much more important is an incredibly subtle detail to the fine structure of the modern magnetic field…

slide27

A closer look…

Anomaly

(I.e., relative to long-wavelength field)

Measured

slide28

The anomalies represent positive and negative interference from

magnetic rocks in the crust

A magnetic ‘reversal’ occurs between these two times

slide31

Calibrating the ocean floor’s ‘strip-chart recorder’

1: Collecting samples with the Glomar Challenger

slide32

K-Ar dating

0.01167 % of natural K

40K

e- emission;  = 4.982x10-10 yr-1

e- capture; e = 0.581x10-10 yr-11

40Ca

40Ar

88.8 %

11.2 %

 = e +  = 5.543x10-10 yr-1

40Ar = e/40K(et-1) + 40Ar0

slide33

Some ‘closure temperatures’ w/r to K/Ar dating:

Amphibole: 500 to 700 ˚C

Biotite: 300 to 400 ˚C

K-feldspar: 200-250 ˚C

slide36

Sometimes it’s fun to pretend that our record of the seafloor’s

magnetic stripes is complete: