earth’s major plates note: boundaries rarely correspond to the contact between oceans and...

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:

1: The earth is divided into continents and oceans

Alfred Wegener proposes continental drift ca. 1912

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 faultingand seismicity

5: when continents rift apart, you find an ocean

between them

RiftHorst Graben

Put it all together, and seethat the continents were once connected but have

drifted apart by ‘sliding’ overor ‘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

• 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

A ridge and its transforms seen in plan view

Ahh, trenches

The real story with seismicityWagener was only partially correct. The ring of fire is a locus

of faulting, but there are other loci not expected if continents drift

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.

Benioff-Wadati zones

Centered on most great belts of seismicity, andis rare elsewhere

Active Volcanism

Terrestrial Magnetism

Intensity(in mysterious units

— 10,000 ’s)

Dip

Preservation in rocks of the orientation of the magnetic field

Variation through time of the apparent location of thenorth 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…

MeasuredAnomaly

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

The anomalies represent positive and negative interference frommagnetic rocks in the crust

A magnetic ‘reversal’ occurs between these two times

How do we know? We’ve seen them appear after volcaniceruptions 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

40K

40Ca 40Ar88.8 % 11.2 %

e- capture; e = 0.581x10-10 yr-11e- emission; = 4.982x10-10 yr-1

40Ar = e/40K(et-1) + 40Ar0

= e + = 5.543x10-10 yr-1

0.01167 % of natural K

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’smagnetic stripes is complete:

Implied rates of plate divergence, convergence and strike-slip motion