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PLATE TECTONICS. Last chapter in Davis and Reynolds. OUTLINE OF LECTURE. Earth engine Plumes Basic ingredients in plate tectonics Plate kinematics In 2-D On a sphere. Review of major questions. Earth layering The composition of the crust

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plate tectonics

PLATE TECTONICS

Last chapter in Davis and Reynolds

outline of lecture
OUTLINE OF LECTURE
  • Earth engine
  • Plumes
  • Basic ingredients in plate tectonics
  • Plate kinematics
  • In 2-D
  • On a sphere
review of major questions
Review of major questions
  • Earth layering
  • The composition of the crust
  • Rheology of the Earth (lithosphere, asthenosphere)
  • Types of plate boundaries
slide5

Heat engine- very efficient

Earth differentiation- primarily by magmatism

Mantle convection-

Mostly solid state

Melting shallow by adiabatic decompression

Lithosphere- the cold

lid at the top

slide8

Spreading at mid-ocean ridges must be compensated by subduction. In addition,there are transform faults in the oceans. Note no volcanism on diagram.

what drives plate motion
What drives plate motion?
  • Most people agree that plates are intimately related to mantle convection;
  • Slab pull?
  • Ridge push?
  • Mantle drag
mantle convection
Mantle convection
  • Time scales
  • Length scales
  • Plume heads and continental breakup
slide13

T - scale ~ plate motions

Length scales - appear much more complicated than the ridge-trench systems

slide15

Continental break-up: plume-caused?

Sometimes clearly not. Other times, major oceans appear to form during times of major flood basalts -short lived, vigorous plume heads that may have broken the continents apart

plate t throughout earth history
Plate T throughout Earth history
  • How far back in the past?
  • Different in the past?
  • How much longer will it last?
slide18

Evidence for PT goes back to the Archean. Faster motions, more melt, smaller continents (the continental nuclei known as cratons or “croutons”)

Granite-greenstone belts; old zircons

slide22

Future is fairly bright as far as PT goes. But after a while, (4 more Ga?), the Earth’s engine won’t have enough power to drive plate. Convection will stop, so will PT.

basic kinematic elements
Basic kinematic elements
  • Plate boundaries, triple junctions
  • Absolute plate motion, relative plate motion
  • Euler poles
  • Worked examples
slide24

Ridges, trenches, transforms

Triple junctions, quadruple j’s

3riple junctions are stable; more plates at a point - not stable

slide25

Absolute plate motions - velocity in an absolute reference frame- say relative to a point outside the Earth. Or an assumed stationary long lived plume…. E.g. Hawaii

Otherwise, one uses a relative velocity reference frame.

One plate is kept stationary; the velocity of the others relative to the “stationary” plate is monitored. The understanding is that the entire system (including the stationary plate) is actually moving on the globe.

In the case of ridges, we use the half spreading rate for velocity calculations.

slide26

Absolute framework - consider Hawaii a stationary plume (it delivers melts in exactly the same spot over its entire history). We can calculate the velocity vector of the Pacific plate.

75-43 - N20W x cm/yr ; 43-0 Ma N70 W, y cm/yr.

slide27

There are very few such long lived plume products and it is questionable whether they remain fixed. The common way of tracking plate motions is in a relative framework.

Some useful rules: 1. Plate motions are transform parallel;

2. Plate moves away from ridge

3. The sum of relative plate velocities is zero*.

*- that is because by definition plates are rigid.

Velocity is a vector: magnitude, direction and sense.

slide36

Complicating a bit- what if the transforms are curved?

We then have to admit there’s some rotation involved.

Any rotation is achieved around a pole. From geometry, this is called the Euler pole. Transforms form arcs that are segments of circles centered in the Euler pole of a plate.

plate tectonics on a sphere
Plate tectonics on a sphere
  • Angular velocity, linear velocity
  • Rotations around Euler poles
  • Projections on stereonets
slide41

Tectonics on a sphere requires that we use angular velocities

v/r and r = R sin g where R is the radius of the Earth.

So what? Check out the fig - predicts motion away from Euler pole. In this case - 2 plates with E at N pole

slide43

The projections used in 3D plate tectonics are stereonets - equal area - however unlike your usual down view with geo structures, this is a side view. All calculations (angles etc) are similar.

what you need to know
What you need to know:
  • The fundamentals of plate tectonics, driving forces; link to mantle convection;
  • Differences between present day and past characteristics of PT;
  • Be able to handle simple 2-3-4-… plate geometry problems in 2D involving only translations.
  • Calculate velocity vectors for such examples;
  • Know what the Euler pole is and angular vs. linear velocity. Be able to find one if you have the other.