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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Last chapter in Davis and Reynolds
Earth differentiation- primarily by magmatism
Mostly solid state
Melting shallow by adiabatic decompression
Lithosphere- the cold
lid at the top
Spreading at mid-ocean ridges must be compensated by subduction. In addition,there are transform faults in the oceans. Note no volcanism on diagram.
Length scales - appear much more complicated than the ridge-trench systems
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
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
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.
Triple junctions, quadruple j’s
3riple junctions are stable; more plates at a point - not stable
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.
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.
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.
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.
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
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.