Lecture

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Lecture 06- Plate Motions
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Tectonic Plates are Rigid Caps not Flat,
Planar Sheets
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Plate Motions on a Sphere

• Mathematically speaking, the most general way to
  describe the movement of a rigid plate (cap) on a
  sphere is through rotation about a fixed pole
• The is known as Eulers Fixed Point Theorem

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Plate Motions on a Sphere
The pole of rotation for plate motion is an
abstract mathematical concept, it is not a
physical feature. The point representing the pole
is known as an Euler pole.
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Euler Poles
Euler poles for plate motions are different than
the geographic North and South Poles. The Euler
pole for the motion of a plate is often located
on another plate
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Example of Euler pole describing the relative
motion of Africa from a fixed, North American
reference frame.
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Euler Poles and Ridges
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Euler Poles and Ridges/Transforms
Ridge Segments point towards the Euler pole
Corresponding transform faults are arcs of
circles having the Euler pole as a center.
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GPS Derived Plate Motions
Where is the Euler Pole ?
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Relative Plate Motions

• It is important to remember that plate motions
  are generally given relative to a reference
  frame.
• Example
• The half-spreading rate vs. full spreading rate
  at a MOR.

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Relative Plate Motions
Hypothetical three plate example We know the
relative velocity of B and C we also know the
relative velocity of A and B. What is the
relative velocity of A and C???
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Relative Plate Motions

• To solve this problem we note the basic law
  relating relative plate velocities
• aVc aVb bVc
• The velocity of plate C relative to plate A
  equals the velocity of plate B relative to plate
  A plus the velocity of plate C relative to plate
  B.

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Remember That Velocities are Vector Quantities
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Remember That Velocities are Vector Quantities
(This is an example of adding two vectors
graphically.)
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Solution to Problem
The velocity of plate C relative to plate A has a
direction that is perfectly horizontal, and a
magnitude of (3)(1.414) cm/yr.
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Plate Motion Summary

• Tectonic plates are spherical caps
• The motion of tectonic plates is best described
  as rotation about an Euler pole
• Plate motion is generally generally given with
  respect to a fixed reference frame
• The velocities of plate are vectors, they have a
  magnitude and direction