PX266 Geophysics (2010/11)
Lecture 14 Handout – Rotation Poles and Tectonic Plate Motion
Dr. Gavin Bell
Plate motions on a ‘flat Earth’ are easily described by vector addition of relative
velocities.
On a spherical Earth, the plates are not planar, but segments of the spherical surface.
Since they are constrained to stay on the surface, their (instantaneous) motion can be
uniquely and completely described by
rotation about an axis passing through the
centre of the Earth
. Formally, this is a consequence of Euler’s ‘fixed point theorem’
for motion of rigid bodies, but it should be fairly obvious from intuition that any
possible motion of a spherical cap segment can be thus expressed.
The intersections of the rotation axis with the Earth’s surface are the
Euler poles
or
instantaneous rotation poles
, one positive and one negative. The sign convention is
that clockwise rotation looking out from the centre of the Earth corresponds to the
positive rotation pole (but don’t worry too much about getting this wrong in an exam).
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 Spring '11
 gavinbell
 Plate Tectonics, rotation poles, Dr. Gavin Bell, positive rotation pole

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