Geoscience Reference
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(a)
(b)
(c)
Geographic
North Pole
Rotation
pole
Positive
rotation pole
longitudes of
rotation
(great circles)
ω
ω
P
θ
O
X
N
latitudes of
rotation
(small circles)
Geographic
South Pole
Figure 2.8.
The movement of plates on the surface of the Earth. (a) The lines of
latitude of rotation around the rotation poles are small circles (shown dashed)
whereas the lines of longitude of rotation are great circles (i.e., circles with the same
diameter as the Earth). Note that these lines of latitude and longitude are
not
the
geographic lines of latitude and longitude because the poles for the geographic
coordinate system are the North and South Poles, not the rotation poles. (b)
Constructive, destructive and conservative boundaries between plates A and B. Plate
Bisassumed to be fixed so that the motion of plate A is relative to plate B. The
visible rotation pole is positive (motion is anticlockwise when viewed from outside
the Earth). Note that the spreading and subduction rates increase with distance from
the rotation pole. The transform fault is an arc of a small circle (shown dashed) and
thus is perpendicular to the ridge axis. As the plate boundary passes the rotation
pole, the boundary changes from constructive to destructive, i.e. from ridge to
subduction zone. (c) A cross section through the centre of the Earth O. P and N are
the positive and negative rotation poles, and X is a point on the plate boundary.
where
is the angular distance between the rotation pole P and the point X, and
R
is the radius of the Earth. This factor of sin
θ
means that the relative motion
between two adjacent plates changes with position along the plate boundary, in
contrast to the earlier examples for a flat Earth. Thus, the relative velocity is zero
at the rotation poles, where
θ
0
◦
and 180
◦
, and has a maximum value of
R
at
90
◦
from the rotation poles. If by chance the plate boundary passes through the
rotation pole, the nature of the boundary changes from divergent to convergent, or
vice versa (as in Fig. 2.8(b)). Lines of constant velocity (defined by
θ
=
ω
θ
=
constant)
are small circles about the rotation poles.
2.4 Present-day plate motions
2.4.1 Determination of rotation poles and rotation vectors
Several methods can be used to find the present-day
instantaneous poles of rota-
tion
and
relative angular velocities
between pairs of plates.
Instantaneous
refers
to a geological instant; it means a value averaged over a period of time ranging
