Geoscience Reference
In-Depth Information
To calculate the rotation vector for the Nazca plate relative to the Antarctic plate
we apply Eq. (2 19):
N
=
A
P
+
P
N
(2.32)
A
Substituting the tabulated values into the equations for the
x
,
y
and
z
components of
N
(Eqs. (2.24)-(2.26)) yields
x
AN
=
8
.
7 cos(
−
64
.
3) cos(96
.
0)
+
13
.
6 cos(55
.
6) cos(
−
90
.
1)
=−
0
.
408
A
(2.33)
y
AN
=
8
.
7 cos(
−
64
.
3) sin(96
.
0)
+
13
.
6 cos(55
.
6) sin(
−
90
.
1)
=−
3
.
931
(2.34)
z
AN
=
8
.
7 sin(
−
64
.
3)
+
13
.
6 sin(55
.
6)
=
3
.
382
(2.35)
The magnitude of the rotation vector
A
N
can now be calculated from Eq. (2.27)
and the pole position from Eqs. (2.28) and (2.29):
0
.
408
2
ω
N
=
+
3
.
931
2
+
3
.
382
2
=
5
.
202
(2.36)
A
λ
AN
=
sin
−
1
3
.
382
5
.
202
=
40
.
6
(2.37)
φ
AN
=
tan
−
1
−
3
.
931
−
=
180
+
84
.
1
(2.38)
0
.
408
Therefore, the rotation for the Nazca plate relative to the Antarctic plate has a
magnitude of 5.2
×
10
−
7
deg yr
−
1
, and the rotation pole is located at latitude
40.6
◦
N, longitude 95.9
◦
W.
2.5 Plate boundaries can change with time
The examples of plates moving upon a flat Earth (Section 2.2) illustrated that
plates and plate boundaries do not stay the same for all time. This observation
remains true when we advance from plates moving on a flat model Earth to
plates moving on a spherical Earth. The formation of new plates and destruction
of existing plates are the most obvious global reasons why plate boundaries and
relative motions change. For example, a plate may be lost down a subduction zone,
such as happened when most of the Farallon and Kula plates were subducted under
the North American plate in the early Tertiary (see Section 3.3.3). Alternatively,
two continental plates may coalesce into one (with resultant mountain building).
If the position of a rotation pole changes, all the relative motions also change.
A drastic change in pole position of say 90
◦
would, of course, completely alter
the status quo: transform faults would become ridges and subduction zones, and
vice versa! Changes in the trends of transform faults and magnetic anomalies on
the Pacific plate imply that the direction of seafloor spreading has changed there,
