Tectonics on a sphere: the geometry of plate tectonics - The Solid Earth - page 38

Latitude Longitude Location

28 ◦ S 74 ◦ E

Southeast Indian Ridge

7 ◦ N 60 ◦ E

Carlsberg Ridge

22 ◦ N 38 ◦ E Red Sea

55 ◦ S 5 ◦ E Southwest Indian Ridge

52 ◦ S 5 ◦ E Mid-Atlantic Ridge

9 ◦ N 40 ◦ W Mid-Atlantic Ridge

35 ◦ N 35 ◦ W Mid-Atlantic Ridge

66 ◦ N 18 ◦ W

Iceland

36 ◦ N 8 ◦ W

Gorringe Bank

35 ◦ N 25 ◦ E E. Mediterranean

12 ◦ S 120 ◦ E Java Trench

35 ◦ N 72 ◦ E Himalayas

35 ◦ S 74 ◦ W

S. Chile Trench

4 ◦ S 82 ◦ W

N. Peru Trench

20 ◦ N 106 ◦ W Middle America Trench

4. Define the pole of relative angular velocity for two plates and explain why it is a useful
concept. Give two practical methods for finding poles of rotation from geophysical data.
When these data are not available, how would you try to locate the poles? (Cambridge
University Natural Sciences Tripos IB, 1977.)
5. Ares is a planet with just two plates, A and B (Fig. 2.24). Plate B comprises the lower
hemisphere and plate A the upper, as shown. Points a, b and c lie on the equator, and
point d is diametrically opposite b. The zero meridian passes through point a. The pole
of rotation of plate A relative to plate B is at 45 ◦ N, 0 ◦ E. The amplitude of the angular
velocity vector is 10 − 10

c

b

radians per terrestrial year. The radius of Ares is 3400 km.

(a) What is the nature of the plate boundary between plates A and B?
(b) State where magnetic lineations might be found and sketch the pattern that would
be observed.
(c) Calculate the relative velocity between plates A and B at locations a, b, c and d.
(d) Sketch possible fault-plane solutions for earthquakes occurring at locations a, b, c
and d.
(e) Discuss the possible existence of such a two-plate planet.
(f) Discuss briefly how the stability or instability of a two-plate tectonic system
depends upon the pole position and/or relative size of the two plates.

e B

Figure 2.24. Planet Ares
has just two plates.

N

6 The lithosphere of Tritekton, a recently discovered spherical satellite of Jupiter, consists
of three plates, as shown in Fig. 2.25. Plate A is a hemisphere, and plates B and C are
half a hemisphere each. Points a, b and c lie on its equator, and point d (not visible) lies
diametrically opposite b. The zero meridian lies midway between b and d and passes
through a. The pole of rotation B to A lies in plate B at latitude 30 ◦ N longitude 0 ◦ .
The amplitude of the angular velocity vector through this pole is 3 × 10 − 9 radians per
terrestrial year. The rotation pole C to B is at the north pole N, and the amplitude of
the angular velocity vector is 6

Pl

c

b

Plate A

Figure 2.25. Tritekton has
three plates. × 10 − 9 rad yr − 1 . The radius of Tritekton is 6000 km.