4 Plate tectonic motion
4.1 Relative plate motions
Plates move relative to one another and relative to a fixed reference frame, such as the rotational axis of the Earth. Plates also move across the curved surface of the Earth and so should not be considered as flat sheets on a flat surface but as caps moving across the roughly spherical surface of the Earth. Consequently, plate motion is not as simple as it might at first appear. This section begins with a consideration of relative plate motions and how they can be measured before moving on to the more complex assessment of plate motions across a sphere and how true plate motions may be measured.
In previous sections you have already tackled the problem of assessing the full and half spreading rates of ocean ridges.
Recall two different methods of determining spreading rates.
The use of dated magnetic anomalies, and the known relationship between ocean depth and age.
A simple calculation dividing the distance from the ridge by the age gives the plate speed and, combined with the direction of travel, its velocity.
Figure 22 shows a section through the Earth from the Atlantic to the Indian Ocean, cutting across three different plates and two constructive plate boundaries. The half spreading rates are shown for each plate at each plate boundary. For the situations in Table 1, estimate the relative rates and relative directions of motion of the African Plate.
Figure 22 (interactive)
The answer to this question reveals two important points about plate motion:
measured plate velocities (speed and direction) must be stated relative to one another;
plates and their boundaries cannot be fixed in relation to the mantle.
The second point derives from a consideration of the African Plate - as it grows in size, at least one of its constructive boundaries must have moved. So how can we determine the true movement of a plate against a truly fixed frame of reference? One possibility would be to assume that one plate is stationary and to determine plate movement relative to that 'fixed' reference (as in Question 6). However, polar wander curves for all of the continents show that all plates bearing continents have moved relative to the Earth's magnetic pole over periods of tens of millions of years. Another way would be to determine plate motion relative to surface features that might be more firmly rooted in the deeper mantle and for this we turn to volcanism that is not dependent on plate boundary interactions - the so-called within-plate or hot-spot volcanism on ocean islands.