5.2.2 Lithospheric plates: forces acting at plate margins
Other forces that act on plates must be generated at their boundaries. These forces push from the ridge, drag the plates down at the trenches, or act along the sides of plates at conservative boundaries (Figure 28).
At constructive boundaries, the upwelling of hot material at ocean ridges generates a buoyant effect that produces the ocean ridge, which stand some 2-3 km above the surrounding ocean floor. Here, oceanic plates experience a force that acts away from the ridge, the so called ridge-push force (FRP in Figure 28), which is a result of gravity acting down the slope of the ridge. The occurrence of shallow earthquakes, resulting from the repeated tearing apart of newly formed oceanic crust, indicates there is also some frictional resistance to this force at ridges. This can be called ridge resistance, shown as RR in Figure 28. Bounding the ridge segments, the oceanic transform faults, where the plate segments slide past each other, encounter resistance to movement, and produce a series of earthquakes: this retarding force is the transform fault resistance, RTF in Figure 28.
The situation at destructive plate boundaries is more complex. A major component is the downward gravitational force acting on the cold and dense descending slab as it sinks into the mantle. This gravity-generated force pulls the whole oceanic plate down as a result of the negative buoyancy of the slab: this is the negative buoyancy force, shown as FNB in Figure 28. The component of this downward-acting force that is transmitted to the plate is the slab-pull force, shown as FSP in Figure 28. The magnitude of the slab pull is related to the angle at which the plate descends, and is greater for steeply dipping plates. However, the sinking slab encounters resistance as it descends, both from the frictional drag on its upper and lower surfaces and from the viscosity of mantle material that is being displaced: this combined resistive force is termed slab resistance, RS (Figure 28). A further complication is that the downwards moving plate must flex at the trench before it begins to slide beneath the opposing plate. This provides a further resistance to the plate motion; this is labelled bending resistance, RB (Figure 28). Due to the pushing of the subducting slab against the overriding plate, there is in addition, frictional resistance that gives rise to shallow and deep earthquakes at subduction zones. These frictional forces can be labelled collectively as overriding plate resistance, shown as RO in Figure 28.
For the overriding plate, another theoretical force analogous to the ocean driving force has been proposed, which is derived from convection induced in the mantle above the subducted plate. Cooling of the mantle wedge against the upper surface of the subducting plate induces convection that sucks more mantle into the wedge. This is the trench suction force, FSU, which serves to pull the plate towards the trench. The collision of plates and the associated deformation processes generate a collisional resistance force, shown as RCR in Figure 28. This acts in the opposite direction within each converging plate, but it is equal in magnitude in both.
The velocities of present-day plate motions appear to be constant, indicating a state of dynamic equilibrium where a balance exists between the driving and resistive forces. However, each plate moves at its own rate - which suggests that the relative importance of the driving and retarding forces must vary from plate to plate. It seems unlikely that any single force is the sole driving mechanism of plate motions. For example, if the ridge-push force is the only driving force, why does the Philippine Plate, with no ridge on its boundary, move at a similar rate (70 mm y−1) to the Indian Plate, which is bounded by both the Carlsberg and South Indian Ocean spreading ridges? Plate motions must, therefore, be controlled by a combination of forces.
Activity 8
For each of the forces listed in Table 3, decide whether they are likely to act as a driving force or a resistive force, and note the correct answer. Some forces can act as either a driving force or resistive force.
| Force | Acts as a driving force | Acts as a resistive force | Might act as either a driving force or a resistive force |
|---|---|---|---|
| oceanic drag | |||
| continental drag | |||
| ridge-push | |||
| transform fault | |||
| slab-pull | |||
| slab resistance | |||
| trench suction |
Answer
| Force | Acts as a driving force | Acts as a resistive force | Might act as either a driving force or a resistive force |
|---|---|---|---|
| oceanic drag |  | ||
| continental drag | | | |
| ridge-push | | ||
| transform fault | | ||
| slab-pull | | ||
| slab resistance | | ||
| trench suction | | |
To investigate which are the most important forces that act on plates, the reasoning process applied in the previous paragraph can be adopted and the plate speed compared quantitatively with factors that relate to the different driving forces. For example, if the dominant driving force is ridge-push (Frp), then the fastest moving plates should be the oceanic plates with the highest ratio of ridge length to surface area. The forces acting at plate boundaries should have magnitudes that are proportional to the length of ocean ridge (in the case of ridge-push), the length of ocean trench (for slab-pull (FSP), trench suction (FSU) and inter-plate resistances) and the length of transform fault (for transform fault resistance (RTF)). Oceanic and continental drag act over the lower surface of the plate, and so they should be proportional to the area of the plate. The rates of true plate motion in relation to the relative lengths of ocean ridge, ocean trench, transform fault, and plate area must, therefore, be examined to estimate the effects of each of the plate driving forces. The motions of the twelve plates listed in Table 5 are shown graphically in Figure 29a-e. Some of the important physical properties of each plate are listed in the table, together with the average true velocity of each plate calculated relative to a hot-spot reference frame. To explore whether or not each of these physical attributes of plate configuration are significant in producing plate motion, the degree and character of data correlation need to be examined.
| Plate | Total plate area × 106/km2 | Total continental area × 106/km2 | Average true velocity /mm y−1 | Circumference × 102/km | Length (effective length)* × 102/km | ||
|---|---|---|---|---|---|---|---|
| Ocean ridge | Ocean trench | Transform fault | |||||
| (a) Eurasian | 69 | 51 | 7 | 421 | 90 (35) | 0 | 56 |
| (b) N.American | 60 | 36 | 11 | 388 | 146 (86) | 12(10) | 122 |
| (c) S.American | 41 | 20 | 13 | 305 | 87 (71) | 5(3) | 107 |
| (d) Antarctic | 59 | 15 | 17 | 356 | 208 (17) | 0 | 131 |
| (e) African | 79 | 31 | 21 | 418 | 230 (58) | 10(9) | 119 |
| (f) Caribbean | 4 | 0 | 24 | 88 | 0 | 0 | 44 |
| (g) Arabian | 5 | 4 | 42 | 98 | 30 (27) | 0 | 36 |
| (h) Indian | 60 | 15 | 61 | 420 | 124 (108) | 91 (83) | 125 |
| (i) Philippine | 5 | 0 | 64 | 103 | 0 | 41 (30) | 32 |
| (j) Nazca | 15 | 0 | 76 | 187 | 76 (54) | 53 (52) | 48 |
| (k) Pacific | 108 | 0 | 80 | 499 | 152(119) | 124(113) | 180 |
| (l) Cocos | 3 | 0 | 86 | 88 | 40 (29) | 25 (25) | 16 |
*Effective lengths (in brackets) are the lengths of plate boundary that are capable of exerting a net driving or resistive force.
Plot 1 in Figure 29 shows the total plate area against average true plate velocity. The graph reveals that whilst it is true that the plate with the largest area (k, the Pacific Plate) has one of the highest velocities, it does not always follow that large plates have high velocities - the plate with the second largest area (e, the African Plate) has a relatively low velocity. Moreover, the plate with the highest velocity (l, the Cocos Plate) is one of the smallest. Whilst it would be possible to draw a best-fit line through some of the data points (for instance, a, b, d, e and k) that showed reasonable positive correlation (i.e. that larger plates have faster velocities and smaller plates have slower velocities), there is little obvious pattern if the data are taken as a whole. Accordingly, it is reasonable to conclude that there is no real correlation between these two variables of plate area and true plate velocity. This logic can be extended to examine the role of oceanic drag: if this were a significant motive force then these same data would show a positive correlation; if it were a significant plate-retarding force then the data would show a negative correlation. Since neither is true, the role of oceanic drag appears to have an insignificant effect on plate motion.
Plot 2 shows the continental area of each plate compared with the average true velocity of that plate. If the continental 'keel' were acting to retard plate motion, those plates with the largest proportion of continental lithosphere might be expected to display the slowest velocities. Interestingly, the plate with the largest continental area (a, the Eurasian Plate) does have the lowest velocity, while the plate with the lowest continental area (l, the Cocos Plate) has the highest velocity; this is the negative correlation predicted above. Clearly, the data points in Plot 2 do not fall on a line, but the general trend is for low speed to be associated with a large continental area. Accordingly, these data appear to indicate that continental drag is an effective resistance to plate movement.
Figure 29 (interactive)
Activity 9
Figure 29 shows that the most striking positive data correlation is between true plate velocity and the length of ocean trench boundary, which includes a subducted slab (Plot 4). In fact, the plates form two distinct data groups:
-
the Indian, Philippine, Nazca, Pacific, and Cocos Plates (h-k), which are all connected to descending slabs and are all moving at an absolute rate of 60-90 mm y−1 relative to the asthenosphere
-
the remaining plates (i.e. Eurasian, North American, South American, Antarctic, Caribbean and Indian Plates (a-g)) are moving much more slowly at 0-40 mm y−1 and do not have significant connections with descending slabs.
It is reasonable, therefore, to conclude that slab-pull is the dominant plate-driving force. By contrast, there is a poorer correlation between ridge length and plate velocity (Figure 29c), from which it can be concluded that the so-called 'ridge-push' may also contribute to plate speed.
The best correlations from the data shown in Figure 29, and summarised in the answer to Question 9, suggest that true plate velocity depends largely on the slab-pull force associated with the descent of oceanic lithosphere, and to a lesser extent on the ridge-push force; continental plate drag is a major retarding force.
Finally, it should be noted that slab-pull and ridge-push forces are largely a consequence of density differences and gravity-slab pull relates to the gravitational sinking of a cold slab of lithospheric material into the mantle whereas ridge-push relates to the gravitational potential energy of the ocean ridge standing 2-3 km above the surrounding ocean floor. So the answer to the question, 'What drives plate tectonics?' is not directly related to mantle convection, but to gravity acting on density differences in the lithosphere that have resulted from its own thermal history.