Plate-Driving Forces

Figure 1. Observed plate motions. Arrow lengths and colors show velocity relative to the average velocity. Note that subducting plates (Pacific, Nazca, Cocos, Philippine, Indian-Australian plates in the center of this Pacific-centered view) move about 4 times faster than non-subducting plates (North and South American, Eurasian, African, Antarctic plates around the periphery).

Figure 2. Diagram showing the mantle flow associated with the "slab suction" plate-driving mechanism in which the sinking slab is detached from the subducting plate and sinks under its own weight. This induces mantle flow that drives both the overriding and subducting plates toward each other at approximately equal rate.

Figure 3. Predicted plate velocities for the "slab suction" plate-driving model. Note that subducting and non-subducting plates travel at approximately the same speed, which is not what is observed (compare to Fig. 1).

Figure 4. The "slab pull" plate-driving mechanism. Here the slab pulls directly on the subducting plate, drawing it rapidly toward the subduction zone. The mantle flow induced by this motion tends to drive the overriding plate away from the subduction zone. This results in an asymmetrical pattern of plate motions.

Figure 5. Plate motions driven by the slab pull plate-driving mechanism. In this case, plates move with about the right relative speeds, but overriding plates move away from trenches, instead of toward them as is observed.

Figure 6. Our preferred model for how mantle slabs drive plate motions. Slabs in the upper mantle pull directly on surface plates driving their rapid motion toward subduction zones. Slab descending in the lower mantle induce mantle flow patterns that excite the slab suction mechanism. This flow tends to push both overriding and subducting plates toward subduction zones.

Figure 7. Predicted plate motions from our combined model of slab suction from lower mantle slabs and slab pull from upper mantle slabs (Fig. 6). This model predicts both the relative speeds of subducting and overriding plates, as well as the approximate direction of plate motions (compare to observed plate motions, shown in Fig. 1).

How Mantle Slabs Drive Plate Motions

C.P. Conrad and C. Lithgow-Bertelloni, "How mantle slabs drive plate tectonics," Science, 298, 207-209, 2002. [abstract] [online version] [reprint] [supporting material] [U. Michigan Press Release] [Geotimes Article]

Subduction zones (where an one plate dives beneath another into the mantle) produce "slabs" of subducted lithosphere in the mantle. These slabs are cold and dense, and their descent within the mantle is thought to provide the primary energy source that drives viscous flow in the mantle and, ultimately, the motions of Earth's surface plates. However, exactly how mantle slabs drive plate motions has been the subject of debate.

To constrain how mantle slabs drive plate motions, we have developed models of viscous flow in the mantle driven by the descent of slabs. By balancing the stresses that this flow exerts on the base of the plates, we can obtain a prediction of plate motions that we can compare to observed plate motions. In this way, we can evaluate a given model's ability of predict observed plate motions. We define two mechanisms by which mantle slabs could drive plate motions: "slab suction" and "slab pull".

Slab Suction. In the slab suction mechanism, we assume that slabs are detached from their surface plates and sink within the mantle. This downward motions induces a symmetrical flow pattern that tends to push both the subducting and overriding plates toward the subduction zone at approximately equal rates (Fig. 2). This pattern is evident in the pattern of plate motions that is predicted by this mechanism (Fig. 3). Here overriding plates move in the correct direction, but with speeds comparable to those of the subducting plates (note the primarily yellow and green colors in Fig. 3). By contrast, subducting plates on today's Earth typically move about 4 times faster than non-subducting plates (note the red and pink colors for the Pacific and Indian basins in Fig. 1).

Slab Pull. If, instead, we assume that the slab is mechanically attached to the subducting plate, then the weight of the slab will pull on subducting plate, drawing it toward the subduction zones. In addition, the trenchward motion of the subducting plate will induce a flow in the mantle that will tend to drive the overriding plate away from the subduction zones (Fig. 4). The pattern of plate motions predicted by this mechanism (Fig. 5) shows the correct ratio of subducting to non-subducting plate speeds, but overriding plates tend to move away from subduction zones instead of toward them, as is observed (Fig. 1).

Combined Slab Pull and Slab Suction. By combining "slab pull" from upper mantle slabs and "slab suction" from lower mantle slabs, we are able to obtain a model for how slabs drive plate motions (Fig. 6) that gives a good prediction of plate motions (Fig. 7). In this model, upper mantle slab (above 670 km) are physically attached to their subducting plates. Their weight pulls subducting plates toward subduction zones. Slabs in the lower mantle (below 670 km) are detached from the slabs above them and are instead supported by viscous stresses associated with flow in the surrounding mantle. This flow excites the slab suction plate-driving mechanism, which contributes to plate motions at the surface. There are several possible reasons that slabs below 670 km do not contribute to the pull force. First, slabs may not be strong enough to support the weight of slabs below 670 km. Second, higher viscosity in the lower mantle will provide support for slabs there. We estimate that upper mantle slab pull and lower mantle slab suction account for 60% and 40% of the forces on plates, respectively.