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Mechanisms of Plate Motion

How can massive volumes of rock move around the surface of the earth?
In order to properly understand this question, the forces that drive plate motion must first be understood.

This is not as simple as it may seem. There has been much debate over the years about the exact mechanisms that drive the lithospheric plates. Presented below are currently the most widely accepted mechanisms.

All the forces acting on the plates can be categorised into two main classes: driving and resisting forces.

Driving:

 - Slab Pull

 - Slab (Trench) Suction

 - Ridge Push

Resisting:

 - Slab Resistance

 - Continental Resistance

 - Transform Fault Resistance

 - Basal Drag

Driving Forces
Slab Pull

This force occurs as a subducting plate sinks into the hot mantle beneath it. The subducting plate, usually basalt, is denser than the material it is subducting into, purely due to its difference in temperature. As the plate sinks into the mantle, it acts to pull the rest of the plate behind it. This force is considered by some to be the primary force driving plate motion at collisional zones (Wilson, 1993). However, there are some plates where there is little or no subduction occurring such as the Antarctic Plate. This plate motion needs to be explained by a different mechanism.

The slab pull force only works when the subducting slab is well attached to the plate it is pulling behind it. When the slab is not well attached it may set up additional circulation patterns in the mantle that serve to suck the plate downwards. It is also interesting to note that plates with a slab subducting into the mantle move faster towards the subduction zone than do plates without a slab. This is thought to be primarily due to the slab pull acting on the plate. This fact tends to support the idea that slab pull is indeed a dominant force in plate motion (Conrad and Lithgow-Bertelloni, 2002).
 

Fig.3.1. Slab Pull and Collisional Resistance

Slab (Trench) Suction

This force occurs between two colliding plates where one is subducting beneath the other. As one plate subducts, it sets up convection currents in the upper mantle that 'exert a net trenchward pull' ie, acts to suck both the plates together (Wilson, 1993).
Associated with the slab suction force is the idea of trench roll-back. As a slab of oceanic crust subducts into the mantle, the hinge of the plate (the point where the plate begins to subduct) tends to regress away from the trench. This occurs because there is effectively no force to hold the hinge in one location.

Fig.3.2 Diagram illustrating Trench Suction and Trench Roll-back


Ridge Push
Before the nineties, this force was considered to be the leading contributor for driving the plates around the earth. However, this idea has now changed with the main mechanism determined to be slab pull. However, ridge push is still considered to be of significance, especially where there is little or no slab pull acting on the plate (eg the Antarctic Plate mentioned above). There have been two main models of ridge push proposed by geologists. Bott (1991) states that the two competing models are that of 'gravity wedging' and 'gravity sliding'. Recently, gravity sliding has emerged as the dominant model.

The name given to this force is actually quite decieving and has led to a misundertanding of this process. The ridges are not pushed apart at their edges as is commonly thought from the gravity  wedging model. The 'pushing' on the plates is actually due to a difference in gravitational potential energy between a plate at its spreading centre and subduction zone. It is known that mid-oceanic ridges rise thousands of metres above the ocean floor. When new sea floor is created, it is hot and relatively thin, as well as being much higher in elevation than the abyssal plains and trenches. As the rock moves away from the spreading centre, the rocks continue to cool. Additional material is pasted onto the base of the crust from the mantle below. This means that as a plate moves away from the spreading zone it gets denser, heavier and thicker.

Beneath the lithosphere is a zone of soft 'plastic' material called the aesthenosphere. This material is less dense than the plate riding above it and acts as a massive shear zone for the over-riding plate. The plate will effectively slide down the slope of the aesthenosphere due to the weight difference between the plate at its spreading centre and subduction zone. Since the plate gets thicker and denser the further away from the spreading centre, the ridge push force will increase towards the subduction zone.

Fig.3.3. Diagram showing the ridge push force
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