Making Plate Tectonics Happen

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Convection Cells

The interior of the earth is hot. The heat is partly left over from the formation of the Earth, but is also due to ongoing decay of radioactive heat producing elements. If more heat is generated in some areas than others, it is possible for convection currents to be set up, as originally suggested by the English geologist Arthur Holmes in 1928. Slow moving convection currents within the asthenosphere or possibly within the whole mantle, move the overlying lithopsheric plates with them. Convection cells are set up when hot, lower density material rises upwards. This then flows sideways and starts to cool down. As the material cools it becomes denser and sinks back down.

Where a convection current rises to the surface at a divergent plate margin or hot spot there is:

high heat flow

rising magma

eruption of lava

Near the surface of the Earth, the current stops rising and spreads out laterally on either side of the MORs, resulting in sea floor spreading away from the ridge. It is this part of the convection cell that carries the rigid lithosphere across the Earth's surface. Beneath the lithosphere, partial melting in the asthenosphere reduces friction and act as a lubricant for the movement of the lithosphere.

Where two of these lateral currents meet at subduction zones, either one or both start to sink, as they are now much cooler and therefore denser. There is:

a deep ocean trench

low heat flow

evidence of compression

Subduction zones are convergent plate margins. This descending flow of material eventually replaces the mantle material, which rises under the MORs.

Other Possible Mechanisms for Plate Movement

Ridge Push at MORs

Rising magma injected along the MORs at divergent plate margins forcibly pushes the lithospheric plates apart. The sheeted dykes found on either side of the ridge axis are evidence for this process. Some geologists argue that this model is flawed because tensional forces are dominant along ridges and it would be more accurate to describe this process as gravitational sliding off the raised ridges.

Slab Pull at Subduction Zones

Recently, attention has focused on the idea that gravity pulls subducted oceanic lithosphere down into the mantle at convergent plate boundaries. This may be the main driving force for the lithospheric plate movement. In this model, the weight of cold, dense lithosphere sinking downwards at ocean trenches pulls the rest of the oceanic lithosphere with it.

Recycling the Earth

We have already seen that no area of oceanic crust is older than 200 Ma, because in geological terms oceans are temporary features that open and close due to the shifting balance between their formation at MORs and destruction at subduction zones. The complete cycle of opening and closing of an ocean is called a Wilson Cycle, after the Canadian geologist J Tuzo Wilson who suggest the idea in 1967.

The Earth will never run of out basalt because the material brought to the surface at a divergent plate margin is returned to the depths of the mantle along the subdcution zone of the convergent margin. The melted material is then carried as the return current, gaining heat from the underlying core over which it passes, back to the 'start' of the convection cell where it rises once again under the MOR. Slab pull probably limits the maximum size of an ocean and one complete Wilson cycle takes about 500 million years to complete.

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