Making plate tectonics happen

• the interior of the Earth is hot
• this heat is parly left over from the formation of 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 Homes in 1928
• shlow-moving convection currents within the asthenosphere, or possibly within the whole mantle, move the overlying lithospheric 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 concection 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 eiher side of the MORs, resulting in sea floor spreading away from the rige
• it is this part of the convection cell that carries the rigid lithosphere across the Earth's surface, like a giant conveyor bely
• beneath the lithosphere, partial melting in the asthenosphere reduces friction and acts as a lubricant for the movement of the lithosphere
• where two of these lateral current 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
• in other words, subduction zones are convergent plate margins
• this descending flow of material eventually replaces the mantle material, which rises under the MORs

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 'mode' 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 (they are huge moutain ranges!)

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 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 

• we have already seen that no area of oceanic crust is older than 200Ma, 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 suggested the idea in 1967
• if you travel along a 'moving walkway', for example, in an airport, there is no danger of running out of floor because the belt is continuous and returns back to the beginning, out of sight, underneath the walkway 
• similarly, the Earth will never run out of basalt because the material brought to the surface at a divergent plate margin is returned to the depths of the mantle along the subduction zone of the convergent margin
• the melted material is then carried as the return current, gaining heat from the underlying core over which is 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 compelte Wilson Cycle takes about 500 million years to complete!!!!!!!