Making Plate Tectonics Happen (Geology)

Convection Cells

The interior of the earth is hot. This heat is partly 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 one place than another then a convection current to occurs.

Convection cells are set up when hot, low density materials rise upwards. This then flows sideways and starts to cool down. As the material cools it becomes denser and sinks back down.

Where 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 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. Like a huge conveyor belt.

Other Possible Mechanisms for Movements

Ridge push at MORs

In this model, rising magma injected along the MORs at divergent plate margins forcibly pushes the lithosphere plates apart. The sheeted dykes found on either side of the ridge axis are evidence for this process.

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 the ocean trenches pulls the rest of the oceanic lithosphere with it.

Recycling the Earth

We have already seen that no area of the oceanic crust is older than 200 Ma, because in geological terms oceans are temporary features that open and close sue 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 Canadian geologist J Tuso Wilcon, whom first suggested the idea.

• One complete Wilson cycle takes 500 million years to complete.