Why does slab pull occur

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. 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 , 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 , Back to Contents. Mechanisms of Plate Motion.

The weight of the lithosphere on this sloping surface produces a downslope force. And since the asthenosphere is weak, the weight of the lithosphere near the ridge sliding down the "slippery slope" of the asthenosphere "pushes" the older part of the plate in front of it.

Note that as the lithosphere slides down away from the ridge, tensional forces and normal fault earthquakes occur at the ridge axis where two plates are sliding pulling apart. They eventually become more dense than the underlying hot mantle. After subducted, cool, dense lithosphere sinks into the mantle under its own weight.

This helps to pull the rest of the plate down with it. And for newly subducting slabs, the resisting force of the mantle as the slab tries to force its way through is stronger than the shearing on the top and bottom of the descending slab. Friction between the converging plates and also the force required to bend a plate resist the movement of the plate at subduction zones. The Earth is made up of different layers. Demonstrating the movement of the Earth's crust and how tectonic plates interact at plate boundaries.

The crust is the outer layer of the Earth. It is the solid rock layer upon which we live. It is either continental or oceanic. Continental crust is typically kilometres thick, whilst oceanic crust is only five to ten kilometres thick.

Oceanic crust is denser, can be subducted and is constantly being destroyed and replaced at plate boundaries. Continental crust is older, lighter and cannot be destroyed.