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Sources of plate motion

The actual cause of plate motion is currently unknown, but seismologists have acknowledged the dissipation of heat from the mantle to be the source of energy driving plate movements. However, this evergy has to be converted into a force to drive this motion.

Two essential forces which are the possibe sources of motion have been identified. They are namely friction and gravity.


Convergence of two oceanic plates.
Source: Wikipedia, the free encyclopedia.

Friction force can be further divided into Mantle Drag and Trench Suction.

The semi-molten rocks in the mantle, known as magma, is intensely heated at depth. It expands, and rises due to a decrease in density. As the magma rises closer to the surface, it begins spreading out just beneath the plates, cools and sinks.

The friction between the magma convection currents and the lithosphere provides the force for the motion of the plates. This is known as mantle drag. As the magma sinks, convection currents exert a downward frictional pull on plates, pulling two plates closer to each other.

Since this usually happens in subduction zones and oceanic trenches, this is known as Trench Suction. One plate may subduct the other plate if there is a great difference in density between the two plates.


The two sources of plate motion that involves gravity are Ridge-push and Slab-pull.

Ridge-push is driven by high elevation of mid-ocean ridges commonly found at divergent plate boundaries. Gravity causes protruding rocks to break away from the ridge towards the subduction zone.

Slab-pull is driven by weight of descending cold, dense plates, which then pulls rest of plate into the mantle. It occurs most commonly at covergent plate boundaries.

Overall Effects

Most early theories of plate tectonics postulated that convection currents were the main driving force of plate movement. They envisioned the plates riding on top of convection currents which is like a large conveyor belt.

However, more current models acknowledge that friction alone is not strong enough to directly influence motion, and Slab-pull and Ridge-push are given greater consideration for the overall driving force for the movement of plates. As of now, no one can provide the real answer.


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