Why do tectonic plates move?

The Earth’s tectonic plates are constantly moving. However, this movement is slow, and rates vary from less than 2.5cm /yr to over 15cm/yr. Tectonic plates move as the asthenosphere, which is ductile, is weak enough to accommodate the movement of the solid and brittle plates above it.

Three theories have been put forward for the movement of tectonic plates on top of the asthenosphere. None have been completely proven. The theories are:

• convection currents
• ridge push
• slab pull

Convection currents and plate movement

The initial explanation for the movement of tectonic plates was convection currents in the mantle. The theory suggests these convection currents are created by heat from within the Earth – much of which is generated by radioactive decay in the core.

One theory of plate movement involves convection currents in the mantle

The idea is that the outer core, which is hotter than the mantle, drives convection currents within the mantle. The mantle is solid. However, the asthenosphere can move (very slowly). The idea is that convection currents reach the asthenosphere and drive the movement of the rigid lithosphere above. The theory proposed as the semi-molten rock in the mantle is heated, it becomes less dense than its surroundings and rises. As it reaches the crust above, it spreads out, carrying the plates above.

This theory gained widespread acceptance for a considerable period despite scant evidence of significant convection currents detected using modern imaging techniques capable of identifying movements sufficient to shift tectonic plates. Consequently, this theory is now largely dismissed.

Ridge push and slab pull

Ridge push and slab pull is now the more widely accepted theory for the movement of tectonic plates.

Why do plates move?

Ridge push and plate movement

Newly-formed plates at oceanic ridges (constructive plate margins) are warm and so have a higher elevation at the oceanic ridge than the colder, more dense plate material further away; gravity causes the higher plate at the ridge to push away, causing the plates to move away from each other.

Slab pull and plate movement

The most recently proposed and most likely explanation for tectonic movements is slab pull. At destructive plate margins, the denser oceanic plate sinks into the mantle under the influence of gravity, pulling the rest of the plate along with it.

Recent research has shown that slab pull is the major driving force for most plate movement because the plates with more of their edges being subducted are the faster-moving ones. However, ridge push has also been presented in recent research as a force that drives plate movement.