The Earth’s tectonic plates are in constantly moving like giant ‘rafts’ on top of the semi-molten mantle below. However, this movement is slow and rates vary from less than 2.5cm /yr to over 15cm/yr.
The movement of the Earth’s crustal plates is believed to be due to convection currents which occur in the semi-molten mantle. These convection currents are created by heat from within the earth – much of which is generated by radioactive decay in the core.
Convection currents in the mantle
So how do convection currents cause plate movements? As 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 with it. As the semi-molten rock then cools, it gradually sinks back down to be re-heated (see diagram above).