What is the driving force behind plate tectonics?
The theory is the heat of nuclear decay in the mantle and core.
The heat produced in the interior of the earth is thought to be the driving force behind plate tectonics. That heat creates convections currents that can be observed in volumes of air and liquid is a known fact. There is substantial evidence that heat is produced in the core and mantle of the earth.
The theory is that the heat from the interior of the earth creates convection currents in the semi liquid mantle. These convection currents push new crust up from the mantle at the divergent boundaries. The new crust creates pressure pushing the plates at the divergent boundary apart. At other places called convergent boundaries the crust that is being pushed by the emergence of new crust is pushed together.
The idea that convection currents created by the heat in the interior of the earth drives plate tectonics is well established.