Tectonic plates are composed of the crust and upper mantle rockwhich is lighter than the less dense mantle. Rising and fallingconvection currents in the mantle cause imperceptibly slowmovements in the plates (on the rate of fingernail growth). Thesource of the heat driving the convection currents comes fromresidual heat from the Earth's formative processes, pressure fromgravity, and from radioactive decay of elements.