Tectonic plates move because of the generation of heat within the Earth from the decay of radioactive elements. Hot materials rise from the mantle. In the upper mantle, they cool, become more dense and sink back into the deep mantle. In the deep mantle, they reheat and rise again. The cycle of rising magmas and sinking cooler materials creates convection currents , which may move the tectonic plates.
Hot water rises, cools and then sinks.
The cycling of rising, cooling, sinking, heating, and rising again forms convection currents that mix the water shortly before it begins to boil. The pressure on the hot asthenosphere materials decreases as the materials rise towards the Earth's surface. Under lower pressure, the asthenosphere largely melts to produce magmas that erupt as lavas at the spreading ridges. The erupting lavas form new oceanic crust. The addition of new crust at the spreading ridges and the convection currents cause the plates at the spreading ridges to spread apart.