Why do the tectonic plates on Earth move?
Tectonic plates move because they are floating on top of the liquid mantle. Because magma is always hot, it will move.
When water is boiled is starts to let off steam. The same could be said about the plates. The atoms are always moving, causing the plates to move. The plates change direction over time, jostling about like toys in a bathtub. The continents come together, move apart, and then together again someday. If you would like a picture it will be below.
When water is heated it also forms convection currents. The hot water rises and then moves outward in both directions. The cold water then sinks at the edges of the pot. It is thought that the hot magma also creates convection currents.
The mid ocean ridges are places when the hot magma comes up and then moves apart in a divergent boundary. The ocean trenches are places where the cooled magma sinks carrying the crust down in these convergent plate boundaries.
Hopefully you found this helpful! I hope you have a wonderful, blessed day. I'll end it here with a nice little quote,
"Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic." -Unknown