Answer:
Plates of the Earth are in motion because of currents in the asthenosphere pushing and pulling the plates around the Earth.
Step-by-step explanation:
Earth's crust is not a single unit, instead, it is divided into numerous tectonic plates. The tectonic plates can be divided into major ones and small ones, or as continental and oceanic ones. All tectonic plates have one thing in common though, they all are in constant motion.
The motion of the tectonic plates comes from the convection currents in the asthenosphere. Basically, the colder and denser material falls deeper, pushing the hotter and less dense material up, thus creating a circular movement. This results in constant pressure on the crust, which is the reason why the tectonic plates have formed in the first place, and why they are moving across the globe.