What starts plate motion?
The forces that cause plate movement never really stop. The Earth is continually moving around, shifting its plates. There are also a number of different reasons the plates move in the first place. The most prominent one is the Earth’s convection currents. The Earth’s core is a very hot place. Its outer core consists of magma. The intense heat from this part of the Earth sends heat upwards towards the surface; this heat causes a force to push upward on the tectonic plates. Sometimes this pressure can be intense enough to cause a rift in a seafloor, or to cause a volcano to form or erupt.
Since our tectonic plates are sitting on a softer layer of Earth called the asthenosphere, it is easy for this force to move the tectonic plates around. This movement is also responsible for earthquakes, and continental drift. Check our link on isostatic equilibrium for more details on this concept.
For more details on the Earth’s plates read our quesiton on earthquakes and rigid plates.