The main driving force of plate tectonics is called slab pull... that is in the vicinity of trenches, a big 100 km thick slab of lithosphere dives BACK INSIDE the earth. Since the surface area of earth is fixed, when this happens there must be a means to create new sea floor; that is what happens along an ocean ridge. Mantle stuff from below upwells and because of slab pull, a crack forms and allows the upwelling mantle stuff, now partially liquid, to fill in. This is how sea floor creates. Each year about 3 square kilometers of new sea floor are created on planet Earth.

Seafloor spreading occurs where two tectonic plates are moving apart. As the rigid plates pull apart from each other, material from the mantle rises up to fill that space, resulting in volcanism and the creation of new ocean crust. Away from plate boundaries, there is little deformation of the crust, and therefore no spreading. When plates are moving towards each other (convergent boundaries), generally whichever plate is less dense is subducted under the other plate. At transform boundaries, plates just slide past each other.