Seafloor Spreading is how the plates move. It is a process in which new oceanic crust (ocean floor) is created from the hot magma that comes to the surface. The magma is cooled by the water. The new floor pushes outward and moves the old floor. This is the force that causes the plates to shift. Harry Hess came up with this theory. John Tuzo Wilson combined Alfred Wegener's theory of continental drift and Harry Hess's theory of seafloor spreading to create a reasonable explanation for how the plates could have moved then created his own theory of plate tectonics.