As the continents move, they pull the seafloor with them.
Explanation:
As the tectonic plates move further and further away from one another they pull the seafloor with them and in that process fractures and cracks appear. During this time hot magma would bubble up and quickly fill in those cracks and in that way the seafloor is spreading and. This is provable because the magnetism on the ocean floor indicates that it was formed in this way.
