Seafloor spreading is caused by diverging plates because that means that the two plates are moving away from each other. This occurs due to convection currents in the mantle that move around causing the tectonic plates which make up the Earth's crust to move around. It can cause one of three things; divergent boundaries, convergent boundaries, and transform boundaries. Divergent means to spread away, convergent means to come together, and transform means to rub against.