How does seafloor spreading move plates?

tectonic plates will slowly move from each other leaving heat from the mantle's currents which makes the crust like plastic and less dense

Seafloor spreading occurs at divergent plate boundaries. As tectonic plates slowly move away from each other, heat from the mantle's convection currents makes the crust more plastic and less dense. The less-dense material rises, often forming a mountain or elevated area of the seafloor.