The seafloor is spreading at rates of around 5 cm usually, though it can go up to 16 cm per year but rarely. The seafloor spreading is caused by the geological activity at the places where there's divergent plate boundaries. As the plates move away from one another they leave thin and fragile crust behind which is easy to break through for the magma. As the magma gets through and reaches the water, it starts to cool off very quickly, constantly pulling up and creating new crust, pushing the old crust sideways and further away from the divergent boundary.
