Sea floor spreading is caused by the continual injection of magma from the sea bed in between ridges found in mid-ocean. Injection of magma causes formation of new sea floor.
Forces cause the sea floor on each side of the mid-ocean ridge to move in opposite directions, each side belonging to a different lithospheric plate.
This process is continuous and results in an ever widening (spreading) sea floor at the expense of area lost elsewhere on the planet. Seafloor spreading ends when mid ocean ridges are subducted.
