Sea floor spreading is a process in which new oceanic crust is created at the mid-ocean ridges as a result of volcanic activity. As tectonic plates are moving away from each other, fracturing of the lithosphere occurs. Magma is then rising up through these fractures, and cools down at the surface, creating new crust.
Age of oceanic crust is symmetrical with respect to the spreading axis and increases with distance from it. This has been confirmed by magnetic measurements, showing pattern of alternating stripes (connected to the past geomagnetic poles reversals) with a divergent plate boundary being a line of symmetry.