planet

Seafloor spreading

The process by which new ocean floor is continually created at mid-ocean ridges and carried away to either side. How this was proved is one of Earth science's finest tricks: the planet's magnetic field reverses polarity now and then, and basalt cooling at a ridge records whichever direction was current at the time. The result is magnetic stripes flanking the axis in mirror symmetry — a tape recording of both the rate and the direction of plate motion, running back hundreds of millions of years.