How ocean basins form

A mountain range that encircles the planet

The global mid-ocean ridge is more than 50,000 kilometres long, in places more than 800 kilometres across, and rises an average of about 4,500 metres above the surrounding sea floor. It is the most prominent topographic feature on the planet’s surface, and almost none of it has ever been seen.^[1,2]

Along it, plates move apart and new oceanic crust is generated continuously, then carried away from the crest.^[1]

How fast

Spreading rates vary by roughly a factor of six across the system: less than 2.5 centimetres a year at the Arctic Ridge, more than 15 on the East Pacific Rise near Easter Island. The Mid-Atlantic Ridge averages about 2.5 centimetres a year — around 25 kilometres in a million years.^[1,2,3]

The sea floor recorded its own formation

Ocean-floor basalt contains magnetite, which makes it magnetically readable. Magnetometers towed across the sea floor from the 1950s found regular magnetic variations — a record left by the crust as it cooled.^[1,2]

Because those stripes record reversals of Earth’s field, and the approximate duration of each reversal is known, average rates of plate movement over a given span can be calculated from them. The ocean floor is a tape recording of its own manufacture.^[1]

Where it goes

None of it lasts. Earth is not growing, so crust must be destroyed as fast as it is made, and destruction happens at subduction zones where one plate sinks beneath another.^[1,2]