Chris: In the world’s ocean basins, new tectonic plate is created at spreading ridges in the middle of the ocean basins, and old plate is returned to the mantle at subduction zones. Studying the rate at which oceanic crust is produced at the major ridges is therefore a good measure of how quickly the planet is turning itself inside out.
To calculate this rate, you need the endpoints of each ridge and the rotation pole for the relative motion of the plates on either side of the ridge. Plotting just the endpoints produces a greatly simplified map of the global ridge system, as shown here.
If you do the calculation, the average spreading rate works out at just under 5 centimetres a year. Put another way, every year these ridges builds about 2.5 square kilometres of new plate.