Seafloor spreading

At mid-ocean ridges, plates pull apart and mantle rises and partially melts as pressure falls; the extracted magma cools into new basaltic crust. The seafloor is a conveyor belt: youngest at the ridge, older the farther you travel from it.

If new crust is constantly made, the Earth is not growing — so old crust must be destroyed somewhere else. It is: at subduction zones, where ocean plates dive back into the mantle. Creation and destruction balance out.