Here's why:

* Divergent Plate Boundaries: At these boundaries, tectonic plates move apart. This creates space for magma from the Earth's mantle to rise and erupt, forming new oceanic crust.

* Mid-Ocean Ridges: These are underwater mountain ranges where this new crust is created. The process of magma rising and solidifying along the ridge causes the seafloor to spread, pushing the plates apart.

Think of it like a conveyor belt: new crust is "made" at the ridge and then moves away from it as the plates continue to diverge.