Here's why:

* Divergent boundaries are where tectonic plates move apart.

* As the plates separate, magma from the mantle rises to the surface, creating new oceanic crust.

* This process builds up the mid-ocean ridge, a long underwater mountain range.

* The ridge is often marked by a central rift valley, where the plates are pulling apart most actively.

Let me know if you have any other questions about plate tectonics!