Here's why:

* Divergent boundaries are where tectonic plates move apart.

* As the plates separate, magma from the Earth's mantle rises to the surface.

* This magma cools and solidifies, creating new oceanic crust.

* The process of new crust formation is called seafloor spreading.

This process is responsible for the creation of mid-ocean ridges, which are underwater mountain ranges found along divergent plate boundaries.