Here's why:

* Tectonic plates move apart: This is a key characteristic of divergent plate boundaries.

* Magma fills the space: As the plates separate, magma from the Earth's mantle rises to the surface, filling the gap and solidifying to form new oceanic crust.

* Under water: This process primarily occurs beneath the ocean, forming mid-ocean ridges, which are underwater mountain ranges.

Let me know if you'd like to learn more about seafloor spreading!