Here's why:

* Divergent Plate Boundaries: At these boundaries, tectonic plates move apart. This creates space where magma from the Earth's mantle can rise to the surface.

* Mid-Ocean Ridges: This rising magma cools and solidifies, forming new oceanic crust. Over time, this process builds up the ocean floor, creating underwater mountain ranges called mid-ocean ridges.

* Volcanic Activity: Volcanoes are common along mid-ocean ridges as magma continues to erupt.

Important Note: While divergent boundaries create mountains, these are typically underwater and not the towering peaks we think of when we say "mountains".