Here's why:

* Divergent Plate Boundaries: At these boundaries, tectonic plates move apart. As they pull away, the Earth's crust thins and weakens, creating a depression or valley.

* Magma Upwelling: The separation allows magma from the Earth's mantle to rise up, often leading to volcanic activity along the rift.

* Examples: The East African Rift Valley and the Mid-Atlantic Ridge are classic examples of rift valleys formed at divergent boundaries.

Let me know if you'd like more details about the process!