Here's how it works:

* Divergent Plate Boundaries: At these boundaries, tectonic plates are moving apart from each other.

* Upwelling Magma: As the plates pull away, the space created allows magma from the Earth's mantle to rise up.

* Fracturing and Faulting: This upwelling magma exerts pressure on the overlying crust, causing it to fracture and break along fault lines.

* Valley Formation: The fractured crust subsides, creating a depression or valley that widens and deepens over time.

Example: The East African Rift Valley is a classic example of a rift valley formed by divergent plate movement. The African Plate is slowly splitting apart, creating a series of valleys, volcanoes, and volcanic lakes.