Here's why:

* Divergent Boundaries: At these boundaries, tectonic plates move apart.

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

* Volcanic Activity: The rising magma often erupts, forming volcanoes along the rift.

* Stretching and Thinning: The pulling apart of the plates causes the Earth's crust to stretch and thin, creating a depression or valley.

Examples:

* East African Rift Valley: A prominent example of a rift valley formed by the separation of the African Plate.

* Mid-Atlantic Ridge: An underwater rift valley that runs along the middle of the Atlantic Ocean, formed by the separation of the North American and Eurasian Plates.

Let me know if you have any other questions about plate tectonics!