Here's why:

* Divergent Plate Boundaries: The Great Rift Valley is formed by the separation of the African Plate. The tectonic plates are moving apart (diverging), creating a gap that is filled by magma rising from the mantle. This process causes the land to split, creating a valley.

* Volcanic Activity: The rift valley is associated with significant volcanic activity. The rising magma from the mantle often erupts, forming volcanoes along the rift.

* Earthquakes: As the plates move apart, there are frequent earthquakes along the rift zone. These earthquakes contribute to the widening of the valley.

The Great Rift Valley is a powerful example of how the Earth's tectonic plates constantly move and reshape the planet's surface.