The Great Rift Valley wasn't "made out of" a specific material in the same way a building is made of bricks. It's a geological formation created by the tectonic movement of Earth's crust.

Here's a breakdown:

* Volcanic Activity: The initial formation of the valley was driven by volcanic eruptions and the upwelling of magma from the Earth's mantle. This magma, upon reaching the surface, created vast flows of lava that cooled and solidified, forming the foundation of the rift valley.

* Crustal Extension: The East African Rift Valley is a divergent plate boundary, where the African tectonic plate is pulling apart. This separation stretches and thins the Earth's crust, causing it to fracture and sink, creating the valleys and escarpments.

* Faulting: The stretching and thinning of the crust leads to the formation of faults—deep cracks in the Earth's surface. These faults further deepen the valley and create the distinctive stepped landscape of the Great Rift Valley.

* Sedimentation: Over millions of years, erosion and weathering have deposited vast amounts of sediment into the valleys, further shaping their landscape.

Therefore, while the Great Rift Valley wasn't "made out of" a specific material, it was shaped by the interaction of volcanic activity, crustal extension, faulting, and sedimentation.