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Why does the floor of a rift valley sink relative to the surrounding land?

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tectonic basins and rift valleys Escarpments of the Great Rift Valley rising above the plain north of Samburu Game Preserve, central Kenya. Beisa oryxes graze in the foreground.

The floor of a rift valley sinks relative to the surrounding land primarily due to the extension of the Earth’s crust. As large crustal blocks or lithospheric plates diverge, the brittle crust fractures, creating a gap. A central block of crust—geologically termed a graben—slides downward into this space.1

This process is often compared to the keystone of an arch falling when the supporting walls move apart. The downward movement occurs along normal faults, where the hanging wall (the valley floor) drops relative to the footwall (the surrounding margins). In many cases, this subsidence is accompanied by the rising of the adjacent crustal blocks.231

While some rift valleys are symmetrical, others are asymmetrical, where one side sinks along a major fault while the other side tilts or flexes downward. Additionally, in regions where the lithosphere is heated and stretched by upwelling material from the asthenosphere, the resulting expansion and thinning of the crust further facilitate the formation of these tectonic depressions.2