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How do rift valleys form when Earth’s crust is stretched?

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continental landform Davis's proposed landscape-development states. The morphology shown is not actually time-indicative. For example, A could be a gully system in soft sediment or a canyon such as the Royal Gorge in Colorado, which is millions of years old. The ridge-ravine topography of B would normally develop under humid conditions, but the river meandering on alluvium indicates a prior or extraneous non-humid aggrading mechanism. The riverine plain of C implies a complex history of planation and aggradation in a current fluvial mode.

Rift valleys form when the Earth’s crust undergoes extension, causing it to fracture and stretch. As large crustal blocks or lithospheric plates diverge, the brittle crust breaks along normal faults, where the rock on the upper side of the fault plane moves downward relative to the rock below.123

This process creates a graben—a depressed block of crust that slides down into the gap created by the separating plates. The down-dropped block becomes the valley floor, while the surrounding margins may remain stable or rise, resulting in the steep, mountainous sides characteristic of these landforms.12

The formation is often compared to the keystone of an arch falling as the walls move apart. While some rift valleys are symmetrical, others are asymmetrical, featuring one steep, faulted wall and a gentler side formed by the tilting or flexing of the lithosphere. This extension may be driven by the upwelling of hot material from the asthenosphere, which can dome and stretch the overlying crust, or by distant tectonic forces.45