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How do normal faults form rift valleys?
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valley
The rugged Atlas Mountains surround a valley in Morocco.
Rift valleys form primarily through the extension of the Earth's crust, which causes the brittle rock to fracture and create normal faults. As large lithospheric plates or crustal blocks diverge, the hanging wall—the block of rock positioned above the fault plane—slides downward relative to the footwall.1234
This downward displacement of a crustal block between two or more normal faults creates a tectonic depression known as a graben. In this process:52
- Crustal Extension: Tensional forces pull the crust apart, creating a gap.62
- Subsidence: A central block of crust drops into this gap, forming the floor of the rift valley.62
- Fault Geometry: These valleys are bounded by steeply dipping normal faults. While some rift valleys are symmetrical grabens, others are asymmetrical, featuring one steep, fault-bounded wall and a gentler side formed by lithospheric tilting or flexing.516
This geological process is a hallmark of divergent plate boundaries, such as the East African Rift System, where the crust is actively thinning and separating.613