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What happens to crustal blocks during the formation of a graben?
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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.
During the formation of a graben, crustal blocks are lowered relative to the surrounding areas due to lateral tension, which causes the crust to stretch or extend.12
This process involves the following geological mechanics:
- Faulting: The block is bounded on at least two sides by steeply dipping normal faults, which typically dip toward each other.132
- Downward Movement: As the crust undergoes extension, the hanging wallsāthe blocks of rock above the faultsāslide downward. These lowered blocks often form the floors of rift valleys.21
- Structural Context: Grabens often occur in conjunction with horsts, which are adjacent blocks that have been raised. Together, these features create a series of raised and lowered blocks, a pattern characteristic of regions like the Basin and Range Province.12
This extensional environment is frequently associated with regional uplift or the upwelling of magma, which forces the lithosphere to stretch and fracture.14