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How do horsts and grabens form in regions of crustal extension?
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valley
The rugged Atlas Mountains surround a valley in Morocco.
In regions of crustal extension, the Earth’s crust undergoes stretching that causes it to fracture into blocks. This process is driven by lateral tension, which leads to the formation of normal faults.1234
As large crustal blocks or tectonic plates diverge, the brittle crust breaks, and blocks of rock move along these steeply dipping fault planes. The formation of horsts and grabens depends on the orientation of these faults:14
- Grabens: These form when a block of crust drops downward between two normal faults that dip toward each other. The resulting down-dropped trough often creates a rift valley or tectonic basin.3241
- Horsts: These are blocks that remain relatively uplifted between two normal faults that dip away from each other.324
These features often occur in series, creating a characteristic landscape of alternating raised and lowered blocks, such as the Basin and Range Province in the United States.34