horsts

A horst is an elongated, uplifted block of the Earth’s crust that remains higher than adjacent blocks. It is bounded on both sides by normal faults that dip away from the horst, while the neighboring blocks between these faults are typically down-dropped grabens or basins.

Formation and characteristics: Horsts form in regions undergoing crustal extension, where large blocks are faulted and moved vertically relative to one another. When faulting is asymmetric or concentrated on parallel faults, some blocks are uplifted while others subside. The resulting topography is often a linear or stepped landscape with alternating highs (horsts) and lows (grabens). Horst blocks can develop as isolated ridges or as components of broader fault-bounded mountain belts.

Geologic context and examples: Horst-and-graben structures are a hallmark of extensional tectonics. They occur in many continental rift systems around the world. Well-known examples include the Basin and Range Province in western North America and the East African Rift system, where numerous fault-block mountains and down-dropped basins create characteristic landscapes.

Relation to graben: The term horst is used in contrast to graben, the down-dropped blocks between normal faults. Together, horsts and grabens form alternating elevated and lowered blocks that reflect the pattern and magnitude of crustal extension in a region.

Significance: Horst structures influence regional geomorphology, drainage patterns, and groundwater flow. They also play roles in sedimentation and, in some tectonic settings, in mineral and petroleum trapping along fault zones. The term derives from German, where horst means a rise or high ground.