Horsts form as elevated blocks of land in regions where the Earth's crust is being stretched and pulled apart, a geological process known as extension.
The Geological Process: Crustal Extension
When the Earth's crust undergoes extension, it experiences significant stretching, which can cause it to thin and lengthen by up to 100% of its original size. This pulling action creates immense stress within the brittle upper crust. To relieve this stress, the crust fractures along a series of parallel breaks known as normal faults.
Role of Normal Faults
Normal faults are characterized by the hanging wall (the block of crust above the fault plane) moving downward relative to the footwall (the block below the fault plane). As the crust continues to pull apart, these normal faults allow sections of the crust to slip past each other.
Formation of Horsts and Grabens
The characteristic topography of horsts is inextricably linked with that of grabens:
- Grabens: As the crust extends, some blocks of land between parallel normal faults drop down relative to the surrounding land. These down-dropped blocks form valleys, known as grabens (from the German word for ditch or trench).
- Horsts: Simultaneously, the blocks of land that remain elevated between these subsiding grabens are called horsts (from the German word for "heap" or "eminence"). Essentially, horsts are the stable, uplifted portions of the crust that are bounded on either side by normal faults, with adjacent grabens having dropped down.
The following table summarizes the key differences:
| Feature | Horst | Graben |
|---|---|---|
| Topography | Elevated landmass, forms a "range" or "ridge" | Down-dropped landmass, forms a "valley" |
| Formation | Remains uplifted between diverging normal faults | Subsides between converging normal faults |
| Movement | Relative upward movement along faults | Relative downward movement along faults |
Key Characteristics of Horst and Graben Topography
- Alternating pattern: Horsts and grabens typically occur in an alternating pattern, creating a distinctive "basin and range" topography.
- Steep fault scarps: The edges of horsts and grabens are often marked by steep slopes called fault scarps, which are direct expressions of the fault lines.
- Regional scale: This type of landscape can cover vast areas, reflecting large-scale tectonic forces.
Examples of Horst and Graben Systems
One of the most prominent examples of horst and graben topography is the Basin and Range Province in the western United States. This region, spanning parts of Nevada, Utah, Arizona, and California, perfectly illustrates numerous mountain ranges (horsts) separated by wide valleys (grabens), all formed by millions of years of crustal extension.
