Tensional stress is the type of geological force that causes fault-block mountains to form. These powerful forces pull the Earth's crust apart, leading to distinctive topographic features.
Understanding Tensional Stress and Faulting
Geological stress refers to the forces acting on rocks, causing them to deform. Tensional stress, specifically, involves forces that pull rock apart, stretching and thinning the Earth's crust. This process is commonly associated with areas where tectonic plates are diverging or moving away from each other.
When the Earth's crust is subjected to significant tensional forces, it eventually breaks or fractures. These breaks are known as faults. The type of fault that results from tensional stress is a normal fault. In a normal fault, the hanging wall (the block of crust above the fault plane) moves downward relative to the footwall (the block below the fault plane).
Formation of Fault-Block Mountains
The formation of fault-block mountains is a direct consequence of tensional stress and normal faulting. As the crust stretches and thins, it breaks into large blocks. Some blocks are uplifted, while others subside, creating a distinctive landscape of parallel mountain ranges and valleys.
- Horsts: These are the uplifted blocks of crust that form the mountain ranges. They are essentially the "shoulders" left standing between two normal faults where the adjacent blocks have dropped down.
- Grabens: These are the down-dropped blocks, forming elongated valleys or basins between the uplifted horsts. The term "graben" comes from the German word for "ditch" or "trench."
This process can lead to dramatic topographic relief over vast areas.
Geological Stress Types at a Glance
To better understand tensional stress, it's helpful to compare it with other types of geological stress:
| Type of Stress | Direction of Force | Resulting Fault Type | Associated Landform Examples |
|---|---|---|---|
| Tensional | Pulling apart (divergent) | Normal Fault | Fault-Block Mountains (e.g., horsts, grabens), Rift Valleys |
| Compressional | Pushing together (convergent) | Reverse/Thrust Fault | Folded Mountains, Volcanic Arcs, Ocean Trenches |
| Shear | Sliding past (transform) | Strike-Slip Fault | Offset Rivers, Linear Valleys |
Examples of Fault-Block Mountains
One of the most prominent examples of fault-block mountain terrain is the Basin and Range Province in the western United States. This vast region, encompassing much of Nevada, and parts of Utah, Arizona, California, and Oregon, is characterized by numerous parallel mountain ranges (horsts) separated by wide, flat valleys (grabens).
The geological evolution of the Basin and Range Province is a classic illustration of how tensional forces have stretched and thinned the continental crust over millions of years, leading to the dramatic faulting and uplift that define its unique topography.
Other regions around the world exhibiting significant fault-block mountain ranges include parts of East Africa (the Great Rift Valley system, which is an early stage of rifting and faulting) and regions in China.
