Tensional stress causes fault-block mountains.
Understanding Tensional Stress
Tensional stress is a type of geological force that pulls rocks apart, stretching and thinning the Earth's crust. Imagine pulling on both ends of a piece of elastic; that's the principle of tensional stress at work within the Earth. This force is commonly associated with areas where tectonic plates are moving away from each other, such as at divergent plate boundaries.
When the Earth's crust is subjected to this pulling force, it begins to fracture and extend. Unlike compressional stress, which pushes rock layers together and causes folding, tensional stress leads to the formation of breaks in the rock, known as faults.
The Formation of Fault-Block Mountains
The distinctive landscape of fault-block mountains arises directly from the effects of tensional stress on the lithosphere. As the crust is pulled apart, large blocks of land are displaced along these newly formed faults.
Normal Faults
The primary type of fault associated with tensional stress and fault-block mountain formation is a normal fault. In a normal fault, the hanging wall (the block of rock above the fault plane) moves downward relative to the footwall (the block of rock below the fault plane). This downward movement accommodates the stretching and extension of the crust.
Horsts and Grabens
The characteristic rugged topography of fault-block mountain ranges is created by an alternating pattern of uplifted and down-dropped blocks, often referred to as horsts and grabens:
- Horsts: These are the blocks of crust that are uplifted or remain relatively high between two adjacent normal faults. They form the prominent mountain ranges.
- Grabens: These are the blocks of crust that drop down between two parallel normal faults. They form the elongated valleys or basins that lie between the mountain ranges.
This process results in a landscape characterized by long, parallel mountain ranges (horsts) separated by wide, flat valleys (grabens). The steep, straight edges of these mountains are often the exposed fault planes, known as fault scarps.
Examples of Fault-Block Mountain Ranges
One of the most classic examples of a region dominated by fault-block mountains is the Basin and Range Province in the western United States. This vast area, encompassing much of Nevada, Utah, Arizona, and parts of California, is characterized by hundreds of parallel mountain ranges separated by arid basins.
Other notable examples of geological features formed by tensional forces include:
- Basin and Range Province, USA: A quintessential example of extensive horst and graben topography, showing how regional extension creates an array of linear mountain ranges and valleys.
- East African Rift Valley: While primarily a rift valley, this region illustrates active continental rifting, where tensional forces are actively pulling the African continent apart, leading to the formation of basins (grabens) and elevated shoulders (horsts) that resemble young fault-block mountains.
Types of Stress and Their Geological Outcomes
Understanding the different types of stress acting on the Earth's crust helps clarify the diverse geological features we observe.
| Type of Stress | Direction of Force | Primary Geological Outcome | Associated Fault Type | Example Feature |
|---|---|---|---|---|
| Tensional | Pulling apart | Stretching, thinning, rifting | Normal Fault | Fault-Block Mountains, Rift Valleys |
| Compressional | Pushing together | Shortening, thickening, folding | Reverse/Thrust Fault | Fold Mountains, Island Arcs |
| Shear | Sliding past | Twisting, tearing | Strike-Slip Fault | Transform Plate Boundaries |
Note: This table provides a simplified overview. Real-world geological processes can involve complex interactions of multiple stress types.
Key Characteristics of Fault-Block Mountains
Fault-block mountains exhibit several defining characteristics that differentiate them from other mountain types:
- Steep Fault Scarps: The mountain fronts are often very steep and straight, marking the location of the active normal faults.
- Alternating Ridges and Valleys: The landscape consists of parallel, elongated mountain ranges (horsts) separated by equally long, flat-bottomed valleys (grabens).
- Evidence of Crustal Extension: They form in regions where the Earth's crust is actively being pulled apart, leading to a significant increase in the surface area of the crust. This often occurs at or near divergent plate boundaries or areas of regional uplift causing extension.
For further reading on geological processes and mountain formation, resources from organizations like the U.S. Geological Survey (USGS) or educational institutions provide comprehensive insights into Earth's dynamic crust.
