Fault block mountains result from the powerful movement of tectonic plates along deep fractures in the Earth’s crust. Unlike folded mountains, which are created when rock layers are squeezed and buckled together, fault block mountains emerge from a process of crustal stretching and breaking. The unique, often linear shape of these mountains is a direct consequence of this distinct geological process.
Tensional Stress and Crustal Extension
The process begins with a powerful, pulling force known as tensional stress acting on the Earth’s lithosphere. This stress occurs where tectonic plates are moving away from one another, causing the rigid and brittle continental crust to stretch.
When the crust is subjected to this stretching, it responds by thinning out and extending horizontally. This crustal extension causes the rock layers to become weakened and fractured, setting the stage for the dramatic vertical movement that defines a fault block mountain.
The Role of Normal Faults
Tensional stress causes the brittle rock layers to rupture, forming a specific type of fracture called a normal fault. Movement along a normal fault is generally vertical, which geologists call dip-slip motion.
Geologists define two blocks of rock separated by the fault plane: the hanging wall and the footwall. The hanging wall sits above the fault plane, while the footwall is the block underneath it.
In a normal fault, the hanging wall moves downward relative to the footwall, directly responding to the crust being pulled apart. The downward slide of the hanging wall along the inclined fault plane creates the necessary extension in the crust. The angle of the fault plane, or dip, is typically steep, often between 45 and 90 degrees, which facilitates this vertical drop.
Constructing the Landscape: Horsts and Grabens
The characteristic landscape of a fault block mountain system is formed by a series of multiple normal faults running roughly parallel to each other. As the crust continues to extend, the alternating vertical movement along these parallel fractures creates a distinctive “range and basin” topography defined by two primary geological structures: the horst and the graben.
The horst is the uplifted block of crust that remains high between two downward-moving fault blocks, forming the mountain ranges themselves. Conversely, the graben is the block that has dropped down between two normal faults, forming the wide valleys or basins.
The continuous, long-term displacement along these faults results in dramatic and linear mountain fronts. The uplifted horsts become the fault block mountains, characterized by steep slopes where the fault scarp is exposed. The adjacent grabens often fill with sediment eroded from the surrounding horsts, creating the flat-bottomed valleys characteristic of these regions.
Notable Fault Block Mountain Systems
The scale of crustal extension required for this type of mountain building is illustrated by major fault block mountain systems around the world. The most extensive example is the Basin and Range Province in the western United States, covering much of Nevada, Utah, and surrounding states. This vast area is composed of an immense network of horst and graben structures, creating a landscape of parallel mountain ranges separated by arid valleys.
Within this province, the Sierra Nevada mountains in California and the Wasatch Range in Utah are prominent examples. In Europe, the Vosges Mountains in France and the Black Forest in Germany represent classic horsts separated by the Rhine Graben, a large rift valley. These examples confirm that the same extensional process has shaped landscapes across different continents.