Fault block mountains represent a distinct landform created by the stretching and breaking of the Earth’s rigid outer layer, the lithosphere. Unlike the soaring, crumpled peaks of fold mountains, which form when continental plates collide and compress the crust, these ranges develop through an expansive process called crustal extension. This type of mountain building is a direct consequence of the Earth’s crust being pulled apart, causing it to fracture into massive, discrete sections. The resulting topography is characterized by a dramatic, alternating pattern of raised mountain blocks and sunken valley floors.
The Necessary Tectonic Environment: Tensional Stress
The initial force required to create fault block mountains is tensional stress, a pulling force that acts to stretch and lengthen a portion of the crust. This stress occurs in regions where tectonic plates are moving away from each other, such as at divergent boundaries, or in areas of broad regional uplift where the crust is spreading out under its own weight. When the crust is subjected to this outward pull, it undergoes thinning and fracturing rather than folding.
The brittle upper crust cannot stretch indefinitely and eventually breaks. This fracturing accommodates the horizontal extension, allowing the crust to cover a wider area. As pulling continues, the fractured sections of the lithosphere begin to move vertically relative to one another along lines of weakness. These areas are often associated with continental rifting, where an entire landmass is slowly being torn apart.
This environment of extension leads to a significant reduction in the overall thickness of the lithosphere. As the crust thins, the underlying mantle material rises closer to the surface, contributing to the heat flow and further weakening the overlying rock. The gravitational forces acting on the stretched and thinned blocks then become a primary driver for the vertical movements that shape the landscape.
The Mechanics of Normal Faulting
The tensional stress is ultimately relieved through movement along specific types of fractures known as normal faults. A fault is essentially a fracture surface where one side has moved relative to the other. Normal faults are defined by the direction of this relative movement: the block of rock above the inclined fault plane, called the hanging wall, moves downward with respect to the block below it, known as the footwall.
The downward slip of the hanging wall is driven by gravity acting on the stretched crust. This movement along the inclined fault plane accommodates the horizontal extension, causing the land surface to lengthen and subside vertically.
Normal faults dip at angles between 45 and 60 degrees, providing an adequate slope for gravitational sliding. When multiple parallel normal faults form, the blocks between them begin to move in opposition to each other. This differential slip creates the vertical displacement necessary to form mountains and valleys. The cumulative displacement along a series of these faults can amount to thousands of meters over millions of years, fundamentally reshaping the landscape.
Defining the Structure: Horsts and Grabens
Movement along parallel normal faults results in the distinctive landform pattern of alternating raised and lowered blocks. The uplifted blocks are termed horsts, which form the mountain ranges themselves. A horst is bounded by normal faults on two sides where the surrounding crust has dropped down.
Conversely, the subsided blocks are called grabens, which create the intervening valleys and basins. A graben forms when a section of crust drops down between two parallel normal faults whose inclined planes dip toward each other. This alternating arrangement of horsts and grabens is often referred to as “basin and range” topography.
This structural pattern is showcased in the Basin and Range Province of the western United States, covering much of Nevada and parts of surrounding states. Thousands of kilometers of crustal extension have produced hundreds of parallel, linear mountain ranges (horsts) separated by wide, flat-bottomed valleys (grabens). Another illustration is the East African Rift Valley, a continental rift zone where the continent is slowly separating, leading to immense grabens that form rift valleys and adjacent horsts that rise as plateaus or mountain flanks.