Mountains are dramatic geological features that arise from the powerful movements of tectonic plates. The way the Earth’s crust responds to immense pressure or tension determines the fundamental architecture of a mountain range. Understanding these formative forces allows for the classification of mountains into distinct types. Fault-block and folded mountains represent two primary categories, distinguished by how rock layers are deformed by tectonic activity.
Fault-Block Mountains: Formed by Tensional Stress
Fault-block mountains result from tensional forces that pull the Earth’s crust apart, often associated with divergent plate movement. This pulling causes the brittle upper crust to fracture, creating normal faults where crustal blocks move vertically.
When the crust is stretched thin, large sections break into blocks that shift up or down between parallel faults. The uplifted blocks, which form the mountain ranges, are termed horsts. The down-dropped blocks, which create the intervening valleys, are known as grabens. These horst-and-graben systems create a characteristic topography of alternating ridges and troughs.
The vertical displacement along these normal faults can be immense, sometimes thousands of meters, defining the height of the fault-block mountains. This mechanism of breaking and shifting, rather than bending, is the defining characteristic of their formation.
Folded Mountains: Formed by Compressional Stress
Folded mountains are created by intense compressional forces, typically generated when two tectonic plates collide at a convergent boundary. This pressure pushes the rock layers inward, causing them to bend and crumple rather than break outright. This deformation process is known as ductile deformation, occurring deep within the crust over millions of years.
The compressional stress forces the rock strata into a series of undulating waves. The upward folds that form the mountain peaks are called anticlines. The downward folds that create the valleys between the ridges are known as synclines.
In areas of extreme compression, the rock layers may also break and stack on top of one another along low-angle thrust faults. This combination of intense folding and faulting allows the crust to thicken significantly. The resulting mountain belt has a complex internal structure of warped and doubled-over rock layers.
Distinguishing Features and Appearance
The difference in formative stress—tension versus compression—gives fault-block and folded mountains distinct appearances and internal structures. Fault-block mountains are defined by sharp, angular features, often displaying one exceptionally steep side. This steep face is a fault scarp, the surface expression of the primary normal fault where the uplift occurred.
The overall appearance of a fault-block range is a series of parallel, elongated ranges separated by wide basins. The internal geology features discrete, tilted blocks of rock with clear breaks along the fault lines. These mountains show little evidence of the deep, pervasive bending seen in folded ranges.
Folded mountains, especially older ranges, tend to exhibit a more rounded or rolling profile due to millions of years of erosion. Even young folded mountains, like the Himalayas, show continuous, bent rock layers in their underlying structure. The rock strata are warped into continuous arches and troughs, revealing long-term plastic deformation. This internal folding results in a massive, linear mountain belt rather than the alternating block pattern of horst and graben systems.
Notable Global Examples
Examples of these mountain types illustrate the forces that shaped them. The Basin and Range Province in the western United States, including the Sierra Nevada and the Teton Range, represents a classic region of fault-block mountains formed by crustal extension. The steep-sided ranges separated by valleys show the horst-and-graben topography.
The world’s highest and longest mountain ranges are typically folded mountains, formed at the convergence zones. The Himalayas, the Alps, and the Andes are all young folded mountains, still actively rising from continental collision. The older Appalachian Mountains and the Ural Mountains demonstrate how long-term erosion softens the profiles of these massive fold belts.