Mountain belts are large, elevated regions of the Earth’s crust formed by tectonic processes. Geologists categorize them primarily by the underlying tectonic mechanism. The two main categories are Collisional (Orogenic) mountain belts, formed by compression, and Extensional (Block-Faulted) mountain belts, formed by extension. The distinction is based on whether the crust was pushed together or pulled apart during formation.
Tectonic Setting and Plate Movement
The fundamental difference between the two types of mountain belts lies in the movement of the tectonic plates. Collisional mountain belts, or orogens, occur at convergent plate boundaries where two continental landmasses push into each other. This sustained pressure results in profound crustal shortening, where the continental lithosphere is squeezed horizontally and forced to thicken vertically. The Himalayas, formed by the collision of the Indian and Eurasian plates, are a primary modern example of this compressive setting.
This convergence drives the formation of exceptionally long, linear mountain ranges spanning thousands of kilometers. The process involves one continental plate being forced partially beneath the other, resisting deep subduction due to its buoyancy. Compressive forces build up over millions of years, creating the highest and most extensive mountain ranges. The resulting tectonic strain is often transmitted far inland, causing deformation hundreds of kilometers from the primary boundary.
Extensional mountain belts form where the Earth’s crust is stretched or pulled apart under tensional stress. This process is associated with continental rifts, where a single continental plate begins to split, or in areas of regional uplift. The divergent forces cause the crust to thin and lengthen, the opposite of shortening and thickening. These belts are often located within the interior of a continent, away from active plate margins.
The Basin and Range Province in the western United States is a classic example of an Extensional belt, where the crust has been significantly stretched. This stretching causes blocks of the upper crust to fracture and drop down along faults, accommodating the extension. Unlike the continuous chains of Collisional belts, Extensional systems are characterized by a series of shorter, parallel ranges separated by wide valleys.
Differences in Crustal Deformation
The opposing forces of compression and tension create dramatically different physical manifestations of deformation. Collisional belts are defined by intense, ductile deformation, where rock layers bend and flow without shattering under pressure. This results in complex, tight folding, forming large-scale structures like anticlines and synclines, which evidence crustal shortening. A defining feature is the presence of low-angle reverse faults, known as thrust faults, where older rock layers are pushed up and over younger layers, stacking the crust.
These thrust faults accommodate significant horizontal shortening, creating thick slabs of rock displaced long distances. The deformation is pervasive, affecting nearly all rock types across the width of the range. The overall structure is characterized by a high degree of internal complexity, reflecting the tremendous energy of the collision.
Extensional belts display brittle deformation, occurring when the crust fractures and breaks under tensional stress. The primary structure is the Normal fault, which is typically steep and allows a block of crust to move down relative to the block beneath it. This mechanism systematically lowers and tilts large segments of the crust to accommodate stretching. The resultant topography is a distinct pattern of alternating high-standing blocks (horsts) and down-dropped, basin-filling blocks (grabens).
The deformation in extensional settings is highly segmented, concentrating along the steep normal faults that delineate the edges of the blocks. The scale of the faulting is smaller and less pervasive than the regional folding and thrusting seen in collisional settings. The movement on these normal faults increases the surface area of the crust, the direct opposite of the shortening observed in orogens.
Variations in Rock Type and Structure
Deep burial and high pressures within Collisional mountain belts lead to profound changes in rock types. Massive crustal thickening forces rocks deep into the Earth, resulting in widespread regional metamorphism. This process transforms existing rocks into new types, such as schists, gneisses, and marbles, which possess a foliated texture recording the intense strain. The partial melting of deep crustal material also generates significant volumes of granite and other intrusive igneous rocks that form the crystalline cores of these ranges.
The overall structure of a collisional belt is characterized by a deep crustal root extending into the mantle, providing isostatic support for the mountains’ great heights. These belts are linear and extensive, representing the fossilized suture zone where two continents joined. The immense crustal thickness, often exceeding 60 kilometers, is a direct consequence of continuous compression and stacking of rock layers.
In Extensional mountain belts, the geological structure reflects the thinner, stretched nature of the crust. While localized metamorphism can occur near magmatic intrusions, the widespread regional metamorphism seen in collisional belts is absent due to the lack of deep crustal burial. Volcanism is a common feature, as the thinning crust allows magma from the mantle to rise more easily to the surface. However, this igneous activity is less voluminous than the deep-seated plutonism of orogens.
The structure of these belts is segmented and blocky, dominated by the horst and graben geometry that creates separate ranges and valleys. The crust is generally of normal or reduced thickness, having been stretched and faulted rather than shortened and thickened. This results in a less continuous, more fractured mountain system. The uplifted blocks expose shallower, less altered rock types compared to the deeply exhumed metamorphic cores of collisional orogens.