Mountain chains are elongate, elevated areas of the earth's surface comprising several sub-parallel mountain ranges. Each mountain range is a connected series of mountain peaks (i.e., large rock masses that rise abruptly above the surrounding landscape). Mountain chains may be a thousand or more kilometers long and hundreds of kilometers wide. Mountain chains are formed by the interplay of endogenic and exogenic processes. Endogenic processes are those that originate within the earth, such as orogenesis and volcanism. Exogenic processes are external processes, such as weathering and erosion due to the action of water, ice, and wind.
Volcanism during subduction of oceanic crust beneath oceanic crust creates an island arc. Island arcs may comprise an arcuate alignment of volcanic island peaks (e.g., the Aleutian Islands) or a continuous land area comprising a central mountain chain formed by volcanic and tectonic processes (e.g., Japan). Subduction of oceanic crust beneath continental crust creates a Cordilleran or Andean-style mountain chain. The best-known example is the Andes in South America, where the oceanic Nazca Plate is subducted beneath the continental South American Plate. In the Barisan Mountains of Sumatra, Indonesia, oceanic lithosphere of the Indo-Australian Plate is obliquely subducted beneath continental crust of the Sunda Plate. A network of faults comprising a transcurrent fault system dissects the resulting volcanic mountain chain.
Collisional mountain chains result from the collision between two continental lithospheric plates or between a continental plate and an island arc. The continental lithosphere is greatly thickened in this process called orogenesis. The resulting belt of uplifted crust forms a mountain chain with a commensurate deep lithospheric keel or root. Such isostatic balance can be likened to an iceberg where only a proportion protrudes out of the water, with a large part of its mass being below water. When material in the mountain is removed by erosion or tectonically through extensional faulting, compensatory uplift will occur while the lithospheric root remains. Rocks previously at greater depths are, therefore, brought closer to the surface in a process called exhumation. Tens of kilometers of rock may be removed by erosion before a collisional mountain chain is eventually flattened. Eroded sediments are deposited in adjacent extensional or foreland basins. Extensive erosion plus or minus tectonic exhumation due to displacement along faults results in exhumation of high-grade rocks. Collisional mountain belts include:
- the European Alps, formed by collision of the European Plate with the Adriatic Plate following closure of part of the Tethyan Ocean,
- the Himalayas, formed by the collision between the Eurasian Plate with island arcs and the Indian Plate.
Folding of rock layers may also occur in the hinterland to a collisional orogenic belt or in a cover sequence where basement rocks slide past one another at the same time as undergoing regional shortening (i.e., a transpressional belt). Trains of folds can also form during gravitationally induced sliding on a weak basal décollement horizon without regional shortening. In all such folded areas, rock layers more resistant to erosion in regional-scale antiforms may define a continuous mountain range along each anticline. Such fold mountain chains generally lack the presence of a deep lithospheric root and will more rapidly disappear due to the effects of erosion over time.
Steep to moderately dipping normal faults formed during regional, horizontal extension during collapse of a collisional mountain belt or rifting may downthrow blocks of rock called graben, leaving elongate, fault-bounded high blocks (horsts). Block-faulted mountain chains are formed if displacements are great enough and erosion rates are low. Rock layers between parallel-dipping normal faults are tilted and this may result in asymmetrical mountain ranges with steep faces and
long dip slopes parallel to layering. Valleys along intervening graben in which the eroded sediments are deposited separate the ridges. Mountain chains formed by extensional faulting occur in areas of widely distributed extension, such as the Basin and Range Province of Utah and Wyoming. Displacement can also take place along shallowly dipping extensional detachment faults that widen to form ductile shear zones at greater depth. The earth's lithosphere is thinned during regional extension. In order to compensate for this, the underlying asthenosphere is arched upward beneath the area of greatest lithospheric thinning. Extensional detachments are folded and deep crustal rocks exhumed, forming metamorphic core complexes. Metamorphic core complexes commonly produce elongate domal mountains as their metamorphic and possibly igneous core is likely to be more resistant to subsequent weathering than the surrounding low-grade rocks.
See also Orogeny; Plate tectonics; Subduction zone
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