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Origin of Mountains
By: Dr. / Zaghloul El-Naggar
Two main hypotheses were put forward to explain the formation of mountains: the vertical —tectonics hypothesis which claims the predominance of vertical movements in the Earth’s crust, and the horizontal-tectonics hypothesis, which states that the major land movements responsible for the building of mountains are primarily horizontal in nature and are directly connected with both plate tectonics and the drifting of continents.
Both hypotheses, however, recognize the close association of orogenesis with geosynclines. As previously mentioned, geosynclines are very large, elongated troughs, several thousands of kilometers long and several hundred kilometers wide, that have been infilled with very thick accumulations of both sediments and layered volcanics (more than 15,000 m thick). Such infill becomes later squeezed and uplifted to form mountains, with or without a crystalline core of igneous and metamorphic rock.
The vertical-tectonics hypothesis postulates that thermal expansion can cause gravity faulting (or sagging) to produce geosynclines in the form of half grabens or full grabens, while plate-tectonics assume that such troughs are formed by the subduction of one lithospheric plate below another as a result of a driving force in the underlying mantle such as convection currents or thermal plumes.
The central idea of plate tectonics is that the solid, outermost shell of the Earth (the lithosphere) is riding over a weak, partially molten, low velocity zone (the asthenosphere). Continents are looked upon as raft-like inclusions embedded in the lithosphere, while only a thin crust (5 km thick) tops the lithosphere in ocean basins. The thickest continental crust, about 70 km, is reported to lie beneath the Alps.
The lithosphere (about 100 km thick) is broken up into about 12, large, rigid plates by rift systems. Each of these plates has been moving as a distinct unit, diverging away or converging towards each other and slipping past one another.
Along divergent junctions, plates spread apart, being accompanied by intensive volcanicity and earthquake activity. The resulting space between the receding plates is filled by molten, mobile, basaltic material that rises from below the lithosphere. This basaltic magma solidifies in the cracks formed by the rift, producing new sea-floor material that adds to the edges of the separating plates and hence, the name “seafloor spreading” for the whole process which is continuously repeated over and over again.
Most basaltic magmas are believed to originate from the partial melting of the rock peridotite, the major constituent of the upper mantle. Since mantle rocks exist under high temperature and high pressure, melting most often results from a reduction in the confining pressure, although the influence of increasing temperature cannot be excluded. This can result from the heat liberated during the decay of radioactive elements that are thought to be concentrated in both the upper mantle and the crust.
Along convergent junctions, plates collide against each other, producing volcanic island-arcs, deep-sea trenches, both shallow and deep earthquakes and volcanic eruptions. In the framework of plate tectonics, orogeny occurs primarily at the boundaries of colliding plates, where marginal sedimentary deposits are crumpled and both intrusive and extrusive magmatism (volcanism) are initiated. However, mountain belts formed at such junctions differ with the different rates of spreading as well as with the nature of the leading edges of the colliding plates (continental or oceanic).
When the abutting edges are ocean floor and continent, the heavy, oceanic lithosphere descends beneath the lighter, continental one to subduct into the underlying mantle. This downbuckling is marked by an offshore trench, while the edge of the over-riding plate is crumpled and uplifted to form a mountain chain parallel to the trench. Great earthquakes occur adjacent to the inclined contact between the two plates, and increasing in depth with the increase in the downward movement of the descending plate, while oceanic sediments may be scraped off the descending slab and incorporated into the adjacent mountains. Such zones of convergence, where the lithosphere is consumed are called subduction zones. Here, the lithospheric material is consumed in equal amount to the production of new lithosphere along the zones of divergence. Rocks caught up in a subduction zone are metamorphosed, but as the oceanic plate descends into the hot mantle, parts of it may begin to melt, and the generated magma may float upwardly, in the form of igneous intrusions and/or volcanic eruptions. The production of magma in the subduction zone may be a key element in the formation of granitic rocks, of which continents are mainly composed.
Granitic magmas are thought to be generated by the partial melting of water-rich rocks, subjected to increased pressure and temperature. Therefore, burial of wet, quartz-rich material to relatively shallow depths is thought to be sufficient to trigger melting and generate a granitic magma in a compressional environment characterized by rising pressures. Most granitic magmas, however, loose their mobility before reaching the surface and hence, produce large intrusive features such as batholiths.
Andesitic magmas are intermediate in both composition and properties between the basaltic and the granitic magmas. Consequently, both andesitic intrusions and extrusions are not uncommon, but the latter are usually more viscous and hence, less extensive than those produced by the more fluid, basaltic magma. A single volcano can, therefore, extrude lavas with a wide range of chemical compositions and hence of physical properties.
Again, when an oceanic plate with a continent at its leading edge collides with another plate carrying a continent, convergence (accompanied by the gradual consumption of the oceanic lithosphere by subduction) gradually closes the oceanic basin in between, producing magmatic belts, folded mountains and mElange deposits on the over-riding continental boundary. This can continue until the two continents collide, when the plate motions are halted, because the continental crust is too light for much of its composition to be carried down to the mantle. Here, the descending oceanic plate may break off, with the complete cessation of subduction at the continent/continent suture, but this can start up again, els1ewhere on the colliding plate. Such continent/continent suture is marked by lofty mountainous chains, made up of highly folded and thrust-faulted rocks, coincident with or adjacent to the magmatic belt. Both giant thrusting and infrastructural nappes lead to considerable crustal shortening and are accompanied by much thickening of the continental crust. An excellent example of continent/continent collision is the Himalayan chain, which began forming some 45 million years ago. This magnificent mountainous chain, with the highest peaks on the surface of the Earth, was created when a lithospheric plate carrying India ran into the Eurasian plate in the Late Eocene time. This can explain how the very thick root underlying the Himalayas was formed.
The plate tectonic cycle of the closing of an ocean basin by continued subduction of an oceanic plate under a continental one until a continent/continent collision takes place and an intra-continental (collisional) mountain belt is formed, has been called the “Wilson cycle,” after J.T. Wilson, who first suggested the idea that an ancient ocean had closed to form the Appalachian Mountain Belt, and then re-opened to form the present-day Atlantic Ocean. As partly mentioned by Dewey and Bird (1970), any attempt to explain the development of mountain belts must account for a large number of common features which are shared by most of the fully developed younger mountain chains such as:
1) Their overall long, linear or slightly arcuate aspect.
2) Their location near the edges of present continents or near former edges of old continents that are presently intra-continental.
3) The marine nature of the bulk of their sediments, and the intense deformation of such sediments.
4) Their frequent association with volcanic activity.
5) Some of their thick sedimentary sequences were deposited during very long intervals, in the complete absence of volcanicity.
6) Short-lived, intense deformation and metamorphism, compared with the lengthy time during which much of the sedimentary succession of mountain belts was deposited.
7) Their composition of distinctive zones of sedimentary, deformational, and thermal patterns that are in general, parallel to the belt.
8) Their complex internal geometry, with extensive thrusting and mass transport that juxtaposes very dissimilar rock sequences, so that original relationships have been obscured or destroyed.
9) Their extreme stratal shortening features and, often, extensive crustal shortening features.
10) Their asymmetric deformational and metamorphic patterns.
11) Their marked sedimentary composition and thickness changes that are normal to the trend of the belt.
12) The dominantly continental nature of the basement rocks beneath mountain belts, despite the fact that certain zones in these belts have basic and ultrabasic (ophiolite suite) rocks as basement and as upthrust slivers.
13) Presence of a thrust belt along the side of the mountainous chain closest to the continent, usually with thrust sheets and exotic blocks (or allochthons).
14) Presence of melange belts (composed of mappable rock units of crumpled, chaotic, contorted and otherwise deformed, heterogeneous mixtures of rock materials, with abundant slumping structures and ophiolitic complexes).
15) Presence of a complexly deformed metamorphic core, with severe metamorphism, magmatization and plutonic intrusions.
16) Presence of magmatic belts of both plutonic, hypabyssal and volcanic igneous activity.
17) Presence of folds of several stages and with unified or divergent trends.
18) Presence of block faulting, especially at the peripheries of the mountainous chain.
19) Presence of deep roots that are proportionately related to both the mass and elevation of the mountainous range, and can be as deep as 5 times the mountain’s height, or even more.
These features are clearly suggestive of geosynclinal deposition, or deposition in mobile belts that are generally referred to as orthogeosynclines and are typically produced by the subduction of an oceanic plate below a continental one. Orthogeosynclines are usually separated into eugeosynclines (characterized by intensive volcanicity) and miogeosynclines (distinguished by being non-volcanic).