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الاثنين، 3 يونيو 2013

Kinds of Mountains

الاثنين, يونيو 03, 2013
Kinds of Mountains
By: Dr. / Zaghloul El-Naggar
  As mentioned above, individual mountains within a range, system chain, or cordillera can be related to individual geological structures such as folding, faulting, igneous activity, or to a combination of such events. However, the development of a whole chain of mountains (orogenesis) has to be interpreted in terms of much larger tectonic episodes (megatectonics or global tectonics).

  Again, regardless of their mode of formation, the present shape of individual mountains is also related to a large number of factors such as its age and the stage it has reached in the mountain-building cycle, the climatic conditions under which it has existed and the resistance of its exposed rock types to erosion. Indeed, mountains are born, grow, achieve youth, maturity and old age, then they are worn down and finally disappear. The oldest known rocks on the Earth’s surface today are believed to be the roots of some ancient mountains. These currently form the relatively stable cratons or shield areas of the continents.

  According to their geometry, structure, rock composition, and/or age, four main kinds of mountains have been recognized; these include: volcanic mountains, folded mountains (or fold belts), fault-block (or block-faulted) mountains and erosional (or upwarped) mountains. These are, indeed, successive stages in the development of mountains, besides being distinctive types. Volcanic mountains represent the initial stage in the development of such gigantic landforms, while folded mountains represent the peak of youthfulness and maturity and erosional (or upwarped) mountains represent the old age. Fault-block Mountains can be produced at any of these stages, but, nevertheless, have traditionally been treated as a specific type of mountain. These four types or stages in the development of mountains can briefly be described as follows:

1)       Volcanic Mountains: (such as Kilimanjaro of East Africa, Paricutin of Mexico, Mauna Loa of Hawaii, Vesuvius of Italy, Fujiyama of Japan, etc.): These are the simplest known mountains and are usually in the form of isolated peaks, constructed from accumulated lava flows, pyroclastic debris and other extruded igneous rocks that might have piled up rapidly (in only a few years), or might have grown slowly (over thousands or even millions of years).

  Such piling-up of eruptive material can take place around volcanic vents producing cinder cones (such as Vesuvius, near Naples) or elsewhere, producing volcanic mountains. It can also flow out at the surface and consolidate in the form of a broad, gently sloping flat topped, volcanic dome, usually several tens or hundreds of square kilometers in extent, being chiefly built of overlapping and interfingering basaltic lava flows (volcanic shield). These can gradually grow into volcanic mountains such as Mauna Loa in Hawaii (which rises from a depth of 4270 m below sea level to a height of more than 3960 m above sea level), Kilauea of the same island, and the Geat basaltic accumulations of Iceland.

  Volcanic mountains seem to have their origins connected to deep faults that extend below the Earth’s crust to the mantle which supplies their building materials. In other words, volcanic mountains are directly related to deep rifling in the Earth’s crust and hence are considered to represent the earliest stage in the development of a mountainous chain.

  In terms of global tectonics, most of the volcanic types of mountains are believed to be associated with movements near the boundaries of lithospheric plate. These are created as a result of downward, subplate disturbances (e.g. the Aleutian and the Cascade volcanoes) or as a direct consequence to the pulling apart of lithospheric plates at mid-oceanic rifts (e.g. both Kilimanjaro and theKenya Mountains which are both directly related to the East Africa rift system).

  Indeed, active volcanoes are most abundant in narrow belts, particularly in the island areas that rim the Pacific Ocean (where it is believed that the Earth’s crust is currently being consumed by descending into the mantle), as well as along mid-ocean ridges (where new oceanic crust has been steadily produced since at least the time between 150 and 200 million years ago).

   The Aleutian Islands are peaks of volcanic mountains that stretch out for 3200 km along the circumference of a circle centered at 62~ 40’ N and l78~ 20’ W Island arcs festoon the western borders of the Pacific Ocean, with great oceanic deeps (trenches) on the outside curve of many of them.

  Similarly, many geologists believe that mid-ocean ridges are true volcanic mountain ranges. These attain heights of as much as 1800 m above the ocean bottom and are covered—in places—by up to 2700 m of water. Nevertheless, in the framework of plate tectonics, such ridges are believed to have “antiroots” rather than roots, and hence their inclusion among mountains can be strongly debated. Antiroots are accumulations of higher-density material in the suboceanic crust that compensates for the low density of oceanic water. These are injected upwardly from the underlying upper mantle by either convection currents or thermal plumes.

    More than 64,000 km of mid-ocean ridges have - so far-been mapped around mid-ocean rift valleys. These have been pouring out fresh basaltic material on both sides of such ruptures in the Earth’s crust, since the early days of their initiation, to build-up new oceanic crusts. The youngest oceanic crust will always be around deep rift valleys and will steadily push older crusts away from it. The oldest existing oceanic crust does not exceed the Mesozoic in age, and is currently being consumed at the convergent edges of the plates with rates almost equivalent to the rate of producing new oceanic crust.

   Few volcanic mountains are found on the continents such as the isolated peaks of Ararat (5100 m), Etna (3300 m), Vesuvius (1300 m), Kilimanjaro (5900 m), andKenya (5100 m). These are also associated with intra-cratonic, deep rift systems that communicate with the upper layer of the Earth’s mantle.

2)       Folded Mountains (or Fold Belts): These represent the peak of the development of mountain belts, and hence are represented by the great mountain systems of the world such as the Andes, Carpathians, Urals, Alps, Juras, Himalayas, etc. Such mountain systems normally comprise broad belts of varied rock types and of structural patterns that involve folding, faulting, over-thrusting and igneous activity. Faults are particularly numerous along the borders of these highly folded belts. Some are normal faults, but the majority is low-angle thrust faults that extend for hundreds of kilometers, pushing gigantic masses of rock over one another for many kilometers (over thrusting).

  Field observations clearly indicate that the development of folded mountains was normally preceded by the formation of geosynclines. A geosyncline is a large basin in the Earth’s crust, usually scores of kilometers wide and hundreds of kilometers long, with sediments of marine origins that do not usually exceed the 300 m depth that alternate with layered volcanic accumulations in complexes of more than 15,000 m thick. Consequently, geosynclines are believed to have been deeply rifled, slowly and steadily subsiding basins to keep pace with the accumulation of such thick sections of sediments and layered volcanic. The formation of a geosyncline must then involve a slow, and continuous downwarping of the Earth’s crust with the continuous deposition of sediments, and a near access to molten basalts. Here, the theory of plate tectonics can provide the clue to the formation of a geosycline. Seismic evidence from many earthquakes confirms the motion of oceanic plates away from mid-oceanic rifts towards and under other plates where inter-oceanic island-arc-trench systems, or oceanic/continental trench systems are formed and the lithosphere of the subducting (or under-gliding) plate is gradually consumed into the mantle at a rate equal to sea-floor spreading. Plate subduction can account for the formation of oceanic trenches, and the partial melting of the descending plate can explain both the availability of molten magma and the formation of volcanic arcs. Such oceanic trenches are ideal sites for the geosynclinal accumulation of sediments, and hence, geosynclines are believed to have developed in such structurally mobile belts, where subsidence is not only produced under the weight of accumulating sediments, but is also maintained by the gradual sliding of one lithospheric plate below another. When the oceanic plate between two continental masses is completely subducted and consumed, continent/continent collision takes place, forming folded mountains and the highest peaks on earth.

  The sediments accumulated in a geosyncline eventually sink to levels where they become surrounded by denser, more viscous rocks, and their own buoyancy sets a limit to the depth to which they can sink under their own weight. At that point, the whole system becomes isostatic, and the thickness of the sediments cannot be increased just by load.

  Both folding and faulting occur continuously while sediments are accumulating. Rocks at the surface are brittle and hence, they break before they flow, but under deep burial, they become plastic and change both their shape and volume by folding and/or slow flowing. When sediments are buried deep enough, they melt. Expansion of such molten rocks causes the whole overlying mass to rise, and their cooling will produce basement rocks that often participate in the folding process.

  Near the edges of the geosyncline, the rocks are squeezed upward and outward along great thrust faults, while in the central area they are pushed upward to form an inter-montane plateau. Evidence of pre­consolidation folding supports the contention that the mountain-building forces were active during sedimentation. Indeed differential downwarping could have produced folding while deposition was in progress, but at this stage, the dominant forces were probably mainly vertical. Thrust faulting along the margins of the geosyncline could have been initiated by a bordering zone of differential subsidence, but as active horizontal and tangential compressive stresses are usually late in the geosyncline’s history (as a result of the collision of plates) they may be the main cause of overthrusting. Such stresses finally elevate the already deformed strata to mountainous heights. Modem examples of geosynclinal zones growing slowly into mountain ranges are thought to exist today between the Pacific border of Asiaand arcs of volcanic islands off the continental coast.

  From the above mentioned discussion, it is obvious that the major mountain systems have evolved as a result of the movement of lithospheric plates. At the boundary of two such plates, one plate can move down relative to the other, a geosyncline develops and island arcs are built by the piling up of eruptive volcanic material initiated by subduction. Later, the geosynclinal infilling of sedimentary and volcanic rocks rises to form a mountainous chain. As it rises, folds and faults develop either through squeezing (the horizontal-tectonics hypothesis) or through gravity sliding of material away from the rising welt (the vertical—tectonics hypothesis) or by both. Mountain ranges could also result from the collision of two continents being rafted along on their lithosphere conveyor belts (e.g. the Alps and the Himalayas). In both cases, folded mountain ranges were not formed by the deformation of only one geosyncline, but rather by the deformation of many.

  Present-day mountain ranges were definitely much higher. These were worn down over time and were left as erosional remnants of the original, sharply folded and faulted uplifts. Isostatic rebounds for the whole mountain range would also intervene to compensate for erosion and keep the isostatic adjustments. This can go on until the mountain roots are exposed to the surface, attain the thickness of the surrounding lithosphere and the mountain chain is almost completely leveled.

3)       Fault-Block Mountains (Block-FaultedMountains) Such mountains are formed by uplifts of the Earth’s crust along steep dipping or almost vertical faults. Differential tilting of blocks of the Earth’s crust along areas of separation such as rifts can produce fault-black (or block-faulted) mountains. These occur in many parts of the world, frequently adjacent to incipient oceans (such as the Red Sea) or at the periphery of fold belts. Subsequent to folding and low-angle thrust faulting in such belts, a period of steep block faulting produces fault-blockMountains at the periphery of the folded mountain range.
  Fault-block mountains are large, uplifted sections of the Earth’s crust that are bounded by faults in the form of alternating horsts and grabens (e.g. the Great Basin and Range province of Oregon, the Sierra Nevada of California, the mountain ranges that border both the Red Sea rift and the rift valleys of East Africa, etc.). Their rocks may be totally crystalline, igneous and metamorphic complexes or may carry a thin or a thick sedimentary cover. The sedimentary cover, being originally deposited in a geosyncline, can sometimes be folded during an earlier cycle of deformation, before the region was broken up into blocks and uplifted by successive movements along the different planes of faults over millions of years to attain mountainous heights. In the North American Cordillera (along the western border of North America), the fault-block mountains began to be elevated about the same time as their neighboring folded mountains and plateaus, indicating that regional deformative forces were acting.
  Many geologists believe that block-faulting is due to either stretching or relaxation in the later phases of a geosyncline/mountain-building cycle. But, according to the theory of plate-tectonics, large scale rifting may be due to intraplate ruptures, followed by the pulling apart of the ruptured lithospheric plate to diverge away from each other as two new separate plates (such as the splitting of the Arabian/Nubian plate). Fault-block Mountains can also be produced at a later stage in the development of subdued folded mountains, when faulting can provide the necessary elevation of the mountainous range.
4) Upwarped (or Erosional) Mountains:  These are the erosional remnants of previously existing mountain ranges and owe their present heights and appearances to broad upwarpings of the Earth’s crust as a result of isostatic adjustment (e.g. the Ozarks, Adirondacks, Appalachians, Rockies, Black Hills, the Highlands of Labrador, etc.). When the old mountain chains were worn down by erosion and reduced to subdued topographies, isostatic re-adjustment brought them to their present-day elevations. Such subdued elevations represent the final stage in the history of a mountainous chain, before they are almost completely leveled and added to a previously existing craton.

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