- A mountain can be defined as an area of land that rises abruptly from the surrounding region and higher than the hill.
- The term ‘orogeny was coined by the American geologist, G.K. Gilbert, in 1890 to describe the process of mountain building. The term was originally used by Gilbert to describe the fold mountain belts of the Alps and the Rockies.
- According to the Oxford Dictionary of Geography, the term ‘orogeny’ may be defined as tectonic “movements of the earth which involve the folding of sediments, faulting, and metamorphism.
- A mountain may have several forms. Important among them are:
- Mountain Ridge,
- Mountain Range,
- Mountain Chain,
- Mountain System,
- Mountain Group, and
- Mountain Ridge: It is a linear, steep-sided high hill, or spur. The slope of one side of a ridge is steep, while the other side is of the moderate slope. A ridge, however, may have symmetrical slopes on both sides. The Shimla Ridge is a good example of a mountain ridge.
- Mountain Range: A mountain range is a linear system of mountains and hills having several ridges, peaks, summits, and valleys.
- Mountain Chain: – A mountain chain consists of several parallel long and narrow mountains of different periods.
- Mountain System: – A mountain system consists of different mountain ranges of the same period. In a mountain system, different mountain ranges are separated by valleys.
- Mountain Group: – A mountain group consists of several unsystematic patterns of different mountain systems.
- Cordillera: – It is a Spanish term referring to a system or major group of mountains. A cordillera consists of several mountain groups and systems. In other words, cordillera is a community of mountains having different ridges, ranges, mountain chains, and mountain systems. It usually refers to an orogenic belt at a continental scale, e.g., the Western Cordillera of the U.S.A., which includes all the ranges between the Pacific and the Great Plains.
Different theories of Mountain Building
1. GEOSYNCLINAL OROGEN THEORY OF KOBER
- The German geologist Kober after taking into account the mountains in the different parts of the world has postulated a typical orogen in which there are two forelands. The mountain ranges are formed as a result of compression of the geosyncline situated on the margins of the forelands of ancient rigid landmasses or kratogen. He calls these border ranges Randkatten, and the zone situated in between the two borders remains virtually unaffected by folding. This unaffected middle part between the mountains has been called Zwischengebirge (betwixt mountains) or the Median Mass by Kober.
- Kober has been an exponent of the contraction by hypothesis. In his view, the contraction has continued since the origin of the earth, which has provided from time to time the necessary force for mountain building. In his view, during each period of mountain building, we find more or less the same sequence of events which may be identified as the general process of mountain formation.
- First of all, there is the formation of the geosyncline in which there is the deposition of sediments and continuous subsidence of the geosyncline resulting in the folding of the sediments and their upliftment into mountains. This latter period of orogenesis is also characterized by volcanism and intense metamorphism. After the formation of the mountains, there is a long period of subaerial erosion in which the mountains are denuded and ultimately reduced to areas of subdued relief.
- The views of Kober are essentially based on the geosynclinal hypothesis of Hall and Dana as expounded later by Haug, and into this, he has tried to fit his own views about mountain building. But while the geosyncline of Haug is a narrow depression, that of Kober is a long and wide sea.
- In addition to the orogenic movements described above, Kober mentions another kind of movement which is found in the landmasses or kratogen. He calls such movements Kratogenic movements. While the orogenic movements cause folding and metamorphism of the geosynclinal sediments up to great depths, the kratogenic movements are responsible for the formation of rifts and fractures in the landmass, of the sea, the kratogenic movements may cause the folding of the sediments.
- For example, the superficial folds of the Jura mountains, the rift valley of East Africa and the Rhine, and the horsts or block mountains of Central Europe have all resulted from kratogenic movements.
Criticism of the theory
- The orogen theory of Kober has been criticized on the ground that the compressive force produced by the earth’s contraction can never be so powerful as to result in the formation of huge mountain chains like the Alps and the Himalayas.
- Further, Kober’s assumption that the two forelands move towards each other and the compressive stresses so produced are responsible for folding, has also remained controversial.
- While his theory is able to explain the formation of mountains like the Alps and the Himalayas which have west to east trends, it fails to explain the origin of north-south trending mountain chains such as the Rockies and the Andes which border the Pacific coast of the American continents.
2. THE THERMAL CONTRACTION THEORY OF JEFFREYS
- H. Jeffreys has presented a detailed discussion of his thermal contraction theory in his famous book, The Earth: Its origin history and physical constitution.
- Jeffreys is a contractionist and seeks an explanation for mountain building within the framework of contraction produced by loss of heat in the earth’s crust.
- According to Jeffrey’s calculations, there has been no change in temperature in the interior of the earth, i.e., in the zone from the centre of the earth to about 700 km below the earth’s surface. But the uppermost 700 km thick layer has experienced temperature decrease. In this upper parts, every successive layer cools more rapidly than the layer below it, and, therefore, the upper layers have suffered more contraction than the lower layers.
- In fact, the hot and less contracting lower layer obstructs and retards the contraction of the upper layer, with the result that the upper layer can contract only by spreading out and thinning itself. The solidified uppermost layer can cool only up to a certain point through this process and therefore its contraction also stops. But the cooling and contraction of the underlying layer continue. The result is that the upper layer becomes larger in comparison to the lower contracting layer, and does not fit in with the underlying layer. In its effort to adjust itself to the contracting lower layer, the upper layer undergoes compression which produces folding and faulting. In other words, the upper layer undergoes crustal shortening.
- On the other hand, the contracting lower crustal layer becomes smaller than the hot interior and does not fit in with the latter and the former has to spread or stretch itself to conform to the internal layer. This generates tension, resulting in the formation of fissures and cracks. These fissures and cracks get filled up with the hot and molten material from below. In between the upper zone of compression and the lower zone of tension, there should be an intermediate zone where the contraction is such that it is able to adjust itself to the lower contracting crustal layer. This intermediate zone is a level of no strain. As the earth has gone on cooling, the level of no strain has been sliding downwards from the earth’s surface. Above this level, on account of the horizontal compressive stress there is buckling and folding and the formation of mountains.
- There is a greater possibility that minor folds and minute puckers will result from thermal contraction and not extensive mountain systems. When we look at the massive Tertiary Alpine orogeny, it appears impossible to believe that only about 200 million years ago there was so much contraction in the earth that mighty mountain systems like the Alps and the Himalayas could come into existence. Several other objections have been raised against the thermal contraction theory and it has now very few supporters. Instead, the drift theories in various forms are now being propounded with greater vigor.
3. RADIO-ACTIVITY THEORY OF JOLY
- J. Joly put forward his theory of thermal cycles or the radioactivity theory in 1925 in his book ‘The Surface History of the Earth’. His main purpose is to present a historical account of the earth’s surface, but he also tries to explain the formation of mountains. His theory is extremely simple and is based on the latest scientific facts available at that time. The continents composed of sial (granite) rest on the heavier sima (basalt).
- An important basis of this theory is the radioactivity of rocks, in comparison to sima, the rocks of the sial are richer in radio-active minerals. Joly is of the view that the amount of heat lost by the earth’s surface by radiation is more t fhan counterbalanced by the heat received from the radioactivity of the sialic rocks. In such a situation there is no need for the transfer of heat to the sial from the substratum (sima).
- Consequently, the heat derived from radioactivity is preserved in the sima, and ultimately the accumulated heat becomes so great that the basaltic rocks begin to melt. The position below the ocean floor is somewhat different. There is no sialic layer here and in the upper layer of the sima, the heat derived from radioactivity is lost in the oceanic waters through conductivity. But there is no loss of heat in the lower layer of the sima and the accumulated heat is sufficient to melt the basalts.
- Joly has shown that assuming the thickness of the sial to be 30 kms, the temperature below the sial should be 1,0500 C. The melting point of basalt is 1,1500 C. In other words, in order that the sima or substratum may melt, the temperature should rise by another 1000 C. Joly has tried to show by mathematical calculation that it may take 33 to 56 million years for the accumulated heat derived from radioactivity to melt the basaltic rocks.
- When the sub-stratum becomes wholly or partially liquid, then its density decreases, and the buoyancies of the continents are reduced and they sink further down into the sima. As a result the oceanic water covers the low continental margins and there is transgression of the sea. In such a situation there is deposition of sediments in the shallow coastal waters. In this manner the geosynclines come into existence, and in course of time it is these sediments which are folded and uplifted to form the mountains.
- When the substratum melts and is in a liquid conditions and the continents are floating over it the
influence of the tidal force is increased,. On account of the tidal effect the sialic blocks start moving
towards the west. Through this process the heat is released from under the continents, and the substratum again begins to solidify as a result of loss of heat. The solidification of the substratum increases its density and the continental blocks are pushed up, and there is a fall in the sea level. In other words, the transgressional seas disappear, and on account of the regression of the sea, the coastal sediments appear on the surface.
- We have seen that the melting of the sub-stratum results in the expansion of the sima, and the expansion is maximum in the oceanic parts. It is obvious that when the substratum again starts solidifying, the maximum contraction will also take place below the ocean floor. The contracting ocean floor exerts pressure on the continents, and the margins of the continents situated between two contracting oceans are subjected to lateral compression with the result that the relatively soft sediments deposited along the coast are folded. This is the first stage of mountain formation. The cooling and re-solidification of the sima take time and the pressure of the ocean floor start much before the sima has completely solidified.
- When the entire sima has solidified, then the sialic blocks (continents) located above them rise up. In this way, it is possible to explain the second stage of mountain formation, namely the uplift of folded coastal sediments. In other words, first of all the folds and nappes are formed as a result of lateral pressure, and later on after the solidification of the sima, the folded, zone is uplifted through the process of isostatic recovery. The theory thus emphasizes the two specific processes of horizontal compression and vertical uplift in the formation of mountains.
- According to this theory, the formation of mountains takes place essentially along the continental margins. Further the bigger the ocean, the greater will be the pressure exerted by the contraction of its floor, and more widespread will be the formation of mountains. Thus, it may be stated as a general rule that the biggest mountains will face the biggest ocean. This is, of course, true up to a certain point as we find an extensive and mighty chain of mountains surrounding the Pacific Ocean. But this theory fails to explain the absence of mountains parallel to the Atlantic coasts or to offer a satisfactory explanation for the location of the great Alpine Himalayan mountain system.
Criticism of the theory
- Jeffreys has been the greatest critic of the theory of Joly and is in complete disagreement with the latter. According to Jeffereys the depth of 30 km for the sial proposed by Joly is excessive and should be around 16 kms on the basis of seismological evidences. If the latter figure is taken as the depth of the sial, then the heat below the sial would be much less than that assumed by Joly. In the absence of adequate heat it will be impossible to proceed further with the theory.
- According to Joly during the liquefaction of the sima, the continental masses are pushed towards the west as a result of the tidal force. Jeffreys has tried to show mathematically that there is no known adequate force which may cause westward displacement of the continents.
- With the help of mathematical laws Jeffreys has shown that once the sima has melted under the influence of radioactivity, it cannot re-solidify. If the distribution of the radioactive substances is such that the temperature of the basaltic layer of the sima is above the melting point then this layer will permanently remain in a liquid state and there is no question of its melting and resolidification at regular intervals.
- The theory is based on radioactivity about which especially in the interior of the earth, our knowledge is inadequate. Further, the theory presents rather an erroneous account of geosynclines.
4. DALY’S HYPOTHESIS OF SLIDING CONTINENTS
- Daly propounded his hypothesis of sliding continents in the book ‘Our Mobile Earth’ in 1926. His hypothesis is based on the view that there has been a downhill sliding movement of the continental masses, the principal cause of which has been gravity. For continental displacement, he does not invoke any tidal or other force besides gravity. His hypothesis is therefore simple and clear.
- Daly assumes that the distribution of land and water was predetermined in the prime days. The landmasses lay near the equator and the poles. In between these three rigid landmasses lay low-lying areas or the seas which was the mid-latitude furrow but in the southern hemisphere, there is no definite speculation about a similar depression. In addition, there was the extensive primeval Pacific Ocean.
- These rigid landmasses as well as the ocean floor were composed of the primitive crust. Thus, in the primeval days the surface of the earth had been divided into land and water. The landmasses were higher than the seas and therefore the equatorial and the polar landmasses sloped towards the Pacific Ocean as well as the two mid-latitude furrows. In the Pacific Ocean as well as the mid-latitude furrows sediments began to be deposited by denudation of the continental domes, and they may therefore be treated as the first geosynclines. The pressure on the ocean floor started rising for two resons the pressure of oceanic water and the weight of the geosynclinal sediments. In this manner the sea floor went on subsiding.
- According to Daly, mountains have been formed by the folding of geosynclinal sediments as a result of lateral pressure exerted by the sliding of the continental crust towards the geosynclinal seas. On this basis the west-east trending Alps Himalayan mountain system has resulted from the sliding of the continental blocks towards the mid-latitude furrow, and the north to south trending the Rockies and the Andes mountains have been formed by the sliding of the continental masses towards the Pacific Ocean. Similarly, the mountain arcs and islands off the coast of eastern Asia owe their origin to the sliding of the Asiatic landmass towards the Pacific Ocean, and on account of the downward pressure exerted by these islands deep fore deeps or trenches have come into existence.
Criticism of the theory
- Several objections have been raised against Daly’s hypothesis. His hypothesis, it is said, is based on a number of self-proved assumptions which are no better than guesses. Daly assumes an uneven earth surface with the separate distribution of land and water from the beginning. In other words, there was the original landmass of Pangaea and the original ocean of Panthalassa. The dome of Pangaea was divided into three parts on account of sliding North Polar landmass, South Polar landmass, Equatorial landmass, and two depressions were formed in between them. He does not go into the question of how all this became possible. It may be assumed that the original crust had two layers on the upper layer of granite and a lower layer of denser rocks.
- Daly also presents a rather confusing concept of geosynclines. Ordinarily, geosynclines are long, narrow and relatively shallow seas but he accepts both the mid-latitude furrows and the Pacific Ocean as geosynclines. Further, his hypothesis anticipates the formation of mountains from every ocean, irrespective of the extent and depth of the oceans and the amount of sediments that may be deposited in them.
- Daly’s hypothesis tries to explain the problem of mountain building in a simple manner with the help of gravitation but fails to present a well-argued and connected account of the problem.
5. THE CONVECTION CURRENT THEORY OF ARTHUR HOLMES
- Arthur Holmes put forward his convection current theory in 1928-29. The main objective of this theory is to present an explanation for the processes of mountain formation, but it also throws light on volcanicity associated with mountain building as well as on continental drift. The importance of this theory lies in the fact that it tries to integrate conflicting views regarding mountain building and continental drift.
- According to Holmes, it is extremely important to bear in mind the distinction between the solid crust and the liquid sub-stratum. The earth’s crust includes the upper layer (sial), the intermediate layer (i.e. the upper part of sima), and the lower layer consisting of crystalline rocks. Below this is the sub-stratum which is liquid but is a continuation of the lower crustal layer. Holmes; theory is based on the possibility of convection currents in the sub-stratum.
- The cause of the origin of convection currents in the sub-stratum is the presence of radioactive substances there. The disintegration of these substances produces enough heat to keep the substratum in a liquid condition and to generate convection currents. Radioactive substances are more abundant in the upper layers of the crust, but heat from these layers is lost by abundant in the upper layers of the crust, but heat from these layers is lost by radiation and conduction, and the temperature, therefore, does not rise.
- Although radioactive substances are less in the sub-stratum, the heat released by them plus the original heat of the sub-stratum are so great that they are able to generate convection currents, as there is no loss of heat from the substratum by radiation or conduction.
- According to A. Holmes, the circulation of convection current is determined by the thickness of the crust and richness in radioactive elements. He states further that the equatorial region is thicker than the Polar region and continental masses have more amounts of radioactive minerals. Hence these are ascending limb under the equator and continental masses because of high temperature while descending limb under the Polar region and a weak ascending limb under the oceanic crust. Diverging currents meet near continental margins and descends.
- In the region of descending currents there is the formation of geosynclines on account of subsidence. The sediments derived from the erosion of the continents will be deposited in them leading to further subsidence. Since in the geosynclinal area the continental and the oceanic currents meet, there is lateral pressure from both sides. In other words, folding of the sediments proceeds together with their deposition and the subsidence of the geosyncline.
- The conventional activity is cyclic, and the three stages may be identified during which there is the formation ofthe geosyncline and the folding and uplift of the mountains.
- First Stage: The first stage is of a very long duration. The convection current are quite strong, and the currents coming from two sources converge and ascend near the continental shelves, resulting in the formation of the geosyncline. Sediments are deposited in the geosyncline and there is subsidence of the geosyncline. The sediments, when they reach great depth, become hot and metamorphosed, and their density increase. In other words, in the zone of the descending currents, the roots of the mountain are formed by geosynclinal subsidence. This is the preparatory stage of mountain building.
- Second Stage: In the second stage the speed of the convection currents becomes even faster, but this stage is of relatively short duration. The convection currents coming from the continental and oceanic crusts descend downward with great force resulting in maximum compression on the geosynclinal sediments and their consequent folding. In this way, the process of mountain building starts.
- Third Stage: In this stage of the convection currents start getting, weaker as the process seems to approach its end. On account of the decrease in the velocity of the descending currents, the downward pressure also decreases and the uplift of the folded sediments begins. The heavier materials which had sunk below start moving up. Eclogite which on account of the pressure had gone down to great depths
melts under the high temperature and rises up. This is the stage when the mountains are uplifted, and the upliftment continues till isostatic equilibrium is attained.
- Holmes also tried to explain the function of geosynclines’ median mass, rift valleys and volcanicity with the help of convection currents through the admits that some of his suggestions are highly imaginative.
- Although the theory of Holmes is quite interesting, some of his assumptions are valid, our knowledge about the existence of convection currents and their behavior is extremely limited, and we do not know whether they are powerful enough to cause the breaking up and drifting of the continents. Though this theory opened a new direction for understanding the problem of mountain formation, the horizontal movements of convection currents below the crust, their rise and descent, their process being periodic instead of continuous, the re-starting of the process from new locations are all assumptions based on speculation and for which we have no proof. At the same time, it is noteworthy that the theory has recently supported and confirmed from studies relating to plate tectonics where convection currents provide the driving mechanism for the movement of lithospheric plates.
6. PLATE TECTONICS AND MOUNTAIN BUILDING
- The theory of plate tectonic as outlined by. Hess and R. Dietz and postulated by W. J. Morgan is the most modern, most scientific, and most acceptable theory. It explains the origin of mountains in a scientific manner with the mechanism of plate movements. It recognizes three types of plate boundaries:
- Divergent boundaries or junctures at the mid-oceanic ridges of rifts,
- Shear boundaries or junctures where two plates pass across each other, and
- Convergent boundaries or junctures where two plates collide against each other and one of the two plates is lost by subduction into the trenches.
- The divergent and the convergent boundaries are particularly important because oceanic ridges and rift valleys are formed on the divergent boundaries, and the folded mountain ranges are built on the convergent boundaries.
- The young folded mountains of the world-the Alps, the Himalayan mountain system as well as the Circum Pacific belt of mountains are located on convergent plate boundaries where there is a state of collision between two plates. It may therefore be stated as a general rule that where there is convergence or collision of two plates, mountains are formed as a result of compression in the earth’s crust. The Convergence of plates is possible under three different conditions:
- Collision between a continental and an oceanic plate or continent-ocean collision.
- Collision between two continental plates or continent-continent collision, and
- Collision between two oceanic plates or ocean-ocean collision.
- This is the most common type of collision, and the mountain ranges encircling the Pacific Ocean are all located at places where there is a collision of continental and oceanic plates. Its simplest and best example is found along the Pacific coast of South America. There exist a steady-state continuous collision between the oceanic and the continental plates, and the oceanic plate is thrust down under the continental plate in the trenches.
- As a result of the intense pressure, the deposition on the continental margin is compressed and folded. As the mobile core of the orogenic belt develops the deformation of the plate margins increases with increasing temperature and intense pressure. There is gravity sliding on account of uplift and thrusting on account of compression. The mobile core pushes the metamorphosed rocks towards the continent and the continental edge is uplifted to form mountains.
- According to the plate tectonic theory the Andes were formed during the early Mesozoic era. The subduction of the oceanic plate beneath the S. Amerian plate started about this time resulting in the deformation of the Palaeozoic marine sediments. After this, the South American plate started moving west which intensified the process of mountain formation in the mid-Mesozoic and early cretaceous times.
- As the oceanic plate descended further the pressure against the South American plate also increased and there was further intensification of these activities and the area of orogenesis also increased.
- The Alpine Himalayan mountain system provides the best example of mountains formed as a result of this type of collision. In Mesozoic times India together with the southern continents formed part of Gondwana land, and there existed the Tethys sea between the main Asiatic landmass (Laurasia) and the Gondwana land. After the Mesozoic, Gondwanaland started breaking up, and India started moving to the north at the rate of 16 cm per year and joined the Asiatic landmass about 30 to 60 million years ago. As a result, the Tethys sea became narrower and ultimately closed.
- At about the same time Africa also started moving north and the portion of the Tethys sea between Europe and Africa became narrower, As a result of the collision of the Indian and the Asiatic landmasses the marine sediments and the crust located in between the two were folded and thrust and the Himalayan mountain chain came into existence from 2 to 30 million years ago.
- On account of the relative buoyancy of the continents, the upper rocks suffered more folding and thrusting. On the uplifted folded sediments are found deposits of flysh derived from the erosion of the former and of molasses in the coastal regions. Similarly, as a result of the collision of the African and European plates, the sediments along the continental margins have been folded and thrusted to form the Alps in Southern Europe and the Atlas Mountain in northwest Africa.
- Where oceanic plates exist on both sides of a convergent plate boundary, the oceanic crust of one plate is subducted under the other plate in the trenches, and the resultant compression leads to the formation of island festoons and islands arcs. This type of mountain is especially found off the western coast of the Pacific Ocean and the northeastern coast of the Indian Ocean. Suess was the first to point out that this arc like island groups are tops of drowned young folded mountain ranges and are extensions of the mountain systems found on the continents.
- Between the continents and the island arcs are shallow seas which are called back-arc basins. The Sea of Japan is a good example of a back-arc basin. Towards the oceanic margin of each island arc is a deep oceanic trench. It appears as if the trenches owe their origin to the descent of the plate. Here the oceanic plate together with the deposited sediments descends under the adjacent oceanic plate and on account of compression on the continental margin of the trenches; there is the formation of metamorphic rocks.
- It may be pointed out that sometimes mountains may be formed where there is a collision of continent and islands arc. This kind of situation exists in New Guinea where the mountains of New Guinea have come into existence about 20 million years ago as a result of the convergence of the island arc lying to the north and the northern edge of Australia.