In this article we will discuss about:- 1. Objectives of the Thermal Convection Current Theory 2. Base of the Thermal Convection Current Theory 3. Mechanism 4. Evaluation.
Objectives of the Thermal Convection Current Theory:
Arthur Holmes postulated his thermal convection current theory in the year 1928-29 to explain the intricate problems of the origin of major relief features of the earth’s surface. Holmes’ major objectives were not confined to search the mechanism of mountain building based on sound scientific background but were also directed towards finding scientific explanation for the origin of the continents and ocean basins in terms of continental drift as he was opposed to the concept of permanency of the continents and ocean basins as envisaged by the advocates of thermal contraction of the earth.
Wooldrige and Morgan have aptly remarked. ‘The only unifying theory which shows a hopeful signs of reconciling certain of the divergent hypotheses of mountain building and continental drift is that due to Holmes’.
The driving force of mountain building implied by Arthur Holmes is provided by thermal convection currents originating deep within the earth. The main source of the origin of convective currents is excessive heat in the substratum wherein disintegration of radioactive elements generates heat regularly. In fact, the whole theory depends exclusively on the mechanism of thermal convective currents.
Base of the Thermal Convection Current Theory:
According to Holmes the earth consists of 3 zones or layers e.g.:
(i) Upper layer of granodiorite (10 to 12 km),
(ii) Intermediate layer (20 to 25 km) of amphibolite, and
(iii) Lower layer of eclogite.
He has further grouped these three layers into two zones e.g.:
(i) Crust consisting of upper and middle or intermediate layers and crystalline upper part of lower layer, and
(ii) Substratum representing molten part of lower layer.
(iii) Crust and substratum are composed of sial and sima respectively. Generally, siai is absent in the oceanic areas.
The origin of thermal convective currents within the earth depends on the presence of radioactive elements in the rocks. The disintegration of radioactive elements generates heat which causes convective currents. According to Holmes there is maximum concentration of radioactive elements in the crust but temperature is not so high because there is gradual loss of heat through conduction and radiation from the upper surface at the rate of 60 calories per square centimetre per year.
‘This is approximately equal to the radioactive energy produced by a layer 14 km thick of granite, 16.5 km of granodiorite, 52 km of plateau basalt or gabbro and 60 km of peridotite’.
According to Holmes the loss of heat from the earth’s surface is compensated by the heat produced by a crustal shell of 60 km thickness. Thus, there is no accumulation of additional heat in the earth’s crust inspite of maximum concentration of radioactive elements. On the other hand, though there is very low concentration of radioactive elements in the substratum but the gradual accumulation of heat produced by the radioactive elements causes convective currents.
The convective currents depend on two factors e.g.:
(i) Thickness of the crust near the equator and the poles, and
(ii) Uneven distribution of radioactive elements in the crust.
Ascending convective currents originate under the crust near the equator because of greater thickness of crust whereas descending convection currents are originated under the polar crust because of its shallow depth.
The rising convective currents originating from below the continental crust are more powerful than the convective currents originating from below the oceanic crust because of greater concentration of radioactive elements in the continental crust.
Mechanism of the Thermal Convection Current Theory:
Convective currents, thus, are generated at some places in the substratum. Because of difference of temperature gradient from the equator (greater) towards the poles (low) rising convective currents are formed under the equatorial crust while downward moving (descending) convective currents are generated under the polar crust.
The convective currents originating under the continental crust are more powerful than the convective currents originating under the oceanic crust. It may be pointed out that the Currents originating under the equatorial crust move towards the poles i.e. towards north and south and thus the crusts are carried away with the convective currents.
There are two situations of rising convective currents when they reach the lower limit of the crustal masses:
(i) The crustal mass, where two rising convective currents diverge in opposite directions, is stretched and thinned due to tensional forces and ultimately the crust is ruptured and broken into two blocks which are carried away by lateral divergent convective currents and the opening between two blocks becomes seas. Thus, divergent convective currents cause continental drifts,
(ii) Where two lateral convective currents originating under the continental and oceanic crusts converge (fig. 11.11), compressive force is generated which causes subsidence in the crustal zones giving birth to geosynclines and closing of sea.
It is apparent that divergent convective currents move the crustal blocks away in opposite directions and thus create seas and oceans while convergent convective currents bring crustal blocks together and thus form mountains.
The convective currents are divided into two groups on the basis of their locational aspect e.g.:
(i) Convective currents of rising columns, and
(ii) Convective currents of falling columns.
The rising convective currents after reaching the lower limit of the crust diverge in opposite directions.
This outward or divergent movement introduces tensional force due to which the crust is stretched, thinned and ultimately broken and the broken crustal blocks are moved apart. The wide open area between two drifting crustal blocks in opposite directions is filled with water and thus an ocean is formed. According to Holmes the equatorial crust was stretched and ruptured due to divergence of rising convective currents which carried the ruptured crustal blocks towards the north and south and Tethys Sea was formed.
This phase is called ‘opening of Tethys’ Again two sets of convergent or downward moving (descending) currents brought Laurasia and Godwnaland together and thus Tethys was compressed and folded into Alpine mountains. This phase is called ‘closing of Tethys’.
‘The convective mechanism is not a steady process but a periodic one, which waxes and wanes and then begins again with a different arrangement of centre’. It means that the convective currents originate at several centres which are not permanent. Geosynclines are formed due to subsidence of crustal blocks mainly continental shelves due to compressive force generated by convergent convective currents moving laterally together under continental and oceanic crusts.
In other words, when the continental and oceanic crusts move together and converge under the continental shelves, they descend downward and thus cause immense compression due to which the crust is subjected to subsidence to form geosyncline. The convective currents of rising columns under continental and oceanic crusts bring materials in the geosynclines which are always located above the descending convective currents of falling column.
Continuous compression and sedimentation causes gradual subsidence of geosynclines. Holmes has described a cyclic pattern of thermal convective currents which includes the origin of convective currents, formation of geosynclines, sedimentation and orogenesis and further rise in the mountains. According to Holmes the cyclic pattern of convective currents and related mountain buildings pass through three phases or stages (fig. 11.12).
First stage is of the longest duration during which convective currents are originated in the substratum. The rising convective currents of two centers converge under the continental shelves and thus form geosynclines due to compression coming from the convergence of two sets of lateral currents. Geonsynclines are subjected to continuous sedimentation and subsidence. As the sediments are pressed downward into geosynclines, these go further downward and are intensely heated and metamorphosed.
Metamorphism of sediments causes rise in their density which further causes downward movement of the metamorphosed materials. Thus, the falling column of downward moving convective currents is the column of increasing density. Amphibolites are metamorphosed into eclogites. A portion of heat is spent during the process of metamorphism and hence the heat does not accumulate to greater extent.
The first stage, characterized by high velocity convective currents, is in fact the preparatory stage of mountain building which is marked by the creation of geosynclines, sedimentation and subsidence of materials partly caused by compression resulting from convergence of convective currents and partly by increase in the density of materials due to metamorphism.
Second stage is marked by phenomenal increase in the velocity of convective currents but this stage is relatively of short duration. The main cause for the phenomenal increase in the velocity of convective currents is the downward movement of cold materials in the falling column and upward movement (rise) of hot materials in the rising column of convective currents.
Increased pressure due to metamorphism of geomaterials in the falling column of descending currents increases the velocity of downward moving convective currents. The high velocity convergent convective currents buckle geosynclinal sediments and thus initiate the process of mountain building (fig. 11.12). This stage, thus, is called the stage of orogenesis.
Third stage is characterized by waning phase of thermal convective currents due to incoming hot materials in the falling column and upward movement (rise) of colder materials in the rising column. Gradually, the rising column becomes a cold column i.e., cold materials are accumulated at the centre of the origin of rising (upward moving) convective currents due to which these currents cease to operate and the whole mechanism of convective currents comes to an end.
The termination of the mechanism of collective currents yields several results e.g.:
(i) The materials of the falling column start rising because of decrease in the pressure at the top of the falling column due to the end of deposition of materials. This mechanism causes further rise in the mountains,
(ii) The depressed and subsided heavier materials in the falling column of descending convective currents start rising due to decrease in the weight and pressure at the top of the falling column,
(iii) Eclogite, which was depressed downward, gets melted due to immense heat and thus it expands.
This expansion in the volume of molten eclogite causes further rise in the mountains. This stage is known as the stage of gliptogenesis. It is, thus, apparent that the thermal convective currents of Holmes explain all the three stages of mountain building e.g., lithogenesis, orogenesis and gliptogenesis.
Griggs through his experiments has validated the mechanism of convective currents and consequent mountain building.
Evaluation of the Thermal Convection Current Theory:
Commenting on Holmes’ thermal convective current theory J.A. Steers (1932) has remarked, ‘the theory is interesting, but it depends upon such factors about which little is known’. It may be pointed out that this comment of Steers about 80 years ago is not valid today as there are ample convincing scientific evidences which validate the mechanism of convective currents originating from within the mantle. Even much appreciated plate tectonic theory is based on thermal convective currents.
The theory was criticised, at the time of its postulation in 1928-29, on the following grounds:
(1) Convective currents theory, no doubt, is a leading theory in a new direction but whole of the theory depends on such factors about which very little is known. Rising and falling columns are doubtful phenomena and therefore doubtful stage can never be taken for the explanation of natural phenomena.
(2) The whole mechanism of convective currents depends on the heat generated by radioactive elements in the substratum (now mantle) but several scientists have raised doubt about the availability of required amount of heat generated by radioactive elements. If heat, thus, is insufficient, convective currents may not be generated and, therefore, the whole mechanism and working of the theory would not be possible. It may be further pointed out that the rising currents pass on their heat into the crust through conduction. This process also causes loss of heat which may weaken the currents.
(3) The horizontal flow of thermal convective currents under the continental and oceanic crusts is also a doubtful phenomenon because of lack of required amount of heat to drive these currents. If horizontal flow of convergent movement of convective currents is not possible, then the falling column would not exist and hence mountain cannot be formed.
(4) The metamorphism of amphibolites into eclogites and resultant downward movement of relatively denser eclogites is also a doubtful phenomenon. Even we accept the metamorphism of amphibolites into eclogites but the resultant increase in density from 3.0 to 3.4 would not be enough to depress and sink eclogites in the falling column. If desired sinking of eclogites is not possible, there would not be proper accommodation of materials brought by the horizontal convergent convective currents into the falling column.
If this is so, the whole falling column would be filled with eclogites and the next stage of the mechanism of the convective currents would not work. It is, thus, argued that the theory does not make proper provision for the accommodation of additional materials.
(5) According to this theory convective currents are originated at few centres only under the continental and oceanic crusts but question arises, why are not they originated at all places? If this so happens, the horizontal movement of these currents would not be possible.
The whole of the continents would be divided into several blocks as the rising convection currents originating from numerous centres would break the crusts and would give birth to volcanic eruptions of various sorts. This observation has been now validated on the basis of plate tectonics as rising convective currents diverge under the mid-oceanic ridges and thus the plate is ruptured and two plates move in opposite directions due to divergent convective currents and fissure flows of lavas occur along the mid-oceanic ridges representing the rupture zone.
It may be concluded that the idea of thermal convective currents conceived by A. Holmes about 80 years ago (in 1928-29) proved its worth in 1960s when the scientists were looking forward to search such a force which can explain the movement of plates. Now, the process of mountain building can be very satisfactorily explained on the basis of convective currents though not in the way as conceived by A. Holmes in 1928-29 but on the lines of plate tectonics.