In this article we will discuss about:- 1. Objectives of Radioactivity Theory 2. Bases of the Radioactivity Theory 3. Mechanism 4. Evaluation.
Objectives of Radioactivity Theory:
Joly postulated his theory based on radioactivity of certain radioactive minerals in the year 1925 in his book, ‘Surface History of the Earth’ to account for the origin and evolution of surface features of the earth. His theory is also known as thermal cycle theory or theory of the surface of the earth.
Though the main objective of Joly’s theory was to present a detailed account of the thermal history of the earth and mathematical explanation of the structure of the interior of the earth but he also attempted to explain the problems of mountain building and the continental drift.
In fact, ‘Joly’s views on the earth’s surface history are based on such reasonable premises, and are so simple in their conception, that they have met with a great deal of favour’. While commenting on Joly’s theory of radioactivity he has remarked, ‘it should not be accepted as proved, but retained as an hypothesis which probably contains a certain elements of truth’.
The driving force of the mountain building as invoked by Joly is provided by expansion and contraction of the substratum of the earth resulting into transgressional and regressional phases of the seas (geosynclines). The expansion and contraction of the substratum are based on the mechanism of heat generated by radioactive elements of the rocks.
It may be pointed out that the theory of A. Holmes and Joly are based on radioactive elements but they sought their help differently e.g., Holmes used radioactive elements to explain the origin of thermal convective currents in the substratum while Joly used them to explain the melting and re-solidification of the substratum. He also implied tidal force and friction to explain continental drift.
Bases of the Radioactivity Theory:
The whole mechanism of Joly’s theory is based on the presence of radioactive elements of the rocks of the earth. In order to explain various aspects of the mechanism of radioactive elements Joly has described first the structure of the earth. According to him continents are made of lighter sialic materials the density of which is 2.67 while the oceanic beds are formed of heavier materials of sima having average density of 3.0. Thus, the crust has been assumed to have been composed of sial and substratum of basalt (sima). Besides a few exceptions, sial is not found in oceanic beds.
According to Joly the rocks of the earth contain radioactive elements but their distribution is not uniform in all zones of the earth. Radioactive elements are found in abundance in sialic zone or the continental rocks but the rocks of sima forming the oceanic crusts are less radioactive. Continuous breakdown of certain radioactive elements like uranium, thorium etc., generates heat.
It may be pointed out that the actual rate of heat production by radioactive elements is exceedingly small but it becomes sufficient enough to produce appreciable result after long period of accumulation. Though the production of heat is comparatively higher in the continental crust because of more radioactive elements than the oceanic beds but there is no large-scale accumulation of heat in the continental crust due to continuous loss of heat through radiation.
Mechanism of the Radioactivity Theory:
According to Joly the disintegration of radioactive elements of sialic or continental rocks produces heat but it does not accumulate in the continents or sial because the total loss of heat through radiation from the sialic crust is more than the total heat produced by the radioactive elements. He has further pointed out that temperature increases with increasing depth.
After detailed mathematical calculation Joly estimated the amount of temperature at the depth of 30km to be 1050°C. He estimated the maximum thickness of sial to be 30km. According to him there is no transfer of heat from sima to overlying sial. He has also estimated the amount of temperature at the outer limit of sima under the continents to be 1050°C.
The conditions under the oceans are rather different. Since there is no sial in the oceanic beds, so the heat produced by radioactive elements, though very small, is lost to the oceanic water through conduction but such situation does not exist at greater depths in the substratum (sima) under the oceans.
Temperature increases with in- creasing depth in the substratum (sima) under the oceans because of accumulation of heat produced by radioactive elements. This mechanism causes temperature gradient at greater depth in sima (substratum). The temperature becomes equal to the melting point of basalt. There is no transfer of heat from the lower part of sima to the upper part of sima so there is accumulation of heat in the lower layers of sima beneath the oceans.
The melting point is 1150°C whereas the temperature at the top of substratum (sima) is 1050°C. If the temperature of the substratum rises to 1150°C it would attain its melting point but the substratum would still remain in solid state unless required amount of latent heat of fusion is provided.
Joly has calculated that the required amount of heat to liquefy the substratum would be available in 33,000,000 to 56,000,000 years. If such conditions become possible i.e. if the substratum reaches the molten condition, several changes take place in the earth’s structure.
Period of Transgressional Sea:
Several interesting events take place when the substratum reaches the molten condition due to accumulation of greater amount of heat produced by the breakdown of radioactive elements.
(1) The expansion of sima due to melting causes increase in the radius of the globe.
(2) Continental masses or sialic masses are raised relative to the centre of the globe.
(3) The density of sima decreases due to melting and hence sialic masses begin to sink in molten sima.
(4) The level of oceanic water rises due to sinking of sialic or continental masses into liquid sima. This mechanism causes extension of oceanic water over the continental margins. This process of expansion of oceanic waters and their encroachment on-continental margins is called transgression of sea and the concerned stage is known as the phase of transgressional sea.
(5) Transgression of sea results in sedimentation on the submerged continental margins. Thus, this theory of radioactivity accounts for the origin of geosynclines due to submergence of continental margins during transgressional phase of sea.
(6) The conditions under the oceans are different because there is absence of sial. The increase in the radius and the circumference of the globe due to melting of sima produces tension in the oceanic beds which causes cracks and faults. Molten materials or molten basalts come upward through these cracks and faults. These molten basalts are then solidified and thus oceanic- islands are formed. The radioactivity theory, thus, explains the islands of the Pacific and other oceans.
(7) Continental masses easily float over molten sima, consequently they are more affected by tidal force which causes westward movement of the continents. It is in this way that the radioactivity theory also describes the process of continental drift.
(8) Continental drift changes the position of the continents and the oceans as the former occupy the positions of the latter. This process allows the escape of heat and thus the transgressional phase comes to an end.
Period of Regressional Sea:
The phase of regressional sea is characterized by the following events:
(1) The temperature of the substratum decreases because of loss of heat due to continental drift. Thus, the cooling of the substratum results in the resolidification of molten substratum. The cooling of the substratum begins from its upper layer and continues downward and ultimately the whole of the substratum becomes solid on cooling.
(2) The density of the substratum, which was relatively decreased during its molten stage, again increases to regain its previous value.
(3) The radius and the circumference of the globe, which were increased due to melting of the substratum, arc again shortened to their previous position, with the result the continents, which were raised relative to the centre of the globe, are again brought to their previous positions.
(4) Relative increase in the density of the substratum due to resolidification causes contraction of the oceanic bed which results in the withdrawal of oceanic waters from the continental margins. This is called the phase of regressional sea. Because of the withdrawal of oceanic water previously submerged continental margins (during the phases of transgressional sea) rise upward and the deposited sediments are exposed above the water level.
(6) It may be remembered that the oceanic beds were subjected to maximum expansion during the period of transgressional phase due to melting of the substratum. Similarly, the oceanic beds are also subjected to maximum contraction during the period of regressional sea due to resolidification of molten substratum.
Thus, contracting beds of two oceans exert lateral compression on the sediments deposited on the continental margins (geosynclines), consequently the sediments deposited during the period of transgressional sea are squeezed, buckled and folded and thus mountains are formed.
Joly has described two parallel processes of mountain building:
(i) The sediments deposited in the shallow seas of the continental margins are squeezed and folded due to lateral compression caused by two contracting oceanic beds,
(ii) Vertical force is produced during the process of resolidification of the substratum. This vertical force raises the whole mountains system formed during the first process.
It is obvious that according to this theory mountains are always formed along the margins of the continents facing oceans. The intensity of lateral pressure and consequent magnitude of folding depend on the amount of contraction of oceanic beds. It may be argued that large oceans would produce more powerful lateral compression and hence greatest mountain would face largest ocean. To some extent this statement is true as the Rockies and the Andes Mountains face the Pacific Ocean.
Joly also explains the period of quiescence between two periods of mountain building. The total period of two solid phases of the substratum (solid phase, molten phase and resoldification phase of the substratum) is called a revolution wherein the melting of substratum (sima) takes total time period of 33,000,000 to 56,000,000 years. It may be, thus, inferred that the process of mountain building occurs in cyclic manner wherein the period of mountain building is alternated by the period of quiescence.
Evaluation of the Radioactivity Theory:
Though the radioactivity theory of Joly based on scientific facts and mathematical calculation was widely appreciated by several scientists but simultaneously it was also severely criticized. A few critics of the theory do not grant theoretical status to the views of Joly rather they take his views as merely descriptive accounts of the earth’s interior.
In fact, the theory of Joly is a well-developed geomorphic story of the earth rather than a theory. J.A. Steers (1932) has remarked that ‘the theory is, at first sight, convincing and it certainly does give adequate explanations of many features of the earth’s surface’.
The following shortcomings have been pointed out by the critics of the theory:
(1) The theory is based on radioactive elements of the rocks of the earth at different depths about which very little is known. Thus, the force of expansion and contraction of the substratum (sima) due to melting and cooling respectively based on radioactive elements is doubtful and perhaps is not enough to form mountains.
(2) Jeffreys did not agree with the 30km thickness of the continental masses as envisaged by Joly. According to Jeffreys the thickness of the continental crust may not be more than 16km. If the thickness of the continental crust is accepted to be 16 km then the whole mechanism of Joly’s theory would come to a grinding halt as required amount of heat of 1150 C would not be possible at the depth of 16km.
(3) The Joly’s concept of cyclic nature of mountain building has been disputed by some critics. The theory envisages uniform periods of quiescence between two periods of mountain building but this concept has also been disputed. J. A. Steers has commented that “in short very essence of the theory, the approximately equally spaced recurrence of similar conditions, seems to be one of its main drawbacks.”
He has further remarked that “there seems to be little doubt that mountain building periods have been recurrent to some extent, but it is very doubtful if they have been so regular as Joly’ s theory would make them”.
(4) This theory envisages two facts about mountain building, (i) ‘The greatest mountains must face the greatest oceanic beds’, (ii) Both the margins of the continent must have mountains of the same period and both the margins should be regular. The first is validated to some extent but the second fact is not validated.
(5) This theory presents erroneous concept about geosynclines. As per this theory geosynclines are always formed due to submergence of continental margins due to transgression of seas. It means that geosynclines should always be located around the continents.
On the other hand, it has been generally accepted that geosynclines are long, narrow and shallow water bodies which are characterized by continuous sedimentation and subsidence but Joly’s geosynclines receive sediments but do not undergo the process of subsidence. Without subsidence the enormous thickness of sediments of the present Alpine mountains cannot be explained.
(6) Jeffreys has demonstrated that sima (substratum) if melted at all cannot resolidify if we accept the capacity of radioactive agencies to liquefy sima.