Brief notes on Radio activity Hypothesis of Joly

Joly’s Radioactivity hypothesis about the thermal history of the earth appeared in 1925 in his most popular treatise ‘The Surface History of the Earth’. His hypothesis is certainly based on radioactivity of the earth’s interior.

Joly’s view regarding the composition of continents and ocean floors are generally accepted. He views the continents as composed of sial, a relatively lighter material, and the ocean floors composed of sima, heavier than sial.

The sima is supposed to be basaltic as concluded from the study of the great basalt flows of Tertiary as well as of earlier periods. The sialic materials have an average specific gravity of 2.67, whereas the sima has specific gravity of 3.00.

According to Joly, the sial masses were about 30 km thick, but in mountainous areas, because of downward compensation, the thickness is even more.

Joly’s hypothesis presumes that all rocks are more or less radioactive. They are producing heat by the atomic disintegration of certain radioactive elements, this being an automatic and continuous process.

However, sialic rocks are rather more active than the sima. Despite the small rate of production of heat by this means, the accumulation of heat thus produced during the very long periods of time may be sufficient to produce great changes in the earth’s constitution. Remember that the rocks forming the oceanic crust are relatively less radioactive.

Joly’s contention is that the total heat loss from the earth surface is much more than the heat produced by the radioactivity of the sial rocks.

He further argues that if the sialic layers have an average thickness of 30 kilometers, the temperature at this base must be about 105°C. It is common experience that temperature goes on increasing as the depth increases.

Joly further argues that there is no transfer of heat from sima or the substratum to the sial rock. Thus, there will be absence of any temperature gradient at the base of the continents. So the temperature of the substratum is likely to be about 1050°C.

According to Joly, conditions under the oceans are quite different because of the absence of sial. It is, therefore, natural that whatsoever radioactive heat is produced in the upper layers of the substratum, it will be lost to the ocean waters.

However, this loss of radioactive heat is confined to the upper sima layers. Thus, at great depth in the substratum, the radioactive heat will accumulate and raise the temperature to level of the melting point of basalt.

It follows that all the subsequent layers below this critical depth must also be at their melting point, and will be in a position to conserve their heat.

It may be pointed out that non-transfer of heat from the lower to the upper part of sima beneath the oceans, and also from the sima to the sial causes the accumulation of heat in the lower substratum with no loss.

The melting point of basalt is 1150°C which is 100°C more than the temperature at the top of the substratum. Even at its melting point i.e. 1150°C the substratum still remains in the solid state.

For substratum to become liquid, the addition of latent heat of fusion to it is necessary. Joly estimated the time required for all this heat to accumulate comes to about 33 million to 50 million years.

Joly considers two phases. One is the molten phase of sima in the earths interior due to increase of radioactive heating. The second phase is the solidified sima substratum after the heat generated during the molten phase has escaped.

If the substratum reaches the molten stage, several important consequences follow. The volume of the globe increases and so the radial distance of the continents from the centre of the earth also increases.

Since the molten sima has become lighter, it will permit the submergence of the continental masses into it. The height of continents relative to the adjoining oceans will decrease with the result that the continental margin will be submerged by transgressional seas.

Thus, the geosynclines will come into existence. In the sima crust of the oceans, because of an increase in the circumference, tensional cracks will be produced from which molten basalt will be poured out. However, these processes are very slow and may take up several million years.

During the molten stage of the substratum when the continents may be regarded as floating, tidal effects would be very important.

The tidal forces of the moon and the sun will carry the continents westward and the oceans will occupy the position formerly occupied by the continents. At this stage the molten sima will lose heat because of its contact with oceanic waters.

Now, the phase of regressional sea begins. Because the heat in the substratum is decreased, the substratum is resolidified. Resolidification of the substratum would mean a reduction of the total volume of the earth.

It means a shortening of the radius of the earth. But now the resolidified sima will have a greater density than that during its molten phase. This will ensure the rising of the continents with reference to the adjoining oceans.

It may be pointed out that in the molten phase of the substratum; the crystallized oceanic crust was under great pressure from below with the result that it was torn into tensional cracks.

During the resolidification phase all sima of the oceanic crust along with the substratum would become a hard non-yielding layer and the oceanic gaps of the oceanic crust will close.

There will also be strong pressure on the margins of the continents. This will cause folding and nappe formation in the soft sediments that had been deposited in the transgressional seas or geosynclines.

The complete resolidification of the sima, which will take its own time, will fully raise the relatively light rocks of the geosynclines. The folded materials in the earlier stages of resolidification will be elevated when the resolidification is complete. Thus, the mountains come into existence.

According to Joly, interval between two molten stages of sima is 33 to 60 million years. Thus, the interval related to radioactivity should be uniform and equal between successive orogenies.

But geologically, it has been proved that the orogenies have not been contempo­raneous. Thus, the mountains of the Tertiary epoch are not of the same age. Probably the Himalayas are younger than the Alps.

There seems to be no doubt that mountain building periods have been recurrent to some extent, but it is doubtful if they have been so regular as Joly’s hypothesis would make them.

Another objection to the theory is based on the great difference between the Pacific and Atlantic coast types.

Joly believes that the island arc in the Pacific represents the volcanic eruption through tensional cracks. These cracks lie at right angles to the longest span of the Pacific according to him.

Jeffreys is strongly critical of Joly’s theory. He does not agree that there is a tidal force which is so powerful as to drag the continents westward.

Another objection is that once sima has melted, it has no chance of resolidification because the amount of heat generated by radioactivity is more than lost by conduction.

According to Steers, the very essence of the theory, the approximately equally spaced recurrence of similar conditions, seems to be one of its main drawbacks.