Brief notes on Jeffreys’ thermal contraction hypothesis

First of all, let us be clear that Jeffrey is a constructionist. His hypothesis is highly mathe­matical and it is not intelligent to a non-mathematician.

Since at that time no force could be found out that was strong enough to cause continental drift, he based his reasoning’s on the contraction of the earth which could have been regarded to have generated a force sufficient for mountain building.

According to Jeffreys’ arguments, cooling of the upper layers as well as the resultant decrease in speed of the earth’s rotation supplied forces necessary for mountain building.

As regards the mechanism of cooling, he opined that the earth comprises several concentric shells which have cooled by different amounts.

There was little change of temperature in the lower 700 kilometers. On the contrary, in the outer 700 kilometers, each layer cooled more rapidly than its counterpart lying immediately below it.

Naturally, there would be more contraction in any layer than that in the adjacent layer. The process of cooling of different layers forming the earth resulted in appreciable fall in the temperature of the upper region.

Thus the upper layers of the earth would solidify first, and there will be the shortening of the crust as well as the reduction in the surface area. Besides, there will be a change in the earth’s volume.

Jeffreys did a lot of tedious mathematical calculations and found out the exact figures in respect of changes in temperatures of the various layers and the resultant changes in the diameter and circumference due to the gradual cooling of the earth. He also calculated the reduction surface area.

Let us bear in mind that the differential cooling and contraction going on in various layers of the outer region of the earth must have had its own repercussion.

Now there was question of adjustment between different layers lying in succession to one another. The greater cooling and contraction of the upper layers of the outer region will bring about volumetric change due to alteration in the outer radius.

According to Jeffreys, an adjustment of this nature means a thinning in the upper region without the resultant reduction of its inner radius. This is so because the inner region being extremely hot tends to cool very slowly.

Between the upper and the lower regions of the earth, there must be some intermediate layer where conditions of contraction are such that they must enable that layer to fit to the interior. This is known as ‘the level of no strain’.

It clearly means that in order to enable the inner shells to fit, they must be stretched horizontally. This involves a lateral spreading and thinning out of the material.

However, above the level of no strain, the horizontal stress caused by compression results in a decrease in the radius of the earth. This fact leads to buckling and folding of the earth’s outer crust. This also causes fissures which will be filled from below.

Beneath the oceans the depth of cooling exceeds that beneath the continents. It is also true that submarine rocks are basic and, therefore, stronger than the continental rocks.

The horizontal compressive forces around the oceans, therefore, have acted towards the land.

Stresses produced due to the thermal contraction must go on increasing until the result that flow and fracture will start. Mountain building will continue until the stresses are relieved. After a period of no activity the process commences again.

Jeffreys argues that five such periods of orogenesis (mountain building) have been recognized. However, this argument does not find support from other geologists.

As stated earlier, the compressive force was directed from the oceanic substratum towards the continental land masses almost at 90°. This is verified by the mountain ranges lying almost parallel to the oceanic area.

However, even though the contraction theory and its mechanism explains fully the alignment of such young folded mountains as the Andes of South America and the Rockies of North America which run in north-south direction along the western margin of the two continents, but this argument of Jeffreys with regard to the compressive forces coming from the oceanic areas fails to explain the formation of the Himalayas and the Alps.

Evaluation of the theory:

The thermal contraction theory has been the subject of criticism on the following grounds:

(i) Jeffreys’ aerial compression of 2 x 10¹⁶ sq. cm. is less than half the calculated available compression. So his calculation of areal compression is a serious underestimate, since it cannot include the older episodes of mountain building. Holmes has concluded that the calculated reduction of area is seriously in deficit of the amount required to explain mountain building.

(ii) As regards periodicity of mountain building, the thermal contraction theory is only partially true.

(iii) It has been argued that on the thermal contraction theory the time interval between various mountain buildings should have increased as cooling progressed, but in fact, judged on other evidence, the interval seems to have remained almost about the same.

(iv) At present the geologists have found out other effective forces of mountain building which have been totally ignored by Jeffreys.

(v) Jeffreys’ earlier quantitative estimates of the contraction due to cooling have been outright rejected because of the fact that the effect of the radio-activity has been totally neglected. In the opinion of Holmes, to ignore the heat produced by radio-active disintegration would be as wrong as to ignore the effect of gravitation.

(vi) Jeffreys’ assumption that there was a rapid decrease of radio-activity with depth, and so in accordance with this assumption, he has given a quantitative account of the cooling of the earth. He has miserably failed to take into account the role of convectional currents in the process of cooling.

(vii) Jeffreys’ view of the earth’s cooling history has been criticized on grounds of the distribution of radio-active elements in depth. Holmes has vehemently criticized the assumption of too complete and unnatural a concentration of radio-activity towards the surface as envisaged by Jeffreys.

(viii) Last but not the least, is the presence of fossils in the rocks of the folded mountains. It clearly indicates the birth of the folded mountains in the seas, a fact which invalidates the thermal contraction theory of the mountain building.