Useful notes on Arthur Holme’s Thermal Convection Current Hypothesis

The Thermal Convection Current hypothesis dates back to Hopkins (1839) and Fisher (1881), but credit goes to Arthur Holmes for presenting more elaborately the same as a force for mountain building, continental drift, volcanism and formation of ocean trenches etc.

Holmes first discussed and illustrated his view in 1928 in his monumental work – Principles of Physical Geology.

This theory of mountain building is now mostly accepted as the most convincing. It is similar to the latest theory, i.e. plate tectonics. Holmes was a ‘Convectionist’ who based his theory on thermal convection currents in the substratum of the earth.

e admitted that such currents that originated in the substratum were subject to periodical wax and wane.

Radioactive heating is the main source of heat flow in the continents. Consequently this heat is higher in those regions where crust is thicker, e.g., in orogenic belts.

However, the amount of radioactive heat transmitted from the mantle to the crust is insignificant. But volcanic activity or igneous intrusion meaning transfer of hot matter from the interior greatly increases heat flow.

The basaltic crust below the oceans is less radioactive than the sialic continental crust. The oceanic crust is thinner than the continental crust.

Thus, high heat flow over mid-oceanic ridges such as, Mid-Atlantic Ridge is indicative of thermal convection currents.

Investigations during the fifties of the twentieth century by E. Bullard and other geologists showed that the heat flow over the crest of the Mid-Atlantic Ridge was about five times the average for the world.

This was a clear proof that there existed hot ascending matter below the ridge. The relatively lighter basaltic magma had risen with the ascending currents through the upper part of the mantle into the fissured and rifted crust of the Ridge.

The heavier ultra basic material flowing sideways near the top of the mantle slowly cool and descend away from the Ridge, where the heat flow will naturally be lower than the average. Such patterns of heat flow across other mid-ocean ridges also suggest the same conclusion.

According to Holmes, Mid-oceanic rise, new oceans, continental borderland, ocean deeps and pattern of heat flow, all are related to thermal convection currents.

An important question is how much radioactivity of the earth may be responsible for thermal convection currents. It is a well known fact that radioactive elements decrease as we pass from granitic through granodiorite, ande- site, basalt, basic rocks to ultra basic rocks.

Thus, heats producing radioactive minerals are concentrated in the sialic crust and very little of radioactive heat flow comes from the mantle.

There is gap between the estimated and necessary temperature for melting of the rocks in the substratum. For bridging this gap, Holmes suggests three possibilities: (i) one such possibility is movement en masse of hot matter in the form of convection currents from the mantle up into the crust. As a result of such currents which start diverging, the crust is ruptured; (ii) the second possibility of large scale heat generation is the to-and fro bending of rocks during an earthquake which generates heat; and (iii) the third possibility is the thickening of the crust as a geosyncline. Radioactive heat may be generated in such a sialic crust. However, this kind of heating is very slow.

The high temperatures in volcanic belts suggest the occurrence of a hot pocket beneath the volcano-prone areas. Here the thinness of the basaltic layer points to stretching and attenuation over hot rising divergent sub-crustal currents.

Thus, the excessive heat flow observed in the mid-oceanic ridges is, therefore, due to thermal convection currents of hot matter and not merely due to radioactive heat.

So it is clear that radioactivity is not the whole cause. Thermal convection currents also are responsible for high heat flow on mid-oceanic ridges.

Therefore, sub-crustal convection currents are supposed to be the probable and primary force for the tangential movement in the crust which causes mountain building or ocean deep formation or the development of mid-oceanic ridges.

As regards the development of the thermal convection current hypothesis, Holmes points out that William Hopkins suggested crustal convection in the earth in 1839.

Osmond Fisher discussed its application in his work Physics of the Earth’s Crust in 1881 pointing out that mountain building and volcanism could be explained in this way.

Wegener who propounded the hypothesis of Continental Drift in the ‘Origin of the Continents and Oceans’ referred to the possibility that convection current in the mantle might provide the transporting agent for continental drift.

Holmes is of the opinion that “whatever may be the source of activating energy, the general consistency of the above estimates (but not quite) raises the convection hypothesis to the dignity of a theory”.

Holmes’ views summarised:

(i) Mountain building and other geophysical phenomena are the result of thermal convection currents of molten magma which occur in the upper parts of the mantle periodically.

These have great transporting power causing convergence and compression of the solid crust or bending it down when the currents descend or tearing it apart where the currents diverge.

(ii) The contribution of radioactive heat is too little to cause the melting of the subcrustal layers. It is, therefore, inferred that additional heat, which is necessary to cause melting of rocks in the substratum, comes from the mantle not by conduction of radioactive heat but by thermal convection currents of molten magma.

(iii)The hypothesis of thermal convection currents has been almost but not quite raised to the status of a theory,. There are indirect evidences such as abnormally high heat flow on mid-oceanic ridges suggesting the existence of convergent sub-crustal currents.

Three stages of the thermal convection currents:

(i) The first phase is that of long period with currents gradually gaining speed. In areas of convergence geosynclines are formed and mountain root begins to develop.

(ii) The second phase is of short duration with rapid currents. Roots of mountains grow. The orogenic belt is thrust, faulted, folded and various features of mountains including nappes, gravity sliding, etc. are developed.

In areas of divergent currents this third phase produces mid-oceanic ridges, volcanic activity, high heat flow, and ocean-floor spreading.

In the last phase which is marked by declining currents, the down-dragging power of currents has waned and the mountain root of geosynclinal sediments, which are obviously relatively light and could not be held down into heavier matter of the interior, is released and rises. Thus, the final uplift of the orogenic belt occurs.

It should be borne in mind that various laboratory experiments support the theory of sub-crustal convection currents.