The Birth-Time of the World and Other Scientific Essays Part 13

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radioactive heat continually given out by such rocks amounts to about one millionth part of 0.6 calories per second per cubic metre of average igneous rock. As we have to account for the escape of about 0.0014 calorie[1] per square metre of the Earth's surface per second (a.s.suming the rise of temperature downwards, _i.e._ the "gradient" of temperature, to be one degree centigrade in 35 metres) the downward extension of such rocks might, _prima facie_, be as much as 19 kilometres.

About this calculation we have to observe that we a.s.sume the average radioactivity of the materials with which we have dealt at the surface to extend uniformly all the way down, _i.e._ that our experiments reveal the average radioactivity of a radioactive crust. There is much to be said for this a.s.sumption. The rocks which enter into the measurements come from all depths of the crust. It is highly probable that the less silicious, _i.e._ the more basic, rocks, mainly come from considerable depths; the more acid or silica-rich rocks, from higher levels in the crust. The radioactivity determined as the mean of the values for these two cla.s.ses of rock closely agrees with that found for intermediate rocks, or rocks containing an intermediate amount of silica.

Clarke contends that this last cla.s.s of material probably represents the average composition of the Earth's crust so far as it has been explored by us.

[1] The calorie referred to is the quant.i.ty of heat required to heat one gram of water, _i.e._ one cubic centimetre of water--through one degree centigrade.

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It is therefore highly probable that the value found for the mean radioactivity of acid and basic rocks, or that found for intermediate rocks, truly represents the radioactive state of the crust to a considerable depth. But it is easy to show that we cannot with confidence speak of the thickness of this crust as determinable by equating the heat outflow at the surface with the heat production of this average rock.

This appears in the failure of a radioactive layer, taken at a thickness of about 19-kilometres, to account for the deep-seated high temperatures which we find to be indicated by volcanic phenomena at many places on the surface. It is not hard to show that the 19-kilometre layer would account for a temperature no higher than about 270 >C. at its base.

It is true that this will be augmented beneath the sedimentary deposits as we shall presently see; and that it is just in a.s.sociation with these deposits that deep-seated temperatures are most in evidence at the surface; but still the result that the maximum temperature beneath the crust in general attains a value no higher than 270 C. is hardly tenable. We conclude, then, that some other source of heat exists beneath. This may be radioactive in origin and may be easily accounted for if the radioactive materials are more spa.r.s.ely distributed at the base of the upper crust. Or, again, the heat may be primeval or original heat, still escaping from a cooling world. For our present purpose it does not much matter which view

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we adopt. But we must recognise that the calculated depth of 19 kilometres of crust, possessing the average radioactivity of the surface, is excessive; for, in fact, we are compelled by the facts to recognise that some other source of heat exists beneath.

If the observed surface gradient of temperature persisted uniformly downwards, at some 35 kilometres beneath the surface there would exist temperatures (of about 1000 C.) adequate to soften basic rocks. It is probable, however, that the gradient diminishes downwards, and that the level at which such temperatures exist lies rather deeper than this. It is, doubtless, somewhat variable according to local conditions; nor can we at all approximate closely to an estimate of the depth at which the fusion temperatures will be reached, for, in fact, the existence of the radioactive layer very much complicates our estimates. In what follows we a.s.sume the depth of softening to lie at about 40 kilometres beneath the surface of the normal crust; that is 25 miles down. It is to be observed that Prestwich and other eminent geologists, from a study of the facts of crust-folding, etc., have arrived at similar estimates.[1] As a further a.s.sumption we are probably not far wrong if we a.s.sign to the radioactive part of this crust a thickness of about 10 or 12 kilometres; _i.e._ six or seven miles. This is necessarily a rough approximation only; but the conclusions at which

[1] Prestwich, _Proc. Royal Soc._, xii., p. 158 _et seq._

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we shall arrive are reached in their essential features allowing a wide lat.i.tude in our choice of data. We shall speak of this part of the crust as the normal radioactive layer.

An important fact is evolved from the mathematical investigation of the temperature conditions arising from the presence of such a radioactive layer. It is found that the greatest temperature, due to the radioactive heat everywhere evolved in the layer--_i.e._ the temperature at its base--is proportional to the square of the thickness of the layer. This fact has a direct bearing on the influence of radioactivity upon mountain elevation; as we shall now find.

The normal radioactive layer of the Earth is composed of rocks extending--as we a.s.sume--approximately to a depth of 12 kilometres (7.5 miles). The temperature at the base of this layer due to the heat being continually evolved in it, is, say, t1. Now, let us suppose, in the trough of the geosyncline, and upon the top of the normal layer, a deposit of, say, 10 kilometres (6.2 miles) of sediments is formed during a long period of continental denudation. What is the effect of this on the temperature at the base of the normal layer depressed beneath this load? The total thickness of radioactive rocks is now 22 kilometres. Accordingly we find the new temperature t2, by the proportion t1 : t2 :: 12 : 22 That is, as 144 to 484. In fact, the temperature is more than trebled. It is true we here a.s.sume the radioactivity of the sediments

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and of the normal crust to be the same. The sediments are, however, less radioactive in the proportion of 4 to 3.

Nevertheless the effects of the increased thickness will be considerable.

Now this remarkable increase in the temperature arises entirely from the condition attending the radioactive heating; and involves something _additional_ to the temperature conditions determined by the mere depression and thickening of the crust as in the Babbage-Herschel theory. The latter theory only involves a _s.h.i.+fting_ of the temperature levels (or geotherms) into the deposited materials. The radioactive theory involves an actual rise in the temperature at any distance from the surface; so that _the level in the crust at which the rocks are softened is nearer to the surface in the geosynclines than it is elsewhere in the normal crust_ (Pl. XV, p. 118).

In this manner the rigid part of the crust is reduced in thickness where the great sedimentary deposits have collected. A ten-kilometre layer of sediment might result in reducing the effective thickness of the crust by 30 per cent.; a fourteen-kilometre layer might reduce it by nearly 50 per cent.

Even a four-kilometre deposit might reduce the effective resistance of the crust to compressive forces, by 10 per cent.

Such results are, of course, approximate only. They show that as the sediments grow in depth there is a rising of the geotherm of plasticity--whatever its true temperature may be--gradually reducing the thickness of that part

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of the upper crust which is bearing the simultaneously increasing compressive stresses. Below this geotherm long-continued stress resolves itself into hydrostatic pressure; above it (there is, of course, no sharp line of demarcation) the crust acc.u.mulates elastic energy. The final yielding and flexure occur when the resistant cross-section has been sufficiently diminished. It is probable that there is also some outward hydrostaitic thrust over the area of rising temperature, which a.s.sists in determining the upward throw of the folds.

When yielding has begun in any geosyncline, and the materials are faulted and overthrust, there results a considerably increased thickness. As an instance, consider the piling up of sediments over the existing materials of the Alps, which resulted from the compressive force acting from south to north in the progress of Alpine upheaval. Schmidt of Basel has estimated that from 15 to 20 kilometres of rock covered the materials of the Simplon as now exposed, at the time when the orogenic forces were actively at work folding and shearing the beds, and injecting into their folds the plastic gneisses coming from beneath.[1] The lateral compression of the area of deposition of the Laramide, already referred to, resulted in a great thickening of the deposits. Many other cases might be cited; the effect is always in some degree necessarily produced.

[1] Schmidt, Ec. Geol. _Helvelix_, vol. ix., No. 4, p. 590

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If time be given for the heat to acc.u.mulate in the lower depths of the crushed-up sediments, here is an additional source of increased temperature. The piled-up ma.s.ses of the Simplon might have occasioned a rise due to radioactive heating of one or two hundred degrees, or even more; and if this be added to the interior heat, a total of from 800 to 1000 might have prevailed in the rocks now exposed at the surface of the mountain. Even a lesser temperature, accompanied by the intense pressure conditions, might well occasion the appearances of thermal metamorphism described by Weinschenk, and for which, otherwise, there is difficulty in accounting.[1]

This increase upon the primarily developed temperature conditions takes place concurrently with the progress of compression; and while it cannot be taken into account in estimating the conditions of initial yielding of the crust, it adds an element of instability, inasmuch as any progressive thickening by lateral compression results in an accelerated rise of the goetherms. It is probable that time sufficient for these effects to develop, if not to their final, yet to a considerable extent, is often available. The viscous movements of siliceous materials, and the out-pouring of igneous rocks which often attend mountain elevation, would find an explanation in such temperatures.

[1] Weinschenk, _Congres Geol. Internat._, 1900, i., p. 332.

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There is no more striking feature of the part here played by radioactivity than the fact that the rhythmic occurrence of depression and upheaval succeeding each other after great intervals of time, and often s.h.i.+fting their position but little from the first scene of sedimentation, becomes accounted for. The source of thermal energy, as we have already remarked, is in fact transported with the sediments--that energy which determines the place of yielding and upheaval, and ordains that the mountain ranges shall stand around the continental borders. Sedimentation from this point of view is a convection of energy.

When the consolidated sediments are by these and by succeeding movements forced upwards into mountains, they are exposed to denudative effects greatly exceeding those which affect the plains. Witness the removal during late Tertiary times of the vast thickness of rock enveloping the Alps. Such great ma.s.ses are hurried away by ice, rivers, and rain. The ocean receives them; and with infinite patience the world awaits the slow acc.u.mulation of the radioactive energy beginning afresh upon its work. The time for such events appears to us immense, for millions of years are required for the sediments to grow in thickness, and the geotherms to move upwards; but vast as it is, it is but a moment in the life of the parent radioactive substances, whose atoms, hardly diminished in numbers, pursue their changes while the mountains come and go, and the

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rudiments of life develop into its highest consummations.

To those unacquainted with the results of geological investigation the history of the mountains as deciphered in the rocks seems almost incredible.

The recently published sections of the Himalaya, due to H. H.

Hayden and the many distinguished men who have contributed to the Geological Survey of India, show these great ranges to be essentially formed of folded sediments penetrated by vast ma.s.ses of granite and other eruptives. Their geological history may be summarised as follows

The Himalayan area in pre-Cambrian times was, in its southwestern extension, part of the floor of a sea which covered much of what is now the Indian Peninsula. In the northern shallows of this sea were laid down beds of conglomerate, shale, sandstone and limestone, derived from the denudation of Archaean rocks, which, probably, rose as hills or mountains in parts of Peninsular India and along the Tibetan edge of the Himalayan region. These beds const.i.tute the record of the long Purana Era[1] and are probably coeval with the Algonkian of North America. Even in these early times volcanic disturbances affected this area and the lower beds of the Purana deposits were penetrated by volcanic outflows, covered by sheets of lava, uplifted, denuded and again submerged

[1] See footnote, p. 139.

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beneath the waters. Two such periods of instability have left their records in the sediments of the Purana sea.

The succeeding era--the Dravidian Era--opens with Haimanta (Cambrian) times. A shallow sea now extended over k.u.maun, Garwal, and Spiti, as well as Kashmir and ultimately over the Salt Range region of the Punjab as is shown by deposits in these areas. This sea was not, however, connected with the Cambrian sea of Europe.

The fossil faunas left by the two seas are distinct.

After an interval of disturbance during closing Haimanta times, geographical changes attendant on further land movements occurred. The central sea of Asia, the Tethys, extended westwards and now joined with the European Paleozoic sea; and deposits of Ordovician and Silurian age were laid down:--the Muth deposits.

The succeeding Devonian Period saw the whole Northern Himalayan area under the waters of the Tethys which, eastward, extended to Burma and China and, westward, covered Kashmir, the Hindu Kush and part of Afghanistan. Deposits continued to be formed in this area till middle Carboniferous times.

Near. the close of the Dravidian Era Kashmir became convulsed by volcanic disturbance and the Penjal traps were ejected. It was a time of worldwide disturbance and of redistribution of land and water. Carboniferous times had begun, and the geographical changes in

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the southern limits of the Tethys are regarded as ushering in a new and last era in Indian geological history the Aryan Bra.

India was now part of Gondwa.n.a.land; that vanished continent which then reached westward to South Africa and eastward to Australia.

A boulder-bed of glacial origin, the Talchir Boulder-bed, occurs in many surviving parts of this great land. It enters largely into the Salt Range deposits. There is evidence that extensive sheets of ice, wearing down the rocks of Rajputana, shoved their moraines northward into the Salt Range Sea; then, probably, a southern extension of the Tethys.