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Underground Temperature and Radium

Is the One Due to the other?

IT would be a matter of the highest interest if we could definitely connect the rise of temperature which is observed in deep borings and tunnels with the radio-activity of the rocks. We are confronted, however, by the difficulty that our deepest borings and tunnels are still too near the surface to enable us to pronounce with certainty on the influence of the radium met with in the rocks. This will be understood when it is remembered that a merely local increase of radio-activity must have but little effect upon the temperature unless the increase be of a very high order indeed. A clear understanding of this point shows us at once how improbable it is that volcanic temperatures can be brought within a very few miles of the surface by local radio-activity of the rocks. To account on such principles for an elevation of temperature of, say, 1,200 deg. at a depth of three or four miles from the surface, a richness in radium must be assumed far transcending anything yet met with in considerable rock masses; and as volcanic materials appear to show nothing of such exceptional richness in radium, we can hardly suppose local radioactivity of the upper crust responsible for volcanic phenomena. When we come to apply calculation to results on the radio-activity of the materials penetrated by tunnels and borings, we at once find that we require to know the extension downward of the rocks we are dealing with before we can be sure that radium will account for the thermal phenomena observed. At any level between the surface and the base of a layer of radio-active materials—suppose the level considered is that of a tunnel—the temperature depends, so far as it is due to local radium, on the total depth of the rock-mass having the observed radio-activity. This is evident. It will be found that for ordinary values of the radium content it is requisite to suppose the rocks extending downward some few kilometers in order to account for a few degrees in temperature at the level under observation. There is, of course, every probability of such a downward extension. Thus in the case of the Simplon massif the downward continuance of the gneissic rocks to some few kilometers evokes no difficulties. The same may be said of the granite of the Finsteraarhorn massif and the gneisses of the St. Gothard massif, materials both of which are penetrated by the St. G-othard tunnel, and which appear to possess a considerable difference in radioactivity. In dealing with this subject, comparison of the results obtained at one locality with those obtained at another is the safest procedure. We must accordingly wait for an increased number of results before much can be inferred. 1 will now lay the cases of the two great tunnels as briefly as possible before you. And first as to the temperature effects observed in the two cases. The Simplon tunnel for a length of some seven or eight kilometers lies at a mean distance of about 1,700 meters from the surface. At the northerly end of this stretch the rock temperature attains 55 deg., and at the southern extremity has fallen to about 35 deg. The temperature of 55 deg. is the highest encountered. The maximum predicted by Stapff, basing his estimates on his experience of the St. Gothard tunnel, was 47 deg. Other authorities in every case predicted considerably lower temperatures. Stockalper, who also had experience of the St. Gothard, predicted 36 deg. at a depth of 2,050 meters from the surface, and Heim 38 deg. to 39 deg.f When the unexpectedly high temperatures were met with, various reasons were assigned. Mr. Fox has suggested volcanic heat. Others point to the arrangement of the schistosity and the dryness of the rocks, where the highest temperatures were read. The latter is evidently to be regarded more as explanation of the lower temperatures at the south end of the tunnel, where the water circulation was considerable, than of the high temperatures of the northern end. The schistosity may have some influence in bringing the isogeotherms nearer to the surface; however, not only are the rocks intensely compact in every direction, but what schistosity there is by no means inclines in the best directions for retention of heat. From the sections the schistosity appears generally to point upward at a steep angle with the tunnel axis.$ Where there is such variability in the temperatures, irrespective of the depth of overlying rock, there is difficulty in assigning any significant mean gradient. The highest readings are obviously those least affected by the remarkable water circulation of the Italian side. The higher temperatures afford such gradients as would be met in borings made on the level—about 31 meters per degree. * Ahstracted from paper read hefore the British Associ ation for the Advancement of Science. † See the account given by Schardt, Ver)mndl . Hchwciy.crijo;chcu Nntur. GCRcllsch., 1904, lxxxvii, “Jahrcsversammlung,” p. 204 ct seq. ‡ Schardt, loco cit. The temperatures read in the St. Gothard rocks were of a most remarkable character. For the central parts of the tunnel the gradients come out as 46.6 meters per degree. Stapff, who made these observations and conducted the geological investigations, took particular pains to ascertain the true surface temperatures of the rock above the tunnel; and from these ascertained temperatures, the temperatures in the tunnel rock and the overlying height of mountain, he calculated the gradients. But this low gradient is by no means the mean gradient. At the north end, where the tunnel passes through the granite of the Finsteraarhorn massif, there is a rise in the temperature of the rock sufficient to steepen the gradient to 20.9 meters per degree. Stapff regarded this local rise of temperature as unaccountable save on the view that the granite retained part of the original heat. This matter I will presently return to. Now, it is a fact that the radium-eontent of the Simplon rocks, after some allowance for what I have referred to as sporadic radium, stands higher than is afforded by the rocks in the central section of the St. Gothard, where the gradient is low. For the Simplon the general mean is (on my experiments) 7.1 bil- lionths of a gramme per gramme. This mean is well distributed as follows: Jurassic and Triassic altered sediments... 6.4 Crystalline schists, partly Jurassic and Triassic, partly Archsean 7.3 Monte Leone gneiss and primitive gneiss.. 6.3 Schistose gneiss (a fold from beneath)... 6.5 Antigorio gneiss 6.8 The divisional arrangement is Prof. Schardt's. Forty-nine typical rocks are used in obtaining these results, and the experiments have been in many cases repeated on duplicate specimens. Including some very exceptional' results, the mean would rise to 9.1 X 10-12 grammes per gramme. Of the St. Gothard rocks I have examined fifty-one specimens selected to be, as far as attainable, representative. Of these, twenty-one are from the central region, and their mean radium content is just 3.3. The portion of the tunnel from which these rocks come is closely coincident with Stapff's thermal subdivision of regions of low temperature/]- This portion of the mountain offers the most definite conditions for comparison with the Simplon results. TTie region south of this is affected by water circulation; the regions to the north are affected by the high temperature of the granite. We see, then, that the most definite data at our disposal in comparing the conditions as regards temperature and radio-thermal actions in the two tunnels appear to show that the steeper gradient is associated with the greater radium-content. It is possible to arrive at an estimate of the downward extension of the two rock masses (assumed to maintain to the same depth their observed radio-activity), which would account for the difference in gradient. In making this estimate, we do not assume that the entire heat-flow indicated by the gradients is due to radium, but that the difference in radium-content is responsible for the difference of heat-flow. If some of the heat is conducted from an interior source (of whatever origin), we assume that this is alike in both cases. We also assume the conductivities alike. Calculating on this basis, the depth required to establish on the radium measurements the observed difference in gradients of the Central St. Gothard and of the Simplon, we find the depth to be about 7 kilometers on the low mean of the Simplon rocks, and 5 kilometers on the high mean. There is, as I have already said, nothing improbable in such a downward extension of primitive rocks having the radio-activi- ties observed; but as a different distribution of radium may, of course, obtain below our point of observation, the result can only claim to be suggestive. * I would like to express here my acknowledgments to the trustees of the “British Museum for granting mo permission to use chips of tin; rocks in their possession; and especially to Mr. Prior for his valuable assistance in selecting the specimens. † Trans. North of England Mining and Mec. Engineers, xxxiii, p. 25 Turning specially to the St. Gothard, we find that a temperature problem of much interest arises from the facts recorded. The north end of the tunnel for a distance of 2 kilometers traverses the granite of the Finsteraarhorn massif. It then enters the infolded syncline of the Usernmulde and traverses altered sediments of Trias-Jura age for a distance of about 2 kilometers. After this it enters the crushed and metamorphosed rocks of the St'. Gothard massif, and remains in these rocks for 7% kilometers. The last section is run through the Tessinmulde for 3 kilometers. These rocks are highly altered Mesozoic sediments. I have already quoted Stapff's observations as to the variations of gradient in the northern, central, and southern parts of the tunnel. He writes: “They (the isotherms) show irregularities on the south side, which clearly depend on cold springs, they bend down rapidly, and then run smoothly inclined beneath the water-filled section of the mountain. Other local irregularities can be explained by the decomposition of the rock; but there is no obvious explanation of the rapid increase in the granite rocks at the northern end of the tunnel (2,000 meters), and it is probably to be attributed to the influence of different thermal qualities of the rock on the coefficient of increase. For the rest these 2,000 meters of granite belong to the massif of the Finsteraarhorn, and, geologically speaking, they do not share in the composition of the St. Gothard. Perhaps these two massifs belong to different geological periods (as supposed for geological reason long ago). What wonder, then, if one of them be cooler than the other.” (Lor. cit., p. 30.) Commenting on the explanation here offered by Stapff, Prestwich* states his preference for the view that the excess of temperature in the granite is due to mechanical actions to which the granite was exposed during the upheaval of this region of the Alps. The means of radium-content in the several geological sections into which the course of the tunnel is divisible are as follows: Granite of Finsteraarhorn 7.7 Usernmulde 4.9 St. Gothard massif 3.9 Tessinmulde 3.4 The central section, however, if considered without reference to geological demarkations, would, as already observed, come out as barely 3.3. And this is the value of the radio-activity most nearly applicable to Stapff's thermal subdivision of the region of low temperature. If we accept the higher readings obtained in the granite as indicative of the radio-active state of this rock beneath the Usernmulde, a satisfactory explanation of the difference of heat-flow from the central and northern parts of the tunnel is obtained. Using the difference of gradient as basis of calculation, as before, we find that a downward extension of about 6,000 meters would account for the facts observed by Stapff, if the outflow took place in an approximately vertical direction. This depth is in agreement with the result as to the downward extension of the St. Gothard rocks as derived from the comparison with the Simplon rocks. We are by no means in a position to found dogmatic conclusions on such results; they can only be regarded as encouragement to pursue the matter further. The coincidence must be remarkable which thus similarly localizes radium and temperature in roughly proportional amounts, and permits us, without undue assumptions, to explain such remarkable differences of gradient. There is much work to be done in this direction, for well-known cases exist where exceptional gradients in deep borings have been encountered—exceptional both as regards excess and deficiency. Fr. Reinold has taken out a German patent for a process for making artificial stone from the sifted ashes obtained by burning household rubbish, employing as a binder a pulp made of the wood, paper and similar ingredients of the rubbish. Artificial stone has already been made by combining burned rubbish with cement, but a large quantity of cement is required, and the mass sets so quickly that only a small quantity can be mixed at a time. The manufacture of papier machS from ashes and paper pulp requires ashes of special quality and the addition of paste as a binder. In the new process the composition is made entirely of materials contained in the rubbish. * Proc. R. s., xii, p. 44.

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