The Earth's Beginning

by Robert S. Ball

We have described these particular observations in some detail because they have been conducted under conditions far more favourable to accuracy than have ever been available in any previous investigations of the same kind. But now we shall omit further reference to this particular undertaking near Leipzig. It is not alone in that particular locality, not alone in Germany, not alone in Europe, not alone on the surface of any continent, that this statement may be made. The statement is one universally true so far as our whole earth is concerned. Wherever we bore a hole through the earth’s crust, whether that hole be made in the desert of Sahara or through the icebound coasts of Greenland, we should find the general rule to obtain, that there is an increase of temperature of about 80° for a mile of descent. This is true in every continent, it is true in every island; and, though we cannot here go into the evidence fully, there is not the least doubt that it is true also under the floor of ocean. If beneath the bed of the Atlantic a hole a mile deep were pierced, the temperature of the rocks at the bottom of that hole would, it is believed, exceed by about 80° the temperature of the rocks at the surface where the hole had its origin. We learn that at the depth of a mile the temperature of the earth must generally be 80° hotter than it is at the level of constant temperature near the surface.

It may perhaps help us to realise the significance of this statement if we think of the following illustration. Let us imagine that the waters of the ocean were removed from the earth. The ocean may in places be five or six miles deep, but that is quite an inconsiderable quantity when compared with the diameter of the earth. The change in the size of the earth by the removal of all the water would not be greater, proportionally, than the change produced in a wet football by simply wiping it dry. Let us suppose that an outer layer of the earth’s surface, a mile in thickness, was then to be peeled off. If we remember that the diameter of the earth is 8,000 miles, we shall see that this outer layer, whose removal we have supposed, does not bear to the whole extent of the earth a ratio even as great as that which the skin of a peach does to the fruit inside. But this much is certain, that if the earth were so peeled there would be a wonderful difference in its nature. For though practically of the same size as it is at present, it would be so hot that it would be impossible to live upon it.

Next comes the very interesting question as to the temperature that would be found at the bottom of a hole deeper still than that we have been considering. Our curiosity as to the depths of the earth extends much below the point to which Captain Huyssen drove down his diamond drill. The trouble and the cost of still deeper exploration of the same kind seem, however, to be actually prohibitive. To bore a hole two miles deep would certainly cost a great deal more than twice the sum which sufficed to bore a hole one mile deep. At a great depth each further foot could only be won with not less difficulty and expense than a dozen, or many dozen feet, at the surface. Mining enterprise does not at present seem to contemplate actual workings at depths much over a mile, so there does not seem much chance of any very much deeper boring being attempted. We do not say that a hole two miles deep would be actually impossible; it may well be wished that some millionaire could be induced to try the experiment. We should greatly like to be able to lower a thermometer down to a depth of two miles through the earth’s crust.

Seeing there is but little chance of our wish for such future experiments being gratified, it is consolatory to find that actual observations of this kind are not indispensable to the argument on which we are to enter. Our argument can indeed be conducted a stage further, even with our present information. The indications already obtained in the hole one mile deep go a long way towards proving what the temperature of a hole still deeper would be. We have already remarked that it was part of Captain Huyssen’s scheme to obtain careful readings of his thermometer at intervals of 100 feet from the surface to the bottom of the hole. A study of these readings shows that the increase of 80° in a mile takes place uniformly at the rate of one degree for each sixty-six feet of depth. As the temperature increases uniformly from the surface down to the lowest point which our thermometers have reached, it would be unreasonable to suppose that the rate of increase would be found to suffer some abrupt change if it were possible to go a little deeper. As the temperature rises 80° in the first mile, and as the rate of increase is shown by the observations to be quite as large at the bottom of the hole as it is at the top, we certainly shall not make any very great mistake if we venture to assume that in the second mile the temperature would also increase to an extent which will not be far from 80°. This inference from the observations leads to the remarkable conclusion that at a depth of two miles the temperature of the earth must be, we will not say exactly, but at all events not very far from, 160° higher than at the level of constant temperature about 100 feet down.

As in the former case, we need not confine ourselves to any particular locality in drawing this conclusion. The arguments apply not only to the rocks underneath Leipzig, but to the rocks over every part of the globe, whether on continents or islands, or even if forming the base of an ocean. No one denies that the law of increase in temperature with the depth must submit to some variation in accordance with local circumstances. In essential features it may, however, be conceded that the law is the same over all the earth. If we take 52° to be the temperature of the level 100 feet down, which limits the seasonal variations, and if we add that at two miles further down the temperature is somewhere about 160° more, we come to the conclusion that at a depth of a little over two miles the temperature of the rocks forming the earth’s crust is about 212° Fahrenheit. Thus we draw the important inference that if, the oceans having been removed, we were then to remove from the earth’s surface a rind two miles thick—a thickness which, it is to be observed, is only the two-thousandth part of the earth’s radius—we should transform the earth into a globe which, while it still retained appreciably the same size, would have such a temperature that even the coolest spot would be as hot as boiling water. This is indeed a remarkable result.

And now that we have gone so far, it is impossible for us to resist making a further attempt to determine what the temperature of the earth’s crust must be if we could send a thermometer still lower. A hole one mile deep we have seen; I do not think we can hope to see a hole two miles deep, but still it may not be absolutely impracticable; but a hole of three or more miles deep we may safely regard as transcending present possibilities in engineering enterprise. Are we therefore to be deprived of all information as to the condition of our earth at depths exceeding those already considered? Fortunately we can learn something. We are assisted by certain laws of heat, and, though the evidence on which we believe those laws is necessarily limited to the experience of Nature as it comes within our observation, yet it is impossible to refuse assent to the belief that the same laws will regulate the transmission of heat in the crust of the earth two miles, three miles, or many miles beneath our feet.

I represent, in the diagram shown in Fig. [23], three consecutive beds of rock—A, B, and C—as they lie in the earth’s crust, a little more than a mile beneath our feet. I shall suppose that the bed B is the very lowest rock whose temperature was determined in the great boring. The drill has passed completely through A, it has pierced to the middle of B, but it has not entered C. The observations have shown that the temperature of the stratum B exceeds that of the stratum A, and we further note that this is a permanent condition—that is to say, B constantly remains hotter than A. From this fact alone we can learn something as regards the temperature of the stratum C which lies in contact with B. Of course we are unable to observe the temperature of C directly, because by hypothesis the boring tool has not entered that rock. We can, however, prove, from the laws of the conduction of heat, that the temperature of C must be greater than that of B; and this appears from the following consideration.

Fig. 23.—At the Bottom of the
Great Bore.

It is plain that C must be either just the same temperature as B, or it must be hotter than B, or it must be colder than B. If C were the same temperature as B, then the law of conduction of heat tells us that no heat would flow from one of these strata to the other. The laws of heat, however, assure us that when two bodies at different temperatures are in contact the heat will flow from the hotter of these bodies into the colder, so long as the inequality of temperature is maintained. As B is hotter than A, then heat must necessarily flow from B into A, and this flow must tend to equalise the temperature in these strata, for B is losing heat while none is flowing into it from C. Therefore B and A could not continue to preserve indefinitely the different temperatures which observation shows them to do. We are therefore forced to the conclusion that B and C cannot be at the same temperature.