THE INTERIOR OF THE EARTH,

Progress, Volume I, Issue 7, 1 May 1906, Page 173

THE INTERIOR OF THE EARTH,

By Berefford Ingram, B. A., F.C.S

It is generally conceded that the present shape of the globe is due to the fact that at one time the earth was a semi-fluid mass which revolved round its axis with a definite uniform velocity. Experiments have been conducted with spheres of plastic material rotating with different velocities round their axes. These have proved indubitably that, under the above conditions, a flatness is always produced at the two extremities at which the axis emerges, while a corresponding increase in -diameter is observed midway between these two extremes It was also noticed that the greater the velocity of rotation, the more oblate the mass became. Nevertheless, there are, in this, many experimental considerations which would not be applicable to the earth m its earliest history ; so that while the plausibility of the demonstration must be fully admitted, it would be unwise to accept the supposition as conclusively established. On the other hand, there does exist absolute proof that the earth is a cooling body. The high temperatures that have been registered at the bottom of mines and deeper borings prove the point beyond all doubt that, as the centre of the earth is approached, the temperature becomes higher. Heat must, therefore, be travelling from the interior to the surface, and thus daily dissipated, in almost inconceivable quantities, into space. Taking the average of all the observations, it would appear that the temperature nses i° Fahr. for every «;i feet of the earth's crust penetrated. It follows that, at a distance of less than two miles, the temperature is the same as that of boilmg water ; while at a distance of 30 miles below the surface, the temperature is sufficient to melt iron ; in fact, at a distance of 50 miles the temperature is higher than any that has yet been artificially produced. There still remains the best part of 4,000 miles to travel before the earth's centre is reached. It would therefore, be meaningless to calculate the temperatures that exist below 50 miles. The existence of enormous temperatures naturally raises the question, what is the condition that prevails in the interior ? Is the rock m a liquid condition, or can it conceivably be in any other state ? On this point geologists seriously disagree. Undoubtedly as the earth's crust is penetrated a temperature must be reached which is sufficient to melt solid rock under ordinary circumstances, but it is equally certain that at that depth an enormous pressure is affecting the underlying mass. Now it is well known that, if the pressure on a solid is increased, the melting-point of that solid becomes higher, and, moreover, that different substances are differently affected by the increase m pressure. As it is not known what is the composition of the rocks that exist at these great depths it becomes impossible to do anything but conjecture what effect the pressure has on the melting-point. When the pressure is relieved in any way (as is supposed to have taken place in volcanic outpourings of lava) it would appear that the condition necessary to keep the rock in a solid state is destroyed, with the observed result. These considerations have led some scientists to consider the earth as a solid mass with its interior at a very high temperature. But it may happen that the increase in pressure at certain depths is not sufficient to prevent the liquefaction of the rock m which case the earth would have a- solid crust with a liquid interior. It may be, however, that, after a certain depth, the pressure becomes great enough to prevent further liquefaction in which case a section through the earth would show three concentric rings, the innermost having a diameter of over 7,000 miles and marking the extremities of the solid nucleus, then a ring, about 7,800 miles in diameter, defining the limits of the liquid substratum, and finally there would be the earth's crust, which, varying according to the position of mountain, land, and water, would probably in no place exceed a depth of over 70 miles A very potent argument has been urged against this second assumption to the effect that, if the crust were so thin as the theory states, it would yield to the deforming influence of the sun and the moon ; in which case the water would be drawn up with the earth, and thus no sensible tidal effects could be produced. In order to counteract this attraction it would be necessary to admit that the thickness of the earth's crust is at least 2,000 miles. An unsatisfactory compromise has thus been suggested, by which it is supposed that the earth is at the present time solid throughout (having passed through the stage of alternate solidity and fluidity), but that large fluid cavities exist throughout the mass.

If the reader shouldjfind himself incapable of coming to any definite conclusion on this question, how much more impossible is it for the student of geology (or mathematics) to settle the difficulty satisfactorily ? Each theory initially commends itself with much plausibility to its reader, but as the more important but less apparent tests are applied, certain defects come into prominence. Fisher'^ Theory. Fisher succeeded in forming a theory that would comprehend and explain all the difficulties that each previous assumption had encountered. He accepted the idea of the existence of a liquid substratum, with this difference, that it consisted of a mixture of fused rock and a dissolved gas (in all probability hydrogen). The origin of earthquakes and volcanoes can be satisfactorily explained on this assumption, which has the additional qualification of accounting for the appearance of vast quantities of that gas (i.e., hydrogen) in all volcanic outbursts. Furthermore, the fused mass, which Fisher supposes to exist, would not give rise to any tides within the earth's crust, and thus one of the most serious objections to the "liquid substratum hypothesis" (which is itself based on the fundamental notions of the effect of heat on solids) is satisfactorily removed. At the same time the theory admits the possibility that the fused mass may communicate its movements to the earth's surface (although not m the form o f " tides "), which deduction, taken with all the other scientific explanations, should strongly recommend the theory to all whom it may interest.