THE EARTH’S INTERIOR

Manawatu Times, Volume XLIX, Issue 2305, 25 November 1925, Page 5

THE EARTH’S INTERIOR

some op rrs mysteries. INTETOTSTTNO INVESTIGATIONS. Ons of the favourite subjects of speculation amongst scientists of many nations has been the composition of the Interior of the earth. In that charmingly written work, Fabre’s “Story Book of Science,” the statement Is made that the apparently solid earth upon which we live Is In reality a molten globe with a cool crust, and that the solid and liquid proportions of that globe bear about the same relation as the liquid and solid contents of an egg. The reasons for this somewhat startling conclusion are as follow: — The weight of the earth has been calculated by astronomical methods, and It comes out in the mass at somewhat a density between that of nickel and Iron, which, of course, is considerably above that of any of the surface rocks of which we have any knowledge. The total calculated weight of this earth is expressed in scientific calculations by the figure 6 followed by 18 zeros or six million billion tons. Of course, figures like these convey no real Import to the human mind, but the fact that they represent a density corresponding to that of a metallic rather than a mineral mass Is certainly a significant one. Interior Temperature. A very wide range of observations In many quarters goes to show that the interior temperature, at any rate of the earth’s crust, steadily Increases In its depth to the extent of about one degree Centigrade for each hundred feet. If that increase In temperature is persistent and consistent, the temperature at a depth of about two miles would be equal to that of boiling water, at ten miles it would rise to that of red-hot iron, and at thirty miles would be sufficiently high to melt any known substance. Accepting these figures as consistent and universal the author of "The Story Book of Science” very naturally came to the conclusion that we were living on a thin cooled crust, covering a molten mass. The phenomena of earthquakes were attributed in accordance with the above reasoning to shrinkage caused by the cooling of the crust, and those of volcanic eruptions to the escape of internal molten material, *1 The boiling geysers of Iceland certainly appear to be qaused by surface water permeating through crevices to regions of subterranean heat. Ths same tiling In a modified form happens in the hot springs district of New Zealand. Considerable attention has been paid In scientific circles to the studs Of earthquake disturbances, and as a result of those Investigations, some new light has been thrown on the matter of the internal composition of the earth. Earthquakes. Some very valuable and interesting contributions to this sphere of knowledge are to be found in the official records of the British Association for the Advancement of Science. One paper In particular, by R. D. Oldham, A.R.S., throws quite a new light on the subject of the mysterious inner composition of the earth. The point of view of the paper in question is that of the Investigation of the rate of speed of earthquake waves. Obviously the rate at which a wave vibration travels will depend on the composition of the medium through which that vibration runs. In the case of earthquakes. It has been found possible to register not only the vibrations which travel round the curved surface of the earth's crust, but also to track the course of vibrations which traverse the Interior of the centre of the globe in straight lines. The records of these Internal vibrations do not correspond with those which would he expected to occur In a globe composed mainly of molten material with but a thin solid crust On the contrary, they show all the indications which would be expected to occur In a globe with an Intensely rigid metal core comprising about four-fifths of its diameter, with a stony crust making up the balance of Its bulk. These new data throw a considerable amount of doubt on the ld.ea of the persistent and insistent increase of the earth’s temperature from Its circumference to its centre, for It Is hard to Imagine how any metal could remain In a state of <fenso rigidity at the temperature earlier scientists believed to exist In the earth’s Interior. Some New Speculation. In order to reconcile the observed Increase In the temperature of the earth’s crust with depth and the apparently rigid nature of the material through whic earthquake vibrations appear to pass, a new suggestion has been put forward in some quarters to the effect that the interior heat of the earth is only a surface condition, and that this heat Is not due to deepseated subterranean incandescence but to radio-activity In the particles of the earth’s crust. We shall need to know a good deal more than we do at present about the laws governing radio-active forces before this new theory can be either definitely confirmed or disproved. Apparently the only thing we do know definitely is that the earth, under conditions of earthquake stress, behaves like a rigid metallic mass, and not like a molten one. A possible explanation of the apparent discrepancy between the internal heat the actual vibrations observed is sought for by some writers in the Idea that matter under conditions of enormous pressure may preserve metallic rigidity, even at an Inconceivable temperature, which, were that factor of pressure absent, might dissolve everything Into a liquid or gaseous form. Enormous pressures do undoubtedly make a great difference in many cases. If the steadily-increasing pressure of the ocean waters persists to the lowest depths, a point would occur! at which even salt water would freeze but, pa far ag we know t nothing of

that sort happens. Of course, we know that when water changes to ice it expands, and if It is under so enormous a pressure that its expansion is Impossible, it may remain liquid. In spite of lower temperatures. On exactly the same lines, It Is, Of course, quite possible that substances may remain rigid, in spile of high temperatures, under conditions of great pressure, but wo have very little certain knowledge on that point In fact there are quite a number of things about this old globe, of ours that we do not know, but which nevertheless, make very fascinating subjects for speculation.