EARTH SECRETS

Hokitika Guardian, 17 July 1931, Page 7

EARTH SECRETS

STRUCTURE OF GLOBE. SCIENTIFIC ATTACK. As tlie second of a series of popular weekly lectures arranged by the Wellington Philosophical Society, Mr W. M. Jones, M.Se:, B.A. gave an interesting address at the Dominion Museum recently on the structure of tue earth. His account of the theories of the earth’s formation and composition and of the manner in which various sciences were solving its riddles were followed with attention by a large audience. Dr. E. Marsden was chairman. Mr Jones commenced with a brief reference to the prevailing theories regarding the beginnings of the earth as a separate body. “The two hypotheses that hold the modern field,’’ he said, “are* the planetesimal and the tidal Theories. These agree in assigning the origin of the earth to the disruption effect of a passing star on the sun. The planetesimal theory, however, postulates an earth originally much smaller that at present, which solidified very soon after its separation, and later increased to its present size by the capture of planetesimals, small solid T bodies, also products of the disruption, which had been left in large numbers rotating around the sun after the cataclysm.

TWO THEORIES,

“The tidal theory assumes that the original earth containing practically as much material as it does now, and that whil@ it attained the liquid stage quickly, it has only gradually solidhed, the, solidification indeed not being yet by any means complete. It is obvious that the two theories must lead to different ideas of the evolution and present condition of the earth. As only an insignificant fraction of the earth, less than 10 miles, is accessible to direct observation, our ideas of its interior must be based on deduction, and in this the sciences of geology, siesmo]ogy, geodesy, astronomy, physics and chemistry all contribute information. “The main conclusions of the geologists, based'- on ■ the • observed distribution and constitution of the materials exposed on the- surface, indicate the following general sequence, from the surface toward the centre: (a) A mantle of sediments, of variable thickness, but not more anywhere than ten to twenty miles, covering (b) a layer of granite, not more than 40, miles thick, which is, however, much thinner | or absent under the ocean basins; (c) a layer of more basic material, basalt or some rock of similar composition, of uncertain thickness, below which is (d) a high pressure facies of the same composition, such... .as eclogite, passing downward into (e) a more basic material, such as periodolite or dunite,” STUDY OF EARTHQUAKES. “The contribution of seismology is of recent date but promises much for the future,” Mr Jones continued. “The genoral method! consists in determining the velocities with which the various types of earthquake waves travel at different depths in the earth. When this is done the results may be used to determine the compressibilities of the rocks concerned, and a comparison of these compressibilities with those observed experimentally in the laboratory for existing rocks gives some idea of what the deep-seated rocks are. Deductions as to t-hei thickness of the different layers can be made in addition. Seismology also reveals a perceptible discontinuity in the nature of the material at a distance from the centre nf rather more than half the earth’s radius. The material of this central core must, from other considerations be very dense, and it is generally believed to be of metallic iron with perhaps an admixture of nickel. “This principle lias received fairly general acceptance, and, of course, accords with the ideas of a denser stratum underyling the granite, but some provision must be made for a flowage of the rock below, which has been : considered solid from its seismologienl properties. However, there is no difficulty jin postulating a state of the rock matter which gives the elasticity of a solid for short period vibrations, but for the age-long stresses involved in crustal movements is able to yield. This ‘hydrostatic’ state of the interior can he shown also to he consistent with the elliptieitv of the earth caused bv its rotation. “Astronomy can show, from the theory of the ‘bodily tide,’ or the amount of yielding of the earth as a whole to the tidal pulls of the sun and moon, and from the existence of a small periodic variation of the position of the pole due to the webbing effect caused by inequalities of distribution of such things as snow, air pressure and vegetation, that the earth as a who’e has a rigidity, of the same order as that of steel. This.' however, does not deny the possibility of a liquid, core, of considerable viscosity.”