HOW MOUNTAINS ARE FORMED — LYTTLETON 355 



any worthwhile degree of accuracy would be an almost impossible 

 task were it not for the occurrence of earthquakes. For study of their 

 wave effects has enabled a great deal to be learned about the pressures, 

 densities, and elastic properties of the material existing at all depths 

 within the present Earth. The pressures inside the Earth are of the 

 order of millions of atmospheres, and far above the strengths of solid 

 materials in all but the extreme outer layers. This great pressure 

 renders the problem tractable, for the internal material must be so 

 distributed that it is supported against gravity entirely by pressure. 

 The times of travel of earthquake waves enable the physical properties 

 of the material, in particular its compressibility, to be found at these 

 enormous pressures. This is obviously essential information if we 

 are to calculate the initial size of the Earth. 



If the Earth grew by accretion of cosmic dust, there would be no 

 reason to suppose any great difference of composition from one part to 

 another, and it would be easy to calculate the uncompressed volume 

 that a mass equal to that of the Earth would occupy if composed of 

 dust. However, the compression squeezes the matter to higher density, 

 the more so the deeper it is inside the Earth, and it is this that makes 

 the calculation awkward. It is necessary to have precise knowledge 

 of how the density varies witli pressure. 



Geophysicists have long since determined the incompressibility at 

 almost all parts of the Earth by their studies of earthquake travel- 

 times. The results show that the incompressibility is almost exactly 

 a linear function of the pressure {see fig 2). The same type of law 

 had also been arrived at quite independently more than a decade ago 

 from purely physical considerations. It therefore seems probable 

 that such a law holds with an accuracy greater than that of the present 

 geophysical data from which it can also be inferred. 



It is found that a straight-line law holds not only throughout the 

 solid mantle and the solid outer shell of the Earth (which is just over 

 400 kilometers deep), but also in the liquid core. The constant of in- 

 compressibility associated with zero pressure is different in each zone, 

 but the slope of the straight-line law is the same. Our first require- 

 ment is to consider an all-solid Earth. Its radius is readily calculated 

 by means of the linear law (and the use of a computer) and comes out 

 to about 350 kilometers greater than the present Earth-radius of 6,371 

 kilometers. This means an initial circumference more than 2,000 

 kilometers greater than the present value, and a surface-area about 60 

 million square kilometers greater ! This is the area that would have 

 been tucked away by folding and thrusting to change the Earth to 

 its present size. These are exactly the kind of changes the geologists 

 need to account for all the epochs of mountain building {see fig. 3) . 



