358 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1964 



central temperature readied the melting point of tlie material there : 

 this would mark the begiiming of the growth of the core. Further 

 release of radioactive energy would increase the temperature, and the 

 core would continue to extend further out. 



The crucial point is that this liquid form, which the material is 

 converted into as a result of both the high pressure and temperature, 

 is more compressible than the solid form constituting the mantle. 

 Thus, as the core mass increases, the Earth gradually gets smaller. 

 Over the whole age of the Earth, the average rate of decrease of the 

 outer radius has been about one-tenth of a millimeter a year — in 3.5 X 

 10^ years this amounts to 350 kilometers. This contraction also re- 

 leases gravitational energy, which will augment the heating by radio- 

 active energy, but in a planet as small as the Earth this additional 

 source of heat cannot be tapped until radioactive heating has first 

 produced liquefaction, so that contraction can begin. 



An indirect consequence of the gradual contraction would be that the 

 rotation of the Earth would have speeded up : the rotatory inertia of 

 the planet would have decreased as the body contracted. It now has 

 only about 4/5 of the original value, and so the present angular velocity 

 would be about 5/4 of the original rate in order to conserve rotatory 

 momentum. This means that the day, if affected by this process only, 

 would initially have been about 30 hours long. It is known that the 

 tides of the sun and moon act to slow the Earth down, but the present 

 process appears to be of comparable importance, and it will need to 

 be taken into account in future discussions of the evolution of the 

 Earth's rotation. 



STRAIN, FRACTURE, BUCKLING 



It is the response of the outer layers of the Earth to this steady lique- 

 faction of the deep interior that is ultimately responsible for the 

 formation of mountains. But the surface does not follow the con- 

 traction entirely uniformly because the strengths of the materials in 

 the outer few kilometers are greater than the pressure. Thus the 

 contraction in the core for a time produces no catastrophic effect at 

 the surface, but only builds up increasing strains. Rocks can be com- 

 pressed by rather more than one part in a thousand of their linear 

 dimensions before they yield altogether and fracture. Thus a spheri- 

 cal Earth could contract down by a few kilometers without serious 

 distortion at the surface, but then any further contraction would re- 

 sult in widespread fracture and buckling of the outer layers. Some 

 catastrophic readjustments would be made as the material gave way 

 under excess strains. This stage would correspond to a period of 

 mountain building. What exactly would take place in any such cata- 

 strophic epoch is almost impossible to consider theoretically, for the 

 Earth's surface layers will have different strengths at different parts, 



