SHAPING THE EAKTH BOWIE 335 



such a depth as 60 miles there could be no voids ; the outer shell, or 

 crust, of the earth must be in intimate contact with subcrustal ma- 

 terial and, therefore, there is no opportunity for the crustal material 

 to collapse on a shrinking interior. Should the interior of the earth 

 be losing heat and contracting in consequence, and should the crust 

 of the earth not be losing heat and, therefore, remaining constant in 

 volume, it is probable that the crust merely thickens locally as the 

 nucleus contracts. Anj^ changes in the volume of the nuclear ma- 

 terial would be so exceedingly slow that the crustal material would 

 yield locally and the crust would continue to be in contact with the 

 nucleus around the whole earth. 



ISOSTATIC xVDJUSTMENTS AND EARTHQUAKES 



If we accept the principle of isostasy — and it is a perfectly logi- 

 cal thing to do — then we are confronted with the problem of how to 

 apply this principle in geological studies and investigations. It is 

 especially important to apply the isostatic principle to the question 

 of earthquakes. 



Earthquakes have been occurring for a billion years, more or less, 

 and probably they will continue to occur as long as the earth has sun- 

 shine and rain. An earthquake is caused by the breaking of the outer 

 portion of the earth's material. Without the break there would be 

 no elastic shock. Where the material of the earth is hard, brittle, 

 and elastic, it will resist deformation due to a force acting on it until 

 the stress is greater than its strength and there will be a sudden yield- 

 ing in the form of a rupture. Any elastic substance necessarily has 

 vibrations when it is struck or broken, and that is exactly what hap- 

 pens to the earth when we have an earthquake. The rock is snapped 

 or broken, and the elastic waves set up by the sudden rupture travel 

 great distances. 



Records of earthquake waves are made with an apparatus called a 

 seismograph. There are many of these instruments scattered over the 

 earth's surface and the number of earthquakes annually recorded on 

 them has been recently estimated at 8,000. There are many quakes of 

 such small intensity that their shocks are not received at the existing 

 seismological stations. It is impossible to state how many earth- 

 quakes actually occur over the earth, but if I might make a guess, I 

 would say from 30,000 to 40,000 a year. 



One of the implications from the proof of isostasy is that the outer 

 portion of the earth is much stronger than the materials that lie 

 somewhat farther down. In order that the irregular surface of the 

 earth may be maintained against the tremendous weight of masses of 

 rock above sea level, this outer portion of the earth must be strong, 

 that is, it must have a strength sufficient to prevent the continental 



