HOW MOUNTAINS ARE FORMED — LYTTLETON 



357 



Figure 3. — The Earth's initial radius was about 350 kilometers greater than it is now. 

 Its surface area must therefore have been reduced by about 60 million square kilometers. 

 This additional material would have been tucked away by folding and thrusting, giving 

 rise to epochs of mountain building which still continue. 



HEATING AND CONTRACTION 



But this is only the beginning of the story. We liave also to explain 

 how the Earth has come to possess its liquid central core, with a radius 

 more than half that of the Earth. Clearly something must have 

 happened to raise the temperature enough to cause the Earth to melt. 

 There is no difficulty here, however, for this could be achieved by only 

 a minute content of radioactive materials : no more than a few percent 

 of the proportion found to be present near the Earth's surface would 

 gradually raise the temperature as these materials — mainly uranium, 

 thorium, and potassium — decayed into other elements, thereby releas- 

 ing energy. Thus, instead of the Earth cooling down, it has in fact 

 been warmmg up and is still probably doing so. It may well have 

 remained entirely solid for a thousand million years or more, until the 



