Page Ten 



EVOLUTION 



March, 1928 



How Old Is the World? 



By Allen S. Broms 



/^OING down a deep well or mine-shaft, 

 one would find the ground tempera- 

 ture rising steadily, — about one degree 

 Fahrenheit for each fifty feet of descent. 

 At this rate, the temperature at twenty 

 miles below the surface would be sufficient 

 to melt some of the rocks. At the center 

 of the Earth everything would be, not 

 merely liquid, but gaseous, though the enormous 

 pressure of the overlying rock matter would keep 

 the whole mass (solids, liquids and gases) as 

 rigid as if made of solid steel. Only a verj- 

 thin outer crust would be solid because it was 

 cool. 



I daresay the Fundamentalists would explain 

 all this as due to "fire and brimstone" and 

 picture (with more or less holy self-satisfaction) 

 that eternal abode for us evolutionists in which 

 we are doubtless to occupy the most scorching 

 place of honor. I fear, however, that we must 

 disappoint by rejecting this pious explanation in 

 favor of one that is sensible. 



Earth Heat From Radioactivity 



Chemical analysis of various kinds of rocks 

 from all parts of the Earth reveal the fact that 

 practically all contain minute portions of the 

 radioactive elements uranium and thorium. By a 

 series of steps, these elements finally break up 

 into helium and lead, the helium being shot out 

 most violently (see illustration). Heat is there- 

 by released, slowly, but steadily. As we can 

 readily measure how fast heat is given off by 

 •each element and we know quite closely the 

 amounts of the elements in the rocks of the 

 Earth it is a simple matter to determine whether 

 enough heat is given off to account for the tem- 

 peratures found underground, keeping in mind 

 that much earth heat slowly works its way to 

 the earth surface and is then lost by radiation 

 into outer space. We find that not only is 

 enough heat thus produced, but too much. This 

 would indicate that the Earth is not cooling off, 

 but actually getting hotter. Thereby, we 

 shall find, hangs a most interesting geo- 

 logical tale. 



Previous Age Estimate Too Loiv 

 On the opposite assumption, that the 

 Earth was gradually cooling off from an 

 original molten state. Lord Kelvin had 

 estimated the age of the Earth at 40,000,- 

 000 years. All he had to do was to meas- 

 ure the rate at which the earth heat was 

 being radiated into space, make due al- 

 lowances for higher rates at higher tem- 

 peratures in the past, and figure how long 

 it would take to cool it from the molten 

 state. 



Geologists claimed that his estimate was 

 too low. They had studied the rates at 

 which sediments were being deposited, 

 they knew what miles upon miles of sedi- 

 mentary rocks had to be accounted for, 

 and they found forty million years too 

 short. They determined the rates at which 



EARTH CRUST 



LIQUID ROCKS 



GASEOUS ROCKS 



(ALL RISIO AS 



SOLID STEEL 



UNDER THE 



ENORMOUS 



PRESSURE.) 



Tracks 



streams were cutting and lowering con- 

 tinental surfaces and about how much of 

 tliis had been done in the past, and again 

 the time allowed was too short. They 

 measured the saltness of the oceans (par- 

 ticularly the sodium content) and how 

 fast the rivers and shore waves were re- 

 moving the salts from the rocks to the 

 seas, and they concluded that the oceans them- 

 selves were more than 100,000,000, perhaps even 

 175,000,000 years old. Of course, this would 

 make the Earth itself much older. 



Heat of th-e Sun Explained 

 Until the discovery of radioactivity, the heal 

 necessary to keep the Earth warm for such a 

 long time could not be explained. The heat 

 from the Sun, though it makes a lot of differ- 

 ence at the surface of the Earth, does not pen- 

 etrate very deeply below that surface. Besides, 

 it had been taken into account. In fact, it 

 presented just another difficulty, for from what 

 source was derived the enormous amount of heat 

 being radiated by the Sun? If the Sun were 

 solid coal, it would all be burned out in a few 

 thousand years. A theory of heat from con- 

 traction extended the time to a few million 

 years. But the heat from radioactivity multi- 

 plied this into billions of years, in closer agree- 

 ment with what we know of the age of the 

 Earth. 



A Peep Into the Geologic Future 



In the Earth itself, it was found that the heat 



from radioactivity more than offsets the loss into 



space and that the Earth is therefore probably 



getting hotter. Some of this heat reaches the 



surface and promptly escapes into space, but the 



temperature deep down inside is slowly rising, 



for the earth-crust acts as an excellent blanket 



in keeping the accruing heat inside. Professor 



Joly has carefully figured the rate of this heat 



accumulation and concludes that in some thirty 



to forty million years the temperature will rise 



high enough to melt and weaken portions 



of the earth crust. If this is true, then 



eventually the lid must blow off. 



What happens when the lid blows off 

 is another story, to be told in the next 

 article. Only one effect need be noted 

 here. According to Joly there ensues a 

 period of rapid heat release so that within 

 six to twelve million years quite all the 

 accumulated heat would be lost. Then 

 the earth-crust blanket would settle down 

 again to its job of holding the heat ac- 

 cumulating for the next great cycle. Every 

 forty to fifty million years these great 

 cycles would repeat themselves. And as we 

 have good reasons for believing that five 

 or six of these cycles have occurred in 

 geological times, we can again estimate 

 the age of the Earth, — roughly between 

 two and three hundred million years, 

 of Helium Atoms shot out from which agrees fairly well with the best 

 Thorium estimates from other evidences. 



CENTER OF I THE EARTH. 

 A slice of the earth 

 according to the 

 Radiactivity Theory 



