EARTH 5 



ten rock, 1200° to 1800° F., will be reached at a depth 

 of some 30 miles, and it is from this depth (or lower) 

 that there comes volcanic lava, escaping through faults 

 in the outer crust. Contrary to what one might expect, 

 however, the temperature of the surface of the earth is 

 not determined by the escape of heat from the interior 

 —because the earth's substance conducts heat very slowly 

 —but rather by solar radiation; while the temperature 

 of the interior of the earth (below a few hundred feet) 

 is determined by the decomposition of radioactive ele- 

 ments, a process liberating much more heat than that 

 gained from the sun. Such has apparently been the case 

 throughout most of geologic time. In consequence of the 

 great pressure at the center of the earth the metallic 

 core has a temperature perhaps not far below that exist- 

 ing at the surface of the sun. 



Paleontologists and geologists have sought by a variety 

 of methods to learn the exact age of the earth, or at least 

 of its superficial rocks. The method that is now beheved 

 to be most accurate depends on the ratio of certain 

 radioactive elements, notably uranium and thorium, to 

 their decomposition products (chiefly various isotopes of 

 lead): assuming that only negligible quantities of these 

 decomposition products were present when the rock first 

 acquired a sohd state, the ratio of the parent radioactive 

 element to its decomposition product indicates the age 

 of the rock as a solid matrix. From such analyses Arthur 

 Holmes in 1947 estimated that the oldest rocks studied 

 up to that date had an age of 3350 million years, the 

 methodologic error in this estimate not exceeding ±10 

 per cent. The figure represents, of course, the time since 

 the crust solidified to such an extent that mixing of iso- 

 topes had ceased. Other data indicate that the total age 

 of the earth as a planet is about 4500 million years. This 

 is almost the age of the solar system, as calculated by 

 several methods, and indeed the galaxy to which our 

 sun belongs is estimated by some astrophysicists to have 

 an age of about 5000 million years. 



