The drawing shows the 

 age of various rock belts 

 within the North Ameri- 

 can continent. From the 

 map, the time sequence 

 of the continent's growth 

 can be determined. 



OLDER THAN 2 V 2 BILLION YEARS 



1 BILLION TO 2V 2 BILLION YEARS OLD 

 | YOUNGER THAN 1 BILLION YEARS OLD 



Atom 



TABLE II 



Product 



Half-Life 

 (millions 

 of years) 



Uranium-238 Lead-206 4,500 



Uranium-235 Lead-207 710 



Thorium-232 Lead-208 13,900 



Rubidium-87 Strontium-87 60,000 



In looking at this list the reader may 

 wonder how half-lives of such long dura- 

 tion are determined. Some of those 

 listed are longer than the age of the 

 earth, and are fantastically longer than 

 the time man has been studying them 

 (it is only about 60 years since the dis- 

 covery of radioactivity). In general, 

 half-lives are determined by chemically 

 extracting a small mass of a radioactive 

 material (for example, uranium-238) and 

 then measuring the very slight drop in 

 activity over a period of several years. 

 When the drop in activity is plotted on 

 a graph against time, the half-life may 

 be measured from the slope of the line. 

 Obviously then for some of the very long 

 half-lives (such as rubidium-87 at 60 

 billion years) this method is less accu- 

 rate than for shorter half-lives (for exam- 

 ple, uranium-235 at only 710 million 

 years). As a consequence some of the 

 half-lives are known only approximately. 

 Some of the current atomic research is 



devoted to the more accurate determi- 

 nation of long half-lives. 



Besides the pairs of atoms listed in 

 Table II, new pairs are being investi- 

 gated by research laboratories all over 

 the world, so that ultimately almost any 

 kind of rock will have some atoms in 

 it that can be used for age determi- 

 nation. We might mention that all of 

 these methods involve extremely deli- 

 cate and costly laboratory procedures. 

 If one were to believe some of the ac- 

 counts mentioned in the popular press 

 one would conclude that there is some 

 magical machine in which you place a 

 sample and an age comes out on a printed 

 tape. In reality, depending on the meth- 

 od used, a single age determination can 

 require the work of two men for as little 

 as a couple of days to as much as several 

 months. 



Finally, we must point out that dating 

 methods cannot be used blindly. Con- 

 sider the case of a granite ledge contain- 

 ing the mineral orthoclase, which crum- 

 bles away by weathering. The ortho- 

 clase grains may be carried in streams 

 down to a lake where they settle to the 

 bottom and are covered by mud. The 

 muddy sediment is compressed and even- 

 tually hardens into rock. Let us as- 

 sume that this rock is one million years 

 old today. If we were to collect some 



of it, pick out the orthoclase, and meas- 

 ure the age by potassium-40, we would 

 conclude that the rock was two billion 

 years old. We would be wrong by 2,000 

 times! For what we would have meas- 

 ured is the age of the original granite 

 which lay miles away. 



Next let's consider a granite that is 

 one billion years old and is undergoing 

 the geologic process called metamor- 

 phism. Suppose that the orthoclase 

 gradually recrystallizes to the potassium 

 mineral called biotite (black mica). In 

 the process, all the argon-40 that was 

 formed in the orthoclase is lost. If we 

 were to take a sample of the biotite and 

 measure its age by the potassium-argon 

 method we would find that it is per- 

 haps only 70 million years old. What 

 we would have dated is the time when 

 the metamorphism took place; that is, 

 when the original granite was metamor- 

 phosed, the potassium-40 clock was "re- 

 set" and started afresh. Thus only by 

 good geologic sense in collecting sam- 

 ples can we know what dates we are 

 getting. Any geologist would know, in 

 the latter case, that he was obtaining 

 the age of the metamorphosed rock, and 

 would use the data accordingly. In the 

 former case, he would probably never 

 have collected the orthoclase from the 

 muddy sedimentary rock in the first 

 place. 



V rom the methods discussed above 

 has emerged, gradually, a set of geologic 

 ages which when plotted on a map illu- 

 minate the time sequence of the growth 

 of whole continents. On this page is a 

 slightly simplified map of the age belts 

 in North America. From it we see that 

 North America started with an ancient 

 core region in central Canada over two 

 and one-half billion years ago. The core 

 has surrounded itself by successively 

 younger belts and has grown larger in 

 the process. 



The extreme ages shown on such a 

 map are generally beyond our ability to 

 comprehend, except in the sense of mak- 

 ing our own lives appear ridiculously 

 short. One can only feel small in a 

 world where the gravel in one's drive- 

 way may be one billion years old ! 

 (end) 



JULY Page 7 



