ABSOLUTE DATING 25 



thus defies the superposition law. And even when poured out from a 

 volcano over surrounding sediments as lava, thus following the prin- 

 ciple of superposition, magma does not contain living organisms. 

 Without fossils, the second guiding principle, that of organic evolu- 

 tion, cannot be applied. So, in normal geologic dating, the age of 

 igneous rocks is doubly relative: it can only be determined relative 

 to the relative age of adjacent sediments. Igneous rocks are always 

 younger than the sediments through which they penetrate. Con- 

 versely, igneous rocks are older than sediments by which they are 

 covered. Geologic dating of igneous rocks consequently is always 

 relative to the age of sediments, whose own age is already relative. 



ABSOLUTE DATING 



Absolute dating, in contrast to the relative dating methods normally 

 used in geology, measures time in years or in larger units. Two of 

 the methods used in absolute dating actually measure time in years. 

 These are dendrochronology and the study of varved clays or other 

 varved deposits. In dendrochronology the year rings of trees, formed 

 through seasonal growth, are counted. In varved clays seasonal layers 

 of finer and coarser clay, deposited during winter and summer in 

 front of an ice-cap, can also be counted. This method has also been 

 successfully applied to, for example, annual layers found in certain 

 rock-salt deposits. But both dendrochronology and the varved sedi- 

 ments can be applied only to time-spans of some tens of thousands 

 of years at the utmost. They are of no importance at all in dating 

 the origin of life on earth. This took place so long ago that we have 

 to measure time in units of millions and even billions of years. 



Physical clocks: radioactive decay series 



Measuring time-spans of such magnitude is only possible by em- 

 ploying 'physical clocks', based on the continuous radioactive decay 

 of natural elements. All heavier natural elements are unstable, and 

 so are several isotopes of the lighter elements. They only exist at 

 this day because their decay is so slow that the age of the earth, or 

 the age of the rocks in which they are found, is small compared with 

 their decay time. 



A single atom of a radioactive isotope decays by a spontaneous 

 process, whose instant of occurrence is not predictable. Only when 



