We Learn about Earth's History from Rocks 525 



PROBLEM I . 



ten rock known as lava. The island of 

 Hawaii was once little more than a few 

 volcanic peaks rising out of the ocean. 

 As the lava flowed out it hardened and 

 built up the shores of the present island. 

 Changes like this in addition to the wear- 

 ing down of mountain ranges by ero- 

 sion, the building up of land, and the 

 laying down of sedimentary rock are 

 constantly going on. This earth of ours 

 which seems to us so steady and so stable 

 has been undergoing changes through- 

 out its estimated three-billion-year life. 



How we estimate the age of the earth. 

 Several methods of estimating the age of 

 the rocks and thus of the earth have been 

 used. What is considered the most accu- 

 rate method was discovered recently. In 

 some parts of the earth there are rocks 

 which contain the element uranium. Ura- 

 nium gradually changes into lead, always 

 at the same rate. The lead remains in the 

 rock and does not change any further. 

 Thus by calculating the proportion of 

 lead and uranium in such rocks one can 

 tell for how long uranium in these rocks 

 has been changing to lead or, in other 

 words, for how long these rocks have 

 been in existence. By this method it is 

 estimated that the oldest rocks are about 

 three billion years old. And if these es- 

 timates are a few million years out of the 

 way it is unimportant. The error is slight 

 in terms of billions of years. 



The history of the earth. As the his- 

 torian speaks of centuries and years, the 

 geologist counts time in eras and periods. 

 An era, as you will read later, is a stretch 

 of time from many millions to a billion 

 years long. Periods are subdivisions 

 within an era just as years are divisions 

 within a century. 



Historians depend on written records. 

 Geologists, too, depend on records but 

 they are not written records. Their rec- 

 ords are in the rocks. By studying the 

 records in the rocks geologists have dis- 

 covered that this changing earth has been 

 inhabited by plants and animals for per- 

 haps two billion vears. How did they 

 know this? They found fossils, the re- 

 mains of plants and animals in rock. The 

 study of fossils soon grew so complex 



■ ' 1 



Fig. 473 How can yon explain structures such 

 as this in a sedimentary rock? (u. s. geological 

 sltrvey) 



