PROBLEM 1 What Can We Learn from Rocks about 



the History of the Earth? 



The forming of our earth. Before consid- 

 ering theories of the origin of the earth 

 we should remember that the earth is one 

 part of the solar system. It is one of sev- 

 eral planets that revolve about the sun. 

 Some of these planets have moons — 

 smaller bodies that revolve around them. 

 The sun itself is considered to be a star. 

 The solar system, then, consists of the 

 sun, its planets, their moons, and other 

 smaller bodies. 



Several theories have been proposed 

 to account for the origin of the earth 

 and the other planets in this system. One 

 theory that seems reasonable to many as- 

 tronomers is that the planets were once 

 part of the sun. According to their the- 

 ory, about three billion years ago another 

 star passed near the sun. Although the 

 distance is thought to have been many 

 thousands of millions of miles, this is rel- 

 atively close as star distances go. These 

 two stars were so close that large and 

 small masses of material were torn out of 

 the sun. The larger masses became the 

 planets. They attracted to themselves 

 most of the smaller masses which were 

 nearby and increased in size. Ever since 

 then the size of each planet has remained 

 about the same. Then came the cooling 

 of the planets, which took place with 

 comparative speed, but a long, long time 

 ago. 



Although we do not know and may 

 never know just how or when the plan- 

 ets came into existence, there is abundant 

 evidence that at one time our planet. 

 Earth, was made up principally of mol- 

 ten rock. When the rock cooled, it hard- 

 ened. Such rock is called igneous (ig'- 

 nee-us) rock. A common example of 

 igneous rock is granite. After a time, 

 water appeared. Many scientists believe 

 this happened before the earth had cooled 

 to its present temperature, so the oceans 

 of those days must have been hot. The 

 rocks that rose above the seas were bare. 

 When it rained, the heavy downpour 

 dashed down the steep slopes, for there 

 was no soil and no vegetation to hold the 

 water back. Pebbles and boulders (large 

 rocks) were broken loose by the rush- 

 ing waters and changing temperatures; 

 slowly they were ground up and changed 

 chemically, forming sand and clay; thus 

 soil was formed. Of course, there were 

 no living things. They could not have 

 existed on such a harsh, forbidding 

 planet. This is what geologists (gee-ol'- 

 o-jists) think happened a long time ago, 

 perhaps three billion years ago. 



Changes in the surface of the earth. 

 What started perhaps three billion years 

 ago is still going on. Heavy rains and 

 running water loosen the soil and smaller 

 rocks. Carried along by a large or small 



