INTRODUCTION: 



crust 



Figure 1.3 Earth's interior, showing three major layers of our planet. 



exploded from the molten interior to form the earliest 

 atmosphere. Finally, the crust solidified, a process 

 that resulted in the additional release of carbon 

 dioxide and water. These vapors formed an ex- 

 tremely dense covering of clouds over the earth — so 

 dense that the crust vi'as then in perpetual darkness. 

 Although rain fell from these clouds, upon contacting 

 the crust the water immediately boiled and returned 

 to the clouds. Finally, as the crust cooled to the boil- 

 ing point of water, 212° F., the water no longer boiled 

 away. Shortly thereafter, the sunlight first broke 

 through the cloud cover. However, there was still a 

 long period of almost constant rains which resulted 

 in the first oceans reaching their present level. 



With this stage of development it is possible to 

 consider the early conditions of the land, waters, and 

 air, and the origin of life itself. 



After the crust formed, cooling of the mantle, and 

 perhaps the crust, may have continued for some time. 

 The consequence of such coolings would be a gradual 

 contraction of the deeper layers of the earth, so that 

 wrinkles (the first mountains) appeared on the sur- 

 face. However, these mountains were flattened rela- 

 tively quickly by erosion caused by runoff from the 

 torrential rains. L'p to now about ten periods of 

 major mountain building have occurred, each period 

 being separated by a cycle of erosion and flattening 

 of the land before the start of the next. 



Although the earliest oceans contained much less 

 salt than do the present ones, today's seas probably 

 are much the same as those of one-half billion (or 

 even more) years ago. It is generally believed that 

 oceans became salty because waters from the land 

 carried salts to the ocean; however, for some time a 

 state of equilibrium has existed in that ocean salt 

 loss has equalled salt gain. 



During the geological phase of our history called 

 the Freactualistic Phase the atmosphere most likely 

 contained much helium and oxygen, plus early addi- 

 tions of hydrogen sulfide, ammonia, methane, chlo- 

 rine, water, and carbon dioxide (Table 7.1, pp. 109- 

 113). Little or no oxygen was present. Gradually the 

 hydrogen and helium escaped to outer space, because 

 the earth's gravitational field was too weak to contain 

 the tremendous energy of the hydrogen and helium 

 molecules. Loss of other materials was probably due 

 to their being used by early life. 



The Freactualistic Phase was followed by the 

 Adualislic Phase. The Actualistic Phase still exists 

 but its atmosphere always had a gaseous composition 

 similar to the one that now exists. The source of most 

 modern atmospheric gases was volcanic activity in the 

 broadest sense. For millions of years, volcanoes, hot 

 springs, and related phenomena supplied gases to the 

 area about the earth. The lightest gases, such as 

 helium and hydrogen, were mostly lost to space; how- 

 ever, heavier nitrogen, carbon dioxide, water vapor, 

 and trace gases tended to remain. 



Note that oxygen was not included among the 

 products of vulcanism. Free oxygen, so necessary for 

 most of earth's life, did not originate from gases re- 

 leased from the earth's interior. Rather, most of the 

 first oxygen was a "waste product" of photosynthesis, 

 the process whereby plants and certain microscopic 

 life use their chlorophyll in the presence of sunlight to 

 combine carbon dioxide and water to form simple 

 sugar. However, for certain life processes most or- 

 ganisms require some oxygen. If one thinks about 

 early life forming oxygen, but also needing some oxygen, 

 he has a classic analogy to "which came first, the 

 chicken or the egg?" However, there is really no 

 problem in regard to early life's source of oxygen. 

 The relatively minute amounts required by ancient 

 organisms could easily have been supplied by either 

 the sun or lightning separating individual water vapor 

 molecules into basic hydrogen and oxygen. 



Although this hypothetical evolution of the present 

 atmosphere would have taken some time, the at- 



