INTRODUCTION: 



miles above the earth's surface. The sun's atmos- 

 phere produces radio waves and X rays which create 

 the lowest layer of the ionosphere and which are the 

 cause of radio and television interference during solar 

 storms. 



About 65 per cent of these solar rays are absorbed 

 by the earth, 47 per cent by the surface, 15 per cent 

 by the atmospheric clouds and moisture, and 3 per 

 cent by atmospheric ozone. About 35 per cent of 

 them are reflected back into space by the atmosphere 

 (air, clouds, and dust) and by the surface, especially 

 those areas covered by water, ice, and snow. 



EARTH'S MAGNETISM AND RADIATION BELTS 



The earth has certain properties similar to those of 

 a magnet. Our planet creates lines of magnetic force 

 that arch away from and back to the earth. This 

 complex of lines of force is called the earth's magnetic 

 field. The arches occur in a series of increasing size 

 that center near the equator, the largest arches pass- 

 ing from one magnetic pole to the other. 



Formation of the magnetic field appears to be 

 associated with the earth's two-layered core — an 

 inner, mostly solid, one and an outer liquid layer. 

 It is believed that the liquid portion of the core cre- 

 ated the magnetic field as the liquid was churned 

 during the earth's formation. This churning move- 

 ment may have produced a small magnetic field that 

 further caused the liquid part of the core to move, 

 thus increasing the original field and forming the 

 general pattern of the present one. The present 

 magnetic field pattern also exists because of tidal 

 pull, or gravitational attraction, of the sun and the 

 moon. Finally, our field is modified periodically by 

 solar storms. 



Earth's magnetic field produces an envelope of 

 radiation, the magnelosphere (Figure 1.5). This enve- 

 lope exists because the magnetic field acts as a mag- 

 netic trap, catching radioactive particles from the 

 sun and interstellar space and holding them in a layer 

 of radiation that surrounds the earth except at the 

 magnetic poles. 



EARTH FEATURES 



If one were to select the best single word to indicate 

 the characteristics of the earth, "organization" might 

 be chosen. Whether one considers physical or bio- 

 logical aspects, organization applies. Moreover, the 



Figure 1.5 The earth and its doughnut-shaped envelope of radiation, 

 the mognetosphere. The diagram is a cross-section through the center 

 of the earth (from pole to pole, the earth's core being in black) and the 

 mognetosphere (cuts ore the two large areas of open stippling). Two 

 arcing lines shown to originate and terminate on each side of the earth's 

 core represent lines of force in the earth's magnetic field. The magnetic 

 field traps solar and other radiation, thereby holding the radiation that 

 typifies the mognetosphere. 



word is very descriptive of actual conditions, despite 

 the great complexity and apparent chaos among the 

 myriad of things upon this planet. 



In future discussions, an attempt will be made to 

 use this basic organization of features to facilitate our 

 understanding of the natural history of earth. At the 

 present time we will consider form and divisions and 

 affecting forces. 



FORM AND DIVISIONS 



Our planet is not a perfect sphere; its polar diam- 

 eter is approximately 7900 miles and its equatorial 

 diameter about 7926.7 miles. Actually its form is 

 pearlike, certain southern latitudes having greater 

 diameters than comparable northern latitudes. 



The division (zonation) of the earth is most funda- 

 mental in the case of global subdivision into core, 

 mantle, and crust. In addition, as we have noted, the 

 surface is zoned into land (lithosphere), air (atmos- 

 phere), water (hydrosphere), and life (biosphere). 

 Each of the surface zones is composed of further units 

 that are easily identified by unique conditions and/or 

 structures. The lithosphere displays inner and outer 

 crust zones plus a distinctive pattern of structural 

 features, such as mountain and valley systems. Also, 



