PART II— DYNAMICS OF THE SOLID EARTH 



upper atmosphere and in the sun; 

 they are largely the concern of space 

 research. The former, the slowly 

 varying field, is the subject considered 

 here. 



Interest in the slowly varying field 

 has been greatly increased by the 

 realization that it has frequently re- 

 versed in the past. (See Figure II-2) 

 The reversals have been helpful in 

 establishing the history of the oceans 

 and the movements of the continents. 

 The study of the magnetic field can be 

 expected to contribute — indeed, is 

 beginning to contribute — to the 

 search for oil and minerals. Measure- 

 ment of the magnetic field has be- 

 come one of the principal tools for 

 studying the earth. 



The Origin of the Earth's 

 Magnetic Field 



These applications lend a new in- 

 terest to the origin of the field itself. 

 A theory as to its origin has proved 

 hard to find. Only in our own day has 

 anything plausible been suggested. 

 Although a theory of the origin has 

 no discernible immediate practical 

 importance, it is a part of the story of 

 the earth without which we cannot 

 be said to understand what is going 

 on. Maybe we can get on very well 

 without understanding, but one feels 

 happier if important practical tech- 

 niques have a proper theoretical un- 

 derpinning. 



The difficulty of discovering the 

 origin of the slowly varying field is 

 due largely to its lack of relation to 

 anything else. It is not related to 

 geology or geography and goes its 

 own way regardless of other phenom- 

 ena. In some places, some of the 

 changes are due to the magnetization 

 of rocks near the surface of the earth, 

 but this is not the case over most of 

 the field. 



Fashionable theory holds that the 

 magnetic field is produced by a dy- 

 namo inside the earth. The earth has 

 a liquid core; the motions in this core 



Figure 11-2— CHRONOLOGY OF EARTH'S MAGNETIC FIELD REVERSALS 



YEARS AGO 



690,000 



890,000 

 950,000 



,610,000 



,630,000 



1,640,000 



1,790,000 



1,950,000 

 1,980,000 



2,110,000 

 2,130,000 



YEARS AGO 



2,430,000 



2,800,000 



2,900,000 

 2,940,000 



3,060,000 



4,380,000 



4,500,000 



LEGEND 



Field as at Present 

 Field Reversed 



This figure shows reversals in the polarity of the earth's magnetic field, a phenome- 

 non of global extent that is known to occur but has never been witnessed. These 

 data are derived from measurements of the direction (N-S) of magnetism frozen into 

 lava as it hardens. The effects of the reversals are unknown. 



24 



