2. CONTINENTAL STRUCTURES AND PROCESSES 

 AND SEA-FLOOR SPREADING 



Continental Drift and Sea-Floor Spreading 



The idea that continents move about 

 on the surface of the earth was ad- 

 vanced about a century ago and has 

 always had adherents outside of the 

 United States. In this country, where 

 no direct evidence existed, the concept 

 was first greeted with skepticism and 

 then, for fifty years, was viewed as 

 nonsense. All American thinking in 

 geology — economic or academic — 

 was built on the alternative concept of 

 a relatively stable earth. 



This is now changed. The past few 

 years have seen a basic revolution in 

 the earth sciences. Continents move, 

 and the rates and directions can be 

 predicted in a manner that few would 

 have dreamed possible but a short 

 while ago. As a consequence, all as- 

 pects of American geology are being 

 reinterpreted, and in most cases they 

 are being understood for the first 

 time. 



The continents are an agglomera- 

 tion of superposed, deformed, melted, 

 and remelted rocks with differing ages 

 covering a span of three billion years. 

 These rocks are eroded in some places 

 and covered with sediment in others. 

 The end result is a very complex con- 

 figuration of rocks with a history that 

 has not been deciphered in more than 

 a century of effort by land geologists. 



The ocean basins are very different. 

 The rocks are all relatively young; 

 they are simply arranged and hardly 

 deformed at all; erosion and deposi- 

 tion are minimal. Consequently, it 

 was logical that their history would be 

 unraveled before that of the conti- 

 nents if only someone would study it. 



Oceanographers have been engaged 

 in just such a study for about two 



decades. It has been enormously ex- 

 pensive compared to continental ge- 

 ology — and trivially cheap compared 

 to lunar exploration. Thus, the haunt- 

 ing possibility exists that the same 

 effort on the continents might have 

 yielded the same results despite the 

 complexities. In the event, the break- 

 through was made at sea by the in- 

 vention of a whole new array of in- 

 struments and the development of a 

 system of marine geophysical ex- 

 ploration. The results have now been 

 synthesized with those from the land 

 to provide the data for the ongoing 

 revolution in the earth sciences. 



Present understanding was achieved 

 in what in retrospect seems a curious 

 sequence. We depart from it to pre- 

 sent the new ideas in more orderly 

 fashion. 



Continental Drift 



First, a little geometry. The move- 

 ment of a rigid, curved plate over the 

 surface of a sphere can occur only as 

 a rotation around a point on the 

 sphere. This simple theorem, stated 

 by Euler, has been the guide for much 

 that follows. Earthquakes on the sur- 

 face of the earth are distributed in 

 long lines that form ellipses and cir- 

 cles around almost earthquake-free 

 central regions. If there are plates, it 

 is reasonable to assume that they cor- 

 respond to the central regions and 

 that the earthquakes are at the edges 

 where plates are interacting. 



The motion of earthquakes confirms 

 the existence of plates with marvelous 

 persuasiveness. Euler's theorem speci- 

 fies the orientation of earthquake mo- 

 tion, and it has been confirmed for 



many plates. Moreover, for any given 

 plate the earthquakes in front are 

 compressional as two plates come to- 

 gether; they are tensional in the rear, 

 where plates are moving apart. Along 

 the sides they are as expected when 

 one plate moves past another. (See 

 Figure II-3) 



The knowledge that plates exist 

 and are moving has immediate im- 

 portance with regard to such matters 

 as earthquake prediction. For ex- 

 ample, one boundary between two 

 plates runs through much of Cali- 

 fornia along the San Andreas Fault. 

 We can measure the rate of offset in 

 some places and we now know that 

 related offsets must occur everywhere 

 else along the plate boundaries. 



Earthquakes indicate that moving 

 plates exist now. But the evidence 

 that they existed in the past is of 

 a different sort. This comes from a 

 vast array of geological and geophysi- 

 cal observations — topographic, mag- 

 netic, gravity, heat flow, sediment 

 thickness, crustal structure, rock 

 types and ages, and so on. Integra- 

 tion of these observations indicates 

 that, where plates move apart, new 

 igneous rock rises from the interior 

 of the earth and solidifies in long 

 strips. These in turn split apart and 

 are consolidated into the trailing edges 

 of the two plates. Because the rising 

 rock is hot, it expands the trailing 

 edges of the plates. The expansion 

 elevates the sea floor into long central 

 ridges, of which the Mid-Atlantic 

 Ridge is but a part. As the new strip 

 of plate moves away, it cools and 

 gradually contracts. The cooling and 

 contraction cause the sea floor to sink. 

 This is why the ridges have gently 

 sloping sides that gradually descend 



26 



