PART II— DYNAMICS OF THE SOLID EARTH 



Most of the remaining topography 

 of the sea floor is in the form of 

 roughly circular volcanoes which can 

 grow as large as the island of Hawaii. 

 These volcanoes remain active for 

 tens of millions of years and during 

 that time they drift as much as a 

 thousand miles. If they develop on 

 young crust, they necessarily sink 

 with the crust as it cools. This seems 

 to be the explanation for the drowned 

 ancient islands commonly found in 

 the western Pacific. Once they were 

 islands, but now they are as much as 

 a mile deep. 



The phenomena that occur where 

 two plates come together are natu- 

 rally different from those that occur 

 where they spread apart. If the plates 

 come together at rates of less than a 

 few inches per year they seem to 

 crumble and deform into young moun- 

 tain ranges. Where they come to- 

 gether faster, the deformation cannot 

 be accommodated by crumbling. In- 

 stead, the plates overlap and one of 

 them plunges deep into the interior 

 where it is reheated and absorbed. 

 This produces the most intense de- 

 formations on the surface of the 

 earth. A line of fire of active vol- 

 canoes, deep depressions of oceanic 

 trenches, and swarms of earthquakes 

 mark the line of junction. Ancient 

 rocks of the continents are now being 

 reinterpreted as having once been 

 deep-sea and marginal-sea sediments 

 that were deposited where plates 

 came together. They occur in central 

 California, the Alps, and many other 

 mountain ranges. Typically, they are 

 highly deformed, which seems quite 

 reasonable considering what must 

 happen when plates smash together. 



Implications of the 

 New Knowledge 



Since the sea floors are young, con- 

 tinental rocks contain what records 

 may exist of ancient plate motions. 

 The present revolution in understand- 

 ing was needed to serve as a guide to 

 geological exploration, however. Land 



geologists had long noted similarities 

 between the rocks on opposite sides 

 of the South Atlantic. In the past few 

 years, many more confirming correla- 

 tions have been discovered. Knowing 

 that the continents were once joined, 

 we can reconstruct, in the mind's eye, 

 a history in which they were once but 

 a small distance apart and the nascent 

 Atlantic was a narrow trough. 



We should pause for a moment to 

 consider how important to economic 

 resource development the new ideas 

 may be. The continental shelves of 

 Atlantic coastal Africa and South 

 America, for example, contain salt de- 

 posits and sediments in thick wedges 

 that seem to lack any dam to trap 

 them. These deposits contain oil. We 

 can imagine the difficulties American 

 oil geologists had in interpreting their 

 records and predicting where to drill 

 when they had no idea of how the 

 oil-bearing rocks accumulated. How 

 easy it may now become, when their 

 origin can be readily explained as 

 occurring in the long narrow trough 

 of the newborn Atlantic! 



Until now exploration has been 

 adequate to demonstrate the existence 

 of continental drifting and global de- 

 formation, but much remains to be 

 done to flesh out the reconstruction of 

 the history. If the exploration at sea 

 continues and is matched by compa- 

 rable effort at continental margins 

 and on land, we may hope to see the 

 beginning of a deep new understand- 

 ing of the earth. 



Continental Structures and 

 Processes 



Our knowledge of continental 

 crustal processes, except in the vicinity 

 of the continental margins, has lagged 

 behind our knowledge of oceanic 

 crustal processes. One reason for the 

 great progress in the study of oceanic 

 crustal processes is the beautifully 

 simple pattern of magnetic anomalies, 

 magnetic-field reversals, and earth- 

 quake and volcanic activity in the 



vicinity of the continental margins 

 that led to the discovery of sea-floor 

 spreading. But another reason must 

 be that the earth scientists who made 

 these advances were not inhibited by 

 the traditions and prejudices of scores 

 of years of separation into highly 

 compartmentalized sub-disciplines. Of 

 course, the continental crust is com- 

 plex, and simple patterns, if they 

 exist, are obscured by the geological 

 and geophysical scars of billions of 

 years of continental damage and re- 

 building. But the attitudes and study 

 methods that led to the discovery of 

 sea-floor spreading and downward- 

 plunging plates will be needed if we 

 are to improve our knowledge and 

 understanding of continental proc- 

 esses during the 1970's. 



Structure of the Continental Crust 



The average thickness of the con- 

 tinental crust of the United States, as 

 determined by seismic measurements, 

 is 41 kilometers; its volume is about 

 40,000 x 10 4 km 3 . The average 

 crustal thickness in the west of the 

 Rocky Mountains is 34 kilometers, 

 while the average thickness to the 

 east of the mountains is 44 kilometers. 



The volume of the western crust is 

 only about 10,000 x 10 4 km :i . Thus, 

 the western crust accounts for only 

 one-fourth of the continental total by 

 volume, as compared to 30,000 x 10 4 

 km 3 for the eastern crust, although its 

 surface area is almost a third of the 

 total. Average seismic velocities also 

 suggest that the western crust is less 

 dense than the eastern crust. Thus, 

 the western crust — the portion of the 

 crust in which continental dynamic 

 processes (earthquakes, volcanic erup- 

 tions, magmatism, ore deposition, and 

 mountain-building) have been active 

 during the past 100 million years or 

 so — is the lesser fraction of the con- 

 tinent in terms of volume and mass. 



Further Western-Crust Data — A 

 recent reinterpretation of a network 

 of 64 seismic-refraction profiles re- 



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