Chapter III 



MARINE GEOLOGY AND GEOPHYSICS 



The last decade has witnessed a revolution in our understanding of 

 the physics of the earth's crust and mantle. Geophysicists now 

 believe that most of the crust of the earth consists of a limited 

 number of large plates and numerous small plates. These plates 

 move apart from ocean ridges, where new material wells up from the 

 mantle, and come together along certain continental margins and 

 island arcs. In these areas one plate may be forced under the other 

 and assimilated back into the mantle. Research into these processes 

 can give new insights into the history of the earth and the movement 

 of continents. It can also provide us with information needed for 

 current practical use. 



Plate-tectonic processes involve earthquakes, geothermal activi- 

 ty, and the deposition of certain minerals. The study of plate 

 tectonics is, therefore, of increasing importance. The findings of 

 these geophysical studies will add to the fundamental understan- 

 ding that is needed to reduce earthquake damage, to tap geothermal 

 energy sources, and to locate and develop nonliving resources of the 

 oceans as those of land areas become inadequate. 



Much of this research is conducted at plate margins, where 

 tectonic processes can most readily be observed. However, studies of 

 the sediments on the continental shelves and ocean bottoms are 

 equally important for finding petroleum and deposits of sand, 

 gravel, and other mineral resources. The mechanical and other 

 physical properties of the shallow sediments must be known for the 

 design and construction of structures resting on the sea floor. In 

 some localities the layering found in undisturbed sediments can 

 provide clues to the history of the earth's climate and aid in research 

 on the possibility of predicting future climate variations. 



A new emphasis is now being given to investigations of 

 sedimentary dynamics the patterns (rates and directions) of erosion, 

 transport, and deposition. The change reflects an increasing concern 



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