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Sea Floor and Coast of Central California. 



The Formation of Canyons 



Submarine canyons are found cutting 

 tiie continental shelves all over 

 tiie world. Altliough tliey liave been 

 l<now/n for a long time, there is no 

 one theory of their origin that is 

 universally accepted. They usually 

 have a V-shaped cross section and 

 extend from the level of the conti- 

 nental shelf down Into the deep 

 ocean. Smaller canyons may join 

 and form a single larger one. 

 Occasionally they are associated 

 with a well-defined river valley on 

 shore, but more often they lead up 

 to a coast that shows no Indication 

 of a valley, either now or earlier. 

 At one time it was thought that 

 submarine canyons were river 

 valleys cut when the sea level was 

 so low that the canyons were 

 above water. Some scientists now 

 believe that the canyons represent 

 the locations of faults in the 

 continental shelf edge. Others 

 believe that the continental margins 

 have been sinking by many thousands 

 of feet and that the canyons may 

 have originated as subaerially cut 

 valleys progressively depressed 

 beneath the sea and cut back. 

 There is more agreement over the 

 idea that they have been modified, 

 enlarged, and kept scoured out by 

 the frequent transport of sedimentary 

 material from the shelf into 

 deep water. These sediments are 

 deposited as fans at the mouth of 

 the canyons and spread out to 

 form the abyssal plains. 



FAciric 



OCEAN 







Mid-Oceanic Ridges 



When the first trans-Atlantic cable 

 was laid, an enormous mountain range 

 was discovered in the mid-Atlantic. 

 Subsequent surveys have shown that 

 the mid-Atlantic Ridge (page 176) 

 forms part of a world-wide system 

 of mid-oceanic mountain ranges 

 encircling the continents and having 

 a total length of some 40,000 miles. 

 The origin of this ridge system is 

 still unknown, although much is 

 being learned about its detailed 

 structure and geophysical character- 

 istics. In many places throughout 

 its length, there is a pronounced 

 valley near the crest of the ridge. 

 This "rift valley," which is associated 

 with intense earthquake 

 activity, suggests that the oceanic 

 crust is under severe tension. 



259 



