342 HEEZEN AND LAUGHTON [CHAP. 14 



(Koczy, 1954). However, these same features have been studied by Dietz 

 (1953) and compared with the deep-sea channels off Cahfornia to which they 

 bear a close resemblance. If the Bengal Abyssal Plain is the result of turbidity 

 currents, then it is plausible that these channels also have a similar origin. 

 Further exploration of them will undoubtedly resolve this problem in the near 

 future. 



Other mid-ocean canyons have been found in other parts of the world, e.g. 

 in the equatorial Atlantic (Heezen, Coughlin and Beckman, 1960) and in the 

 Hikurangi Trench, and many more will no doubt be discovered. 



The mechanism by which the mid-ocean canyons can be formed is still under 

 discussion. Tectonic movements have already been discussed, but it is hard to 

 see how such long, uniform and continuous channels can have been formed in 

 this way when there is little evidence of vertical movements either side of the 

 channels. Their association with abyssal plains, their similarity with the delta 

 channels and interplain channels, and the deep-sea sands found in the canyon, 

 argue a turbidity-current origin. However, it is difficult to envisage a turbidity 

 current confined to a channel 2 mi wide and 50 fm deep travelling without 

 decay for more than a thousand miles, Ewing et al. (1953) suggested a mecha- 

 nism of successive filling of confined basins starting from the north, each one 

 breaking through to the next by an interplain channel, and that these 

 channels eventually joined to form one continuous one. However, there is 

 little topographic evidence of these filled basins. Heezen et al. (1959) question 

 whether the process is erosional or depositional. Dietz (1958) believes that the 

 channels may be due to localized streams of clear dense water of high salinity 

 and low temperature derived from Arctic regions. This explanation does not 

 require the turbulence to keep j)ai"ticles in suspension and therefore permits 

 lower current velocities, but fails to explain the mid-ocean canyons in the 

 Indian Ocean and is difficult to apply in the equatorial Atlantic. The inde- 

 pendence of canyon location and basement structure and the evidence of 

 lateral migration shown by recent seismic reflection profiles support a deposi- 

 tional versus a tectonic or erosional origin. 



C. Abyssal Gaps and Interplain Channels 



If two adjacent but distinct abyssal plains have no through passage at or 

 below the level of the higher plain, they are said to be separated by a sill, a 

 ridge or a rise depending on the size of the feature involved. However, it has 

 been found that several plains are interconnected by constricted passages 

 which have been called variously "abyssal gaps" (Heezen et al., 1959), and 

 "interplain channels" (Laughton, 1960). However, both terms can be retained 

 if "abyssal gap" is used to describe tJie larger features of a passage cutting 

 across a ridge, and "interplain channel" more specifically to describe the 

 continuously graded channels formed by turbidity currents travelling in an 

 abyssal gap between two abyssal plains. Thus an abyssal gap may or may not 

 contain interplain channels. 



