750 HEEZEN [chap. 27 



denies that they have the abihty to erode. Unable to accept the high velocities 

 indicated by the Grand Banks sequence of cable breaks, Shepard (1954) sug- 

 gested that an unrelated series of local and independent submarine landslides 

 had broken the cables. He wrote: "Is it not possible that slides developed more 

 slowly on the gentle outer slopes and built up pressure gradually against the 

 cables until they broke? . . . otherwise the sequence could be explained as at 

 least in part a coincidence and in part due to the slower development of slides 

 at an increasing distance from the epicenter." 



Jones (1954) also believed the breaks were "... much more likely to have 

 been caused by slumping than by the thin stream of material carried in a 

 turbidity current (flowing at) an incredible velocity"", 



A. Spontaneous Liquefaction 



Encouraged by the objections to the turbidity-current hypotheses raised by 

 Shepard and others, but not satisfied with their alternate explanations, Terzaghi 

 (1957) offered a new explanation for the cable breakage which followed the 

 1929 Grand Banks earthquake. He supposed that the sediment in the epicentral 

 area and over the whole area of cable breakage to the south existed in a meta- 

 stable state. He supposed that the earthquake shock caused spontaneous 

 liquefaction over the epicentral area and that the apparent velocity of the 

 turbidity current was instead: "the velocity of the advance of the boundary of 

 the liquified portion of the sediments underlying the ocean floor and not the 

 velocity at which the slide material moved over the floor". 



The only fact he attempts to explain is the velocity indicated by the sequence 

 of cable breaks. He is concerned neither with the problem of how the sediment 

 came to be in the supposed metastable state, nor with why the sequence of breaks 

 was downslope and not in all directions from the epicenter. Nor was he con- 

 cerned with the emplacement of the shallow-water sediments in deep water or 

 the grading of the deposited beds. It is not readily apparent why the passage 

 of such a wave of liquefaction should break submarine cables with at least 10% 

 slack. One might think that the cable would be required to sink thousands of 

 feet into the sediment before breakage would occur. Although it cannot be 

 denied that the mechanism he suggests may locally affect sediments, the weight 

 of evidence seems definitely to point away from Terzaghi 's theory of propaga- 

 tion of spontaneous liquefaction as an explanation of the Grand Banks cable 

 breaks, 



B. Seismo-volcanic Catastrophe 



One of the most severe critics of the turbidity-current concept is Professor 

 Hans Pettersson (1954), the well-known leader of the Albatross Expedition. 

 To explain the occurrence on the equatorial Atlantic abyssal plains of graded, 

 deep-sea sands which contained "twigs, nuts, and the bark of dicotyledonous 

 trees", he offered the following alternative to turbidity-current deposition: "In 

 the event that a large island, harbouring vegetation, and with a fairly extensive 



