356 



HEEZEN AND LAUGHTON 



[chap. 14 



floor of oceanic trenches range from -^o to | this vahio. Sufficient measurements 

 have not been made to determine whether there is any significant difference in 

 heat flow between the abyssal plains and the abyssal hills, but except for eff'ects 

 of very recent turbidity currents, no such contrast woulrl he expected. 



8. Theories of the Origin of the Abyssal Plains 



Any hypothesis or theory of the origin of abyssal plains must account for 

 several critical characteristics. These are : (a) flatness of the plain ; (b) con- 

 tinuity of gradient over large distances, and the exponential shape of the 

 plain ; (c) the terrigenous sediments and shallow-water detritus found on the 



LOWER STEP 



zrsrN 



35"W 



ABYSSAL HILLS 



CAPE VERDE '-°*^'' continental rise upper continental rise 



ABYSSAL PLAIN CONTINENTAL RISE 



Fig, 



32. Total magnetic intensity variations over the Madeira Abyssal Plain. Data 

 obtained with a fluxgate magnetometer from the Research Vessel Vema. Note that 

 the amplitude of the magnetic anomalies over the abyssal plain is not significantly 

 different from the amplitude over the adjacent Mid-Atlantic Ridge and Madeira Rise. 

 This apparently indicates that the structure lying beneath the abyssal plain is 

 essentially similar to that lying beneath the adjacent features and that the only basic 

 difference between these adjacent provinces is that the abyssal plain has been 

 smoothed by turbidity-current sedimentation. 



plain. Since the discovery and beginning of systematic investigation of abyssal 

 plains in 1947, several explanations of their origin have been offered. Each of 

 these is discussed in the following paragraphs, with special reference to the facts 

 upon which it is based. 



A. Atecfonic Areas 



'fJie suggestion that abyssal plains represent atectonic areas in which the 

 original crust of the Earth has been luiaffected by diastrophism since the origin 

 of ocean basins was made by Tolstoy and Ewing (1949). This hy]wthesis was 

 offered in a tentative fashion largely on the basis of two facts: (1) "the jilain 

 was flat and nearly level" ; (2) "the seismic reflections from the plain were of a 



