14. ABYSSAL PLAINSi 



Bruce C. Heezen and A. S. Laughton 



1. Introduction 



One of the most striking discoveries of the 1947 Mid-Atlantic-Ridge Expedi- 

 tion was that vast areas of the deep-ocean floor in the North Atlantic are 

 occupied by flat, nearly level plains. In 1948 the Swedish Deep-Sea Expedition 

 discovered a similar plain in the Indian Ocean south of the Bay of Bengal. ^ 

 Subsequently, more abyssal plains were discovered throughout the world. 



Although a rough idea of the extent of abyssal plains could be gained from 

 a study of records made by standard echo-soiniders, the large errors inherent 

 in these instruments made a study of abyssal-plain gradients im])ossible. All 

 irregularities observed in abyssal plains on standard echo-sounding records 

 could be ascribed to errors and inaccuracies in the recording system ; thus, it 

 was not possible to determine exactly how smooth the abyssal plains were. 



Soon after the discovery of abyssal plains, it became obvious that a much 

 more accurate echo-sounder recorder was needed before an effective study of 

 these features could be made. To fill the specific need, a Precision Depth 

 Recorder (P.D.R.) was developed (Luskin, Heezen, Ewing and Landisman, 

 1954). 



The ocean-basin floor is divided into two parts, the oceanic rises and the 

 abyssal floor. ^ The abyssal floor, in turn, is divided into the abyssal plains, 

 the abyssal hills and areas of gently undulating topograjjliy. An abyssal plain 

 is an area of the ocean-basin floor in which the ocean bottom is flat and the slope 

 of the bottom is less than 1 : 1000. An abyssal hill is a small, relatively sharply 

 defined liill that rises in the ocean-basin floor to an elevation a few fathoms to 

 a few hundred fathoms in height and is from a few hundred feet to a few miles 

 in width. The term "abyssal -hills province" is applied to those areas of the 

 abyssal floor in which nearly the entire area is occujDied by hills, i.e. the province 

 lies at approximately the depth of the adjacent abyssal plain but lacks a smooth 

 floor. Isolated, sharply defined abyssal hills and grou])s of abyssal hills also 

 protrude from the abyssal plains. The areas of undulating topography may 

 either represent lower tectonic activity than the abyssal hills, or greater age 

 and consequently greater thickness of pelagic sedimentation. 



There are a few areas of the ocean-basin floor which approach abyssal-plain 

 smoothness but are apparently not a part of the modern abyssal plains. These 

 areas may represent relic abyssal plains. Large areas of the oceanic rises are 

 extremely gentle and, although these areas contrast sharply with the rugged 

 relief of the abyssal hills, they do not reach the featureless smoothness of 

 abyssal plains. 



1 Lamont Geological Observatory (Columbia University) Contribution No. 591. 



2 In 1951 H.M.S. Challenger made a second crossing (Gaskell and Ashton, 1954). 



3 The major physiographic regions of the ocean are described by Heezen and Menard 

 in Chapter 12. 



[MS received December, 1960] 312 



