Shape and Structure of Ocean Basins* 



MAURICE EWING and MARK LANDISMAN 



Lamont Geological Observatory, Columbia University, Palisades, New York 



Shape of Ocean Basinst 



Continents and Oceans 



WE shall take as a starting point the familiar idea that the 

 continents are blocks of low-density sialic rock, remarkably 

 uniform in thickness and other properties, which are in isostatic 

 or floating equilibrium on a heavier substratum, the sima. These 

 blocks cover about 35% of the earth's surface. The peculiar shapes 

 and the distribution of the blocks have given rise to many specu- 

 lations about their origin and their permanence in size, shape, and 

 relative position. Such speculations obviously relate directly to 

 the shape, structure, and history of ocean basins. 



We shall assume familiarity with general information about the 

 shapes of the ocean basins and the distribution of oceans on the 

 globe. For example, the earth's surface may be divided into a 

 Pacific or "water hemisphere," which is about 89% ocean, and a 

 "land hemisphere," which is only about 53% ocean. The pole of 

 the land hemisphere is at about 47°N Lat, 2°W Long (Dietrich, 

 1957, p. 1). We also assume knowledge of the fact, much used by 

 proponents of continental drift, that if the Americas should be 

 displaced eastward to close up the Atlantic Ocean, their boundary 

 would neatly fit the Mid- Atlantic Ridge at the halfway mark, and 

 then the boundary of Europe and Africa. In this connection the 

 reconstructions of Koppen and Wegener, Du Toit, and others, in 

 which all of the land areas are fitted neatly together, may be called 

 to mind. 



* Lamont Geological Observatory, Columbia University, Contribution No. 444. 

 t This section by Maurice Ewing. 



