SHAPE AND STRUCTURE OF OCEAN BASINS 5 



plains, and continental lowlands. It results from subaerial erosion 

 which effectively reduces the elevation of land surfaces and fills 

 marginal seas, eventually to sea level. Its elevation above the 

 floors of the ocean basins depends primarily on the volume of 

 water in the oceans. Through the action of isostasy, however, the 

 sea level influences the thickness of the continental crust. The 

 + 100-m level includes much of the shield area of the earth, and 

 the fact that shield areas are generally so well adjusted to present 

 sea level is strong evidence that the volume of sea water has been 

 essentially constant for more than 500 million years. If we extend 

 the +100-m level somewhat (to include regions of low hills up to 

 700 m), this level accounts for about 20% of the total earth 

 surface. 



Deep sea trenches account for 1 to 2% of the surface, and 

 mountain chains and highlands for about 15%. 



The remaining 18% includes depths from about 200 m to 3500 m, 

 contains parts of the mid-ocean ridge system, but consists prin- 

 cipally of the transition zone between continents and oceans — the 

 continental borderlands including continental slopes and conti- 

 nental rises. 



Continental Borderlands 



There are many patterns of transition from continent to ocean, 

 but the great majority of them can be put usefully into three 

 classes (Fig. 2). In the first pattern, the sequence of form is shelf, 

 slope, trench, ridge, and basin (westward from Peru). The second 

 pattern differs in having a marginal sea and island arc between 

 slope and trench (north from Venezuela). The third is represented 

 by the sequence shelf, slope, rise, and basin (east from New Jersey). 



Seismic refraction measurements have shown that in some 

 instances the three patterns are structurally similar, apparent 

 differences ha\-ing arisen from burial by younger sediments (Drake 

 etal., 1959). 



Mid-Oceanic Ridge System 



The Mid-Oceanic Ridge system, typified by the Mid-Atlantic 

 Ridge, is an Important and extensive feature of the earth's crust 



