16. THE MID-OCEANIC RIDGEi 



Bruce C. Heezen and Maurice Ewino 



1. Introduction 



The knowledge that there is a median ridge in the Atlantic gradually emerged 

 from the observation of a few shoal soundings almost a century ago and became 

 definite when the early oceanographic expeditions measured bottom-water 

 tem])eratures in the eastern and western basins. Recent investigations of the 

 morphology and seismicity of the ocean floor have revealed the wide extent and 

 continuity of the mid-oceanic ridge, an active geotectonic featureof equal rank, 

 in terms of areal extent and probable global significance, to the continents and 

 the ocean-basin floor. After a ten-year study of the topography and geology 

 of the North Atlantic, Heezen, Tharp and Ewing (1959) found that several 

 major morphotectonic provinces, mapped in moderate detail in the North 

 Atlantic, could be traced with certainty along a world-encircling belt, 40,000 

 miles long, passing through the South Atlantic, Indian, and South Pacific 

 Oceans, the Norwegian Sea and the Arctic Basin. The mid-oceanic ridge, much 

 longer and broader than the Alpine-Himalayan Mountain system, is morpho- 

 logicalh^ identical to the East African rift valley system, which in fact forms a 

 continental extension of the mid-oceanic ridge. The structure and origin of the 

 mid-oceanic ridge is fundamental to the origin of oceans in general, as well as 

 to the particular tectonics of the individual oceans. 



2. North Atlantic 



A typical trans-Atlantic profile between North America and Africa is shown 

 in Fig. 1. The threefold division of the ocean into continental margin, ocean- 

 basin floor and mid-oceanic ridge is well expressed in this profile. Each major 

 physiographic division occupies approximately one-third of the area of each 

 of the oceans which possesses a well -developed mid-oceanic ridge. The most 

 thoroughly investigated part of the mid-oceanic ridge is that between 20°N and 

 .^O'^N latitude in the Atlantic Ocean. It is in this area that the major mor- 

 phological divisions of the ridge were first delineated. Cross-sections through 

 this area are considered "type profiles"' (Fig, 2). The lateral boundaries of the 

 mid-oceanic ridge are defined by the first abrupt scarp, or abrupt gradient 

 change, between the abyssal hills of the ocean-basin floor and the mid-oceanic 

 ridge. The physiographic provinces of the mid-oceanic ridge can be divided into 

 the crest provinces and the flank provinces 



A. Crest Provinces 



The most rugged to])ography of the Mid-Atlantic Ridge is found along the 

 axis of the crest provinces. A steep cleft or rift valley is formed by the inward- 

 facing scarps of the rift mountains which rise 500-2000 fm above the valley 



1 Lamont (Geological Observatory Contribution Xo. 597. 

 [MS reeeived August, 1962] 388 



