THE ATLANTIC FLOOR* 



Bruce C. Heezen and Marie Tharp 



Columbia University, Department of Geology and Lamont Geological Observatory, 



Palisades, New York 



Former continental connections across present seas have been frequently 

 proposed by botanists, zoologists and paleontologists striving to understand 

 the affinities and routes of dispersal of land biota. Land connections across 

 present epicontinental seas have clearly occurred during the Pleistocene due 

 to eustatic fluctuations of sea level. More difficult is the question of earlier 

 connections across the deep seas (Heezen et al., 1959). 



FORMER LAND CONNECTIONS ACROSS THE DEEP SEA 



Geophysicists and oceanographers have long been skeptical of sunken 

 ancient continents or sinuous isthmian links across the deep sea (Bucher, 1952 ; 

 Ewing, 1952). The structure of the ocean floor in depths greater than 4000 m 

 is fundamentally different from that beneath the continents. The Mohorovicic 

 discontinuity, the boundary between the crust and mantle, lies at some 30 to 

 40 km below the continents. However, below the ocean's surface the Mohoro- 

 vicic discontinuity lies at a depth of only 10 to 12 km. The rocks lying im- 

 mediately above the Mohorovicic discontinuity are known entirely on the 

 basis of their seismic-wave velocities. In a typical deep-sea crustal section i to 

 1 km of sediment lies above a thin 1 to 2 km layer in which the seismic-wave 

 velocity is between 4 and 5 km/sec (Hill, 1957). This second layer is sometimes 

 ascribed to altered basalt and sometimes to lithified sediments, although it 

 seems more likely that the material is igneous rather than sedimentary. 

 Beneath this lies a layer 3 to 5 km thick which has a velocity of about 6.5 

 km/sec. The mantle below the Mohorovicic discontinuity has a velocity of 

 about 8.2 km/sec. Hundreds of measurements beneath the deep-sea floors 

 have revealed this column to be essentially universal. However, in the 

 central part of the Mid-Oceanic Ridge, in the Norwegian Sea, in the northern 

 North Atlantic and in other areas (primarily where the depth is less than 4000 

 m), the crustal structure is quite different, the seismic velocity of the deepest 

 observed layer ranging between 6.8 and 7.5 km/sec. Recent seismological 

 studies in Iceland have led Tryggvason (1962) to conclude that the 7.4 km/sec 

 layer reaches to tens of kilometers depths below Iceland, an emerged portion 



* Lamont Geological Observatory Contribution No. 577. 



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