STUDIES OF THE ATLANTIC DEEP-SEA FLOOR: 

 THE STATE OF THE ART 



Dr. D. H. Matthews 

 (Department of Geodesy and Geophysics, Cambridge) 



THE " HEROIC AGE " : 1945-55 



The growth of submarine geophysics is closely bound up with 

 the development of new techniques. It has recently been said that 

 the first decade after World War II was the heroic age of marine 

 geophysics: the techniques available at the time were echo sound- 

 ing, seismic refraction (which enables one to determine the 

 thickness and velocity of sound in layers of rock below the sea 

 floor), measurement of gravity at discrete points by swinging 

 pendulums in a submerged submarine, and bottom sampling by 

 coring for soft sediment and dredging for hard rocks. During the 

 period it became possible to measure the rate of efflux of heat 

 through the sea floor, and the accuracy and convenience of echo 

 sounding was greatly improved with the advent of better timing 

 and straight-scale recording. Echo sounding is the most revealing 

 of all single techniques and the results of many years of work were 

 summarised by Heezen and Tharp in 1 957 with the first publication 

 of a chart showing the physiography of the North Atlantic Ocean 

 and the definition of distinct physiographic provinces (Fig. 1): 

 the continental margin (shelf, slope and rise), the ocean basin floor 

 (abyssal plains with slopes of less than 1: 1000 and areas of abyssal 

 hills), and the Mid-Ocean Ridge (Heezen, Tharp and Ewing, 1959). 



Towards the end of that first decade, in 1954, two symposia 

 were held. The first, with the avowed intention of crystallizing 

 present knowledge of the earth's crust, was held at Columbia 

 University (Poldervaart, 1955). The papers concerned with the 

 crust beneath the deep ocean, by Ewing and Press, Gutenberg, and 

 Worzel and Shurbet were primarily concerned to drive home the 

 difference between continent and ocean — the vital discovery that 

 the Moho (the abrupt discontinuity between less dense crust and 

 denser mantle rock) is shallower under the oceans which accounts 

 for the general equality of gravity measured at sea level over 

 continents and oceans. This discovery is certainly the greatest single 

 achievement of submarine geology. 



In the same year there was a one-day discussion on the floor 

 of the Atlantic held at the Royal Society (Bullard 1954). The 

 essential difference between continent and ocean was summarised 

 by Hess (Fig. 2). The evidence for it, presented by Hill, Laughton 

 and Gaskell, was primarily from explosion seismology. Browne 

 and Bullard examined its consequence, that the oceans must be 



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