MAJOR ACCOMPUSHMENTS 75 



include long-range underwater sound transmission, the gravi- 

 tational and magnetic fields of the earth, and emergency solu- 

 tions to problems of salvage and installation of deep-sea en- 

 gineering structures. A sufficiently large number of geophysical 

 measurements have now been made to allow typical charac- 

 teristics of the mantle, crust, and sediments in the oceans to 

 be fairly well known. In fact, these broader structures Eire as 

 well or better known at sea, as a result of these investigations, 

 than they are on land. 



In the most common type of oceanic province, the abyssal 

 plain, the sediments beneath the ocean floor have been found 

 to be flat and undisturbed and about one to three kilometers in 

 thickness. They are commonly underlain by a layer of high 

 surface relief having an intermediate seismic velocity. In other 

 cases they are supported directly by main crustal rock about 

 five kilometers thick. The base of the crust, the Mohorovicic 

 discontinuity, is about ten kilometers below sea level beneath 

 these plains. The lower crustal material has about the same 

 seismic velocity as it does on continents, and is usually con- 

 sidered to have the same composition in both tj^pes of crusts. 

 The mantle beneath the crust also appears to have about the 

 same composition in oceanic and continental regions, since 

 seismic velocities in both regions have the same average values. 



In oceanic ridges and over sea mounts the sediments are 

 usually thin or absent, except in sharp, narrow valleys. Ocean 

 ridges are also characterized by a relatively thin oceanic crust 

 underlain by a mantle layer having a relatively low seismic 

 velocity and low density. Magnetic anomalies over the ridges 

 show characteristic variations, believed by some to be a result of 

 sea-floor spreading. Heat-flow rates are high over the ridges 

 (seismicity is also usually higher than in abyssal plains). 



Similar characteristic features have been determined for other 

 tjrpes of provinces. In oceanic trenches, where water depths are 

 between five and ten kilometers, sediments vary greatly in 

 thickness, as do the depths to mantle rocks. A large negative 

 gravity anomaly exists over the typical trench, indicating either 

 a relative mass deficiency in the mantle, or a great thickness of 



