Rock Outcrops 



Soundings, dredge samples, rock-damaged core 

 barrels, and photographs have proved the existence of 

 hard rock and other hard surfaces on the sea floor even in 

 deep water. Acoustic probing has verified the occurrence 

 of isolated pinnacles, lava flows, flat-lying buried strata, 

 and partially covered boulders and loose rock at, or not 

 far below, the sediment-water interface. The presence of 

 exposed rock is related to major earth movements, erosion, 

 volcanic activity, changing rates of sediment accumulation, 

 bottom current, and wave action, and it also depends on the 

 major underwater topographic features and superimposed 

 intermediate relief. Usually, rock appears on ledges, 

 ridges, topographic highs, and upper surfaces of seamounts; 

 in trenches, canyons, fault scarps, and rift mountains; 

 and on nearshore shelves, banks, and terraces. But it is 

 probable that outcroppings of nearly all magnitudes occur 

 throughout the entire range of underwater environments. 



Rock relief above existing sediment surfaces varies 

 in height from a few centimeters to thousands of meters. 

 Although still within the definition of microrelief, many 

 outcroppings examined photographically were more than 

 50 cm high. Partially buried boulders can appear as 

 outcrops in bottom photographs, but probably are not 

 continuous for any distance. Distribution of rock outcrop 

 is highly irregular. Laughton 11 found that surface features 

 of seamounts varied considerably over short distances. On 

 a topographic high in the Indian Ocean, the surface at one 

 location changes from cross -rippled sands to a manganese - 

 encrusted bed of nodules or bedrock in a distance of not 

 over 70 meters (fig. 9). Many small isolated local outcrops 

 of a few meters in extent exist in a variety of surrounding 

 areas of different origins. They are in sharp contrast to 

 great underwater mountain ranges many kilometers in 

 length. Depth of water is not a limiting factor. 



Unless turbidity flows are active in an area, currents, 

 wave and eddy action, and gravity pull tend to remove the 

 finer-grained materials from isolated highs by a constant 

 washing action. 19 It is not unusual for these agencies to 

 keep areas clean of fine sediment cover for intermittent 



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