Coral, when it occurs as a secondary foirmation, as for instance 

 atop a non-submerged, subaerially-formed basalt flow, will grow usually 

 as separate and distinct coral heads with spaces two feet across and up 

 to four feet deep between. Again, positioning of level, stable founda- 

 tions on such surfaces would be most difficult; it would be much simpler 

 to relocate to nearby, more level areas. 



Other Seafloor Rock 



Rock is found in other seafloor environments: in submarine canyons, 

 in deep-sea trenches, on fault scarps, and as erratics dropped to the 

 seafloor. In the case of submarine canyons, rock often crops out on 

 the walls. Rock types range from intrusive igneous, such as granite, 

 through soft sedimentary. Sediment creep, sediment flow, and turbidity 

 currents are serious problems in submarine canyons and largely preclude 

 canyons as sites for bottom-resting installations. Rock outcropping in 

 the deep sea trenches is primarily igneous, ranging from basalts to 

 peridotite and dunite, with some outcrops of fragmented or brecciated 

 rock. Trench slopes are locally steep with pockets of angular talus. ^^ 

 Design considerations will include those for seamounts and ridges with 

 the addition of earthquake and turbidity current considerations.^ 



Ice-rafted rocks occur often on the seafloor. Photographs from 

 the Arctic show rocks occurring more often on higher ridges, presumably 

 because higher current velocities minimize sediment deposition.'^-'- 

 Photographs of the Thresher search area, at 2500 meters, from the base 

 of the continental slope to the upper part of the continental rise, 

 indicate that ten percent of the bottom area is made up of rocks, pre- 

 sumably ice-rafted. Most of the rocks are gravel to cobble-sized; 

 boulders larger than one meter are not uncommon, and one photo shows a 

 boulder exceeding three meters in diameter.-*- Foundation design in 

 areas of ice-rafted rocks would appear similar to that in nodule areas. 



Rock is also exposed in areas where permanent currents and tides 

 prevent deposition or cause erosion. The Gulf Stream in crossing the 

 Blake Plateau has had sufficient force to prevent deposition to 600 

 fathoms; sedimentary rocks of Tertiary age have been recovered. Tides 

 have swept the floors of trenches in the Gulf of California, exposing 

 a bed of boulders four meters in diameter at 400 fathoms. 2 The topog- 

 raphy of these environments is essentially no different from that to be 

 found on submarine volcanoes; however, the large currents do pose a 

 significant lateral loading problem. 



ROCK FOUNDATION CONCEPTS 



The placement of a bottom-resting structure — whether it be a 

 500-pound hydrophone or a 15-ton, 10-foot-high radioisotope thermo- 

 electric generator — on one of the seafloor rock environments described 

 earlier, requires a foundation system to: (1) support the structure, 

 (2) restrict and possibly prevent rocking motions, and (3) prevent 



