SECT. 2] OCEANIC ISLANDS, SEAMOUNTS, GUVOTS AND ATOLLS 377 



wind and waves bring a continuous flow of ocean water over the windward 

 reefs to the lagoon. Much of the wind-driven surface water is returned by a 

 slower and thicker bottom current, producing closed circulation. \^on Arx 

 measured these currents in the lagoons of Bikini and Rongelap where he also 

 found evidence of a secondary circulation composed of two counter-rotating 

 compartments (von Arx, 1954). 



Some of the lagoons that have been accurately mapped by close soundings 

 show a shallow terrace (Fig. 5) that separates the encircling reef from the 

 deeper parts of the basin. Irregular depressions on the floors of such terraces 

 may represent sinks that were formed by solution when sea-level was lowered 

 during glacial times (Emery et al., 1954). 



Widely scattered soundings may suggest that the floors of lagoons are fairly 

 flat but in lagoons that have been covered by dense soundings numerous 

 isolated mounds of coral are revealed. These mounds were referred to long ago 

 as coral knolls (Darwin, 1889). Some are small, measuring 10-15 ft across and 

 3-20 ft in height, but much larger ones are numerous and these may be several 

 hundred feet to more than a mile in width and may rise to near low-tide level 

 even from the deeper parts of the lagoon. The 1 8,000 soundings made in the 

 24-mi lagoon of Eniwetok Atoll in the Marshall Islands (Emery et al., 1954) 

 revealed more than 2000 coral knolls. Detailed maps of individual knolls show 

 that the structin*es vary in outline as well as in size. Summits are broad, the 

 sides having slopes of 45-60° with, in some cases, vertical slopes for as much as 

 10 fm of their depth (Fig. 5). Living corals cover up to 75% of the knoll crest. 

 No coral knoll has ever been drilled but Nesterofif (1955) used T.N.T. to blow 

 off the tops of several, finding concentric coral growth. There was no evidence 

 to suggest that the knolls were residuals. In the Maldive Islands coral knolls 

 shaped like miniature atolls are called "faros". 



The coral knolls contribute sediment to the lagoon as do the marginal reefs. 

 Additional sediment is derived from coral thickets on the lagoon floor and 

 from large areas of the bottom that are covered by algal growth {Halimeda). 

 Since very little sediment escapes from the lagoon its floor is slowly raised with 

 the passage of time. 



D. Subsurface Geology 



Most atolls that rise from the deep sea have probably been developed on the 

 tops of volcanoes that may or may not have been truncated by erosion prior 

 to the initiation of reef growth. In the Pacific Basin (inside the circum-Pacific 

 andesite line) the volcanic rock of the atoll foundation is probably all basalt. 

 Rock of this sort has been dredged from the slopes of Bikini, Wotje and Ailuk 

 Atolls in the Marshall Islands (Ladd and Schlanger, 1960) and has been re- 

 covered from a deep drill hole on Eniwetok (Fig. 6). Though the atolls of the 

 Pacific Basin may all have basaltic foundations it is not safe to assume that 

 they have all had similar histories of subsidence and reef growth. The drill holes 

 of Bikini can be tied to those of Eniwetok and the volcanic foundations of 

 these two atolls apparently subsided at similar rates. The same cannot be said, 



