Atolls are coral reefs found in the open ocean consisting of an annular rim surrounding a central lagoon. There is a general presumption that atolls have volcanic foundations and some 425 have been recognized around the world (Wiens 1962; McLean and Woodroffe 1994). However, because of similar superficial morphology many shallower water coral reefs have been termed “shelf atolls” (Ladd 1977). In Indonesian waters 55 such reefs have been recognized though many will not have the volcanic foundations of the majority of atolls found in the Pacific and Indian Oceans.
The largest atoll is Kwajalein in the Marshall Islands (120 × 32 km2) followed by Rangiroa in the Tuamotus (79 × 34 km2), though many smaller atolls are only a few kilometers in diameter. In a sample of 99 atolls, Stoddart (1965) indicated a mean area of 272.5 km2, and atolls worldwide are considered to have a total area of 115,000 km2.
The Contribution of Atoll Studies to Marine Geology
reef-building corals grow in shallow (<100 m) water
many atolls rise from depths of several thousand meters
there is a similarity of morphology (annular rim and lagoon) wherever atolls are formed.
Darwin’s hypothesis of atolls evolving from fringing reefs around a volcanic island via a barrier reef stage was the catalyst for subsequent mainstream investigations into glacio-eustatic sea-level change, vertical and horizontal tectonic movements, and the foundations and internal structure of oceanic atolls. Atoll investigations have subsequently played a major part in research into carbonate diagenesis, isostasy, continental drift, plate tectonics, and seafloor spreading. They have generally confirmed the remarkable insight of Darwin regarding the origin of atolls.
The Internal Structure and Origin of Atolls
Darwin recognized that resolution of the problem of atoll development required deep drilling and, after several aborted attempts, T. Edgeworth David of Sydney University in 1896–1898 drilled the atoll of Funafuti to a depth of 339.7 m, all in shallow water coral, but so fragmented that those who opposed subsidence argued that the core passed through a detrital slope deposit. It was to be almost 50 years before drilling totally penetrated the coral of an atoll to the basaltic basement at a depth of more than 1,300 m on Enewetak atoll. Subsequent drilling, much of it associated with atomic tests, at Enewetak and Bikini atolls but also at Midway by the US Geological Survey, and at Muroroa and Fangataufa in French Polynesia, by the French, confirmed both the volcanic foundations of atolls and the depth of in situ coral far below the photic limits of both modern coral growth and growth at glacially lowered sea levels. Volcanic basement was determined at 1,283 and 1,408 m at Enewetak, 415 and 438 m on Mururoa (though seismic results show basalt as shallow as −170 m, Guille et al. 1996), 153 and 503 m at Midway and 270–400 m at Fangataufa. These figures are typical of those from many other atolls, which have now been investigated by drilling or seismic survey.
While shallow water corals have been recognized to depths greater than 1000 m, clearly confirming widespread subsidence, considerable lithification, cementation, and diagenesis has taken place in the lower sections of the atoll foundations, which have been determined as up to 30,000,000 years in age. Alteration of the carbonate rocks results from fluctuations of more than 100 m in sea level with vertical migration of phreatic water table favoring the transformation of the original aragonite of the biogenic structure to calcite. At depths greater than 500 m cold permeating sea water dissolves both aragonite and calcite leaving behind only low-Mg calcite. Dolomitization takes place within thick aquifers of mixed fresh and saline waters and is common in the lowest portions of the atoll foundations.
Like other reefs, atolls were most recently exposed during the last glaciation when karstic processes prevailed on the atoll surface, as in previous low sea-level phases, forming a clearly identifiable “solution unconformity” beneath Holocene reef material which is commonly only 10–20 m thick. Karstic features, including the saucer-like shape of atolls, have long been considered to be the major determinant of the gross morphology of atolls (e.g., see Purdy 1974).
The internal structure and radiometric ages of basalts beneath atolls have been integral parts of the evidence to develop the ideas of plate tectonics and seafloor spreading (e.g., Scott and Rotondo 1983; Guille et al. 1996). Volcanoes develop at oceanic “hot spots” from which they migrate through tectonic plate movement, acquiring coral reefs if they are in warm enough waters. Subsidence takes place due to both volcanic loading and cooling, and reefs and islands pass through the classic Darwinian sequence of fringing, barrier, and atoll formations producing linear chains along the direction of plate movement, such as the Hawaiian and Society Islands. If they are out of the zone of coral growth, subsidence continues and atolls become submerged as guyots, for example, the Emporer Seamounts to the northwest of the Hawaiian chain.
The Surface Morphology of Atolls
The high energy situation of most atolls means that once they reached sea level after their last period of exposure, they have quickly developed strong zonational patterns. Reef slopes commonly display groove and spur structures, reef crests are surmounted by significant algal ridges and reef flats of about 500 m width are a conduit for sediments which form accumulating aprons of sand within the lagoon. Lagoons, with or without patch reefs are often of similar depth, within the range of 10–30 m with only a thick Holocene sediment veneer over the older Pleistocene foundations. Water circulation within the lagoons is complex, with wave setup raising swell side water levels and strong flow over the rim. Water escapes through leeward passes but in many instances a return bottom current back towards the swell side may be present.
Particularly on the swell and/or windward side of atolls, are linear islands termed “motus” separated by narrow passages or “hoa.” The majority are constructed of coral gravel broken from the reef front and deposited a set distance back on the reef flat as the entraining waves lose their transportational capacity. Spectacular episodes of deposition have been described during hurricanes or tropical cyclones but these are not absolutely necessary for motu formation as equatorial atolls such as the Maldives, not experiencing strong, tropical revolving storms, also have significant island development. These islands may be only a metre or so above high tide and are regarded as the coastal land-forms most susceptible to sea-level rise associated with global climate change.
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