Atoll

Atoll Formation

Charles Darwin first hypothesized the subsidence theory for the origin of atolls in 1842. Convinced of the existence of seabed subsidence to counterbalance tectonic uplift, his theory of subsidence suggested that atolls are a product of an evolutionary sequence of coral reef formation around the perimeter of subsiding oceanic seamounts and volcanoes. These fringing reefs maintain growth close to the sea's surface, the optimum depth for coral growth being less than 25 m. Over time and keeping pace with the subsiding landmass, these fringing reefs gradually evolve into barrier reefs and with the final submergence of the landmass, result in an atoll (Figure 1). Darwin's theory was supported by the results of drilling in Bikini Atoll in 1947 and Eniwetok Atoll in 1951, both in the Marshall Islands, which found volcanic rock beneath reef limestone at depths of 1,600 and 1,405 m, respectively.

Although Darwin's explanation is widely promulgated, there have been two theories put forth as alternatives to his model of atoll formation by subsidence. Reginald Daly's “glacial control theory” took into account the effects of sea level rise as a consequence of historical glaciations and suggested that atolls were the product of weathering and erosion of marine terraces at times of low sea level. In contrast to Daly's proposition, the “karst control” or “antecedent karst” theory developed by Edward Purdy suggested that at times of low [Page 167]sea level, the exposure of the marine terraces to rainwater would actually result in karst topography. The rim of the marine terrace would result in rainwater runoff from the edge of the marine terrace and concentration of rainwater at the center. These differential rates of solution experienced at the rim and center would result in a depression at the center of the platform that, with subsequent sea level rise, would flood. Coral growth on the peripheral rim would thus form the barrier reef, and flooding of the interior depression in the central lagoon (Figure 2). Debates over atoll formation are ongoing.

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