One hundred and thirty years since Darwin: ‘Reshaping’ the theory of atoll formation

Abstract

April 2012 marked the 130th anniversary of the death of Charles Darwin. One of many significant contributions he made to science was the subsidence theory of atoll formation, which he penned on 12 April 1836 during the voyage of the Beagle through the Pacific. Darwin’s elegant theory, founded on the premise of a subsiding volcano and the corresponding upward growth of coral reef, was astonishing for the time considering the absence of an underpinning awareness of plate tectonics. His theory has endured until modern times in spite of a number of opposing ideas and permutations and has an enviable longevity amongst paradigms in geomorphology. Darwin frequently alluded to the generally circular morphology of the atoll shape, yet the reality is that many atolls are neither circular nor elliptical, instead possessing irregular morphologies. In particular, many exhibit major arcuate ‘bight-like’ structures (ABLS) in their plan form. These departures from the circular form are indicative of geomorphological processes that cannot be ignored. ABLS are the morphological expression of large submarine failures that are common on the slopes of volcanic edifices. Such failures can occur during any stage of atoll formation and are a valuable addition to Darwin’s theory because they indicate the instability of the volcanic foundations. Moreover, ABLS have fundamental implications for hazard research in the context of oceanic islands. Not only does our extension to the theory explain the diversity of atoll shape, but it also provides a mechanism for identifying a vast number of potential local tsunamigenic sources, which is critical for advancing modern understanding of tsunami hazards in oceanic environments.