Introduction

Coral atolls consist of an annular reef rim surrounding a central lagoon. On the atoll rim there may be reef islands either sandy cays or shingle motu (Stoddart & Steers, 1977). They appear particularly fragile constructions, exposed to a range of oceanographic, atmospheric and anthropogenic processes, and yet the prototypes of present day sea-level atolls have endured in a variety of forms for millions of years. Of the 425 atolls in the world (Stoddart, 1965), most are in the Indian and Pacific Oceans, though isolated atolls do occur in the Caribbean (e.g. Stoddart, 1962a)

The evolution of atolls is of particular relevance in the context of coastal evolution studies, because one of the most central theories of atoll evolution dates back to Charles Darwin and the voyage of HMS Beagle. This voyage gave rise to the concept of natural selection and evolution of species, with the publication of On the Origin of Species in 1859, but Darwin's view on evolution of atolls, was published much earlier with his observations on the geology of reefs (Darwin, 1842).

Darwin had formulated his theory of reef development shortly after leaving South America, and before seeing the reefs of the Pacific. He refined his ideas as a consequence of visiting reefs in the Society Islands, and had completed a draft of his theory before reaching New Zealand (see Stoddart, 1962b). The only atoll that Darwin landed on was the Cocos (Keeling) Islands in the Indian Ocean, and at that stage he was keen to verify his intuition.