The Small Islands of Almost-Atolls

Abstract

THE familiar inductive series of fringing reefs, barrier reefs, and atolls may be further subdivided so as to contain six members: Normal fringing reefs, offshore fringing reefs, narrow-lagoon barrier reefs, broad-lagoon barrier reefs, almost-atolls, and atolls. Almost-atolls, or atoll-like reefs encircling lagoons in which one or several small islands rise, are of interest as affording a critical test of certain competing coral-reef theories, as follows: Murray's theory of outgrowing reefs around still standing islands explains a completed atoll by supposing that the original volcanic island is slowly worn down as the encircling reef grows outward and the lagoon is excavated behind the growing reef by solution, the degraded central island eventually disappearing in away not clearly explained, perhaps by outwash of its detritus from the lagoon by currents which are fed by the influx of surf over the windward reef and discharged by outflow through passes in the leeward reef. Under this theory the small island of an almost-atoll would be a nearly worn-down central island, which would exhibit rolling hills of low relief surrounded by delta flats; or in a later stage, after the delta deposits had been swept away, the low hills of the vanishing island would be encroached upon by the lagoon waves and cut back in low bluffs fronted by low-tide rock platforms that gradually deepen into the lagoon.