Chapter 15 The rock coasts of oceanic islands

Abstract

AbstractOceanic plates are geologically young, forming at mid-ocean ridges and becoming deeper and older with distance away from these spreading centres, to be subducted into ocean trenches. Most of the islands that occur on these oceanic plates are basaltic, formed at hot spots, and carried into deeper water as the plate migrates. In tropical reef-forming seas, volcanic islands are usually protected by coral reefs, and undergo transition from fringing reefs, to barrier reefs, to atolls, as envisaged by Darwin. Linear island chains comprise volcanic islands at successive stages in the progression from volcano through coral reefs to seamounts and guyots. Erosion occurs rapidly in the early stages once eruption has ceased, and older islands are conspicuously dissected by fluvial action, as observed by Dana. Many are subject to submarine slumping. In the absence of coral reefs, marine abrasion truncates islands, producing near-vertical cliffs, and islands may be entirely bevelled; Balls Pyramid in the southern Pacific appears to be at the penultimate stage of this planation with a broad shelf around it. Coral reefs protect the shoreline, which is usually deeply embayed, with progressive subsidence until volcanic residuals are all that remain on ‘almost-atolls’. Reef limestones indicate earlier phases of reef formation, and there are limestone cliffs around many tropical islands composed of Last Interglacial limestone often veneering older reef terraces. In some cases, the morphology of these limestone coasts contains prominent notches or surf benches reflecting different degrees of exposure to wave energy, or subtle flexure and vertical displacement. Islands provide discrete examples of rocky coasts, with contrasts between adjacent islands, or islands of different ages, providing many insights into the evolutionary stages and the morphodynamics of bold coasts.