Island Origins and Environments

Islands in different parts of the world often show similarities in origin although, conversely, islands that appear superficially similar may sometimes have quite different origins. Commonalities of origin and physical development occur mostly among the community of 'oceanic islands', those islands that originated within the ocean basins (Nunn, 1994). Older 'continental islands' are parts of the continents that have become islands through submergence of continental margins. Most oceanic islands develop either along convergent plate boundaries or in intraplate (mid-plate) locations. Convergent plate boundaries are places where one slab (or 'plate') of oceanic crust (or 'lithosphere') is being thrust reluctantly beneath another. The downgoing plate is eventually pushed so far beneath the Earth's surface that it begins to melt, producing magma that sometimes finds its way back to the ocean floor where it erupts and may eventually produce a volcanic island. There are intra-plate locations ('hotspots') where the Earth's crust is uncommonly thin and where liquid rock from the layer below may push its way to the surface to form a volcanic island. Origins can not always be readily determined from examination of modern, above-sea islands. Often the key to an island's origin lies buried deep beneath a thick cover of younger rocks, sometimes far below sea level, so the use of models of island genesis is common. ... This section discusses the origins of islands by appearance and composition, beginning with nascent ocean-floor islands - from which all oceanic islands developed - through mature above-sea oceanic islands and older sunken islands. These are not primarily age distinctions but developmental stages that may not be attained by every oceanic island. Most oceanic islands are younger than continental islands, the origin of which is that of the continental masses of which they are part.