Oceanic islands

(geology) Any island which rises from the deep-sea floor rather than from shallow continental shelves.

Islands rising from the deep sea floor. Oceanic islands range in size from mere specks of rock or sand above the reach of tides to large masses such as Iceland (39,800 mi² or 103,000 km²). Excluded are islands that have continental crust, such as the Seychelles, Norfolk, or Sardinia, even though surrounded by ocean; all oceanic islands surmount volcanic foundations. A few of these have active volcanoes, such as on Hawaii, the Galápagos islands, Iceland, and the Azores, but most islands are on extinct volcanoes. On some islands, the volcanic foundations have subsided beneath sea level, while coral reefs growing very close to sea level have kept pace with the subsidence, accumulating thicknesses of as much as 5000 ft (1500 m) of limestone deposits between the underlying volcanic rocks and the present-day coral islands. See also Reef; Volcano.

Oceanic islands owe their existence to volcanism that began on the deep sea floor and built the volcanic edifices, flow on flow, up to sea level and above. The highest of the oceanic islands is Hawaii, where the peak of Mauna Kea volcano reaches 14,000 ft (4200 m). Most volcanic islands are probably built from scratch in less than 10⁶ years, but minor recurrent volcanism may continue for millions of years after the main construction stage. See also Volcanology.

Islands in regions of high oceanic fertility are commonly host to colonies of sea birds, and the deposits of guano have been an important source of phosphate for fertilizer. On some islands, for example, Nauru in the western equatorial Pacific, the original guano has been dissolved and phosphate minerals reprecipitated in porous host limestone rocks. The principal crop on most tropical oceanic islands is coconuts, exploited for their oil content, but some larger volcanic islands, with rich soils and abundant water supplies, are sites of plantations of sugarcane and pineapple. Atoll and barrier-reef islands have very limited water supplies, depending on small lenses of ground water, augmented by collection of rainwater. See also Atoll; Island biogeography; Reef.