Oceanic islands

Article By:

Winterer, Edward L. Geological Research Division, Scripps Institution of Oceanography, La Jolla, California.

Last reviewed:December 2019

DOI:https://doi.org/10.1036/1097-8542.464200

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; and with this exclusion, 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. On some of these, for example Enewetak Atoll in the western Pacific, the foundations are as much as 8 × 10⁹ years old. See also: Reef; Volcano

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