General DiscussionNone can avoid the visions brought to mind by the mention of tropical isles. Palms whispering in the trade winds, blue lagoons, white surf on the rim of a reef, the everpresent fragrance of flowers, and everywhere the warmth, azure skies, and towering cumulus capping mountain peaks of green, red, and black. It is little wonder that much research has been done on the tropical islands of the world. Yet, even today, hundreds of these pockets of paradise remain little known, as they lie scattered across 190,000,000 square kilometers of the ocean.
Oahu Yap Island Penhryn Atoll
Rangiroa Atoll Groupe Acteon Iles Gambier Malden Atoll
Christmas Island Society Islands Maldive Islands Diego Garcia
Oceanic islands are of two basic types: volcanic, and coral reefs. In the tropical oceans, every volcanic island has some reef formation on its shores. Coral reefs alone are more common, however, than the combination of volcanoes and reefs. In each case, the type of island provides information on the structure and stability of the sea floor on which it lies (Shepard, 1973).
Coral atolls provide good oceanographic indicators. The reefs differ with varying kinds of organic growth, which, in turn, respond to changes in the surrounding ocean. Living reefs lie in waters with mean annual temperatures between 23 and 25 degrees Celsius. Coral reefs are confined, therefore, to the tropical oceans, a benign, gracious, attractive environment.
There are five basic types of coral reefs: (1) fringing reefs, which are connected to an island's shore, (2) barrier reefs, separated from the land or island, (3) atolls, which are somewhat oval-shaped, rise from deep water, and surround a lagoon in which there is no land, (4) faros, ring-shaped reefs and rings within rings, and (5) table reefs, or coral banks, which rise above the sea floor as coral plateaus without rims.
Scientific interest in coral reefs was sparked by the return of Charles Dalwin from his flve-year voyage aboard HMS Beagle. Recognizing that corals grew only in shallow, warm, tropical waters, Darwin could not conceive thousands of sea-floor platforms rising from the depths of the Paciflc and Indian oceans. So, in 1837, he presented a paper to the Royal Society in which he postulated the development first of a fringing reef, which became a barrier reef as the island subsided and the coral continued to grow upward. The flnal stage in Darwin's sequence was the atoll, the remaining remnant of what had previously been an island-coral reef complex.
Darwin's proposal created an astounding uproar among British geologists. The Anglican Church joined in, bitterly and vociferously. The geologists claimed that the ocean basins had been in place since the beginning of time and in no way could have had any vertical movements. The church cried, "Blasphemy!"
Time proved Darwin correct. More than a hundred years later, when U.S. Navy scientists drilled deep holes through the Eniwetock and Bikini atolls, they encountered the exact sequence of rock layers Charles Darwin had predicted.