Atoll - Formation


See also: Formation of coral reefs

In 1842 Darwin explained the creation of coral atolls in the southern Pacific Ocean based upon observations made during a five-year voyage aboard the HMS Beagle from 1831 to 1836. Accepted as basically correct, his explanation involved considering that several tropical island types—from high volcanic island, through barrier reef island, to atoll—represented a sequence of gradual subsidence of what started as an oceanic volcano. He reasoned that a fringing coral reef surrounding a volcanic island in the tropical sea will grow upwards as the island subsides (sinks), becoming an "almost atoll", or barrier reef island, as typified by an island such as Aitutaki in the Cook Islands, Bora Bora and others in the Society Islands. The fringing reef becomes a barrier reef for the reason that the outer part of the reef maintains itself near sea level through biotic growth, while the inner part of the reef falls behind, becoming a lagoon because conditions are less favorable for the coral and calcareous algae responsible for most reef growth. In time, subsidence carries the old volcano below the ocean surface and the barrier reef remains. At this point, the island has become an atoll.

Atolls are the product of the growth of tropical marine organisms, and so these islands are only found in warm tropical waters. Volcanic islands located beyond the warm water temperature requirements of reef building, or hermatypic, organisms become seamounts as they subside and are eroded away at the surface. An island that is located where the ocean water temperatures are just sufficiently warm for upward reef growth to keep pace with the rate of subsidence is said to be at the Darwin Point. Islands in colder, more polar regions evolve towards seamounts or guyots; warmer, more equatorial islands evolve towards atolls, for example Kure Atoll.

  • Darwin’s theory starts with a volcanic island which becomes extinct

  • As the island and ocean floor subside, coral growth builds a fringing reef, often including a shallow lagoon between the land and the main reef

  • As the subsidence continues the fringing reef becomes a larger barrier reef farther from the shore with a bigger and deeper lagoon inside

  • Ultimately the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon

Reginald Aldworth Daly offered a somewhat different explanation for atoll formation: islands worn away by erosion, by ocean waves and streams, during the last glacial stand of the sea of some 900 feet (270 m) below present sea level developed as coral islands (atolls), or barrier reefs on a platform surrounding a volcanic island not completely worn away, as sea level gradually rose from melting of the glaciers. Discovery of the great depth of the volcanic remnant beneath many atolls such as at Midway Atoll favors the Darwin explanation, although there can be little doubt that fluctuating sea level has had considerable influence on atolls and other reefs.

Coral atolls are also an important place where dolomitization of calcite occurs. At certain depths water is undersaturated in calcium carbonate but saturated in dolomite. Convection created by tides and sea currents enhance this change. Hydrothermal currents created by volcanoes under the atoll may also play an important role.

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