When Does an Atoll Start to Form?

An atoll or coral reef begins to form when tiny marine animals called coral polyps attach themselves to rocks on the sea bed. The reef, which eventually takes the shape of a ring or horseshoe enclosing a lagoon, is made up of the lime-based skeletons of innumerable polyps. After building skeletons round themselves, coral polyps produce new polyps, which in turn surround themselves with skeletons.

The young polyps remain attached to the parents so that succeeding generations combine to produce a great mass of coral. Atolls occur only in shallow water where the temperature is over 65 degree Fahrenheit (18.3 degree Celsius). They are found in the West Indies, the Indian Ocean, along the coast of Brazil and, notably, in the Pacific. North-east of Australia huge reefs have formed and some atolls may stretch 40 miles in diameter and more than a thousand feet in depth.

Atolls are rings of coral that create protected lagoons and are usually located in the middle of the sea. Atolls usually form when islands surrounded by fringing reefs sink into the sea or the sea level rises around them (these islands are often the tops of underwater volcanoes). The fringing reefs continue to grow and eventually form circles with lagoons inside.

In 1842, Charles Darwin explained the creation of coral atolls in the southern Pacific Ocean based upon observations made during a five-year voyage aboard 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 hermatypic (reef-building) 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.

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. [Dolomitization is a geological process by which the carbonate mineral dolomite is formed when magnesium ions replace calcium ions in another carbonate mineral, calcite.]

At certain depths water is under saturated 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.

In 1896, 1897 and 1898, the Royal Society of London carried out drilling on Funafuti atoll in Tuvalu for the purpose of investigating the formation of coral reefs to determine whether traces of shallow water organisms could be found at depth in the coral of Pacific atolls. This investigation followed the work on the structure and distribution of coral reefs conducted by Charles Darwin in the Pacific.

The first expedition in 1896 was led by Professor William Johnson Sollas of the University of Oxford. The geologists included Walter George Woolnough and Edgeworth David of the University of Sydney. Professor Edgeworth David led the expedition in 1897. The third expedition in 1898 was led by Alfred Edmund Finckh. In conclusion, now we know when does an atoll starts to form.

Content for this question contributed by Natalie Abram, resident of Omaha, Douglas County, Nebraska, USA