An atoll is a ring-shaped coral reef which has broken the ocean surface and formed a barrier around a small, relatively shallow part of the ocean. The enclosed part of the ocean is known as an atoll lagoon. The atoll may be a string of coral islands which rises from a submerged but continuous coral reef. Older atolls may have risen above the ocean surface enough to close off the lagoon from the outside ocean. Over time, parts or all of the reef may rise high enough to sustain terrestrial ecosystems.
Conditions under which an atoll can form
A coral atoll can only form in warm, tropical saltwater, ideally between 65 and 86 degrees F. These sea temperatures are most stable between 30 degrees north and 30 degrees south of the equator.
Reef-building coral depends on a symbiotic relationship with photosynthetic microalgae, so the reef cannot form below the photic zone. If the water is too deep for sunlight to penetrate, there can be no photosynthesis. At the same time, coral reefs depend on wave action and surf to bring in nutrients.
Thus, before a reef can grow, there must first be a sloping land mass near the surface of the ocean to anchor that reef. For a ring-shaped atoll to result, that landmass can’t be bigger than a single mountain, which must also be clearly separated from other oceanic islands by ocean below the photic zone. This will keep the growing reef from interacting with other reefs.
However, the central land mass can’t remain stable or growing. This would result in a growing island, not an atoll. For an atoll to form, the central land mass must subside below the ocean surface after the coral reef has been initially established.
The most common land formation which meets all these requirements is an oceanic volcano which grew up from the ocean floor. The volcano can grow slowly or quickly into its maximum height, which must break the ocean surface so that the coral can grow in a ring in the shallow sloping seafloor around the volcano. After its major growth, the volcano must then become extinct or dormant enough to be stable for the purpose of anchoring coral formation. The coral atoll itself cannot form until the volcano is completely extinct, so that it can subside back down below the ocean surface.
This subsidence should not be confused with the collapse of land into the ocean in a volcanic caldera. While this process also results in a ring-shaped archipelago, the islands of a calderic archipelago, such as Santorini, are made of igneous rock, not coral limestone. The volcanic slopes of Santorini do anchor reef coral, but it can’t grow into an atoll as long as Santorini is an active volcano.
Formation of a coral atoll
After a suitable seamount has reached the photic zone, reef coral can begin to grow on its slopes. The first stage in the formation of a coral atoll happens when the seamount breaks the ocean surface. This limits the area available for the growth of reef coral to a ring around the seamount.
This coral ring is known as a fringing reef. It can take up to 10,000 years for a fringing reef to grow completely around its anchoring island, and as much as 100,000 years for the coral to grow into a barrier reef around the island. The barrier reef is the second stage in the formation of a coral atoll.
The third stage happens when the central volcano begins to subside. The barrier reef must be established before this point, so that the underlying coral limestone is capable of becoming the primary support which keeps the coral in the photic zone. For the coral reef to remain viable, the volcano must not subside faster than the reef coral can grow toward the sunlight. A coral atoll cannot form at all if the barrier reef has not been established before subsidence begins.
It can take as long as 30 million years before the extinct volcano vanishes again below the ocean surface. During this time, the coral is constantly vulnerable to extreme storms and tectonic activity. If either of these lowers the living part of the coral below the photic zone, the reef will die.
After the volcano has sunk completely below the waves, all that remains above the ocean surface is a ring of coral with a lagoon of protected water in its center. This is the coral atoll.
Young coral atolls are extremely vulnerable to storms. However, coral atolls which survive for long enough can build up deposits of sand and organic matter, which also raise it higher above sea level and give it an increasing amount of protection from wave action. In time, all or parts of the raised coral atoll may grow high and wide enough to sustain fully terrestrial ecosystems.
Alternate theories of coral atoll formation
The dominant theory of coral atoll formation is volcanic island subsidence, as previously described. This theory was originally proposed by Charles Darwin, and is generally supported by the discovery of submarine mountains in the middle of coral atolls.
However, other factors may also play a role in coral atoll formation. Fluctuating sea levels definitely have an influence on coral reefs. Rising sea levels may even have directly caused the formation of some coral atolls.
In the absence of a central volcano, reef coral may colonize submarine limestone karst formations, similar to those found in limestone caves. In karst, the interaction of mild acidity with limestone makes the limestone even more porous than usual, to the point where light can filter into protected pockets of rock. This would enable reef coral to use the karst instead of an extinct volcano as an anchoring point.