An atoll reef is essentially a type of reef that was formed surrounding a body of water such as a lagoon. These types of ring shaped coral formations often form tiny islets that protect the lagoons and the central island behind it.
Formed through a complex process initiated by underwater volcanoes, atolls develop slowly, over millions of years, and are now among the most prevalent types of coral reef formations in the world.
Seamounts, or underwater volcanoes, play an important role in the formation of atoll reef formations. These ring shaped corals are formed after a volcanic island is built up gradually by cooling lava on the seabed. Once the island has formed, breaking the surface of the water, the island is then ready for the next stage in which the slow growth of atolls can begin.
That being said, the stages of an actual atoll reef’s development are as follows:
- The first step is made by tiny animals known as corals that begin to form the reef structure around the island. These are known as hard corals, due to their ability to form hardened, limestone-based exoskeletons that maintain the structural integrity of the reef over time.
- Fringing reefs are the first that form, remaining relatively close to the island’s shoreline, and being maintained close to the surface of the water. A shallow strip of water known as a lagoon forms between the island and the reef structure.
- The atoll reef is then enhanced to form a structure similar to a barrier reef, as the seamount slowly erodes over the course of millions of years, and subsides into a flat-topped structure known as a guyot.
- Finally, the reef is pushed farther out from the shoreline, and the lagoon grows in size. As waves and storms pound the coral formation over time, pieces and parts of the reef are dislodged, and the sand is later deposited by waves to form the small islets that are the major characteristic of what makes atolls unique.
One of the main characteristics of an atoll reef is that the entire process of its formation may take as long as 30 million years to complete. This is significantly longer when compared to many other types of reefs, being accounted for by the reefs’ unique structure and form.
Atolls are also roughly circular in all cases, being formed around a seamount, and in many places in the Pacific, they can also occur mid-ocean. A few examples of these types of reefs can be found in the Chagos and Maldives island groups, as well as the Cocos Islands and Seychelles.
Another important trait of atoll reef formations is that they are quite difficult to spot. While still keeping the shoreline relatively protected over time, they have also presented significant problems to ships sailing to volcanic islands, many of them having become shipwrecked due to being unable to avoid the shallow, but sharp reef structures and islets hidden by the waves.
Atoll reef formations are significantly more well-known and prevalent than barrier reefs and other common types of reefs, being among the unique characteristics that set volcanic islands apart from other land masses.