Atoll

An atoll is a ring island.

It sits in warm ocean water. Most atolls are in the Pacific Ocean. There are about 440 atolls in the world.

A ring of coral makes the island. This ring holds a blue pool. This pool is a lagoon.

Some atolls are very small. They are low to the sea. The islands are made of coral.

Atolls are special places in the sea. They look like bright rings of land.

An atoll is a ring-shaped island. It is made of a rim of coral. This coral ring circles a pool of water. We call this pool a lagoon.

Atolls live in warm ocean waters. They stay in tropical and subtropical areas. Most of the 440 atolls are in the Pacific Ocean. You can find them in the Indian Ocean, too.

How do they form? One idea comes from Charles Darwin. He thought a volcanic island slowly sinks. As it sinks, the coral grows upward. This leaves only a ring of coral and a lagoon.

Another idea is the karst model. This says rain dissolves coral on a flat island. This makes a saucer shape. When the sea rises, the rim becomes an atoll.

Atolls are often very low. Most are less than 10 meters high. Some are very large. Kiritimati is one of the largest atolls by land. Huvadhu Atoll has many islands. It has 255 islands in total.

An atoll is a special kind of ring-shaped island. It is made of a rim of coral that circles a body of water called a lagoon.

These islands only grow in warm tropical or subtropical parts of the ocean. This is because the corals that build them need warm water to thrive. There are about 440 atolls in the world today. Most of them are found in the Pacific Ocean. You can also find them in the Indian Ocean near places like the Maldives.

Scientists have different ideas about how these islands form. One famous idea is the subsidence model. This idea was explained by Charles Darwin. He thought a volcanic island slowly sinks beneath the sea over a long time. As the island sinks, the coral reef grows upward to stay near the sunlight. Eventually, the volcano disappears completely under the water. All that remains is the ring of coral and the lagoon.

Another way atolls might form is through the antecedent karst model. This model suggests that a flat coral reef is first exposed to the air. When it is above the water, rainfall begins to dissolve the coral. The rain dissolves the center of the reef faster than the edges. This creates an island that is shaped like a saucer with a raised rim. When the sea level rises again, the middle floods to create a lagoon.

Atolls are usually very low to the sea. Most of them are less than 10 meters high. Because they are so low, they are mostly made of coral and reef debris. Some atolls are much larger than others. Kiritimati is known as one of the largest atolls by land area. Another atoll called Huvadhu Atoll in the Maldives is unique because it has many islands. It is made up of 255 individual islands.

Even though they look similar, not every reef is an atoll. For example, Bermuda is sometimes called a pseudo-atoll. It looks like an atoll, but it formed in a very different way. In Bermuda, the water is kept warm by the Gulf Stream. This allows coral to grow even though it is not in the tropics. Atolls are amazing examples of how living things and changing oceans work together.

An atoll is a unique, ring-shaped island system found in the ocean. It consists of a coral rim that encircles a central body of water known as a lagoon.

These structures are composed of coral islets or cays located on the rim. Modern definitions describe them as annular reefs that enclose a lagoon. In these lagoons, there are no land features except for reefs and small islets made of reef detritus. Because they rely on reef-building corals, atolls only exist in warm tropical or subtropical waters.

There are two primary scientific models used to explain how atolls develop. The first is the subsidence model, famously proposed by Charles Darwin. This model suggests a sequence of gradual changes starting with a high volcanic island. A fringing reef grows around the edges of this volcano. As the volcanic island becomes extinct, it begins to sink or subside beneath the ocean surface. Over geologic time, the reef grows upward to stay near the sunlight. The reef eventually becomes a barrier reef that is detached from the sinking island. Eventually, the volcano disappears entirely, leaving only the coral ring and a lagoon.

For an atoll to remain stable, the coral growth must match changes in sea level. This includes the sinking of the island or the rising of the oceans. If the coral cannot keep pace, the structure may become a seamount or a guyot. An island where reef growth just barely matches the rate of subsidence is called the Darwin Point. In colder regions, islands evolve into seamounts. In warmer, more equatorial regions, they evolve into atolls, such as Kure Atoll.

The second explanation is the antecedent karst model. This model suggests that atolls are Pleistocene features resulting from sea-level changes. First, a flat-topped coral reef develops during the subsidence of an oceanic island. When the sea level drops, this reef is exposed to the atmosphere. Rainfall then begins to dissolve the limestone through a process called karst dissolution. This dissolution is not uniform across the reef. The rate of dissolution is lowest along the rim. The rate increases toward the center, reaching its maximum there.

This creates a saucer-shaped island with a raised rim. When the sea level rises again, the rim provides a rocky core for new coral growth. The flooded center of the saucer then becomes the lagoon.

Atolls are widely distributed across the globe, with approximately 440 existing today. Most are located in the Pacific Ocean. Large concentrations exist in the Marshall Islands, the Tuamotu Islands, and the Caroline Islands. The Indian Ocean also holds many atolls, including the Maldives and the Chagos Archipelago. Indonesia has several atolls in the Raja Ampat and Thousand Islands regions. The Atlantic Ocean has very few, with only eight atolls near Nicaragua belonging to Colombia.

Atolls vary greatly in size and physical characteristics. Most are very low-lying, with elevations usually less than 10 meters. Kiritimati in Kiribati is cited as one of the largest atolls by land area. In the Maldives, Huvadhu Atoll is notable for its complexity, containing 255 individual islands. Some atolls are even classified as "raised coral atolls," such as Lifou in New Caledonia. Others, like the Great Chagos Bank in the Indian Ocean, are massive submerged structures covering 12,642 square kilometers.

Beyond their shape, atolls are important for complex chemical processes. They serve as sites where the dolomitization of calcite occurs. This is a process where magnesium from seawater replaces calcite to form dolomite. Scientists study various models for this, including the seepage-reflux and mixing-zone models. This shows that atolls are not just landforms, but active parts of the ocean's chemical and biological systems.