Atolls – Special Coral Islands

Indonesia is the worlds largest archipelago. This means it’s a country of thousands of individual islands spread over thousands of kilometers. Although reports vary, the number of islands in the country numbers over 17,000 with around 1000 inhabited islands. The islands measure from big to small, including several of the worlds largest islands. The islands are a mix of volcanic and karst as well as the large continental islands such as Papua and Borneo. However, one of the most interesting island formations is the atoll. Indonesia is home to many atolls throughout the country.

What is an Atoll?

An atoll is a large ring of islands surrounding a lagoon. It can be made of one ring of unbroken land or it can be made up of dozens or even hundreds of small islands surrounding the lagoon. Atolls vary in size from a few hundred square meters to thousands of square kilometers of lagoon size. There are only 400 + atolls in the world and they are mainly located in the Indian and Pacific Oceans, with only eight in the Caribbean. As Indonesia is located at the boundary of the Pacific and Indian Oceans, it’s also home to a variety of these interesting islands.

Coral Atolls
NASA/Johnson Space Center, Image Science & Analysis Laboratory, Public domain, via Wikimedia Commons

How do they Form?

It was Charles Darwin who first postulated how atolls form due to information he collected during his famous five year voyage on the Beagle. He noted the existence of many different kinds of islands including atolls and near atolls such as Bora Bora. What he noticed was that these islands consisted of coral limestone, meaning they were made of coral reefs. As coral reefs are alive, they are constantly growing toward the surface. This led to his theory of island formation which states that atolls start life as a volcano with a coral reef growing along the edges. As eons pass the volcano becomes extinct and slowly subsides while the coral continues to grow toward the surface. Eventually the peak of the islands subsides below the surface and all that is left is the barrier reef. The reef creates a ring like shape with a protected lagoon inside.

Coral Atoll Formation
Coral Atoll Formation: Susan Mayfield and Sara Boore, modified by Eurico Zimbres, Public domain, via Wikimedia Commons

As the millenniums pass the coral ring continues to grow but the wave action affects the land mass. Often breaks are created in the ring making passes in and out of the lagoon. In some cases entire islands disappear due to erosion and the atoll is no longer a perfect ring but may only be a semi circle or a dotted line of islands.
One interesting note about atoll formation is the fact they are only located in the tropics. Atolls are made of corals and corals only grow in tropical water. The northernmost atoll in the world is Kure Island which is part of the Hawaiian Islands. The southernmost is along on the east coast of Australia, north of Lord Howe Island.

Atolls in Indonesia

Coral growth on Atolls
Typical Coral Growth Outside of an Atoll

There are not many of the large atolls in Indonesia that are commonly found in the Marshall Islands, French Polynesia, or the Maldives. However, Indonesia does boast one of the largest in the world. The atoll of Sabalana lies north of Sumbawa and south of Sulawesi and measures over 2500km2. Other smaller atolls dot the landscape in the Flores Sea due to the history of volcanism in this area. The lagoons often offer a calm anchorage with great conditions for water sports. One of the features of an atoll is they often offer good diving.

Diving on Atolls

There are two main features of diving on an atoll. The first is the coral reefs surrounding the entire area. They are often steep slopes or vertical walls that scuba divers love. Sea fans and soft corals are the main attractions of walls with sighting of large marine life in the blue. The other feature of an atoll is diving in the channel. Most of these island structures feature a pass that cuts through the island forming a channel.

These channels are one of the best places to dive, especially when the tide washes into the lagoon. The strong currents are a magnet for large fish and sharks as well as eagle rays and mantas. The nutrients that wash in with the tide are the base of the food chain and all of the resident fish gather to feed. These channel dives are some of our favorite dives anywhere and we think you will enjoy them too!

Atolls and coral
Atolls Feature Channels leading into the Lagoon

If you are interested in geography or oceanography then you will be interested in atolls. Their formation is amazing to understand and exploring one in person is something everyone should have the chance to do. If you are a diver then coral atolls are something you have heard about and would love to explore. Why not join us on the Samata Luxury Yacht and explore the ocean in style? We are taking bookings for 2022 and would love to share the wonders of Indonesia with you.

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