Coral reefs and atolls are where we love to dive. Tropical water, vibrant marine species, and a gorgeous vacation vibe make for the perfect destination. We use these terms all the time, but what exactly is an atoll, and how are they formed?
The process of atoll formation can take up to 30 million years, and it all begins with a submerged volcano. Each time the volcano erupts it spews lava out onto the seabed. Over time these lava piles build up on top of one another until they beak the surface of the ocean creating a volcano oceanic island.
The submerged walls of these islands provide the perfect substrate for microscopic corals to attach and start the process of reef-building. Billions of hard corals create a ring around the volcano top, their hard limestone skeleton providing the building blocks for still more corals and other creatures to attach. Of course, this only occurs in tropical waters where the water temperature is warm enough for corals to survive.
A fully developed reef that encircles an island is known as a fringing reef, and it can extend from just below the water surface to depths of 165 feet (50m). Within the fringing reef, nutrients are sparse and conditions less favorable for coral growth, and the bright teal color of the inner lagoon is due to decaying limestone from the natural decomposition of the inner corals.
The volcanic island, now extinct, will erode over millions of years and eventually disappear back below the surface of the ocean. The top of the volcano becomes flattened due to constant wave action, and the ring-shaped fringing reef left behind becomes a barrier reef. The barrier reef protects the now deeper inner lagoon from the worst of the ocean weather and waves, however, these conditions are also what keeps the outer reef healthy.
In the final stages in atoll formation, the constant pounding of the ocean starts to break apart the corals, eroding them into grains of sand which are deposited and build up against the reef. Over time, the sand and other organic matter build-ups to form a ring-shaped island around the reef, known as an atoll.
Today, there are thousands of diveable atolls in different stages of formation around the world. Some of the newest volcanic islands are in Tonga, where Metis Shoal was formed in 2019, and the Solomon Islands, where the submarine volcano Kavichi last erupted in January 2021.
Far older atolls exist in regions of the Philippines or Indonesia. Tubbataha Reef is an atoll in the final stages of development, characterized by extremely well-colonized reefs and a thriving marine ecosystem.