Step 1 – Volcanism
Do you know what Coral Atoll Islands are? If you want to make your own atoll, or just understand how existing ones were formed, the first step is to find a volcano. Out in the ocean, volcanic islands form when molten rock spews up (from hot spots or the edges between tectonic plates) and builds up through the seawater and higher than the surface of the ocean. Remember that volcanic islands are usually more or less round if seen from above!
Step 2 – Coral Reef Formation
Now that you have an island, the next thing you need is for coral reef animals (corals, shellfish, sea urchins, sponges, etc.) to start building a reef in the shallow water around the volcanic island, usually after the volcano stops being active. This is called a fringing reef, since it will be built by all of these small creatures on the fringe or outer edge of the island.
Step 3 – Subsidence
When the volcano is good and extinct, two things start to happen to it to make it shrink back down. First, normal wind and rain will erode the volcano, slowly changing it from a tall pointy mountain, to a shorter, rounded hump. Second, the hot chamber of molten rock (magma chamber) under the volcano will move or simply go cold, and as it cools it will shrink. This causes the volcano to subside, or shrink back down into the ocean water. As the volcano slowly disappears back into the water, a ring of coral reef is left behind – and that’s how to make an atoll!