Answer: Atoll
Step-by-step explanation:
An atoll is a group of islets surrounding a water body. It is formed when a volcano erupts beneath water. The process of an atoll creation start from a piling up of lava from a volcano erupting until the lava is elevated enough to emerge from the water. This forms an island.
After several thousands of years, the island wears away and parts of it break apart to descend into the ocean. The remaining islets form an atoll.