A ring-shaped coral island found far from land is called an atoll.

Here's how atolls form:

1. Volcanic Activity: An underwater volcano erupts and builds up a cone.

2. Coral Growth: As the volcano becomes dormant or extinct, coral reefs begin to grow around the rim of the island.

3. Subsidence: The volcanic island slowly sinks back into the ocean, often due to tectonic activity.

4. Atoll Formation: The coral reef continues to grow upwards, eventually forming a ring of islands with a lagoon in the center.

Let me know if you have any more questions about atolls!