Book 6, Question 93
Why Do Some Reefs Grow in Rings?
A ring reef is called an atoll. It starts as coral growing around a volcanic island. Over a very long time the island sinks, but the coral keeps growing up, leaving a ring behind.
From the book
Page from A Million Whys Book 6
More fun facts
- Ring-shaped reefs called atolls start as reefs that fringe volcanic islands.
- If the island sinks below sea level while the coral keeps growing upward, a ring of coral is left behind.
- Charles Darwin came up with an early idea about how atolls form.
- Some geologists think rising and falling seas shaped atolls, with corals growing most on the raised rim where light is best.
- Past Kauai in Hawaii, islands are slowly sinking, and coral reefs have grown around their volcanic cores.
Sources: U.S. Geological Survey, NOAA Ocean Service, Smithsonian Magazine
Questions kids ask
What is an atoll?
A ring-shaped coral reef, often with a lagoon in the middle.