Ring-shaped reefs, atolls, are usually explained like this: corals grow along the edge of a volcanic island while the island slowly sinks. What's left is a hollow ring that can stretch for kilometers. But satellite images show similar rings around very small reefs where there are no volcanoes and no subsidence. Sometimes small rings appear inside larger atolls, like rings within rings.
The authors of the new study decided to test whether rings can arise on their own — from the interplay of coral growth and ocean currents. They built a computer model: corals grow by simple rules, die from lack of food or too strong a flow, and water moves according to the laws of physics. They ran the simulation, and got ring-shaped reefs of various sizes: from small patches to large atolls. The shapes matched the real ones.
The model didn't just draw similar pictures. It reproduced numerical patterns from global databases: how reef size relates to its shape, and what its fractal dimension (a measure of contour ruggedness) is. This means ring-shaped reefs aren't always a volcanic legacy. Sometimes the ocean itself sculpts them.
This mechanism explains why rings appear at very different scales — from meters to tens of kilometers. Nature uses the same recipe: growth plus current. And then an atoll isn't a monument to a sunken volcano, but a living pattern drawn by water.
Fractal dimension is a number that shows how rugged a contour is. For ring-shaped reefs, it's close to 1.3—similar to Norway's coastline.