Coral atolls are supposed to require a volcano. Darwin's classical explanation demands that a reef accretes around a volcanic island, the island subsides, and the ring remains -- a geological process spanning millions of years. But satellite imagery reveals annular shapes in tiny patch reefs where no volcano ever existed, and nested rings within lagoons that no subsidence model can explain. A new numerical model demonstrates that the coupling between coral growth and water currents alone produces ring-shaped reefs across every spatial scale, from meters to kilometers. Local colonization and mortality depend on resource supply and hydrodynamic stress; the fluid dynamics is solved explicitly. The simulations reproduce not just the qualitative shape but the precise scaling laws and fractal dimensions measured in global reef datasets.
The structural insight is that certain geometries are not artifacts of specific historical causes but attractors of coupled growth-flow systems. The ring is not a fossil record of a vanished island; it is what happens whenever something that grows interacts with something that flows around it. The annular form is overdetermined -- multiple unrelated pathways converge on the same shape because the shape is dictated by the physics of the interaction, not the biography of the substrate. When we see a recurring form in nature and attribute it to a shared ancestor or a shared history, we may be looking at a shared dynamical inevitability -- a geometry that any growth-flow coupling will find, regardless of where it starts.
(arXiv:2603.16933)