Volcanic islands can develop a fringing reef of coral that persists as the volcano submerges, transitioning first to a barrier reef and later to an atoll at the island sinks below the waves. But how does the coral manage to keep even with the sea surface?

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Moorea Island has a reef that is mostly a barrier, but occasionally fringes the volcano. Image credit: NASA

As a living colony that builds mineral structures, coral reefs toy with the overlap between biology and geology. They need light and clear water to grow, yet cannot survive above the ocean waves. How do reefs constantly stay at sea level when that level rises and falls, or the ocean floor subsides below them?

If the reef is left poking above the waves during a drop in sea level, the exposed surface is quickly eroded. But how does it grow vertically to keep up with a rising sea level? Jason Goldman, the Animals Recruit, wrote a piece for Conservation Magazine on new research about the evolution of island reefs over time. He writes that coral reefs don't grow vertically up, but creep along the sea floor, climbing slopes to stay even with sea level:

[A]s sea level rises, reefs follow the shorelines, growing along the seafloor itself as it rises towards the island. The increase in sediment that results from the upslope movement prevents a type of fast-growing corals called acroporids from dominating the reef, being replaced by other slower-growing corals.

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Eventually as sea levels rise and drop, the creeping reef is going to encounter the remnants of a reef-gone past. When it does, it's arrested from further creeping, but at the same time acts as a barrier slowing the influx of sediment from the eroding island. Without smothering sediment, the fast-growing acroporids surge upwards, transforming the structure from a fringing reef to a barrier reef with an internal lagoon.

Darwin got the broad-strokes right, but the details are more complex than he imagined. Read the full story here.

You might also be interested in the beauty of islands, or about the geologic end-point of reefs, limestone.

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