Darwin predicted a sunken volcano lies beneath every atoll, and he was right
On the Beagle voyage, Charles Darwin reasoned that ring-shaped coral atolls grow on volcanic islands that slowly sank, the coral climbing upward to stay in the light. He predicted old volcanic rock beneath each lagoon. Later research bore him out, and his logic even explains the Great Barrier Reef.
Coral reefs are built by colonies of tiny polyps, relatives of sea anemones and jellyfish in the phylum Cnidaria, that secrete hard calcium carbonate skeletons. Most grow best in warm, shallow, clear, sunlit and turbulent water, paradoxically thriving where nutrients are scarce. The first coral reefs appeared around 485 million years ago at the start of the Ordovician, replacing Cambrian reefs of microbes and sponges.
Darwin described three stages. A fringing reef grows around a volcanic island; as the island and seabed subside, the reef keeps pace by building on older dead coral, becoming a barrier reef with a lagoon behind it; when the island disappears, a ring of living coral remains as an atoll. If the bottom sinks or the sea rises too quickly, corals that depend on photosynthesising partners can fall too deep for light, a fate called coral drowning.
Most existing reefs are under 10,000 years old, having grown after the last glacial period as meltwater flooded continental shelves. The Great Barrier Reef began roughly 20,000 years ago, when the sea stood 120 metres lower. Rising water turned hills on Australia's coastal plain into islands and then submerged them, letting corals overgrow them. Sea level there has barely changed in 6,000 years, and the living structure is thought to be 6,000 to 8,000 years old.
Reefs cover under 0.1 percent of the ocean, about half the area of France, yet house at least a quarter of all marine species, earning the nickname rainforests of the sea. Estimates of their yearly economic value, from tourism, fisheries and coastal protection, range from tens of billions to several trillion US dollars. Shallow tropical reefs have declined by half since 1950 under pressure from nutrient runoff, warming and acidifying water, destructive fishing and sunscreen.
Source: Coral reef
