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If the land subsides slowly, the fringing reefs keep pace by growing upwards on a base of older, dead coral, forming a barrier reef enclosing a lagoon between the reef and the land.
A barrier reef can encircle an island, and once the island sinks below sea level a roughly circular atoll of growing coral continues to keep up with the sea level, forming a central lagoon.
The corals could then overgrow the hills, forming the present cays and reefs.
Sea level on the Great Barrier Reef has not changed significantly in the last 6,000 years, Although the Great Barrier Reef formed along a continental shelf, and not around a volcanic island, Darwin's principles apply.
Like sea level rise, a rapidly subsiding bottom can overwhelm coral growth, killing the coral polyps and the reef, due to what is called coral drowning.Where the level of the underlying earth allows, the corals grow around the coast to form what he called fringing reefs, and can eventually grow out from the shore to become a barrier reef.Where the bottom is rising, fringing reefs can grow around the coast, but coral raised above sea level dies and becomes white limestone.Often called "rainforests of the sea", shallow coral reefs form some of the most diverse ecosystems on Earth.
They occupy less than 0.1% of the world's ocean surface, about half the area of France, yet they provide a home for at least 25% of all marine species, Paradoxically, coral reefs flourish even though they are surrounded by ocean waters that provide few nutrients.They are under threat from climate change, oceanic acidification, blast fishing, cyanide fishing for aquarium fish, sunscreen use, Most of the coral reefs we can see today were formed after the last glacial period when melting ice caused the sea level to rise and flood the continental shelves.