Up Earth Quakes Volcanoes Atoll
Nature Gallery (Plate Tectonics)
Atoll

This takes around 6 million years, beginning with the formation of a coral reef around the island between 0 and  200 meters deep (0 to 656 feet). Beyond that depth, the coral does not have enough light to survive. At the same time, the island sinks slowly, or subsides, under its own weight.

Over time, the volcanic mountain disappears completely

As the volcano slides back into the ocean, constantly eroded by torrential tropical downpours, the coral proliferates. New coral colonies grow on the skeletons of old, constantly renewing the calcareous crown on the surface.
Eventually, all that remains of the volcanic mountain is an underwater basalt platform, covered with a thick calcareous coral crust.

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GEOTHERMIC ENDO-UPWELLING
This is the phenomenon where water rises up from the depths of the ocean and passes through the porous calcareous crust.

This recent theory seeks to explain the richness of plant and animal life around a lagoon: unlike surface water, which is warmed by the sun to about 200 meters, water from the deep down in the ocean is full of nutritious salts.

Channels in the coral mix lagoon water with the ocean

An atoll usually remains open to the ocean thanks to a number of channels known as hoa. These channels mean that the shallow waters of the lagoon are constantly mixed with the deeper waters of the ocean.

This mixture of nutrient-rich deep ocean water and the warmer lagoon waters in the isolated atolls provides the Tahitian pearl oyster with an ideal environment to produce the most beautiful pearls in the world.