Making of an atoll

The Making of an Atoll: The Subsidence Phenomenon

Every single island in the Tuamotus, and a few islands in the other archipelagos, are atolls.  So, what is an atoll?

An atoll begins to form when a volcanic island becomes inactive.  All of the islands in French Polynesia are slowly sinking and moving west.  Some have just been sinking a lot longer than others.  The volcanic islands in the Tuamotus are some of the oldest in the region.

The volcanic island sinks or subsides under its own weight.  At the same time, a coral reef forms around the island.  This takes about 6 million years.  The coral reef can be between 0 and 200 meters deep.  Anything deeper than 200 meters soon dies as it does not have enough light to survive. 

Over time, the volcanic mountain disappears completely.  The coral proliferates as the volcano slips into the ocean.  New colonies spring up on the skeletons of the old ones, constantly renewing the calcareous crown of the surface.  Eventually, what remains of the mountain is an underwater basalt platform covered with a thick calcareous crust.

This display is a good example of the making of an atoll.  The image up front shows the volcano, the center shows the mountain sinking and the furthest one is an atoll.

Making of an atoll

Making of an atoll

Typically, there is a passe that allows vessels to enter the lagoon that is surrounded by the coral reef.  Some atolls have no passe which makes them impossible to visit.  Other atolls have a dangerous passe which has to be navigated during a certain time (slack tide) to avoid damaging your vessel.

Teti’aroa’s atoll

Teti’aroa has no passe.  There is no way to bring the big boat into the protected lagoon.  However, there are 5 mooring balls located near the reef buried in over 50 meters of water.  We were able to secure one of these moorings.  The photo below is from our Navionic app which shows a light gray reef all the way around the atoll.  The yellow spots indicate the motus or islets and the blue (inside the gray) is the lagoon.  The gray represents the coral around the motus.

Teti'aroa motus

Teti’aroa motus

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