Life and death of a volcano
All the islands of French Polynesia are of volcanic origin even though they are not issued of the same phenomenon. Actually, two different phenomena give birth to islands : some volcanoes appear “along dorsals”, and some other are issued of “hot zones”.
After their birth, all the volcanic islands follow the same destiny and their dissimilarity can be explained by theirdifferent ages. Actually, the first stage is the volcanic eruption followed by the extinction of the volcanowhich sinks slowly into the ocean and erodes to the contact of the air. Before long, the high volcanic island that is formed is surrounded by a coral reef. Only this reef will remain and form an atoll after the volcano has completely disappeared under the sea.
The Society, the Marquesas, the Austral and the Gambier archipelagos are mainly composed by high volcanic islands with a lagoon or not : it means they are pretty young. Bora Bora and Maupiti are in a transitional period, between high island and atoll : the moutain in the center only rises of a few hundred meters high. Tetiaroa and Mopelia are older and the genuine volcano has completely sunk, leaving a ring of coral and a wonderful lagoon.
Volcanos born “along dorsals”
The constant movements between the Nazca and the Pacific plates create a strong seismic activity that gives birth to a gigantic underwater volcanos ridge called the East Pacific ridge. The Tuamotu archipelago, the oldest of French Polynesia with an age estimated between 40 to 63 million years, has come to life thanks to this phenomenon. This advanced age explains the huge number of atolls that can be found in this archipelago.
The play of the lithospherical plates engenders a north-west directed drift of the Pacific plate. As a consequence, million years ago, the shelf that supported Polynesia was located more south-east.
“Hot zones” volcanism
Independently to the phenomenon of dorsal volcanos, fixed and isolated expulsions of magma out of the earth’s crust provoked many volcanic eruptions. When it emerges into the air, this magma solidifies and after the mass has cooled, becomes an island. Then, the newly formed island drifts westwards while the rising colum of magma continues its work and gives birth to a new island, and so on. As a consequence, the further of this column the islandsare situated, the older they are. Thus, strings of islands are created and, before long, will become real archipelagos.
The Society, the Marquesas, the Austral and the Gambier archipelagos have come to life thanks to those isolated anf fixed columns of magma. The appearance of the Austral is spread over 0 to 25 million years while the birth of the Gambier, the Society islands (25 million) and the Marquesas (17 million) occurred during shorter periods.