Follow us on
Nature and environment

Formation of islands and atolls

Comment s'est formé les îles polynésiennes
© Philippe BACCHET

Formation of the islands

The Polynesian Islands came about as a series of underwater volcanic eruptions. They are grouped in five archipelagos, facing the same direction from the south-west towards the north-east. In their variety they have diverse world famous landscapes for which the Society, Tuamotu, Gambier, Marquesas and Austral islands are renowned.

Scattered over 5.5 million square kilometres at the centre of the Pacific Ocean, the 118 French Polynesian Islands of which only 67 are inhabited, make up a very scattered territory that in total is only 3,521 km2 of emerged land (Tahiti alone, the largest island, represents one third of this surface area). The lagoons, on the other hand, stretch over 12,800 km2 and are made up of 12,000 km of coral reefs. They extend as a colourful and fascinating universe that has a wealth of certain species of fish, shells, crustacean, sea urchins, star fish, sea cucumbers, algae etc.

Formation of the high islands

The origin of all these islands is a hot spot under the earth's crust. Regularly, at an interval of several million years, this becomes active, forms a new volcano that pierces the surface of the ocean and becomes, once extinguished, a high island. The islands recede one by one into the distance, the oldest in front, and are part of the Pacific shelf. This moves towards the north-west by 10 cm a year. Between them, the bottom reaches an extraordinary depth of 4,000 metres.

The high islands represent a mountainous backdrop that is more or less rugged according to their age and they are surrounded - or not, as the case may be -by a lagoon or coral barrier reef:

  • The islands surrounded by a lagoon or barrier reef: apart from the real wealth represented by the presence of the spectacular aquatic environment or the lagoon, the coral reef protects the island from attacks by the ocean.
  • The high islands without a lagoon: such as the Marquesas islands, Maketea and Rurutu. The ocean beats the shore that it sculpts into a cliff. Deep bays can shelter ports that have strong draughts from the waterway.

Formation of the atolls

Géologie des atolls et motu de Polynésie française
© Lucien PESQUIE - Bleu Lagon Production

Over many thousands of years, the very heavy volcanic high island subsides slowly under its own weight and is eroded by winds and rains, whereas the coral reef that surrounds it, grows due to the continual formation of new coral. The more the island subsides, the bigger the lagoon becomes. When there is no longer any trace of the engulfed volcano, the island continues to exist in the form of an atoll, a coral ring surrounding a lagoon, with several sand islets, the motu. The atolls, the last emerged stage of this development, are nearly 10 million years old and are the oldest islands.
Polynesia has many islands (about 80). Extremely varied in their contours and with or without passes (linking the waters of the lagoon with those of the ocean) or hoa (passes with shallow waters), they all have similar geological and biological features: more or less circular in shape, they present an extremely steep edge, beaten by the waves of the ocean, on which the coral structure is formed that is the origin of the cliff, and they shelter at their centre a sub-marine life of exceptional abundance. Not forgetting the superb, white sand beaches, shaded by coconut trees that decorate each of their motu.