How oceanic islands are formed
Most oceanic islands rise from the sea as a result of volcanic activity on the ocean floor. They do not have any links with the continents. Some oceanic islands are built up around one or two volcanic vents while others, like Tahiti, are formed by a whole series of vents. At first such islands are uninhabitable but gradually, as the lava cools, they begin to support life.
It may, however, take hundreds of years for plants and animals to become established. Sometimes, coral islands develop from reefs that form around the volcanic island. The original volcano may sink following movements in the Earth’s crust, or because the sea level rose. The coral reefs continue to grow upwards until only a coral atoll remains.