Sinking volcanoes in the Pacific Ocean

How is it possible that tiny little circular islands exist in the middle of the Pacific Ocean? Even on a sea floor which is 4 or 5 kilometer deep? These little round islands are atolls. They have been build up from volcanic rock and corals for over millions of years. Charles Darwin already published a theory on the formation of atolls over a century ago. His vision is still generally accepted.

The atoll Marakei, which was visited by curators
 from Museon in the Hague in October 2012.

The theory is in fact that a volcano slowly sinks down into the ocean and the island slowly changes shape. At first this mountainous island is a separate volcano at sea. When the (millions of) years pass this island slowly sinks down because the sea floor sinks down as it becomes older. Also erosion helps to break down the volcano. It changes into an smaller mountain with a barrier reef around it. This process continues until the mountain is long gone and the circular coral reef is only thing left over.This is called an atoll.

The steps in which a volcanic island
changes into an atoll.

The island Bora Bora. The growing coral reef around the island
is already visible as a ring around a shallow lagoon. In the
middle are still the remains of an old volcano.

Darwin reasoned that corals which are located around the volcanic island, slowly grow and follow the sea level. At the same time the island slowly disappears under water. Once the volcano is completely under water, the corals form an atoll. The ring of corals is called a barrier reef, because the lagoon in the middle is an shallow area. There corals cannot grow as good as at the edge of the island. The lagoon often contains brackish or even sweet water.

The islands of Hawaii show from East to West this development towards
becoming atolls. The youngest island, Hawaii, is a very active volcano.
The slightly older islands not anymore. Towards the West the islands are
older, eroded and further down in the ocean. The original volcano is not
visible above the water anymore.

More information:
- Nice explanation of the development of atolls on the beach: YouTube.com/watch?v=btRCAQHqbdY
- Colleagues of Museon in The Hague visited an atoll in Kiribati for in preparation for an exhibition. The text in this blog item is inspired on their request for more information about the development of atolls. Check their work for Museon on: Klimaatverhalen.nl

Nederlandse versie van dit artikel: Wegzinkende vulkanen in de stille oceaan.