An upscale Atoll has all the characteristics of a typical atoll, for example, a Riffgürtel and the remains of a central lagoon, which remain well above the present sea. Upscale atolls are first "ordinary" atolls, which are raised by the Riffbildung over the sea beyond. The reason for this improvement is the re -emergence of tectonic or volcanic activity. As a rule, falls the central lagoon of the atoll dry, silted up and settle it with time to plant. In some islands could form a coral reef then at sea level again. Upscale atolls often have rich phosphate deposits, which date back to the deposition of guano by seabirds.
The French term for an upscale Atoll is Makatea, in the Anglo - American world, it is called a raised reef Iceland or Makatea Iceland. Makatea is an ancient Polynesian word meaning " white stone " (limestone) means. The name also refers to a island in the Tuamotu Archipelago, an upscale Atoll.
Already James Cook made his thoughts on their origin:
"It fell on us mighty coral rocks jutting fifteen feet from the water. The coral animals can not get away over the water. How have you been thus raised, whether by an earthquake or some other way, I can not decide. "
According to present knowledge, are possible mechanisms for the uplift of atolls:
- The resurgence of volcanic activity
- The bending of the oceanic crust by vertical load due to the formation of volcanic buildings
- Large-scale uplift by the presence of mantle plumes
- Large-scale uplift by plate tectonic processes.
The cause of the uplift is controversial in some cases, such as in the case of elevated atolls of the Cook archipelago.,
Atolls are formed in tropical waters by the growth of a coral reef around the tip of a submarine volcano emerged. After the end of volcanic activity from the volcano mountain falls slowly because of the absence nachdringenden magma and due to its own weight, but also by the decrease of the oceanic crust in the plate tectonic drift. At the same time he is removed by erosion, up above the water surface, only the steadily growing upward Riffkranz is visible. In some cases, however, the volcanic activity revives and nachdringende magmatic masses lead to an enhancement of the atoll.
Also with volcanic processes in the context is the so-called lithospheric uplift by bending ( en: lithospheric flexure ). Here reacts thin compared with the continental crust oceanic crust when it is loaded by a volcanic complex newly formed in two ways: a depression in the immediate vicinity of the volcano and - of a bulge in the distance - as compensation. A good example of this process is Hawaii. It may happen that existing coral atolls to be raised in this radius over the ocean beyond. Such processes are mainly observed in relatively young volcanoes that are younger than 2 million years. For example, the mass of the volcano that formed the island of Pitcairn 900,000 years ago, caused the uplift of the island located 200 km northeast Henderson. This process is assumed as for the islands ' Atiu, Ma'uke and Miti'āro in the Cook Islands.
Another process that could lead to the uplift of extensive areas of oceanic crust, the large-scale ascent of magma in a mantle plume or diapir - is discussed. So shall the raising among others, the Cook, Marquesas, Pitcairn and Society Islands on the so-called "South Pacific Super Well" ( roughly: " South Pacific United curvature " ) go back.
The position of some fine atolls at the edges of tectonic plates and near subduction zones indicates that you upgrade must be triggered by large-scale uplift processes related to plate tectonic activity.
- Makatea ( Tuamotu )
- Mangaia (Cook Islands)
- ' Atiu (Cook Islands)
- Ma'uke (Cook Islands)
- Miti'āro (Cook Islands)
- Rimatara ( Austral Islands)
- Rurutu ( Austral Islands)
- Iceland Henderson ( Pitcairn Islands)
- Malden ( Line Islands, Kiribati)
- Flint ( Line Islands, Kiribati)
- Starbuck ( Line Islands, Kiribati)
- Swain Iceland (American Samoa)
- Banaba (Kiribati )
- Lifou ( Loyalty Islands, New Caledonia )
- Maré ( Loyalty Islands, New Caledonia )
- Aldabra ( Aldabra group, Outer Islands )
- Assomption ( Aldabra group, Outer Islands )
- Astove ( Aldabra group, Outer Islands )