Upscale atoll

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Nauru , an upscale atoll.

An upscale atoll has all the characteristics of a typical atoll , for example a reef belt and the remains of a central lagoon , which, however, are well above today's sea level. Upscale atolls are initially “ordinary” atolls that are raised above the sea surface after the reef has formed. The cause of this uplift is the resurgence of tectonic or volcanic activities. As a rule, the central lagoon of the atoll dries up, silts up and plants settle in over time. A coral reef was subsequently able to form again at sea level on some islands. Upscale atolls often have abundant phosphate deposits that go back to the deposition of guano by seabirds.

The French term for an upscale atoll is makatea , in the Anglo-American language area it is referred to as raised reef island or makatea island . Makatea is an old Polynesian word that means "white stone" ( limestone ). The name also refers to an island in the Tuamotu Archipelago , an upscale atoll.

Emergence

This aerial photo of Navassa clearly shows the former reef rim lifted completely out of the water and the silted lagoon (top right)

James Cook already thought about their creation:

“We noticed huge coral rocks protruding fifteen feet from the water. The coral animals cannot get over the water. So how they were lifted, whether by an earthquake or in some other way, I cannot decide. "

- James Cook : Cook's trips around the world - report based on his diaries, Leipzig 1966, p. 244

As far as we know today, possible mechanisms for atoll uplift are:

  • the resurgence of volcanic activity
  • the bending of the oceanic crust due to load as a result of the formation of new volcanic buildings
  • large-scale uplift due to the presence of a mantle plume
  • large-scale uplift through plate tectonic processes.

The cause of the uplift is controversial in some cases, for example in the case of the upscale atolls of the Cook Archipelago .

Atolls are formed in tropical waters by the growth of a coral reef around the surface of a submarine volcano. After the end of volcanic activity, the volcanic mountain slowly sinks due to the lack of penetrating magma and due to its own weight, but also due to the sinking of the oceanic crust due to the plate tectonic drift. At the same time, it is being removed by erosion until only the reef rim, which is constantly growing up, is visible above the water surface. In some cases, however, volcanic activity resumes and penetrating igneous masses uplift the atoll.

Elevation caused by so-called lithospheric flexure is also related to volcanic processes . Here the oceanic crust, which is thin compared to the continental crust, reacts in two ways when it is burdened by a newly formed volcanic complex: a depression in the immediate vicinity of the volcano and - as compensation - a bulge some distance away. Hawaii is a good example of this process . It can happen that existing coral atolls in this radius are lifted above the sea surface. Such processes can be observed especially in relatively young volcanoes that are younger than 2 million years. An example: The mass of the volcano that formed the island of Pitcairn 900,000 years ago caused the uplift of the island of Henderson, 200 km northeast . This process is also assumed for the islands of 'Ātiu, Ma'uke and Miti'āro in the Cook Archipelago.

Another process that could lead to the uplift of large areas of oceanic crust is the large-scale rise of magma in a plume or mantle diapir . The uplift of the Cook, Marquesas, Pitcairn and Society Islands is said to go back to the so-called “South Pacific Superswell” (roughly: “South Pacific Large Vault”).

The position of some uplifted atolls on the edges of tectonic plates and near subduction zones indicates that their ascent through large-scale uplift processes is related to plate tectonic activity.

Examples

Individual evidence

  1. ↑ Patrick Vinton Kirch: On the Roads of the Winds - An Archaeological History of the Pacific Islands Before European Contact, Berkeley 2000
  2. ^ The Cook-Austral volcanic chain , Alain Bonneville, Institut de Physique du Globe de Paris
  3. ↑ Geology of Cook Islands , based on: George Szentes: Caves of the Cook Islands , The British Caver Vol. 127, 2005
  4. ↑ C. Chauvel, W. McDonough, G. Guille, R. Maury, R. Duncan: Contrasting old and young volcanism in Rurutu Island, Austral chain . In: Chemical Geology, Vol. 139, June 25, 1997, pp. 125-143, ISSN  0009-2541
  5. ^ T. Spencer: The Pitcairn Islands, South Pacific Ocean: plate tectonic and climatic contexts. In: Biological Journal of the Linnean Society, Vol. 56 (1-2), September 1995, pp. 13-42
  6. ↑ Gerhard H. Eisbacher: Introduction to Tectonics . 1st edition. Ferdinand Enke Verlag, Stuttgart 1991, ISBN 3-432-99251-3 .
  7. ↑ T. Spencer and TG Benton: Structure, topography and vegetation: the significance of raised reef islands. In: Biological Journal of the Linnean Society, Vol. 56 (1-2), September 1995, p. 11
  8. ^ Steve G. Blake: Late Quaternary history of Henderson Island, Pitcairn Group. In: Biological Journal of the Linnean Society, Vol. 56 (1-2), September 1995, pp. 43-62
  9. ^ R. Stoddard, CD Woodroffe, T. Spencer: M auke, Mitiaro and Atiu: Geomorphology of Makatea Islands in the Southern Cooks. In: Atoll Research Bulletin No. 341, National Museum of Natural History, Smithsonian Institution, Washington DC, September 1990, pp. 2-62
  10. ↑ Carol A. Stein & Seth Stein: The Superswell and Darwin Rise: Thermal no longer? , mantleplumes.org