An atoll is a ring-shaped coral reef enclosing a lagoon. The word atoll comes from atolhu, the equivalent term in the Maldivian language ( Dhivehi ). The coral reef forms a fringe of often extremely narrow islands are called Motu after the Polynesian word for "island" usually. In the lagoon are still remnants of former volcanic summit may as islands rise above the sea level.
Charles Darwin made his thoughts to the formation of atolls. According to his theory arise from fringing reefs atolls that arise around a volcanic island. The island can over time sink into the sea, either by erosion or because the sea floor drops and the sea level rises, the reef continues to grow upward. At the end only the reef extends up to the surface and forms a ring of small islands.
Another theory of Hans Hass, however, comes from without volcanoes. According to her is atolls of cone-shaped reefs where the corals die off in the center due to inadequate water supply and only the corals on the edge continue to grow, so that is also a ring-shaped structure is formed form.
By subsequent action (raising the earth's crust, lowering of the sea level ), it may happen that an atoll " lifts " and the area enclosed by the ring of coral lagoon largely, sometimes completely dry out. This is called an " upscale Atoll ". Examples include Niue, Nauru or Henderson Iceland.
Location and size
Atolls are found exclusively in tropical waters, mainly in the Pacific Ocean and the Indian Ocean, the Caribbean Sea, there are only four atolls. A state which consists only of atolls that form the Maldives.
The northernmost atoll is the Kure Atoll to the northwest of Hawaii at 28 ° 24 ' north latitude, located at the southernmost atoll Ducie the Pitcairn Islands is at 24 ° 41' S. lat atoll -like structures further south can be found at most undersea Elizabeth Reef in the Tasman Sea at 29 ° 58 ' S. lat
The largest total area ( lagoon and reef ) has 3850 km ² atoll Thiladhunmathi - Miladhunmadulu the Maldivian ( two names, but geographically an atoll ). Of this 51 km ² land area. The largest contiguous atoll structure is the Great Chagos Bank in the Chagos archipelago with a total area of 12,642 km ², with only 4.5 sq km land area. The Saya de Malha Bank submarine, a ring-shaped coral reef which lies entirely below sea level and is therefore not classified as atoll has a total area of approximately 35,000 km ² (without the separate northern Ritchie Bank).
However, many atolls have only a small area of land and no natural fresh water sources and are therefore uninhabited islands.
Impacts of global climate change
The inhabitants of many atolls are concerned about their livelihoods due to the feared rise in sea level due to global warming. The increasing because of the thinning of the ozone layer, UV radiation is suspected to interfere with the growth of the corals. The steady increase in CO2 concentration in the atmosphere causes beyond hyperacidity near-surface seawater. The carbon dioxide dissolves in seawater to hydrogen, which attacks the lime, which gives the reef stability.
- Ahunui, a circular atoll of the Tuamotu Archipelago
- Kwajalein, the largest Pacific Atoll
- Wake, the islands are formed only from the former volcano
- Malé, the capital of the Maldives, a densely populated Motu
- Nauru, an upscale Atoll
- Kure is the northernmost atoll of the Earth
- Ducie, the southernmost atoll of the Earth
- The Clipperton Island, the most remote atoll