Atolls: Difference between revisions
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:: REEF FLAT is a shallow structure up to 1,000-2,000 m wide that is most exhibited at low tide. When there is no island, the reef flat is the emerging structure that separates the open ocean and the lagoon.<ref name="alevizon" /> | :: REEF FLAT is a shallow structure up to 1,000-2,000 m wide that is most exhibited at low tide. When there is no island, the reef flat is the emerging structure that separates the open ocean and the lagoon.<ref name="alevizon" /> | ||
:: REEF ISLANDs often develop on the windward portions of atolls. The high energy waves wash up coral debris and calcareous sediment from the oppossite reef rims which slowly build up to form an island.<ref name="alevizon" /> | :: REEF ISLANDs often develop on the windward portions of atolls. The high energy waves wash up coral debris and calcareous sediment from the oppossite reef rims which slowly build up to form an island. If uninhabited, these islands often develop huge colonies of seabirds. <ref name="alevizon" /> | ||
'''Central Lagoon'' | '''Central Lagoon''' slopes gently to form a shallow, water-filled canyon at the center of the rim of corals.<ref name="palaces"/> It can range from several to 70 meters deep and is most often characterized by an accumulation of coral sediment but may contain scattered patch reefs and coral spurs. In the formation of the atoll, the limestone reefs that had previously grown on the inner side of the barrier reefs begin to decay. Without the central island or connection between the ocean and lagoon, the resources and nutrients within the lagoon become very limited. The water in the lagoon often begins to look brighter teal as the limestone corals die and decay. <ref name="alevizon" /> | ||
==Location== | ==Location== | ||
[[File:atolldistribution.gif]] | |||
Atolls are much more prevalent in the Indo-Pacific than in the Caribbean because of the relative abundance of submarine volcanoes.<ref name="palaces"/> Both the Pacific and Indian oceans have multiple series of atolls including the Marshall, Cook, and Caroline Islands in the Pacific and the Maldives, Chagos, Seychelles, and Cocos Islands in the Indian. <ref name="mark"> | |||
Many of these larger island atolls have been inhabited by humans who built island nations. The Marshall Islands are made up of 29 coral atolls which together account for more than 10 percent of all the atolls in the world. The capital of the Republic of the Marshall islands, Majuro, has a population of almost 30,000. <ref name="marshall"> | |||
==History and Tourism== | ==History and Tourism== | ||
Revision as of 12:57, 21 April 2013
Atolls
Atolls are ring-shaped reef formations with a central lagoon. At this stage in darwinian development, the island young fringing and barrier reefs surrounded has now been completely submerged and a ring of coral 1-20 miles in diameter is left. [1] Atolls are often uninhabited desert islands, relatively untouched and unexplored by humans. However, some atolls grow beyond short reef formations, forming substantial landmass.[2] These islands and islets have become the home of several island nations including the Republic of the Marshall Islands in the Pacific Ocean and the Maldives in the Indian Ocean. [3]
Formation
Atolls are the third and final stage of Darwin's subsidence theory. After corals have colonized a newly developing island forming a fringing reef, the volcano gradually sinks as it cools and condenses leaving the ring of corals behind.[4] This middle stage where the top of the island still emerges from the ocean but a lagoon has formed in the separation of the reef and island is known as a barrier reef. The corals continue to build upwards to stay in the sunlit range. Eventually the island will submerge completely leaving one large lagoon and a ring of reef formations. This is an atoll.[1]
Morphology and Marine Species
- Atolls can range in size from very small to expanding over 130 km with land and lagoon areas of up to 3,000 sq. km . They also vary greatly in rim width. Some ranging only a few hundred meters while others form large islands. Additionally, because atolls are so old (they began as fringing reefs), their reef formations are built on thick layers of dead coral reefs. Scientists have drilled up to 1400 meters towards the ocean floor before hitting volcanic rock. [4]
- While the expanse and colonization of atolls varies greatly, there are three distinctive characteristics that all atolls share: an outer reef, a perimeter (reef) rim, and a central lagoon.
Outer Reef is the ocean-facing side of the circle of corals. The lower portions slope steeply towards the ocean floor while the upper region forms a system of spur-and-groove formations.[5] These alternating buttresses and channels help to stabilize the reef and dissipate the force of the waves while the channels drain away accumulating coral debris and sediment. The region of shallow, sunlit, and oxygen abundant water is home to many large corals, coralline algae, and small fish. Larger fish like jacks, sharks, barracudas, and tuna often frequent the area in hopes of catching these smaller fish. [6] Additionally, the windward portion of the outer reef often forms a prominent algal ridge habited by an abundance of calcareous algae. [1]
Perimeter Rim is the portion of the reef formation that protrudes the water. It is the part that is visible from the air and sea level, forming the distinct circular structure. The perimeter rim, or reef rim, can vary greatly in width and may be broken at points, forming small channels that connect the lagoon to the outer ocean. If these canals form, the daily tidal cycles of the ocean allow circulation of the water within the lagoon. If the rim is continuous, the water in the lagoon is must rely on strong waves to push water over the rim or the rain cycle to recycle water. [5] The perimeter rim may either form a reef flat or build into a reef island.
- REEF FLAT is a shallow structure up to 1,000-2,000 m wide that is most exhibited at low tide. When there is no island, the reef flat is the emerging structure that separates the open ocean and the lagoon.[5]
- REEF ISLANDs often develop on the windward portions of atolls. The high energy waves wash up coral debris and calcareous sediment from the oppossite reef rims which slowly build up to form an island. If uninhabited, these islands often develop huge colonies of seabirds. [5]
Central Lagoon slopes gently to form a shallow, water-filled canyon at the center of the rim of corals.[1] It can range from several to 70 meters deep and is most often characterized by an accumulation of coral sediment but may contain scattered patch reefs and coral spurs. In the formation of the atoll, the limestone reefs that had previously grown on the inner side of the barrier reefs begin to decay. Without the central island or connection between the ocean and lagoon, the resources and nutrients within the lagoon become very limited. The water in the lagoon often begins to look brighter teal as the limestone corals die and decay. [5]
Location
Atolls are much more prevalent in the Indo-Pacific than in the Caribbean because of the relative abundance of submarine volcanoes.[1] Both the Pacific and Indian oceans have multiple series of atolls including the Marshall, Cook, and Caroline Islands in the Pacific and the Maldives, Chagos, Seychelles, and Cocos Islands in the Indian. <ref name="mark">
Many of these larger island atolls have been inhabited by humans who built island nations. The Marshall Islands are made up of 29 coral atolls which together account for more than 10 percent of all the atolls in the world. The capital of the Republic of the Marshall islands, Majuro, has a population of almost 30,000. <ref name="marshall">
History and Tourism
References
- ↑ 1.0 1.1 1.2 1.3 1.4 Strykowski, Joe and Rena M. Bonem. Palaces Under the Sea. Crystal River, FL: Star Thrower Foundation, 1993, p. 28-9. Print.
- ↑ Evers, Jeannie. (Editor). “Atoll.” Education.nationalgeographic.com. National Geographic Society, 2013. Web. 19 Apr. 2013.
- ↑ “Republic of the Marshall Islands.” CoRIS. National Oceanic and Atmospheric Association. Jan. 15, 2013. Web. 19 Apr. 2013.
- ↑ 4.0 4.1 Mark McGinley (Lead Author);J. Emmett Duffy (Topic Editor) "Atoll". In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth March 29, 2010; Last revised Date October 2, 2012; Retrieved April 19, 2013
- ↑ 5.0 5.1 5.2 5.3 5.4 Alevizon, William. “Atoll: Oceanic Ring of Coral.” Coral-reef-info.com, 2012. Web. 20 Apr. 2013.
- ↑ “What are Coral Reefs.” CoRIS. National Oceanic and Atmospheric Association. Feb. 2012. Web. 21 Apr. 2013.