- Key People:
- James D. Dana
- Related Topics:
- island
coral island
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What is a coral island?
A coral island is a tropical island that is made of organic material from the skeletons of corals as well as other animals and plants that live with corals. These islands are low-lying, typically a few meters above sea level, and are known for their white coral sand beaches and coconut palms.
Geologically, a coral island or cay is a small part of a larger coral reef system, usually located on the relatively flat top of the reef. These islands can form through the uplift of a reef system above sea level or through the accretion of reef rock and detritus from storms and wave action. Winds and currents play a role in shaping the island.
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How do coral islands form?
Coral islands are tropical islands that consist of low land formations built from the skeletons of corals, plants, and animals associated with corals.
There are two ways that coral islands can form:
* Accretion Coral islands can form through the accumulation of reef rock, broken off from the reef by storms and waves, which mixes with finer reef detritus. Beaches develop, and wind may heap material into dunes. Rainwater dissolves and redeposits lime, cementing the material together. Plants and animals colonize the land, contributing remains that help soil develop.
* Uplift Part or all of a reef system may rise above sea level due to crustal movements. The submarine reef top becomes a plateau with a shallow or dry lagoon.Charles Darwin suggested that atolls, a type of coral island, are the final stage of a reef's upgrowth around a sinking, extinct volcano.
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What role do coral reefs play in making coral islands?
Coral reefs play a fundamental role in the creation of coral islands. These islands are made of organic material, mainly from the skeletons of corals and other reef-associated organisms.
Reef building happens below the high-tide level, and islands or cays usually sit on top of the reef system.
Winds and currents shape the reefs and determine the position of coral sand cays, also known as "low islands," that form on reefs. Sediment is lifted from the reef surface and deposited where water velocity decreases. Wind action on these deposits can then build dunes.
Storms and surf can also pile up material into beaches and shoals, which may then develop into islands or cays on top of the reef.
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What are the main features of a coral island's geology?
Coral islands are tropical islands composed of the skeletal remains of corals and related organisms. They are low-lying, rarely more than a few meters above sea level, and feature white coral sand beaches and coconut palms.
Geologically, coral islands are small parts of larger reef systems that may have originated through uplift or accretion. Uplift occurs when crustal movements raise a reef system above sea level, creating rocky islands with cliffs and sculptured surfaces. Accretion occurs when reef rock is broken off by storms and waves, then mixed with finer detritus. This material accumulates through wave action, forming beaches and dunes, which are then cemented together by dissolved lime and colonized by plants and animals, contributing to soil development.
Atolls are ring-shaped coral reefs enclosing a lagoon. They may evolve from reefs fringing a subsiding volcanic island, as explained by Charles Darwin's subsidence theory, or from reef platforms uplifted above sea level.
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How do waves and currents affect the shape of coral islands?
Waves and currents play a crucial role in shaping coral islands. They transport sediment from the reef surface, depositing it on the leeward side where water velocity decreases, leading to the formation of coral sand cays or low islands.
Wind-generated currents and tidal effects influence the orientation, shape, and position of these islands. Storm waves can carry coral fragments, forming shingle banks that add stability when cemented. However, hurricanes can also erode shorelines.
Accretion is another process where reef rock broken off by storms and waves mixes with finer detritus. Beaches develop, and wind forms dunes. Rainwater dissolves and redeposits calcium carbonate, cementing the material together, while plants and animals colonize the land, contributing to soil development.
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Why are coral islands important for studying Earth's geological history?
Coral islands are valuable for studying Earth's geological history because fossil corals can serve as absolute geochronometers, preserving a record of the number of days in a year when they were growing.
By examining the fine external ridges of calcium carbonate in fossil corals, scientists can estimate the number of days per year, which has decreased over time due to the slowing rotation of the Earth. Additionally, the carbon and oxygen isotopic composition of coral skeletons and shells can provide insights into past water temperature changes and offer a method for assessing the age of a sample. Through the study of coral reefs and islands, the effects of plate tectonics and sea level changes can be better understood.
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coral island, tropical island built of organic material derived from skeletons of corals and numerous other animals and plants associated with corals. Coral islands consist of low land perhaps only a few metres above sea level, generally with coconut palms and surrounded by white coral sand beaches. They may extend dozens of kilometres and include almost any tropical limestone island whose structure is integrally part of a living or relatively recent coral reef. Reef building takes place mostly below high-tide level, and a typical coral island or cay usually surmounts the relatively flat top of the whole reef system. Geologically, the island is just one small part of the whole coral reef.
Coral reefs take four main forms. Fringing reefs consist of a flat reef area directly skirting a nonreef island, often volcanic, or a mainland mass. Barrier reefs are also close to a nonreef landmass but lie several kilometres offshore, separated from the landmass by a lagoon or channel often approximately 50 m (160 ft) deep. Some barrier reefs are more or less circular, surrounding an island, but larger barrier reefs, such as those along the Red Sea coast and the Great Barrier Reef of Australia, are complex linear features consisting of chains of reef patches, some of them elongated into ribbon reefs. The third category of reefs consists of atolls, which are like circular barrier reefs but without their central landmass. Finally, there are patch reefs, which have irregular tablelike or pinnacle features. Smaller patches occur inside atoll lagoons. Larger patches occur as isolated parts of larger developments of any of the other three reef categories. They sometimes occur completely separate from other kinds of reefs.
Coral reef islands occur in association with all categories of reef but especially on reefs whose flat tops are well developed, perhaps a kilometre or more in width. Reef islands may occur in isolation or in a chain along the length of the reef. Sometimes they take the form of long strips of land occupying most of the length of the central area of a reef top.
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There are two completely different kinds of origin of reef islands: uplift and accretion. In the first, part or all of a reef system may become land as a result of crustal movements raising it above sea level (e.g., the Aldabra Islands in the western Indian Ocean). The previously submarine reef top becomes a low plateaulike feature, and such islands are typically rocky, with cliffs, and with land surfaces pitted and sculptured by solution weathering (karst). They are often still recognizable as atolls with a lagoon, now much shallower or even completely dry, as an interior basin. If present sea levels were to fall again, as they have done in the recent geological past due to an increase in polar ice, most of the world’s coral reefs would, in effect, become raised features. It is only because present sea levels are the highest for many thousands of years that there are no raised reef islands of this kind now.
Coral islands created by accretion have developed from rubbly reef rock broken off from the reef by storms and waves and mixed with finer reef detritus. The exceptional conditions of cyclonic storms are sometimes sufficient to create reef-top shoals in a single event. Other material accumulates by more regular methods such as normal currents and wave action. Beaches develop around the shoal, and wind may heap up the lighter, finer material into dunes. Rainwater can now reach all this material, which, being almost entirely of calcium carbonate, is readily dissolved by it, and the dissolved lime is redeposited around the loose material, cementing it together. The newly formed land is soon colonized by plants and animals, which also contribute their own remains to the island, helping soil to develop. Many of the reef islands in the central and southern Pacific and of the Maldive Islands in the Indian Ocean originated in this way.
Reef islands, especially those close to sea level, are not very stable. The cyclones which help to create them may also damage and destroy them. Waves may attack one side and redeposit the material on the other. Precarious though reef islands are, they have nevertheless long been the homes of peoples like the Polynesians and Micronesians in the Pacific and the Maldivians in the Indian Ocean. These people have been able to survive by their seafaring skills, fishing the reef waters, rearing animals and crops on the land, and using for drinking water the thin lens of rainwater held within the reef rock.