Coral Islands
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A coral island is a type of
island An island or isle is a piece of land, distinct from a continent, completely surrounded by water. There are continental islands, which were formed by being split from a continent by plate tectonics, and oceanic islands, which have never been ...
formed from
coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...
detritus In biology, detritus ( or ) is organic matter made up of the decomposition, decomposing remains of organisms and plants, and also of feces. Detritus usually hosts communities of microorganisms that colonize and decomposition, decompose (Reminera ...
and associated organic material. It occurs in tropical and sub-tropical areas, typically as part of a
coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in group ...
which has grown to cover a far larger area under the sea. The term low island can be used to distinguish such islands from
high island Geologically, a volcanic island is an island of volcanic origin. The term high island can be used to distinguish such islands from low islands, which are formed from sedimentation or the uplifting of coral reefs (which have often formed on ...
s, which are formed through volcanic action. Low islands are formed as a result of
sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to th ...
upon a
coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in group ...
or of the uplifting of such islands.


Ecosystem

Coral reefs are some of the oldest ecosystems on the planet, over geological time, they form massive reefs of
limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
. The reef environment supports more plant and animal species than any other habitat. Coral reefs are vital for life in multiple aspects some of which include structure, ecology, and
nutrient cycle A nutrient cycle (or ecological recycling) is the movement and exchange of inorganic and organic matter back into the production of matter. Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cyc ...
s which all support
biodiversity Biodiversity is the variability of life, life on Earth. It can be measured on various levels. There is for example genetic variability, species diversity, ecosystem diversity and Phylogenetics, phylogenetic diversity. Diversity is not distribut ...
in the reefs. Coral reefs build massive
calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime (mineral), lime or being chalky. The term is used in a wide variety of Science, scientific disciplines. In zoology ''Calcare ...
skeletons that serve as homes for animals such as fish hiding inside the crooks and crannies of the reef and barnacles attaching themselves directly to the coral's structure. The structures also help plants that need the sun to photosynthesize, by lifting the plants to the ocean's surface where the sunlight can penetrate the water. The structures also create calm zones in the ocean providing a place for fish and plant species to thrive. Over geological time a reef may reach the surface and can become a coral island, where it begins a whole new ecosystem for land-based creatures.


Formation

A coral island begins as a
volcanic island Geologically, a volcanic island is an island of volcanic origin. The term high island can be used to distinguish such islands from low islands, which are formed from sedimentation or the uplifting of coral reefs (which have often formed ...
over a hot spot. As the volcano emerges from the sea, a
fringing reef A fringing reef is one of the three main types of coral reef. It is distinguished from the other main types, barrier reefs and atolls, in that it has either an entirely shallow backreef zone (lagoon) or none at all. If a fringing reef grows direc ...
grows on the outskirt of the volcano. The volcano eventually moves off the hot spot by means of
plate tectonics Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...
. Once this occurs, the volcano can no longer keep up with the wave
erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...
and undergoes
subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope mov ...
. Once the island is submerged, the coral must keep growing to stay in the
epipelagic zone The photic zone (or euphotic zone, epipelagic zone, or sunlight zone) is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological ...
. This causes the coral to grow into an
atoll An atoll () is a ring-shaped island, including a coral rim that encircles a lagoon. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical parts of the oceans and seas where corals can develop. Most ...
with a shallow
lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') an ...
in the middle. The lagoon undergoes accretion and creates an island completely made of
carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...
materials. The process is later enhanced with the remains of plant life which grows on the island.


Low vs. high island

The term "low island" refers to geologic origin rather than a strict classification of height. Some low islands, such as
Banaba BanabaThe correct spelling and etymology in Gilbertese should be ''Bwanaba'' but the Constitution of Kiribati writes Banaba. Because of the spelling in English or French, the name was very often written Paanapa or Paanopa, as it was in 1901 A ...
,
Makatea Makatea, or Mangaia-te-vai-tamae, is a raised coral atoll in the northwestern part of the Tuamotus, which is a part of the French overseas collectivity of French Polynesia. It is located southwest from Rangiroa to the west of the Palliser Islan ...
,
Nauru Nauru, officially the Republic of Nauru, formerly known as Pleasant Island, is an island country and microstate in the South Pacific Ocean. It lies within the Micronesia subregion of Oceania, with its nearest neighbour being Banaba (part of ...
, and
Niue Niue is a self-governing island country in free association with New Zealand. It is situated in the South Pacific Ocean and is part of Polynesia, and predominantly inhabited by Polynesians. One of the world's largest coral islands, Niue is c ...
, rise several hundred feet above
sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...
, while numerous high islands (those of volcanic origin) rise a few feet above sea level, often classified as "
rocks In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's ...
". Low islands ring the
lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') an ...
s of
atoll An atoll () is a ring-shaped island, including a coral rim that encircles a lagoon. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical parts of the oceans and seas where corals can develop. Most ...
s. The two types of islands are often found in proximity to each other. This is especially the case among the islands of the South Pacific Ocean, where low islands are found on the fringing reefs that surround most high islands.


Distribution

Most of the world's coral islands are in the
Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...
, but they are also found in the
Atlantic The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for se ...
and
Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or approximately 20% of the water area of Earth#Surface, Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia (continent), ...
s. The American
territories A territory is an area of land, sea, or space, belonging or connected to a particular country, person, or animal. In international politics, a territory is usually a geographic area which has not been granted the powers of self-government, ...
of Jarvis,
Baker A baker is a tradesperson who baking, bakes and sometimes Sales, sells breads and other products made of flour by using an oven or other concentrated heat source. The place where a baker works is called a bakery. History Ancient histo ...
and
Howland Island Howland Island () is a coral island and strict nature reserve located just north of the equator in the central Pacific Ocean, about southwest of Honolulu. The island lies almost halfway between Hawaii and Australia and is an Territories of the ...
s are clear examples of coral islands in the Pacific. Atolls in the Atlantic are found in Colombia's
Archipelago of San Andrés, Providencia and Santa Catalina The Archipelago of San Andrés, Providencia and Santa Catalina (, ), or San Andrés and Providencia, is one of the departments of Colombia, and the only one located geographically in Central America. It consists of two island groups in the Caribb ...
. The
Lakshadweep Islands Lakshadweep () is a union territory of India. It is an archipelago of 36 islands divided into three island subgroups: the Amindivi Islands in the north, the Laccadive Islands (separated from Amindivi roughly by the 11th parallel north), and ...
union territory Among the states and union territories of India, a Union Territory (UT) is a region that is directly governed by the Government of India, central government of India, as opposed to the states, which have their own State governments of India, s ...
of
India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...
is a group of 39 coral islands, along with some minor
islets An islet ( ) is generally a small island. Definitions vary, and are not precise, but some suggest that an islet is a very small, often unnamed, island with little or no vegetation to support human habitation. It may be made of rock, sand and ...
and
banks A bank is a financial institution that accepts deposits from the public and creates a demand deposit while simultaneously making loans. Lending activities can be directly performed by the bank or indirectly through capital markets. As banks ...
. Some of the islands belonging to
Kiribati Kiribati, officially the Republic of Kiribati, is an island country in the Micronesia subregion of Oceania in the central Pacific Ocean. Its permanent population is over 119,000 as of the 2020 census, and more than half live on Tarawa. The st ...
are considered coral islands. The
Maldives The Maldives, officially the Republic of Maldives, and historically known as the Maldive Islands, is an Archipelagic state, archipelagic country in South Asia located in the Indian Ocean. The Maldives is southwest of Sri Lanka and India, abou ...
consist of coral islands. St. Martin's Island is an coral island located in
Bangladesh Bangladesh, officially the People's Republic of Bangladesh, is a country in South Asia. It is the List of countries and dependencies by population, eighth-most populous country in the world and among the List of countries and dependencies by ...
. Coral islands are located near
Pattaya Pattaya is a city in Eastern Thailand, the second-largest city in Chonburi province and the List of municipalities in Thailand, eighth-largest city in Thailand. It is on the east coast of the Gulf of Thailand, about southeast of Bangkok, and h ...
and
Ko Samui Ko Samui (or Koh Samui), often locally shortened to Samui (, ), is an island off the east coast of Thailand. Geographically in the Mu Ko Samui, Chumphon Archipelago, it is part of Surat Thani Province, though as of 2012, Ko Samui was granted munic ...
,
Thailand Thailand, officially the Kingdom of Thailand and historically known as Siam (the official name until 1939), is a country in Southeast Asia on the Mainland Southeast Asia, Indochinese Peninsula. With a population of almost 66 million, it spa ...
.


Ecology

Coral is important for biodiversity and the growth of fish populations, so maintaining coral reefs is important. Many coral islands are small with low elevation above sea level. Thus they are at threat from storms and
rising sea levels The sea level has been rising from the end of the last ice age, which was around 20,000 years ago. Between 1901 and 2018, the average sea level rose by , with an increase of per year since the 1970s. This was faster than the sea level had e ...
. Through chemical and physical changes humans can cause significant harm to reef systems and slow the creation of coral island chains. Coral reefs are threatened by numerous
anthropogenic Anthropogenic ("human" + "generating") is an adjective that may refer to: * Anthropogeny, the study of the origins of humanity Anthropogenic may also refer to things that have been generated by humans, as follows: * Human impact on the enviro ...
impacts, some of which have already had major effects worldwide. Reefs grow in shallow, warm, nutrient-poor waters where they are not outcompeted by
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
. By adding fertilizers into the water runoff, phytoplankton populations can explode and choke out coral reef systems. Adding too many sediments can cause a similar problem by blocking out the sun, starving the
zooxanthellae Zooxanthellae (; zooxanthella) is a colloquial term for single-celled photosynthetic organisms that are able to live in symbiosis with diverse marine invertebrates including corals, jellyfish, demosponges, and nudibranchs. Most known zooxanthell ...
that live on coral causing it to undergo a process known as
coral bleaching Coral bleaching is the process when corals become white due to loss of Symbiosis, symbiotic algae and Photosynthesis, photosynthetic pigments. This loss of pigment can be caused by various stressors, such as changes in water temperature, light, ...
. The ocean's acidity is also a factor. Coral is made of calcium carbonate and is dissolved by carbonic acid. With the increase in carbon dioxide from combustion reactions in the atmosphere through precipitation, carbon dioxide mixes with water and forms
carbonic acid Carbonic acid is a chemical compound with the chemical formula . The molecule rapidly converts to water and carbon dioxide in the presence of water. However, in the absence of water, it is quite stable at room temperature. The interconversion ...
, raising the ocean's acidity which slows coral growth. Although low islands may have fewer potential habitats than high islands, thus lower species diversity, studies of both types of islands in
Palau Palau, officially the Republic of Palau, is an island country in the Micronesia subregion of Oceania in the western Pacific Ocean. The Republic of Palau consists of approximately 340 islands and is the western part of the Caroline Islands ...
found that species diversity, at least in the waters around the island, is more affected by island size than by its origin.


Climate and habitability

Low islands have poor, sandy soil and little fresh water, which makes them difficult to farm. They cannot support human habitation as well as high islands. They are also threatened by
sea level rise The sea level has been rising from the end of the last ice age, which was around 20,000 years ago. Between 1901 and 2018, the average sea level rose by , with an increase of per year since the 1970s. This was faster than the sea level had e ...
due to
global warming Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes ...
. The people that do live on low islands survive mostly by fishing. Low islands usually have an
oceanic climate An oceanic climate, also known as a marine climate or maritime climate, is the temperate climate sub-type in Köppen climate classification, Köppen classification represented as ''Cfb'', typical of west coasts in higher middle latitudes of co ...
. File:Moruroa atoll.JPG, The
Moruroa Moruroa (Mururoa, Mururura), also historically known as Aopuni, is an atoll which forms part of the Tuamotu Archipelago in French Polynesia in the southern Pacific Ocean. It is located about southeast of Tahiti. Administratively Moruroa Atoll i ...
atoll, composed of ''low islands'' on the reef encircling a central lagoon File:Niue ISS004.jpg, Satellite image of
Niue Niue is a self-governing island country in free association with New Zealand. It is situated in the South Pacific Ocean and is part of Polynesia, and predominantly inhabited by Polynesians. One of the world's largest coral islands, Niue is c ...
, a ''low island'' and former atoll, where reef and lagoon have risen above sea level due to uplifting


See also

*
Raised coral atoll A raised coral atoll or uplifted coral atoll is an atoll that has been lifted high enough above sea level by tectonic forces, protecting it from scouring by storms and enabling soils and diverse – often endemic – species of flora and fauna to ...


References


External links


''Micronesian culture: High island and low island cultures'' at Britannica.com.
Retrieved 2011-09-22. {{DEFAULTSORT:Coral Island Atolls Islands by type