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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 part of a continent. Oceanic islands can be formed from volcanic activity, grow into atolls from coral reefs, and form from sediment along shorelines, creating barrier islands. River islands can also form from sediment and debris in rivers. Artificial islands are those made by humans, including small rocky outcroppings built out of lagoons and large-scale land reclamation projects used for development.

Definition
An island is an area of land surrounded by water on all sides that is distinct from a continent. Islands can occur in any body of water, including lakes, rivers, seas. Low-tide elevations, areas of land that are not above the surface during a high tide, are generally not considered islands. Islands that have been bridged or otherwise joined to a mainland with land reclamation are sometimes considered "de-islanded", but not in every case. == Etymology ==
Etymology
The word island derives from Middle English '', from Old English igland, itself from ig or ieg'', similarly meaning 'island' when used independently, and with the suffix -land carrying its contemporary meaning. Old English ieg is actually a cognate of Swedish ö and German Aue, and more distantly related to Latin '''' (water). The spelling of the word with was modified in the 15th century because of a false etymology caused by an association with the Old French loanword isle, which itself comes from the Latin word insula. == Geology ==
Geology
through Earth at the Hawaii hotspot. Magma from the mantle rises into the lithosphere, creating a chain of volcanoes as the lithosphere moves over the hotspot. Formation in oceans Islands often are found in archipelagos or island chains, which are collections of islands. These chains are thought to form from volcanic hotspots, areas of the lithosphere where the mantle is hotter than the surrounding area. An example is the Hawaiian Islands, Other island chains form due to being separated from existing continents. The Japanese archipelago may have been separated from Eurasia due to seafloor spreading, a phenomenon where new oceanic crust is formed, pushing away older crust. It was estimated that Zealandia, the continent-like area of crust that New Zealand sits on, has had 93% of its original surface area submerged. When these coral islands encircle a central lagoon, the island is known as an atoll. The formation of reefs and islands related to those reefs is aided by the buildup of sediment in shallow patches of water. In some cases, tectonic movements lifting a reef out of the water by as little as 1 meter can cause sediment to accumulate and an island to form. Antarctic islands, are sometimes permanently connected to another land mass by sea or glacial ice. An example of this is Ross Island in Antarctica. Formation in freshwater A fluvial island is an island that forms from the erosion and sedimentation of debris in rivers; almost all rivers have some form of fluvial islands. These islands may only be a few meters high, and are usually temporary. Changes in the flow speed, water level, and sediment content of the river may affect the rate of fluvial island formation and depletion. Lakes form for a variety of reasons, including glaciers, plate tectonics, and volcanism. Lake islands can form as part of these processes. == Life on islands ==
Life on islands
The field of insular biogeography studies the ecological processes that take place on islands, with a focus on what factors effect the evolution, extinction, and richness of species. Scientists often study islands as an isolated model of how the process of natural selection takes place. Island ecology studies organisms on islands and their environment. It has yielded important insights for its parent field of ecology since the time of Charles Darwin. Areas with high lives of biodiversity are a priority target of conservation efforts, to prevent the extinction of these species. Despite high levels of endemism, the total species richness, the total number of unique species in a region, is lower on islands than on mainlands. The level of species richness on islands is proportional to the area of that island, a phenomenon known as the species-area relationship. This is because larger areas have more resources and thus can support more organisms. Populations with a higher carrying capacity also have more genetic diversity, which promotes speciation. Oceanic islands, ones that have never been connected to shore, are only populated by life that can cross the sea. This means that any animals present on the island had to have flown there, in the case of birds or bats, were carried by such animals, or were carried in a sea current in what is known as a "rafting event". This phenomenon is known as oceanic dispersal. Tropical cyclones have the capacity to transport species over great distances. Animals like tortoises can live for weeks without food or water, and are able to survive floating on debris in the sea. One case study showed that in 1995, fifteen iguanas survived a 300 km journey to Anguilla in the Caribbean, an island which no iguana had lived on previously. They survived floating on a mass of uprooted trees from a storm. Plant species are thought to be able to travel great distances of ocean. New Zealand and Australia share 200 native plant species, despite being separated by 1500 km. The classical example of this is Darwin's finches, a group of up to fifteen tanager species that are endemic to the Galápagos Islands. These birds evolved different beaks in order to eat different kinds of food available on the islands. The large ground finch has a large bill used to crack seeds and eat fruit. The Genovesa cactus finch prefers cacti as a food source, and has a beak adapted for removing pulp and flowers from cacti. The green warbler-finch (in the habit of true warbler species) consumes spiders and insects that live on plants. In the case of smaller animals, it has been hypothesized that animals on islands may have fewer predators and competitors, resulting in selection pressure towards larger animals. Larger animals may exhaust food resources quickly due to their size, causing malnutrition in their young, resulting in a selection pressure for smaller animals that require less food. Having fewer predators would mean these animals did not need not be large to survive. He first traveled to the islands as a naturalist on HMS Beagle in 1835, as part of a five-year circumnavigation of Earth. He wrote that "the different islands to a considerable extent are inhabited by a different set of beings". Through the study of the finches and other animals he realized that organisms survive by changing to adapt to their habitat. == Humans and islands ==
Humans and islands
History of exploration The first evidence of humans colonizing islands probably occurred in the Paleolithic era, 100,000 to 200,000 years ago. Reaching the Indonesian islands of Flores and Timor would have required crossing distances of water of at least . Some islands, such as Honshu, were probably connected to the mainland with a land bridge that allowed humans to colonize it before it became an island. Between 1100 and 800 BC, Polynesians sailed East from New Guinea and the Solomon Islands and reached the islands that make up the modern-day Fiji and Samoa. The furthest extent of this migration would be Easter Island in the East, and New Zealand in the South, with New Zealand's first settlements between 1250 and 1300. Historians have sought to understand why some remote islands have always been uninhabited, while others, especially in the Pacific Ocean, have long been populated by humans. It has been hypothesized that since the inhabitants had little incentive for trade and had little to any contact with the mainland, they had no need for boats. Others compare the motivations of Polynesian and similar explorers with those of Christopher Columbus, the explorer who sailed westward over the Atlantic Ocean in search of an alternate route to the East Indies. The study of the culture of islands is known as island studies. The interest in the study of islands is due to their unique cultures and natural environments that differ from mainland cultures. Polynesians were known to fish close to shore, as well as in deep water. It was reported that Rapa Nui people were known to fish as far as from shore at coral reefs. Spear, line, and net fishing were all used, to catch tuna as well as sharks and stingrays. Different island climates made different resources more important, such as the Hawaiian islands being home to irrigated fields of taro, There is archeological evidence that Canary Islanders would chew the roots of ferns for sustenance, a practice that wore heavily on their molars. Three others, Palau, Federated States of Micronesia, and the Marshall Islands, are island nations of the Pacific region that maintain a defense, aid, and immigration agreement with the United States called a Compact of Free Association. Colonization leaving due to nuclear testing. Since the first discoveries of Polynesian, Micronesian, and other islands by Westerners, these nations have been the subject of colonization. Islands were the target of Christian missionaries. These missionaries faced resistance, but found success when some local chiefs used European support to centralize power. Beginning in the 16th century, European states placed most of Oceania in under colonial administration. Pohnpei was colonized by Spain as early as 1526. It changed hands from Germany to Japan to the United States before joining the Federated States of Micronesia in 1982, maintaining a "free association" status with the U.S. Guam was a Spanish territory until 1898, and now is an unincorporated territory of the U.S. The decolonization era saw many island states achieve independence or some form of self-governance. Nuclear weapons testing on the Marshall Islands left many atolls destroyed or uninhabitable, causing the forced displacement of people from their home islands as well as increases in cancer rates due to radiation. Islands in colder climates often rely on seasonal tourists seeking to enjoy nature or local cultures, and may only be one aspect of an island's economy. In contrast, tourism on tropical islands can often make up the majority of the local economy and built environment. These islands sometimes also require consistent foreign aid on top of tourism in order to ensure economic growth. This reliance can result in social inequality and environmental degradation. During tourism downturns, these economies struggle to make up the lost inflow of cash with other industries. == Threats to islands ==
Threats to islands
Climate change threatens human development on islands due to sea level rise, more dangerous tropical cyclones, coral bleaching, and an increase in invasive species. For example, in 2017 Hurricane Maria caused a loss of almost all the infrastructure in Dominica. Sea level rise and other climate changes can reduce freshwater reserves, resulting in droughts. Coral bleaching is expected to occur with more frequency, threatening marine ecosystems, some of which island economies are dependent on. Tuvalu signed a cooperation agreement with Australia agreeing to annually allow 280 of its citizens to become permanent residents of Australia. The Marshall Islands, a country of 1,156 islands, have also been identified as a country that may be existentially threatened by rising seas. Increasing intensity of tropical storms also increases the distances and frequency with which invasive species may be transported to islands. Floodwaters from these storms may also wash plants further inland than they would travel on their own, introducing them to new habitats. Agriculture and trade also have introduced non-native life to islands. These processes result in an introduction of invasive species to ecosystems that are especially small and fragile. One example is the apple snail, initially introduced to the U.S. by aquarium owners. It has since been transported by hurricanes across the Gulf Coast and neighboring islands. These species compete for resources with native animals, and some may grow so densely that they displace other forms of existing life. == Artificial islands ==
Artificial islands
in Osaka is built on an artificial island. For hundreds of years, islands have been created through land reclamation. One of the first recorded instances of this when people of the Solomon Islands created eighty such islands by piling coral and rock in the Lau Lagoon. One traditional way of constructing islands is with the use of a revetment. Sandbags or stones are dropped with a barge into the sea to bring the land level slightly out of the water. The island area is then filled with sand or gravel, followed by a construction of this revetment to hold it together. Islands have also been constructed with a permanent caisson, a steel or concrete structure built in a closed loop and then filled with sand. These atolls were previously low-tide elevations, landmasses that are only above water during low tide. The United Nations Convention on the Law of the Sea indicates that these islands may not have the same legal status as a naturally occurring island, and as such may not confer the same legal rights. == See also ==
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