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Island

Island

An island is a relatively small area of land that is completely surrounded by water. It is impossible to give a total number to the islands that exist on the surface of the planet. As a result of erosion or rising sea level, some islands drown over time. The longest surviving islands usually last no more than 5 to 10 million years (Earth is 4.5 billion years old). Ongoing volcanic activity continues to create new islands and to add to existing ones. On November 14, 1963, a underwater volcanic explosion off the southern coast of Iceland created the island of Surtsey, which continues to gain land as the ongoing lava flows cool.

The eight largest islands on Earth are (in descending order): Greenland, New Guinea, Borneo, Madagascar, Baffin Island, Sumatra, Honshu (largest of the Japanese islands), and Great Britain. Islands can be divided into two types: continental or oceanic.

Continental islands

Continental islands are parts of the continental shelvesthe submerged, gradually sloping ledges of continents. These islands are formed in one of two ways: rising ocean waters either cover coastal areas, leaving only the summits of coastal highlands above water, or cut off a peninsula or similar piece of land jutting out from the mainland. Continental islands lie in shallow water, usually less than 600 feet (180 meters) deep. Greenland and Newfoundland (off the eastern coast of Canada) are examples of continental islands. A drop in sea level would be sufficient to connect these islands to the North American continent.

Words to Know

Barrier islands: Long, thin, sandy stretches of land that lie in shallow waters parallel to a mainland coast.

Continental shelves: Submerged, gradually sloping ledges of continents.

Hot spot: Plumes of magma welling up from Earth's crust.

Island arc: Curved row of islands of volcanic origin that develops where two plates converge, usually near the edge of a continent.

Magma: Hot, liquid material that underlies areas of volcanic activity; magma at Earth's surface is called lava.

Plate tectonics: Geological theory stating that Earth's crust is divided into a series of vast platelike sections that move as distinct masses over the planet's surface.

Long, thin, sandy stretches of land that lie in shallow waters parallel to a mainland coast are called barrier islands. These are technically not continental islands since they are formed by the erosion of mainland rock (sand). The sand is carried to coastal areas by rivers and then carried offshore by strong waves and other ocean currents.

Oceanic islands

Oceanic islands are not scattered haphazardly about the deep ocean waters. They arise from volcanic activity on the ocean floor. Over time, the cooled lava forms mountains, the tops of which rise above the surface of the ocean as islands. According to the geological theory of plate

tectonics, Earth's crust is divided into a series of vast platelike sections that move as distinct masses over the planet's surface. Most oceanic islands are formed as oceanic plates move over fixed hot spots (plumes of magma or lava welling up from the crust). Some oceanic islands are situated above the boundaries where oceanic plates converge or come together, while others arise where plates diverge or spread apart (a process called seafloor spreading).

The Hawaiian-Emperor island chain in the north Pacific Ocean formed as a result of a plate moving over a thermal plume of magma from a fixed hot spot. The hot spot is believed to be causing the currently active volcanoes of Mauna Loa and Kilauea on the island of Hawaii.

When two plates converge, the plate carrying the heavier crust dips under, or subducts, the plate carrying the lighter crust. At the point of subduction, a deep trench develops. Parallel to it, on the lighter plate, volcanic action produces a row of islands. These island groups are called island arcs, after their curved pattern. The Aleutian Islands, off the southwest coast of Alaska, are such islands.

Island ecosystems

Islands often contain a strange mix of plants and animals. Because oceanic islands are isolated by their surrounding waters, they are home to only a few species of animals. Many of these animal species are found nowhere else on the planet. The small size of islands also prevents them from supporting a larger number of animal species. A few seabirds and insects exist in greater numbers since they are able to migrate over the waters separating islands. Plant life on islands is most abundant, as seeds are carried by winds, water currents, and birds from remote lands.

Island ecosystems (communities of plants, animals, and microorganisms) are delicate and balanced. Over time, they have reached a steady statewhat is taken out of the environment is replaced. The relationship between predators and prey remain constant: those that die are replaced by newborns. The introduction of other life-forms into the closed system of an island, therefore, can have dramatic immediate effects. Changes brought about by humans is particularly devastating to islands. For example, domestic goats and rabbits introduced by human colonizers can completely rob a small island of succulent vegetation in less than a year. Dogs can turn every small mammal into prey.

Such changes to an island's ecosystem can result in the extinction of animal or plant species, many of which are not even known. Scientists are increasingly concerned about raising awareness of the special features of islands and their contributions to geological and evolutionary knowledge.

[See also Coast and beach; Plate tectonics; Volcano ]

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island

island, relatively small body of land surrounded entirely by water. (As the oceans form a continuous mass of water on the earth's surface, all continents are islands in the strict sense of the word.) The largest islands on earth are, in descending order of size, Greenland, New Guinea, Borneo, Madagascar, Baffin Island, Sumatra, Honshu (largest of the islands of Japan), and Great Britain. Depending on their origin, islands are either continental or oceanic. Continental islands are created by rise in sea level where only the summits of coastal highlands remain above water; or by the sea breaking through an isthmus or peninsula and cutting the land from the mainland. Typical continental islands are Great Britain and Martha's Vineyard. Other islands emerge along coasts as barrier islands, such as the Outer Banks, off North Carolina. Oceanic islands can result from volcanic islands rising above the water, especially on or near a mid-ocean ridge, as when the island of Surtsey appeared along the Mid-Atlantic Ridge S of Iceland in 1963. Island arcs, such as the Aleutian Islands, result from magmatic activity associated with the convergence of lithospheric plates (see plate tectonics). Oceanic islands may also be the emergent tips of volcanoes (seamounts) formed by hotspots. Oceanic islands that result from coral growth on the summit of seamounts are called coral islands or atolls (see coral reefs). These low islands only occur in tropical ocean areas. Oceanic islands are generally characterized by low faunal diversity, consisting of a few sea birds and insects. Vegetation is usually more abundant, as seeds are carried from remote lands by wind, water currents, and birds.

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Island

ISLAND

A land area surrounded by water and remaining above sea level during high tide.

Land areas exposed only during low tide are called low-tide elevations or drying rocks, reefs, or shoals. The existence of islands has generated numerous disputes, centering primarily on the size of the territorial sea surrounding an island and the determination of what state has sovereignty over a particular island. The size of the territorial sea has become an important question affecting fishing rights and the right of unrestricted passage for foreign vessels. Although the territorial sea of an island is usually determined by reference to its coastal baseline, some adjustments have been recognized in the cases of archipelagoes and islands located close to the mainland.

Determination of what state has title to an island has traditionally depended upon an open and continuous assertion of sovereignty over the island, which is usually, but not always, accompanied by physical presence of some representative of the state.

cross-references

Territorial Waters.

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island

is·land / ˈīlənd/ • n. a piece of land surrounded by water. ∎ fig. a thing resembling an island, esp. in being isolated, detached, or surrounded in some way: the university is the last island of democracy in this country. ∎  a freestanding kitchen cupboard unit with a countertop, allowing access from all sides. ∎  Anat. a detached portion of tissue or group of cells. Compare with islet. • v. [tr.] make into or like an island; place or enclose on or as on an island; isolate: islanded among the new stores, these houses were valuable property the house where she has been islanded.

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island

island. OE. (Angl.) ēġland, (WS.) ī(e)ġland, later īland = MDu., MLG. eilant (Du. eiland), ON. eyland; f. OE. ī(e)ġ island, in comp. water, sea, OHG. ouwa stream, watery meadow, island (G. au(e) brook, meadow, pasture), ON. ey island :- Gmc. *aujō for *aʒwjō, adj. formation on *azwō-stream, water (whence OE. ēa, OS., OHG. aha, ON. á, Goth. ahwa), rel. to L. aqua water (cf. AQUATIC). The present sp. (from XVI) is due to assim. to next.

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Island

Is·land / ˈēˌslän(t)/ Icelandic name for Iceland.

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island

islandunironed, viand •prebend •beribboned, riband •husband • house husband •unquestioned • escutcheoned •brigand, ligand •legend •fecund, second, split-second •millisecond • nanosecond •microsecond • rubicund • jocund •Langland • garland • parkland •Cartland, heartland •headland • Shetland • Lakeland •mainland •eland, Leland, Wieland, Zealand, Zeeland •Greenland • heathland • Cleveland •Friesland • Queensland • midland •England • Finland • Maryland •dryland, highland, island •Iceland • Holland • dockland •Scotland •foreland, Westmorland •Auckland, Falkland •Portland • Northland •lowland, Poland, Roland •Oakland • Copland • Newfoundland •woodland • Buckland • upland •Jutland, Rutland •Ireland • moorland •Cumberland, Northumberland •Sunderland • Switzerland •Sutherland • Hammond •almond, Armand •Edmund, Redmond •Desmond, Esmond •Raymond • Grimond • Richmond •Sigmund • Sigismund • Osmond •Dortmund • unsummoned •diamond • gourmand • unopened •errand, gerund •reverend • Bertrand • dachshund •unchastened •old-fashioned, unimpassioned •unsanctioned •aforementioned, undermentioned, unmentioned •unconditioned • unsweetened •unenlightened • unleavened •self-governed • unseasoned •wizened • thousand

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Island

Island

Island types

How many islands?

Island formation

Coral islands

Island biogeography

Island economics

Resources

An island is a land mass smaller than a continent and is completely surrounded by water. That distinction, although somewhat artificial, suggests that different geologic forces act to create and maintain islands versus continents. Because of their size and degree of isolation, islands further differ from continents in their natural environmentsin the biological systems they support, in their rate of response to change, in their ability to recover from ecological disaster.

The plants and animals found on islands can be unusual in the sense that they live nowhere else. The seemingly skewed distribution of populations on islands, compared with that on a continent, results in part from the small size of islands; they cannot carry a representative zoo, and, without migration in from

outside, the extinction of one life form might leave a gaping hole in the biota. Also, the water surrounding islands acts as a barrier to the passage of some life forms, particularly large mammals, but encourages the migration of others, such as birds and insects. Because of their relative isolation and the potentially unique biota that may be established on them, islands have been known, at least since the time of Charles Darwin, as natural laboratories of evolution.

Island types

The islands discussed here are of three kinds: continental islands, oceanic islands, and coral islands. Not discussed are inland islands, such as islands found in the middle of a lake.

Continental islands

Continental islands are parts of the continental shelf that rise above the surrounding water. That is, they are situated on the shallow water margin of a continent, usually in water fewer than 600 ft (200 m) deep. Greenland, the largest island in the world, and Newfoundland are examples of continental islands. A drop in sea level would be sufficient to connect these islands to the North American continent.

Another, rarer kind of continental island consists of small pieces of continental material that broke away from a land mass. These islands are now part of a separate crustal plate that is following an independent path. The Seychelles in the Indian Ocean were once associated with the Madagascar-India portion of the supercontinent Pangaea. With the breakup of Pangaea about 200 million years ago, the Seychelles began their independent existence. Their continental basement structure, however, clearly associates them with the continents rather than with oceanic islands of volcanic origin.

Oceanic islands

Oceanic islands arise from volcanic action related to the movement of the lithospheric, continent-bearing crustal plates. Unlike continental islands, oceanic islands grow from oceanic crust. Oceanic islands are not scattered haphazardly about the deep ocean waters, but are aligned along tectonic plate boundaries where crust is being created or subducted. In addition, some arose as oceanic plates moved over fixed hot spots in the deeper mantle.

Coral islands

Coral islands are distinct from both continental islands and oceanic islands in that they are formed of once living creatures, the corals, which colonize in place to form coral reefs.

Barrier islands

Unrelated to this three-part classification of islands are islands of a fourth kind, barrier islands. Barrier islands occur in shallow-water coastal areas and are composed of unconsolidated sediment, usually sand. Barrier islands form 15% of all coastline in the world, including most of the coastline of the continental United States and Alaska, and also occur off the shores of bays and the Great Lakes. Some barrier islands are stable enough to support houses or an airport runway; others are short-lived, moved annually by winter storms and reestablished by wave and tidal action. As their name suggests, they afford some protection to the mainland from erosion.

How many islands?

Islands are intrinsically impermanent. The more stable oceanic islands last a relatively brief time of 5-10 million years. Some islands drown, a result of erosion, subsidence of the ocean crust, or rising sea level. Sea levels are related in part to the amount of water bound up in the polar ice caps or released into the oceans; and the size of the polar ice caps is related to a variety of factors including variations in the positions of continents, the orientation of Earths axis, and the amount of cloud cover.

Sea level is fairly high now; it was lower during the Little Ice Age, circa fourteenth to nineteenth century, and even lower about 18,000 years ago. A lowering of sea level brings back into view drowned islands.

Ongoing volcanism continues to add to existing islands and create new ones. An example is Surtsey, off the southern coast of Iceland, which came into existence with a submarine volcanic explosion on November 14, 1963, and has continued to accrete surface area as the ongoing lava flows cool. There are also islands that appear intermittently.

Because islands come and go, the number of islands in existence cannot be established except in relation to a proscribed time perioda human generation, or a century or two. With the discovery of some islands in the Russian Arctic in the mid-twentieth century, however, it is thought that no islands remain to be discovered in our time. Satellite and ship-based scanning equipment is now being used to search for islands whose positions appear on nautical maps but which have themselves disappeared, and to identify underwater sites of new island formation.

Island formation

The classification of islands based on their foundationcontinental crust versus oceanic crust versus coralsas given above has been around in some form since it was first addressed by Darwin in 1840. A study of island formation, however, shows different geologic events contributing to the genesis of different kinds of islands. The following discussion of island origins is limited to islands that do not have the geological characteristics of continents, namely, oceanic islands and coral islands.

Oceanic islands

The development of plate tectonics theory in the 1960s greatly aided scientists understanding of the genesis of islands. Oceanic islands originate in volcanic action typically associated with the movement of the lithospheric plates.

The lithosphere is the major outer layer of the earth. It consists of the crustboth continental and oceanic-and upper mantle, and ranges from the surface to 62 mi (100 km) deep, although subducted crust has been remotely detected at depths of 620 mi (1,000 km). (For comparison, the average radius of Earth is 4,173 mi [6,731 km].) The lithosphere is divided into rigid, interlocking plates that move with respect to one another.

There are 11 major plates (two of which seem to be fracturing) and many smaller ones. The plates move over the next lower layer, the asthenosphere (a sort of crystalline sludge), perhaps by thermal convection, at an average rate of 4-4.5 in (10-11 cm) per year. The plate boundaries tend to move away from each other at mid-ocean ridges and to approach each other at the edges of continents.

At the mid-ocean ridges, magma wells up and cools, forming mountains. At the same time, existing sea floor spreads apart, and new sea floor is created. As sea-floor spreading continues, the mountains, which sit on ocean crust, are carried away from the mid-ocean ridge and therefore away from the source of new lava deposits. Many of these undersea mountains, as a result, never grow tall enough that their tops could emerge as islands. These submarine mountains are known as seamounts. It is possible to date with some accuracy the age of seamounts by measuring their distance from the ridge where they were born.

A major exception to the nonemergence of mountains formed at mid-ocean ridges is the volcanic island of Iceland, which has had a more complicated history; it was formed both by upwelling magma from the mid-Atlantic ridge and by volcanism over a hot spot deeper in the mantle, which also contributed to upwelling magma in the same area. The hot spot, active for about 55 million years, has since cooled, and the mid-Atlantic ridge has shifted abruptly from its previous position to a position somewhat eastward. It is still active, producing lava from volcanoes, the flows of which sometimes close harbors. Iceland is one of the few places on Earth where a mid-ocean ridge has risen to the lands surface and become visible.

Volcanoes producing lava flows and occasional seamounts, and more rarely emerged islands, characterize divergent plate boundaries at the mid-ocean ridges. However, converging plate boundariestwo plates coming togetherare characterized by volcanoes that often produce emerged islands, as well as by forceful earthquakes and deep oceanic trenches, such as the Marianas Trench. Most plates that converge do so at the edges of continents.

When two plates meet, the plate carrying the heavier oceanic crust is subducted as the plate carrying the lighter continental crust is pushed over it. At the point of subduction a deep trench develops; and parallel to it, on the lighter plate, volcanic action produces a row of islands. (The magma involved in the volcanism comes from melting of the oceanic crust as it is subducted.) These island groups are called island arcs, after their curved pattern. An island arc with active volcanism is called a back arc. Between the back arc of the system and its associated trench there may be a second, nonvolcanic arc of islands, called the front arc, that is thought to be caused by upthrust of crust from the lighter plate. The front arc may lie below the surface of the water and not be readily visible.

Island formation occurs at intraplate locations (anywhere between the boundaries) as well as at plate boundaries. It is reasoned, with strong scientific support, that some of these mid-plate volcanic islands resulted from passage of a lithospheric plate over a thermal plume rising from a fixed hot spot deeper in the mantle layer.

The formation of the Hawaiian-Emperor island chain in the north Pacific is attributed to this mechanism. The hot spot is thought to be located near the Loiki Seamount and to be causing the currently active volcanoes of Mauna Loa and Kilauea on the island of Hawaii. The chain trends northwestward, with the oldest islands at the northwestern end. As the Pacific plate drifts, at a rate of 4 in (10 cm) per year, areas that had been positioned over the hot spot and so subject to volcanism, lava accumulation, and island formation, move off the hot spot and cool down. Their position is then taken by new areas of lithosphere that drift over the thermal plume and a new island begins to form on the sea floor.

This scenario would explain why some of the animals populating the Hawaiian islands are older than the islands themselves. Because of the relative closeness of the islands to each other and the slow pace of the Pacific plates drift, animals theoretically could have had time to raft, swim, or fly from the older, now submerged islands at the northern end of the chain toward the younger, more southerly islands, which are not submerged. By island hopping over geological time, some of the original species may eventually have made their way to islands that had not yet come into existence at the time the animals established a presence on the islands.

The hot spot model works well for the Hawaiian-Emperor island chain but does not explain all intraplate clusters or chains of islands. Indeed, one of the problems facing oceanographers is the association of islands whose formation can easily be described by global and local conditions with islands that appear not to have been produced by the same processes.

Coral islands

Coral islands are (usually) low-lying islands formed by hermatypic, or reef-building, corals, chiefly scleractinian corals and hydrocorallians. Reef-building corals occur in a broad band stretching around the globe from 25 degrees north of the equator to 25 degrees south of the equator and require an average water temperature of about 6877°F (20-25°C). They do not grow below 165 ft (50 m) in depth. They also have specific needs for water salinity, clarity, calmness, and sunlight. Sunlight aids in formation of the living corals exoskeleton, and so aids in reef-building. Corals anchor on somethingseamounts, submarine slopes of islands, or debris such as abandoned army vehicles and bedspringsand therefore are generally found at the edges of continents or existing islands. If the surface of a reef emerges into the airthrough, for example, a slight drop in sea levelthe creatures dry up and die. The exposed, dead surface of the reef then serves as a platform for the accumulation of sediment, which may in turn become sufficient to support plant and animal life. Thus, offshore islands in tropical and semitropical zones around the world often have a core of emerged, dead coral reef. For example, a reef that emerged in about 3450 BC provided the base on which all of the islands in the Maupihaa Atoll, in the Society Islands, are founded. Indeed, study of the rate of uplift of emerged coral reefs has helped scientists determine local sea levels in past eras.

Island biogeography

Islands may be regarded as closed ecosystems. Although this is not true in every casewitness the island-hopping of species on the Hawaiian-Emperor island chainor at all times for any given island, the relative isolation of islands has made them an ideal setting in which to explore theories of evolution and adaptation.

Two words frequently used in relation to island environments are equilibrium and change. Ecosystems in equilibrium are assumed to have reached steady state, with very slow rates of change. No more is taken out of the ecosystem than is replenished; predator-prey relationships remain constant, and die-offs are balanced by new colonization. Whereas the individual species involved in these interactions may change, the overriding patterns do not. The equilibrium model of insular biogeography was formally stated in the 1960s by R. H. MacArthur and E. O. Wilson and has been used to, among other things, establish and manage natural preserves on both islands and mainlands.

Environments in equilibrium are, of course, subject to change. A catastrophic storm can destroy a large sector of the biota; land bridges come and go. The changes introduced by the entry of other life forms into the closed system of an island, however, can have dramatic immediate effects. Island environments may have permissive or controlling effects on organisms that attempt to establish a presence. Among the permissive effects, or opportunities for colonization, is the availability of unoccupied biological niches. Unfilled niches appear to hasten organismal radiation, the evolutionary branching of species. On the Hawaiian islands, for example, the native, or endemic, family of birds known as honeycreepers has branched into 23 species, many adapted to different feeding nichesseeds, insects, and so on. Further, the beaks of the birds have adapted to extracting the different diets from different tree speciesa remarkable series of adaptations.

Among the controlling effects that islands have on would-be immigrants is simple inhospitality. Volcanic or coral islands lacking sufficient layers of sediment to grow plants, for example, would not be attractive or even feasible as a home for many kinds of animals, including agricultural humans.

Once a breeding pair of immigrants has successfully penetrated the isolation of an island and taken up residence there, it can profoundly affect the existing dynamics of the islands ecosystem. Human-induced change is particularly devastating to islands. The domestic goats and rabbits introduced by human colonizers can denude a small island of succulent vegetation in less than a year, and dogs can turn small mammals into prey. If plants to a goats liking are not available or if steep ravines effectively corral dogs activity, the ecosystem effects may be finiteif one does not consider the ticks and other disease carriers that may be introduced with the immigrants. Thus, the interactions between islands and migrant species are a two-way street; migrant species propose entering (and potentially changing) the closed system of an islands environment, and the environment permits or controls the success of such entry.

The question of closed versus open systems becomes highly interesting in the case of endemic island speciesspecies native to an island, and perhaps found nowhere else. Did they evolve in place from an extinct ancestor? Did they island-hop from now drowned islands? If a land bridge was ever available, were the species around to use it? Local conditions rather than grand theories are usually called on to answer such questions, although the answers may in turn support grand theories. In many cases the answers remain perplexing. For example, it is estimated that more than 40% of the species of marine molluscs on the shores of Easter Island and a neighboring island are endemic. This is a startlingly high figure, for the islands are considered too young for evolution alone to have resulted in such prolific and successful branching. The molluscs may have originated from the shores of drowned islands in the region. The finding of old endemic species on young islands has led to some changes in the temporal boundaries of the geologic time scale, which are linked to index species.

The closed-system model of islands is useful for making inferences about evolution and adaptation but does not necessarily agree completely with reality. Immigrant species do colonize islands, with the colonization rate correlated with closeness to the mainland and size of the animal. Animals reach islands by swimming, rafting (on floating logs or matted leaves), flying, transport by carriers (a tick on a dog), or walking on frozen ice. The success rate need not be very high to develop thriving animal populations on islands. It is estimated that the arrival of one breeding pair every few hundred thousand years would have been enough to build the rich species diversity of the Philippine Islands. Over geological time, that amounts to a large number of accidental tourists.

A second metaphor has therefore arisen, that of islands (more specifically, interisland distance) as a filtering mechanism. The filter has, again, permissive and controlling effects. In a hypothetical series of five islands extending outward in a line from a species-rich mainland, large mammals may never get beyond the first island, small mammals and tortoises may be filtered out by the third island, and the fifth island may be colonized only by fliersbirds, bats, and insects. Such a filtering effect has been recorded in islands extending eastward from New Guinea; the last wallabies and marsupials occur on the close islands New Britain and New Ireland, the last frogs on the Solomon islands, the last snakes on Fiji, and the last lizards on the island of Tonga. Biologists studying the biota on filtering islands consider the energy expenditure needed for animals to reach a distant goal and the adaptations species may have had to make to consume local food.

Island economics

Islands provide a variety of economic features. In addition to fish (and animals that feed on fish) as a food source, shells have been used as money and exported in jewelry. Coral has many uses, including manufacture into road-building material, jewelry, and small implements. Harbors promote ocean trade. Snorkeling draws tourists, and some tropical woods are in high demand.

Island ecosystems, however, are coming under intense pressure from human use as industrialization continues. Management of island resources by legislation that prevents or limits certain activities has not worked well in developing countries, where individuals increasingly rely on harvest of local resources for subsistence or to improve their standard of living. Islands with developing economies may also lack scientists and government ministers trained in the long-term

KEY TERMS

Biogeography The distribution and relationship of plants and animals to a geographic locale.

Island arc An curved row of islands of volcanic origin that develops where two lithospheric plates converge, usually near the edge of a continent, and associated with the formation of a deep trench parallel to the arc as oceanic crust is subducted.

Magma Hot, liquid material that underlies areas of volcanic activity and forms igneous rock; magma at Earths surface is called lava.

care of island ecosystems. Such situations are being addressed on several fronts. International attention has been directed toward the renewable use of resources and the training of island biologists. Island and marine parks have been proposed. As some island species are approaching extinction before their origins are known, scientists are increasingly concerned about raising awareness of the special features of islands and their contributions to geological and evolutionary knowledge.

Resources

BOOKS

Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, NJ: Prentice Hall, 2004.

Marjorie Pannell

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Island

Island

An island is an area of land, smaller than a continent , that is entirely surrounded by water . That distinction, although somewhat artificial, suggests different geologic forces acting to create and maintain islands versus continents. Islands further differ from continents in their natural environments—in the biological systems they support, in their rate of response to change, in their ability to recover from ecological disaster.

The plants and animals found on islands often seem an odd assemblage. Some in fact are odd, in the sense that they live nowhere else. The seemingly skewed distribution of populations on islands, compared with that on a mainland, results in part from the small size of islands; they cannot carry a representative zoo, and, without migration in from outside, the extinction of one life form might leave a gaping hole in the biota. Also, the water surrounding islands acts as a barrier to the passage of some life forms, particularly large mammals , but encourages the migration of others, such as birds and insects . Because of their relative isolation and the potentially unique biota that may be established on them, islands have been known, at least since the time of Charles Darwin, as natural laboratories of evolution .

Island types

The islands discussed here are of three kinds: continental islands, oceanic islands, and coral islands. Not discussed are inland islands, such as islands found in the middle of a lake .


Continental islands

Continental islands are parts of the continental shelf that rise above the surrounding water. That is, they are situated on the shallow water margin of a continent, usually in water less than 600 ft (200 m) deep. Greenland, the largest island in the world, and Newfoundland are examples of continental islands. A drop in sea level would be sufficient to connect these islands to the North American continent.

Another, rarer kind of continental island consists of small pieces of continental material that broke away from a land mass. These islands are now part of a separate crustal plate that is following an independent path. The Seychelles in the Indian Ocean were once associated with the Madagascar-India portion of the supercontinent Pangaea. With the breakup of Pangaea about 200 million years ago, the Seychelles began their independent existence. Their continental basement structure, however, clearly associates them with the continents rather than with oceanic islands of volcanic origin.


Oceanic islands

Oceanic islands arise from volcanic action related to the movement of the lithospheric, continent-bearing crustal plates. Unlike continental islands, oceanic islands grow from oceanic crust. Oceanic islands are not scattered haphazardly about the deep ocean waters but are aligned along converging oceanic plate boundaries or along the mid-ocean ridges, or diverging oceanic plate boundaries, associated with sea-floor spreading. In addition, some arose as oceanic plates moved over fixed hot spots in the deeper mantle.


Coral islands

Coral islands are distinct from both continental islands and oceanic islands in that they are formed of once living creatures, the corals, which colonize in place to form coral reefs.


Barrier islands

Unrelated to this three-part classification of islands are islands of a fourth kind, barrier islands . Barrier islands occur in shallow-water coastal areas and are composed of unconsolidated sediment, usually sand . Barrier islands form 15% of all the coastline in the world, including most of the coastline of the continental United States and Alaska, and also occur off the shores of bays and the Great Lakes. Some barrier islands are stable enough to support houses or an airport runway; others are short-lived, ripped up annually by winter storms and reestablished by wave and tidal action. As their name suggests, they afford some protection to the mainland from erosion .


How many islands?

Islands are intrinsically impermanent. The more stable oceanic islands last a relatively brief time of 5-10 million years. Some islands drown, a result of erosion, subsidence of the ocean crust, or rising sea level. Sea levels are related in part to the amount of water bound up in the polar ice caps or released into the oceans; and the size of the polar ice caps is related to a variety of factors including variations in the positions of continents, the orientation of the earth's axis, and the amount of cloud cover.

Sea level is fairly high now; it was lower during the Little Ice Age, circa fourteenth to nineteenth century, and even lower about 18,000 years ago. A lowering of sea level brings back into view drowned islands.

Ongoing volcanism continues to add to existing islands and create new ones; an example is Surtsey, off the southern coast of Iceland, which came into existence with a submarine volcanic explosion on November 14, 1963, and has continued to accrete surface area as the ongoing lava flows cool. There are also islands that appear intermittently.

Because islands come and go, the number of islands in existence cannot be established except in relation to a proscribed time period—a human generation, or a century or two. With the discovery of some islands in the Russian Arctic in the mid-twentieth century, however, it is thought that no islands remain to be discovered in our time. Satellite and ship-based scanning equipment is now being used to search for islands whose positions appear on nautical maps but which have themselves disappeared, and to identify underwater sites of new island formation.


Island formation

The classification of islands based on their foundation—continental crust versus oceanic crust versus corals—as given above has been around in some form since it was first addressed by Darwin in 1840. A study of island formation, however, shows different geologic events contributing to the genesis of different kinds of islands. The following discussion of island origins is limited to islands that do not have the geological characteristics of continents, namely, oceanic islands and coral islands.

Oceanic islands

The development of plate tectonics theory in the 1960s greatly aided scientists' understanding of the genesis of islands. Oceanic islands originate in volcanic action typically associated with the movement of the lithospheric plates.

The lithosphere is the major outer layer of the earth . It consists of the crust—both continental and oceanic-and upper mantle, and ranges from the surface to 60 m (100 km) deep, although subducted crust has been remotely detected at depths of 620 mi (1,000 km). (For comparison, the average radius of the earth is 4,173 mi [6,731 km].) The lithosphere is divided into rigid, interlocking plates that move with respect to one another.

There are 11 major plates (two of which seem to be fracturing) and many smaller ones. The plates move over the next lower layer, the asthenosphere (a sort of crystalline sludge), perhaps by thermal convection , at an average rate of 4-4.5 in (10-11 cm) per year. The plate boundaries tend to move away from each other at mid-ocean ridges and to approach each other at the edges of continents.

At the mid-ocean ridges, magma wells up and cools, forming mountains . At the same time, existing sea floor spreads apart, and new sea floor is created. As sea-floor spreading continues, the mountains, which sit on ocean crust, are carried away from the mid-ocean ridge and therefore away from the source of new lava deposits. Many of these underseas mountains, as a result, never grow tall enough that their tops could emerge as islands. These submarine mountains are known as seamounts . It is possible to date with some accuracy the age of seamounts by measuring their distance from the ridge where they were born.

A major exception to the nonemergence of mountains formed at mid-ocean ridges is the volcanic island of Iceland, which has had a more complicated history; it was formed both by upwelling magma from the mid-Atlantic ridge and by volcanism over a hot spot deeper in the mantle, which also contributed to upwelling magma in the same area. The hot spot, active for about 55 million years, has since cooled, and the mid-Atlantic ridge has shifted abruptly from its previous position to a position somewhat eastward. It is still active, producing lava from volcanoes, the flows of which sometimes close harbors. Iceland is one of the few places on Earth where a mid-ocean ridge has risen to the land's surface and become visible.

Volcanoes producing lava flows and occasional seamounts, and more rarely emerged islands, characterize divergent plate boundaries at the mid-ocean ridges. However, converging plate boundaries—two plates coming together—are characterized by volcanoes that often produce emerged islands, as well as by forceful earthquakes and deep oceanic trenches, such as the Marianas Trench. Most plates that converge do so at the edges of continents.

When two plates meet, the plate carrying the heavier oceanic crust dips under, or subducts, and the plate carrying the lighter continental crust rides over it. At the point of subduction a deep trench develops; and parallel to it, on the lighter plate, volcanic action produces a row of islands. (The magma involved in the volcanism comes from melting of the oceanic crust as it is subducted.) These island groups are called island arcs, after their curved pattern. An island arc with active volcanism is called a back arc. Between the back arc of the system and its associated trench there may be a second, nonvolcanic arc of islands, called the front arc, that is thought to be caused by upthrust of crust from the lighter plate. The front arc may lie below the surface of the water and not be readily visible.

Island formation occurs at intraplate locations (anywhere between the boundaries) as well as at plate boundaries. It is reasoned, with strong scientific support, that some of these mid-plate volcanic islands resulted from passage of a lithospheric plate over a thermal plume rising from a fixed hot spot deeper in the mantle layer.

The formation of the Hawaiian-Emperor island chain in the north Pacific is attributed to this mechanism. The hot spot is thought to be located near the Loiki Seamount and to be causing the currently active volcanoes of Mauna Loa and Kilauea on the island of Hawaii. The chain trends northwestward, with the oldest islands at the northwestern end. As the Pacific plate drifts, at a rate of 4 in (10 cm) per year, areas that had been positioned over the hot spot and so subject to volcanism, lava accumulation, and island formation, move off the hot spot and cool down. Their position is then taken by new areas of lithosphere that drift over the thermal plume and a new island begins to form on the sea floor.

This scenario would explain why some of the animals populating the Hawaiian islands are older than the islands themselves. Because of the relative closeness of the islands to each other and the leisurely pace of the Pacific plate's drift, animals theoretically could have had time to raft, swim, or fly from the older, now submerged islands at the northern end of the chain toward the younger, more southerly islands, which are not submerged. By island hopping over geological time, some of the original species may eventually have made their way to islands that had not yet come into existence at the time the animals established a presence on the islands.

The hot spot model works well for the Hawaiian-Emperor island chain but does not explain all intraplate clusters or chains of islands. Indeed, one of the problems facing oceanographers is the association of islands whose formation can easily be described by global and local conditions with islands that appear not to have been produced by the same processes.


Coral islands

Coral islands are (usually) low-lying islands formed by hermatypic, or reef-building, corals, chiefly scleractinian corals and hydrocorallians. Reef-building corals occur in a broad band stretching around the globe from 25 degrees north of the equator to 25 degrees south of the equator and require an average water temperature of about 68–77°F (20–25°C). They do not grow below 165 ft (50 m) in depth. They also have specific needs for water salinity, clarity, calmness, and sunlight. Sunlight aids in formation of the living corals' exoskeleton, and so aids in reef-building. Corals anchor on something—seamounts, submarine slopes of islands, or debris such as abandoned army vehicles and bedsprings—and therefore are generally found at the edges of continents or existing islands. If the surface of a reef emerges into the air—through, for example, a slight drop in sea level—the creatures dry up and die. The exposed, dead surface of the reef then serves as a platform for the accumulation of sediment, which may in turn become sufficient to support plant and animal life. Thus, offshore islands in tropical and semitropical zones around the world often have a core of emerged, dead coral reef. For example, a reef that emerged in about 3450 b.c. provided the base on which all of the islands in the Maupihaa Atoll, in the Society Islands, are founded. Indeed, study of the rate of uplift of emerged coral reefs has helped scientists determine local sea levels in past eras.


Island biogeography

Islands may be regarded as closed ecosystems. Although this is not true in every case—witness the island-hopping of species on the Hawaiian-Emperor island chain—or at all times for any given island, the relative isolation of islands has made them an ideal setting in which to explore theories of evolution and adaptation .

Two words frequently used in relation to island environments are equilibrium and change. Ecosystems in equilibrium are assumed to have reached steady state, with very slow rates of change. No more is taken out of the ecosystem than is replenished; predator-prey relationships remain constant, and die-offs are balanced by new colonization. Whereas the individual species involved in these interactions may change, the overriding patterns do not. The equilibrium model of insular biogeography was formally stated in the 1960s by R. H. MacArthur and E. O. Wilson and has been used to, among other things, establish and manage natural preserves on both islands and mainlands.

Environments in equilibrium are, of course, subject to change. A catastrophic storm can destroy a large sector of the biota; land bridges come and go. The changes introduced by the entry of other life forms into the closed system of an island, however, can have dramatic immediate effects. Island environments may have permissive or controlling effects on organisms that attempt to establish a presence. Among the permissive effects, or opportunities for colonization, is the availability of unoccupied biological niches. Unfilled niches appear to hasten organismal radiation , the evolutionary branching of species. On the Hawaiian islands, for example, the native, or endemic , family of birds known as honeycreepers has branched into 23 species, many adapted to different feeding niches—seeds, insects, and so on. Further, the beaks of the birds have adapted to extracting the different diets from different tree species—a remarkable series of adaptations.

Among the controlling effects that islands have on would-be immigrants is simple inhospitality. Volcanic or coral islands lacking sufficient layers of sediment to grow plants, for example, would not be attractive or even feasible as a home for many kinds of animals, including agricultural humans.

Once a breeding pair of immigrants has successfully penetrated the isolation of an island and taken up residence there, it can profoundly affect the existing dynamics of the island's ecosystem. Human-induced change is particularly devastating to islands. The domestic goats and rabbits introduced by human colonizers can denude a small island of succulent vegetation in less than a year, and dogs can turn every small mammal into prey . If plants to a goat's liking are not available or if steep ravines effectively corral dogs' activity, the ecosystem effects may be finite—if one does not consider the ticks and other disease carriers that may be introduced with the immigrants. Thus, the interactions between islands and migrant species are a two-way street; migrant species propose entering (and potentially changing) the closed system of an island's environment, and the environment permits or controls the success of such entry.

The question of closed versus open systems becomes highly interesting in the case of endemic island species—species native to an island, and perhaps found nowhere else. Did they evolve in place from an extinct ancestor? Did they island-hop from now drowned islands? If a land bridge was ever available, were the species around to use it? Local conditions rather than grand theories are usually called on to answer such questions, although the answers may in turn support grand theories. In many cases the answers remain perplexing. For example, it is estimated that more than 40% of the species of marine molluscs on the shores of Easter Island and a neighboring island are endemic. This is a startlingly high figure, for the islands are considered too young for evolution alone to have resulted in such prolific and successful branching. The molluscs may have originated from the shores of drowned islands in the region. The finding of old endemic species on young islands has led to some changes in the temporal boundaries of the geologic time scale, which are linked to index species.

The closed-system model of islands is useful for making inferences about evolution and adaptation but does not necessarily agree with reality. Immigrant species do colonize islands, with the colonization rate correlated with closeness to the mainland and size of the animal. Animals reach islands by swimming, rafting (on floating logs or matted leaves), flying, transport by carriers (a tick on a dog), or walking on frozen ice. The success rate need not be very high to develop thriving animal populations on islands. It is estimated that the arrival of one breeding pair every few hundred thousand years would have been enough to build the rich species diversity of the Philippine islands. Over geological time, that amounts to a large number of accidental tourists.

A second metaphor has therefore arisen, that of islands (more specifically, interisland distance) as a filtering mechanism. The filter has, again, permissive and controlling effects. In a hypothetical series of five islands extending outward in a line from a species-rich mainland, large mammals may never get beyond the first island, small mammals and tortoises may be filtered out by the third island, and the fifth island may be colonized only by fliers—birds, bats , and insects. Such a filtering effect has been recorded in islands extending eastward from New Guinea; the last wallabies and marsupials occur on the close islands New Britain and New Ireland, the last frogs on the Solomon islands, the last snakes on Fiji, and the last lizards on the island of Tonga. Biologists studying the biota on filtering islands consider the energy expenditure needed for animals to reach a distant goal and the adaptations species may have had to make to consume local food.


Island economics

Islands provide a variety of economic features. In addition to fish (and animals that feed on fish) as a food source, shells have been used as money and exported in jewelry. Coral has many uses, including manufacture into road-building material, jewelry, and small implements. Harbors promote ocean trade. Snorkeling draws tourists, and some tropical woods are in high demand.

Island ecosystems, however, are coming under intense pressure from human use as industrialization continues. Management of island resources by legislation that prevents or limits certain activities has not worked well in developing countries, where individuals increasingly rely on harvest of local resources for subsistence or to improve their standard of living. Islands with developing economies may also lack scientists and government ministers trained in the long-term care of island ecosystems. Such situations are being addressed on several fronts. International attention has been directed toward the renewable use of resources and the training of island biologists. Island and marine parks have been proposed. As some island species are approaching extinction before their origins are known, scientists are increasingly concerned about raising awareness of the special features of islands and their contributions to geological and evolutionary knowledge.


Resources

books

Bakus, Gerald J., et al. Coral Reef Ecosystems. Rotterdam: A. A. Balkema, 1994.

Davis, Richard A., ed. Geology of Holocene Barrier IslandSystems. New York: Springer-Verlag, 1994.

Dubinsky, Z., ed. Coral Reefs. Ecosystems of the World 25. (series ed., David D. Goodall). Amsterdam: Elsevier, 1990.

Nunn, Patrick D. Oceanic Islands. Oxford, England: Blackwell Publishers, 1994.


Marjorie Pannell

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Biogeography

—The distribution and relationship of plants and animals to a geographic locale.

Island arc

—An curved row of islands of volcanic origin that develops where two lithospheric plates converge, usually near the edge of a continent, and associated with the formation of a deep trench parallel to the arc as oceanic crust is subducted.

Magma

—Hot, liquid material that underlies areas of volcanic activity and forms igneous rock; magma at the earth's surface is called lava.

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"Island." The Gale Encyclopedia of Science. . Encyclopedia.com. 10 Sep. 2018 <http://www.encyclopedia.com>.

"Island." The Gale Encyclopedia of Science. . Encyclopedia.com. (September 10, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/island-0

"Island." The Gale Encyclopedia of Science. . Retrieved September 10, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/island-0

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http://www.mla.org/style

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http://www.chicagomanualofstyle.org/tools_citationguide.html

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http://apastyle.apa.org/

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