What are the islands by origin. Platform continental islands. See what "mainland islands" are in other dictionaries

The islands are divided into two large groups: continental and oceanic.

mainland islands- these are sections of the mainland that separated from it (i.e., became islands) due to the flooding of parts of the mainland by the sea that experienced splits or subsidence under the influence of tectonic forces. In most cases, the separation is purely external, reflected in the fact that the surface of the island and the surface of the mainland are separated by a water surface, and do not merge into one another without interruption. In fact, by their foundations, the continental islands continue to be connected with the body of the continent, since these foundations rest either on the continental shelf ( New Earth, O. Wrangel, Newfoundland, Great Britain with Ireland, Tasmania, etc.), or on the continental slope (Madagascar, Ceylon, Sardinia, Corsica, etc.).

Oceanic islands have nothing to do with the continent, that is, they have never been part of it and arose completely independently of the continents. According to the method of occurrence, oceanic islands are volcanic and coral.

Start volcanic island gives a volcanic eruption at the bottom of the sea. In shallow places, an island may appear in one go, but those islands that are now rising from great depths, appeared on the day surface, probably as a result of repeated eruptions and successive accumulation of volcanic products. The growth of the island continues after its appearance above sea level, and volcanic islands are almost always of considerable height. In this respect, Fr. Hawaii, consisting of five merged volcanoes, of which Mauna Loa and Kilauea are still active. Mauna Loa is submerged by its base to a depth of 4600 m, and its height above the sea is 4166 m; thus, this gigantic volcanic structure rises 8766 m above the seabed, that is, almost to the height of Everest.

Volcanic islands are scattered throughout the oceans under the most diverse latitudes. In the Mediterranean Sea, the most famous islands are the Aeolian, Pantelleria, Santorini, Atlantic Ocean- Jan Mayen, Azores, Madeira, Ascension, St. Helena, Gough, Tristan da Cunha, Bouvet, etc. There are fewer of them in the Indian Ocean (Crozet, Kerguelen, Amsterdam, etc.).

The Pacific Ocean is especially rich in volcanic islands. Here they are located both in the center (many islands of Oceania, Hawaiian, etc.) and along the outskirts (Galapagos, Juan Fernandez, a number of small Sunda, Easter Island, etc.).

Coral islands are one of the varieties of buildings erected by corals - marine colonial organisms with a calcareous skeleton. Calcareous algae, foraminifers, pelecypods, gastropods, etc. also participate in the construction of coral structures. The structure grows due to the fact that new animals settle on the skeletons of dead animals, which in turn are replaced by other generations. The growth of one meter of polyp forest requires, depending on the conditions of the coral habitat, from 35 to 335 years.

Corals are very demanding on habitat conditions, and therefore the geographical distribution of coral structures is strictly limited. Reef-building corals can exist only in warm (with a temperature of at least 18 °) waters, as a result, neither in polar nor in temperate latitudes coral formations do not arise unless these temperate latitudes are heated by warm sea currents, such as Bermuda (the existence of these coral islands is associated with the Gulf Stream). Due to the high demand for heat and light, coral colonies at depths exceeding 40-60 m also do not live, because there is not enough heat and light. In addition, for the favorable development of corals, normal or increased salinity of sea water and its sufficient saturation with oxygen are necessary. There are no coral colonies against the mouths of the rivers, because the river brings fresh water. They are also absent in such stagnant bays, the shores of which are covered with mangroves, for there is not enough oxygen; for the same reason, any coral structure grows more vigorously on the side facing the ocean than on the opposite side: on the ocean side, the water, set in motion by waves and surf, is continuously mixed with air and is better saturated with oxygen.

Corals, as marine organisms, can only build their structures to the level of the ocean. Coral buildings above the water rise only a few meters, and this is due for the most part heaping of fragments of coral limestone by the action of the surf. On the other hand, the thickness of a coral structure cannot exceed 40-60 m, i.e., the greatest depth of coral habitat. However, in a number of cases, structures were observed that rose excessively high above the water, as well as their thickness, significantly exceeding 60 m (over 300 m in the Funafuti Atoll in the Ellis Islands group). A satisfactory explanation of these cases can only be found on the assumption that either the seabed, on which the base of the coral structure lies, was subjected to uplifts and subsidence, or the ocean level slowly rose and fell.

If the level of the ocean does not change, but oscillations of the bottom occur, then with a slow sinking of the bottom, the coral structure, continuing to grow upwards, will continuously increase its thickness; the maximum possible value of this power is determined by greatest depth reached by lowering the bottom. It is important that this sinking proceed slowly enough - no faster than the growth of coral upwards; rapid sinking will cause the death of corals, as it will take them to a depth that is not favorable for their existence. When the seabed rises, the coral structure is pushed above sea level; however, its further natural growth upwards is stopped due to drying.

A similar result will be obtained under the condition of the immobility of the seabed, but with fluctuations in the level of the ocean. Here, the rise of the level in its consequences is equivalent to the lowering of the bottom in the first example, and the lowering of the level is equivalent to the rise of the bottom.

Eustatic fluctuations in the level of the ocean (that is, fluctuations covering the entire World Ocean) are quite possible and have been proven in a number of cases. But the assumption of the rise and fall of the seabed seems more convincing, since the fluctuations of the solid earth's crust are a universal and indisputable phenomenon for all periods of the existence of our planet.

Coral islands in their distribution are very often associated with volcanic structures (both those that appeared on the daytime surface, and a little undergrown to it). Without this, the existence of coral islands in the open ocean, in the area of great depths, would be impossible. First, a volcanic cone grows from the bottom of the sea; when its top is so close to the level of the ocean that the depth of the sea in this place becomes favorable for the settlement of corals, colonies of coral polyps begin to develop.

There are three main types of coral structures: coastal reefs, barrier reefs and atolls (lagoon islands). Coastal, or fringing, reefs are closely adjacent to the coast of the mainland or island. The barrier reef is separated from the land by a more or less wide strip of water. An atoll is an island in the form of a ring, closing on all sides the inner calm lagoon; at the same time, the slopes of the atoll descend steeply towards the ocean (up to 45 °), in some places sheer (for example, part of the slopes of Bikini Atoll in the Marshall Islands), and gently into the lagoon.

Coral structures are usually broken. Coastal reefs off New Guinea Solomon Islands, in the New Hebrides are absent in the places of bringing fresh water from land, as well as in areas abounding in mangroves. The same must be said about the barrier reefs that exist around almost all the Pacific islands. volcanic origin, - in Tahiti, Fiji, etc.; there are especially many of them in the Coral Sea. Broken in many places and the Great Barrier Reef, stretching for 1900 km along the Australian mainland from Cape Sandy to the Torres Strait; the inner channel separating the barrier from the mainland is not deep (35-70 m), but its width varies from 35 to 125 km.

There is usually one or more gaps in the atoll ring. The inner lagoon is shallow - no more than 100 m (in Tuamotu, for example, the depth is not more than 25-35 m), and the water in it is exceptionally warm; Thus, Agassiz on Funafuti (in the Ellis Archipelago) at a depth of 27 m found a temperature of + 31 °. Some of the lagoons are really small. inland seas; individual lagoons on the Tuamotu have a length of up to 55-70 km, and in the northern part Maldives up to 140 km (with a width of 30 km). At the same time, the average width of the ring bordering the lagoon usually does not exceed 0.5 km.

There are coral islands in the Atlantic Ocean (Bahamas, Bermuda), in the Red Sea, Indian Ocean (Maldives, Laccadives, Chagos Archipelago), but the classic area of their development is the Pacific Ocean. North and east of coral sea atolls are becoming more and more numerous and in some archipelagos they are already the only type of coral buildings. The most numerous atolls are in the Gilbert, Ellis, Tuamotu archipelagos and on the Marshall and Caroline Islands. Tuamotu has about 70 atolls of regular (round or elliptical) shape; their inland lagoons are themselves often dotted with coral islands.

Of particular interest are the "imperfect atolls", i.e., those in which, among the lagoons, islands are not coral, but composed of igneous rocks. An example is Mangareva (Tuamotu group), Fr. Ermite (west of the Admiralty Islands), in whose lagoon there are four volcanic islets, as well as Truk Lagoon (in the center of the Caroline Group); in the Truk lagoon, with a diameter of about 60 km, there are at least 16 islands resembling mountain peaks, and one of them reaches a height of 480 m above the sea.

Coral islands in the Pacific Ocean are located on underwater heights, elongated, as a rule, from northwest to southeast (Caroline, Marshall, Marquesas, Tuamotu, Samoa, Tubuai, etc.).

The mainland islands (examples will be given below) are part of the land that was previously part of the continent, and later separated from it. This occurs as a result of various hydrological or geological processes. As a rule, the mainland and the island have a fairly similar relief. They are separated by water areas, such as shelf seas and straits. The observation of scientists shows that the distance between the main landmass and the island can vary. This is due to the mobility of the earth's crust.

Islands of mainland origin are divided into several types. All of them are connected with their continents at the genetic level. However, despite this, the flora and fauna of such islands can differ significantly. So, let's look at the types of islands depending on the origin.

Platform mainland islands

The platform islands are, in fact, an extension of the continent. They lie on the continental shelf and are separated from the main landmass by various water areas, such as straits and seas. Islands have this origin. Canadian archipelago, Severnaya Zemlya, Svalbard and British. These land areas practically do not differ from the mainland in flora and fauna. And this is due to the fact that they were formed relatively recently.

Islands of the mainland slope

The second type is the islands of the continental slope. They are not much different from the first ones, but their break with the continent took place a little earlier. Unlike platform ones, their separation from the main land occurred due to deep tectonic splits, and not troughs, as in the first case. mainland island of this type is separated from the continent by an oceanic strait. Vivid examples are about. Greenland and about. Madagascar.

Orogenic islands

The third type is orogenic islands. These land areas are formed from the continuation of the mountain folds of the mainland. These include New Zealand, Tasmania, about. Nova Zemlya, which, in fact, is a continuation Ural mountains. They are all orogenic mainland islands. Examples can go on and on. Sakhalin, which is a continuation of the Far Eastern mountain range.

island arcs

And finally, the most active view mainland islands - island arcs. They are found in large numbers on the coast. East Asia, Central America and between South America and Antarctica. These include the Japanese island arc, the Aleutian, the Philippine and others. It is worth noting that it is these land areas that are currently in the area of \u200b\u200bhigh activity of the earth's crust.

Peculiarities

Due to its remoteness from the main continent and complete isolation from other lands, the mainland island has a high level of endemic flora and fauna. The earlier it separated from the mainland, the more peculiar its plant and animal world. Islands such as Hawaii, Novaya Zemlya are located at a considerable distance from their continents. This led to the formation of more than 70% of endemics in the flora and fauna of these lands. Also, representatives of the animal world live on the islands, which have some deviations from the usual norms. For example, gigantism in reptiles and, conversely, island mammals are usually smaller than on the mainland. The Galapagos tortoises can also be attributed to the first group - they are unusually large sizes. The second includes different kinds hoofed animals.

tasmania island

The mainland island of Tasmania is separated from the mainland by Bass Strait. His geological structure and relief allow us to say that this is a continuation of the East Australian mountains. The island has a unique fauna. Animals from Antarctica are found here, as well as a number of some representatives that have long been destroyed on the mainland.

New Earth

The Novaya Zemlya archipelago is also classified by scientists as a continental type. The main islands are separated from each other by the narrow strait Matochkin Shar. Another archipelago is washed by the strait separating it from the island of Vaygach.

Sakhalin island

Sakhalin Island is a mainland island. Located at east coast Asia. It separates from Fr. Hakkaido, whose minimum width is 40 km, as well as Tatar (from the mainland) and Nevel. The latter freezes in winter and has a width of no more than 8 km.

Islands of New Zealand

The islands of New Zealand have a mainland origin. The arc on which they are located is located from the entire length of Australia. There are many faults in New Zealand, accompanied by volcanic eruptions and earthquakes.

After reviewing the information in the article, everyone will be able to correctly answer which islands are mainland.

general characteristics

The biotas of the islands of the seas and oceans are extremely diverse. Closely related to the conditions of formation of the islands themselves, they, however, do not have a single feature that would be characteristic of all the islands without exception. The diversity of the island's biota depends on its origin, age, size, distance from the mainland and its natural conditions.

According to their origin, islands in the seas and oceans are divided into two main groups - continental and oceanic. .

1. Continental represent a part of the continent that separated in one or another geological epoch. They protrude above the water level within the underwater margin of the continents and are usually composed of bedrock (Great Britain, Greenland, Madagascar, New Zealand, New Caledonia, Hawaiian, etc.). Small ones can be formed as a result of the accumulative activity of waves and surf.

2. Oceanic islands are distributed within the ocean floor and on mid-ocean ridges. These include coral (atolls and reefs) and volcanic islands (Galapagos, Kerguelen, Mascarene, Canary, Easter, etc.).

Between these two groups of islands, continental and oceanic, there are intermediate forms - geosynclinal islands, or islands of the transition zone and island arcs. They differ greatest variety natural conditions and have a more complex structure of landscapes - from flat to mountainous areas. As a rule, all geosynclinal islands form into archipelagos or form island arcs (Great Sunda, Philippine, Japanese, etc.).

The complexity of the ecosystems of the islands can be low, including biogenic (atolls, reefs and mangroves) and shelf, and high (volcanic, geosynclinal and continental slope, or shelf). high islands, in turn, may have a weakly or well-defined altitudinal zonality.

The conditions for the formation of biota on continental and oceanic islands are completely different.

1. The mainland islands separated from the continents with the species diversity of vegetation and animal population that was inherent in this part of the continent.

2. Oceanic islands were inhabited only by organisms capable of crossing large expanses of water. In the first case, the type of biota formation can be defined as relic , in the second - as an immigration .

Biomes of mainland islands

On islands of continental origin, the biota gradually becomes depleted over time due to the extinction of some forms and even species. The extinction of a species can be explained by its small numbers at the time of the separation of the island from the mainland, which does not ensure a long existence in conditions of isolation. Moreover, the death of the species inherited by the island is not compensated by the appearance of immigrants who enter the island, as a rule, in a limited number of individuals and do not always survive.

The gradual extinction of species on the islands can be judged from the fact that small islands of continental origin, in contrast to large ones, have almost purely oceanic fauna. Thus, among the Pearl Islands, about 1/3 of continental species live on the large island of Rey, and only 1/10 on the small island of Kondatora (Voronov, 1987). If two islands of different sizes separated from the mainland at the same time, then the original biota can be almost completely preserved on the larger one and completely or partially disappear on the smaller one.

Features of the biota of the mainland islands

1. Isolation and remoteness from the mainland determine the high endemism of flora and fauna. How ancient island, the more endemic species and forms. For example, species endemism of the flora of New Zealand, New Caledonia, Hawaiian Islands reaches 70-80%. At a lesser distance from the mainland (the islands of Great Britain, Ireland, Japan, Sri Lanka), endemism is less pronounced.

2. On the islands, deviations are often observed in the appearance of certain groups of animals. For example, large mammals are usually smaller than on the mainland (ponies, Philippine buffalo, etc.). In birds and reptiles, on the contrary, island gigantism is expressed (lizards on Komodo Island, turtles on Galapagos Islands). The reason for this phenomenon has not yet been elucidated.

3. Often the islands are characterized by flightless birds and insects. The origin of flightless birds is associated with the absence of mammals on the islands that could exterminate them. In the selection of flightless insects, their drift by wind and hurricanes into the ocean played an important role. For many species of flying insects, an equilibrium is established between the number of individuals blown away by the wind and the number of individuals brought to the island, provided that the island is part of an archipelago.

4. In their distribution, the biocenoses of the islands follow the same zonal patterns as the communities of the continents. However, their structure and energy connections are simpler with a smaller species composition. Only on the big islands mountain systems(Madagascar, New Zealand, Cuba, Great Britain, etc.) the vegetation cover and animal population are no less complex than on adjacent continents. On islands with a simplified landscape characteristic, the communities are more uniform, and the depletion of the species composition is more significant.

Biomes of oceanic islands

On islands of oceanic origin, communities arise on the products of volcanic activity or coral limestones. The flora and fauna of these islands is entirely immigrant, but may be older than the islands themselves.

Features of the biota of oceanic islands

1. The penetration of species from the mainland to one or another island is facilitated by the so-called "land bridges" and by a chain of islands of volcanic or other origin. On some of these islands, the species may have had a temporary home and migrated to the newly formed island as a relic that arose as a result of extinction in neighboring land areas. Thus, the possibility of an element of relict origin of flora and fauna on the islands of oceanic origin is not excluded.

2. The transfer of organisms through body of water for each individual is random. With long-term existence migration process this transfer acquires a certain statistical probability. So, when crossing a space 100 miles wide, only one individual out of a thousand survives, the next 100 miles - again one individual out of a thousand, and so on. The chance of reaching an island located 200 miles from the source of migration is one in a million.

3. The most common ways organisms colonize islands are hydrochory ( sea currents), anemochory (winds, storms, and hurricanes), and zoochory (transport by birds). In the colonization of the islands by plants and animals, man plays an important role (anthropochory).

4. Mostly birds can actively populate the islands, but this process is restrained by the “nesting conservatism” of the birds themselves. Mature plants washed ashore by waves, as a rule, rarely take root. Epiphytes that are on the trunks survive. Spores and light seeds are transported by wind over long distances, and therefore, for example, ferns on the islands are widespread. Insects generally do not tolerate salt water well and colonize the islands if they are carried by wind or birds. perish in sea water amphibians, reptiles and freshwater fish. Of the reptiles on the islands, only geckos and skinks are common. Ultimately, death in sea water leads to a rather poor species composition of these groups of animals. So, in South America there are many amphibians, and on the neighboring Galapagos there are neither frogs nor salamanders, there are no bird species characteristic of the mainland, and from land mammals one genus of bats, rats and a rodent resembling a hamster are represented.

5. Great importance in the settlement of the island by organisms carried by the wind and, to a lesser extent, water, has its "trapping angle" - the location of the island in relation to the flow of migrants. So, if the island is located perpendicular to the flow of migrants, then the probability that the migrant will end up on the island is greater.

The flora of the islands of oceanic origin is characterized by a small number of species composition. Their fauna lacks mammals, amphibians and snakes.

6. The process of speciation on oceanic islands proceeds faster than on the mainland and continents, since, as a rule, a small population of a species with a depleted gene pool is formed on oceanic islands. In addition, this local small population is in geographical isolation. As a result, on different islands, differences arise in the set of genes of the same species, leading to the emergence of intraspecific forms on each island, and subsequently species. In this case, the unfilled ecological niches are also of great importance.

As evidence that reliably confirms the originality of speciation on islands of oceanic origin, one can cite finches living in the Galapagos.

C. Darwin in his diary "A naturalist's journey around the world on the ship" Beagle "" noted that natural history of these islands is highly interesting and well worthy of attention. Most of the organisms living here are natives, not found elsewhere. This "satellite of America", separated from the mainland by space open ocean at 500 - 600 miles, received a few random colonists from the continent. Obviously, one species of finch came to the Galapagos, eating its own food. Finches that did not have enough food died or switched to other food. So, they learned to catch insects, pull out larvae from cracks in tree trunks, crack nuts, eat plant leaves, etc. Ultimately, natural selection led to the formation of specialized forms that filled all the ecological niches on the island intended for songbirds. In the Galapagos, there are 3 genera, 13 species and 37 island forms of finches: large, medium and small ground finches, cactus, thick-billed arboreal, woodpecker, parrot, coconut, mangrove, warbler, etc. Interestingly, the Darwin woodpecker finch, which does not have a long tongue, uses a cactus needle to get an insect from under the bark of a tree.

Quite exotic representatives of the endemic fauna also live on the Galapagos Islands: giant tortoises, ancient marine and terrestrial iguanas, sea lions, lava snakes, flightless cormorants and the Galapagos penguin. Most representatives of the biota of the islands have relatives on the South American continent.

7. A characteristic feature of the biota of the islands is the constant "delivery" of settlers. Species that arrived on the island earlier have an undeniable advantage over ecologically close species that arrived on the island later. The first settlers have more time to reproduce and increase in numbers in the absence of competition or the presence of weaker competition.

8. Each species can naturalize on the island only under the following three conditions:

– successful passage of ecesis (a full development cycle from the appearance on the island to the bringing of viable offspring, inclusive);

– survival in competition with species that previously settled the island;

- adaptation to new conditions of existence.

Stages of species adaptation

A) Already at the stage of ecesis, the formation of a community begins, which consists in the adaptation of organisms to living together.

b) At the next stage, the stage of complex grouping, vegetation with several tiers is formed. At the same time, tall plants protect the lower ones from excess sunlight, wind and salt spray. At the same time, the processes of soil formation are activated.

V) At the final stage, despite the absence of many groups of plants and animals characteristic of the land, a biocenosis is established on the islands of oceanic origin. However, the food chains in it are shortened (usually 2–3, less often 4 links). The poverty of the species floristic composition leads to a low diversity of herbivorous animals and the monotony of predators with their low abundance.

Many representatives of the animal population have a connection with the ocean, going through one or more of their stages of development in it or getting food in the aquatic environment.

9. Insular biotas are characterized by a process of cosmopolitanization, caused by the fact that animals and plants that most successfully overcame oceanic spaces inhabited many islands and are found even in several climatic zones and belts. Most cosmopolitans are on low-lying atolls with the most homogeneous ecological conditions and the lowest species diversity of biota. Paradoxically, cosmopolitanization island biota contribute scientists transporting animals and plants from one island to another.

10. The limited number of populations of the island biota and its isolation lead to the rapid extinction of many species due to direct extermination by humans and violation of habitat conditions. This is how the Steler cow (Commander Islands), the wingless guillemot (Newfoundland), the moa (New Zealand), the dodo (Mauritius), etc., died out. The Javanese and Sumatran rhinos, the Ceylon elephant, Galapagos tortoises and other representatives of a peculiar island biota.

11. The most catastrophic for the flora and fauna of many islands are the deliberate or accidental importation of goats, pigs, dogs, cats, rats, house mice and other synanthropic species, as well as unsuccessful attempts to acclimatize animals - objects of hunting. The introduction of European red deer to New Zealand led to the destruction of forests over a large area. In addition, the possum (herbivorous marsupial) brought from Australia to this archipelago destroyed forests in many areas. Pigs exterminated kiwi, owl parrot and tuatara, which survived only on adjacent small islands. The flora of Saint Helena and Kermadec suffered irreparable losses as a result of the importation of goats.

Unfortunately, there are many examples of senseless destruction of both island ecosystems and representatives of their organic world. In just a few millennia of the development of agriculture and centuries of industrial development, man has made noticeable changes in the biological values of the Earth created by nature over millions of years.

MAINLAND ISLANDS

Land areas that once formed part of the continents, then separated from the latter. At the same time, the continents themselves either still exist, or have long disappeared.

Samoilov K.I. Marine vocabulary. - M.-L.: State Naval Publishing House of the NKVMF of the USSR, 1941

mainland islands

land areas separated from continents as a result of geological or hydrological processes.

Edwart. Explanatory Naval Dictionary, 2010

See what "MAINLAND ISLANDS" is in other dictionaries:

Areas of the mainland that have separated from it due to abrasion or the sinking of part of the land below sea level. To M. o. include naib. major islands of the world: Greenland, Kalimantan, Madagascar ... Natural science. encyclopedic Dictionary

Land areas surrounded on all sides by the waters of the oceans, seas, lakes, rivers. They differ from the mainland in relatively small sizes. There are single islands and their groups (archipelagos). Islands in the oceans and seas are divided into mainland ... ... encyclopedic Dictionary

- (Islands) small parts of the land compared to the continents, surrounded by the sea on all sides. By origin, the islands are divided into mainland, i.e., separated parts of the land, and original, never connected to the mainland, alluvial and oceanic ... ... Marine Dictionary

Land areas surrounded on all sides by the waters of oceans, seas, lakes or rivers. In the seas and oceans, there are single islands and groups (archipelagos). By origin, they are divided into continental (separated from the continents) and oceanic ... ... Marine Dictionary

Land areas surrounded on all sides by the waters of the ocean, sea, lake or river. They differ from the mainland in relatively small sizes. There are single O. and groups of O. Archipelagos. O. in the oceans and seas are divided into continental, ... ... Great Soviet Encyclopedia

O. are called relatively small areas of land, surrounded on all sides by water. They differ from the continents only in size, but the difference between the two is very conditional, since the ratio between the sizes of the penultimate largest ... ... Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron Encyclopedic reference book "Latin America"

In the process of the development of the earth's crust and its interaction with the World Ocean, large and small islands were formed. The total number of islands is constantly changing. Some islands appear, others disappear. For example, deltaic islands are formed and eroded, ice massifs are melting, taken for islands (“lands”), sea braids acquire an island character and, conversely, the islands join the land and turn into peninsulas. Therefore, the area of the islands is calculated only approximately. It is about 9.9 million km 2. About 79% of all island land falls on 28 large islands. Of these, the largest is Greenland (2.2 million km 2).

Among the 28 largest islands the globe includes the following:

1.Greenland

2. New Guinea

3.Kalimantan (Borneo)

4. Madagascar

5. Baffin's Land

6.Sumatra

7.UK

9.Victoria (Canadian Arctic Archipelago)

10. Ellesmere Land (Canadian Arctic Archipelago)

11.Sulawesi (Celebes)

12. South Island of New Zealand

14.North Island of New Zealand

15. Newfoundland

18. Iceland

19. Mindanao

20. New Earth

22. Sakhalin

23. Ireland

24.Tasmania

25.Banks (Canadian Arctic Archipelago)

26. Sri Lanka

27. Hokkaido

Both large and small islands are located either singly or in groups. Groups of islands are called archipelagos. Archipelagos can be compact (for example, Franz Josef Land, Svalbard, Greater Sunda Islands) or elongated (for example, Japanese, Philippine, Greater and Lesser Antilles). Elongated archipelagos are sometimes called ridges (for example, the Kuril ridge, the Aleutian ridge). Archipelagos of small islands scattered across the expanses Pacific Ocean, are combined into the following three large groups: Melanesia, Micronesia (Caroline Islands, Mariana Islands, Marshall Islands), Polynesia.

By origin, all the islands can be grouped as follows:

I. Mainland Islands:

1) platform islands,

2) islands of the continental slope,

3) orogenic islands,

4) island arcs,

5) offshore islands: a) skerry, b) Dalmatian, c) fjord, d) spits and arrows, e) delta.

II. Independent islands:

1) volcanic islands, including a) fissure lava outpouring, b) central lava outpouring - shield and conical;

2) coral islands: a) coastal reefs, b) barrier reefs, c) atolls.

mainland islands genetically related to the continents, but these connections are of a different nature, which affects the nature and age of the islands, their flora and fauna.

platform islands lie on the continental shelf and geologically represent a continuation of the mainland. The platform islands are separated from the main land mass by shallow straits or shallow straits. Examples of platform islands are the following: British Isles, Svalbard archipelago, Franz Josef Land, Severnaya Zemlya, New Siberian Islands, Canadian Arctic Archipelago.

The formation of straits and the transformation of part of the continents into islands date back to recent geological time; therefore, the nature of the island land differs little from the mainland.

Islands of the mainland slope are also parts of the continents, but their separation occurred earlier. These islands are separated from the adjacent continents not by a gentle trough, but by a deep tectonic fault. Moreover, the straits are oceanic in nature. The flora and fauna of the islands of the continental slope is very different from the mainland and is generally insular in nature. Examples of continental slope islands are the following: Madagascar, Greenland, etc.

Orogenic islands are a continuation of the mountain folds of the continents. So, for example, Sakhalin is one of the folds of the Far Eastern mountain country, New Zealand - the continuation of the Urals, Tasmania - the Australian Alps, islands mediterranean sea- branches of alpine folds. The archipelago of New Zealand is also of orogenic origin.

island arcs garlands border East Asia, America and Antarctica. Most large area island arcs is located off the coast of East Asia: the Aleutian ridge, the Kuril ridge, the Japanese ridge, the Ryukyu ridge, the Philippine ridge, etc. The second region of the island arcs is located off the coast of America: the Greater Antilles, the Lesser Antilles. The Tritium Region is an island arc located between South America and Antarctica: an archipelago Tierra del Fuego, Falkland Islands and others. In tectonic terms, all island arcs are confined to modern geosynclines.

Mainland offshore islands have different origins and represent different types of coastline.

Independent islands have never been part of the continents and in most cases formed independently of them. The largest group of independent islands are volcanic.

Volcanic islands are found in all oceans, especially in the areas of mid-ocean ridges. The size and features of volcanic islands are determined by the nature of the eruption. Fissure outpourings of lava create major islands, not inferior in size to the platform ones. The largest island of volcanic origin on Earth is Iceland (103 thousand km 2).

The main mass of volcanic islands is formed by eruptions central type. Naturally, these islands cannot be very large. Their area depends on the nature of the lava. The main lava spreads over long distances and forms shield volcanoes 9 for example, Hawaiian Islands). The eruption of acidic lava forms a sharp cone of a small area.

Coral islands are waste products of coral polyps, diatoms, foraminifera and other organisms. Coral polyps are quite demanding on habitat conditions. They can only live in warm waters with a temperature not lower than 20 0 C, therefore, coral buildings are common only in tropical latitudes and go beyond them only in one place - Bermuda washed by the Gulf Stream.

Depending on their location in relation to modern land, coral islands are divided into the following three groups:

1) coastal reefs,

2) barrier reefs,

3) atolls.

coastal reefs begin directly at the coast of the mainland or island in the low tide and border it in the form of a wide terrace. Near the mouths of rivers and near mangroves, they are interrupted due to low salinity of the water.

barrier reefs located at some distance from land, separated from it by a strip of water - a lagoon. The largest reef at present is the Great Barrier Reef. Its length is about 2,000 km; the width of the lagoon ranges from 35 to 150 km at a depth of 30-70 m. Coastal and barrier reefs border almost all the islands of the equatorial and tropical waters of the Pacific Ocean.

atolls located among the oceans. This - low islands in the form of an open ring 100-200 m; the diameter of the entire atoll ranges from 200 m to 60 km and even more. Inside the atoll there is a lagoon up to 100 m deep. The depth of the strait between the lagoon and the ocean is the same. The outer slope of the atoll is always steep (9 to 45 0) and falls to a depth of hundreds of meters, and sometimes more than a kilometer. The slopes facing the lagoon are flat and inhabited by a variety of organisms.

The genetic relationship of the three types of coral structures is still an unresolved scientific problem. According to Charles Darwin's theory, barrier reefs and atolls are formed from coastal reefs with the gradual subsidence of islands. At the same time, the growth of corals compensates for the lowering of its base; a lagoon appears on the site of the top of the island, and the coastal reef turns into a ring atoll.