How are continental islands formed? Mainland Island - what is it? Definition, types and examples. Patterns of the location of the continents

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 almost always differ in 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 are the Aeolian Islands, Pantelleria, Santorini, Atlantic Ocean- Jan Mayen, Azores, Madeira, Ascension, St. Helena, Gough, Tristan da Cunha, Bouvet, etc. IN Indian Ocean there are fewer of them (Krozet, 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 represent 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, normal or high salinity is necessary for the favorable development of corals. sea ​​water and adequate oxygen saturation. 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 open ocean, in the area of ​​distribution of great depths. 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 of 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.).

Large and small islands formed - land areas in the sea, ocean, lake, river, surrounded on all sides by water. The area of ​​all the islands of the planet is 9.9 million km2, which is equal to the area. However, this figure is approximate, as total number Islands are constantly changing: some islands appear, others disappear. About 79% of all island land falls on 28 large islands. Of these, the largest -.

The islands are located singly or in groups. Groups of islands lying at a short distance from each other and having a common underwater base are called archipelagos (, Svalbard, Aleutian, Japanese, Greater and Lesser Antilles).

By origin, all the islands can be divided into groups:

mainland islands.These are separated parts of the mainland, similar to it in and. They are separated from the main land mass by straits or shelf seas ( , Canadian archipelago). Many mainland islands are a continuation of the mountain folds of the mainland. So, - one of the folds of the Far Eastern mountain country; - continuation ; - . The time of formation of these islands is different.

Volcanic islands.These are independent islands, they have never been part of the mainland. there are islands in all oceans, with the exception of. The largest island of volcanic origin on Earth -. Many volcanic islands appeared during the spreading of lava. This group includes such islands as the Hawaiian archipelago, Ascension Island.

The typical form of which is atolls. This ring island in the form of a narrow ridge, continuous or broken, surrounding a shallow and well-heated lagoon. The chief builder of these islands giant size are small marine inhabitants - coral polyps. They extract lime from sea water and build shell houses from it, which, when joined together, form straight vertical columns or coral thickets similar to dense bushes. These tiny animals live shallow, because they love warm clear water, - either off the coast or on the tops of seamounts. Having lived their short life, the corals die, and new polyps erect their thickets on the empty houses. Under the weight of the upper buildings, the lower ones are compressed, forming a solid foundation. Centuries pass, and on this foundation there are chains of coral rocks - reefs. If coral buildings were erected along the edges of a round crater underwater volcano, then an island is formed, having the shape of a ring with a lagoon in the middle. In it, the ocean water, which enters here at the time, acquires a surprisingly pure azure color, which creates a special charm for which the atolls are famous.

Most coral structures (reefs and atolls) are found in warm waters and, they are often found in entire archipelagos. Atoll islands can be from 2 to 90 km across.

Coral structures provide convincing evidence that some areas of the ocean floor are rising and others are sinking. If the ocean floor were motionless, then the height of such buildings would not exceed 50 meters. But studies prove that they often reach a thickness of many hundreds of meters and are located at depths up to 1600 m. The only reason for such an arrangement of these structures may be the sinking of the ocean floor, along with which the entire coral structure sinks. Thanks to this, the upper, youngest parts of the buildings get the opportunity to gradually build on. In other places, coral structures rise tens of hundreds of meters above sea level. This means that the seabed has risen in these areas.

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 most big 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, Kuril ridge, Aleutian Ridge). archipelagos small islands scattered across the expanse Pacific Ocean, are combined into the following three large groups: Melanesia, Micronesia (Caroline Islands, Mariana Islands, Marshall Islands a) 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 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 mountainous country, New Zealand- continuation of the Urals, Tasmania - 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 are off the coast East Asia: Aleutian ridge, Kuril ridge, Japanese ridge, Ryukyu ridge, Philippine ridge, etc. The second region of 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 are different types 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 live only in warm waters with a temperature of at least 20 0 С, 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.

An island is a piece of land that is washed on all sides by water - ocean, sea, lake or river. In the case of the ocean, the island is smaller than the mainland.

With the separation of land and waters of the oceans, islands arose. These are areas of land that rise above the surface of the water that surrounds them. Their number is constantly changing - some islands rise from the abyss, others sink under water, ceasing to exist. Some islands are in splendid isolation in the middle of the expanses of water, others are united in companies - groups. If the islands of the group are close to each other and stand on a common underwater basis, in this case they are called archipelago. Such, for example, are Svalbard, the Kuril Islands, Franz Josef Land, the Aleutian Islands, the Greater and Lesser Antilles, and other island groups.

Freshwater islands

Let us immediately separate the islands located in freshwater reservoirs - lakes and rivers - from ocean and sea ones. Lake islands are formed as a result of sedimentation, tectonic processes (bottom uplift), volcanic activity, or changes in water levels. An example of a well-known lake island is Lindau Island, on which one of ancient cities Germany, which is the namesake of the island. Lindau is located on Lake Constance.

River islands are formed in the channel, more often in the lower reaches, where the current is smoother. Distinguish between alluvial and floodplain river islands. The stream carries sand, small pebbles, clay, which are deposited over time, forming an island. Floodplain islands are separated by formed separate branches in the main channel of the river. The largest river island in the world is Bananal, located on the Araguaia River in Brazil.

Salt water islands are divided into continental (continental) and oceanic. mainland islands- These are areas of land that are part of the mainland. They lost contact with him as a result of lowering the surface of the earth or raising the level of the ocean. They are located on the shallows that connect them to the mainland, separated from it by straits or shelf seas. The depth of the seas lying between them is small. usually does not exceed two hundred meters. Such islands separated from the mainland relatively recently. For example, the British Isles became isolated in the Anthropogen, with rising sea levels due to melting glaciers. Sometimes the mainland islands are a continuation of the mountain range. For example, the island of Sakhalin is a continuation of the Far Eastern mountainous country, the island of Novaya Zemlya is a continuation of the Urals, the island of Tasmania is a continuation of the Australian Alps. And sometimes sea currents deposit sand deposits on the shelf, eventually forming an island.

continental islands separate from parent continent deep seas, with depths of hundreds, and often thousands of meters. Such islands may be part of existing continents or the remnants of long-vanished ones. They separated from the continental plates long ago, millions and tens of millions of years ago, usually as a result of a sharp subsidence of a land area and flooding it with ocean waters. In this case, not a gentle depression is formed, but a stepped failure. Continental islands arise when the earth's crust breaks, when a plume rises and rifts form. The earth's crust above the plume is cracking. If the ocean is nearby, the breakaway piece of land is squeezed into it and an island is formed. This is how Madagascar was born.

oceanic islands are not connected to the continents in any way and are located on the oceanic crust. They can be formed as a result of the activity of a volcano and then they belong to the group of volcanic. Or as a result of the work of marine organisms - polyps-corals - and, accordingly, are included in the category of coral islands. But - in order.

Volcanic islands

They are formed when an erupting volcano increases the height of its cone with erupting lava, eventually rising more and more above the sea surface. Similar processes are usually observed at the boundaries of lithospheric plates. By total area created by volcanoes islands larger than Europe.

There are enough representatives of this species among the islands. These include Iceland, Kunashir (the largest in the family Kuril Islands), Easter Island, White Island in New Zealand (pictured). The volcano also created the notorious Krakatau in the group of the Greater Sunda Islands. In the Mediterranean Sea of ​​volcanic origin, the island of Sicily, with famous volcano Etna "on board". There are about four thousand islands of similar origin in Japan, and people live on some of them.

But active volcanoes they know how to create completely different islands, where their labor is used only for initial stage formations. And then marine organisms get down to business, equipping everything to their liking in their new home.

coral islands

The foot of the calmed volcano is found by coral polyps to be very suitable for the construction of their dwellings and are actively developing this, for the time being, free territory. Each new generation of these animals builds its own calcareous formations on buildings of the past generation. The lower layers of buildings under the weight of the upper ones are compacted, forming coral reef. In height, such a colony is up to fifty meters.

As time goes by, as a result of erosion or collapse of the walls of the volcano, it hides under water. And the corals remain, forming a ring of reefs. Its outer side is constantly receiving new portions of nutrients from the ocean, and the polyps located there are happily growing. But the inhabitants of the inner side are less fortunate, there is little food and they stop development. A lagoon filled with water is formed. Sometimes the ring can be open, sometimes solid. Such an island is called an atoll. In diameter, it usually ranges from 2 to 90 kilometers. Over time, the lagoon fills with alluvial sand and the remains of marine organisms. It turns out the island. For example, the islands of the Maldives (pictured), Marshall Islands, Tuamotu Island, Bikini Atoll have a similar origin. All of them are in the warm seas that coral polyps love.

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1. General characteristics.

2. Biomes of mainland islands.

3. Biomes of oceanic islands.

1. General characteristics. The biotas of the islands of the seas and oceans are extremely diverse. Closely related to the conditions for the formation of the islands themselves, however, they 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.

By origin islands in the seas and oceans are divided into two main groups - mainlandAndoceanic.

Mainland represent a part of the continent 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.

oceanic islands 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 are the most diverse 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.).

By complexity of ecosystems 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 weak or well-defined altitudinal zonality.

Conditions for formation of biota on continental and oceanic islands is 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 overcoming large expanses of water. In the first case, the type of biota formation can be defined as relict, in the second, as immigration.

2. Biomes of mainland islands. On islands of mainland origin, the biota gradually evolved over time. impoverished by extinction parts of forms and even species. The extinction of the species can be explained by its small number at the time of the separation of the island from the mainland, which does not ensure long-term existence in conditions 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. So, among the Pearl Islands on big island Rei is inhabited by about 1/3 of continental species, and on the small island of Kondator - only 1/10 (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.