How are coral islands formed? If you had a bad dream.

Biogenic islands are found only in the tropical and equatorial latitudinal zones of the ocean with warm waters. According to the composition of the substrate, atolls, coral reefs and mangrove islands are distinguished. However, the latter are small and have a very limited distribution in the coastal zone. Coral formations are fringing reefs stretching along the coast, or barrier reefs located at a distance from the coast and separated from them by lagoons. Most of the reefs are under water, and only their tops protrude above the ocean level in the form of small islands of intricate outlines, for example, on the Great Barrier Reef off the eastern coast of Australia. Atolls in the ocean arise on the tops of large underwater volcanic mountains or in the process of long-term evolution of an annular fringing reef around volcanic islands, subsequently submerged below sea level and overlain by a layer of coral limestones. As a result, ring low islands are formed, composed of coral sand - a product of the destruction of reefs that surround the inner shallow lagoon, for example, the Caroline, Marshall, Gilbert, Line, Tuamotu islands in the Pacific Ocean, the Malvinas and Chagos islands - in Indian Ocean, the islands of Albuquerque, Saint-Andres, Roncador - in the Atlantic Ocean (Caribbean Sea) and others. These islands are young formations resulting from the superstruction of coral reefs during the Holocene.

From the name it is clear that such names were given to the islands that "grew" from coral reefs. It looks like this. First, an active underwater volcano, having made its last eruption, rises above the surface of the water and fades away. It is surrounded on all sides by coral reefs that reach the ocean floor with their roots. Over time, the volcano subsides or collapses, but the reefs remain in place, repeating its shape, continuing to grow. In the end, only the “deck” of the island remains above the surface with a shallow central lagoon, which shows the mouth of the former volcano.

The central lagoon of the island is the most beautiful place of the island, which is deservedly its attraction.

This type of island is favorite place recreation for people from all over the world, being calling card beautiful Pacific Islands (Figure 4)

Coral reefs grow not only by turning hard corals into sand that raises the level of the seabed. An equally important source of their formation is a sticky substance that is secreted by both polyps and individual algae that settle on them. This substance cements all calcareous residues into an indestructible rock surface.

Figure 4. - Coral Islands. Maldives.

In the tropics, it rains quite often. Then the concentration of salt in the surface layers of sea water drops sharply, and many polyps die. Sometimes clouds of silt and sand float up, which, settling, bury animals under them. Dead coral colonies crumble and turn into coral sand.

Thus, coral formations arise as a result of endless processes of creation and destruction.

People have long been interested in how reefs are formed, especially atolls that occur right in the open ocean.

The famous Russian navigator F. F. Bellingshausen expressed a number of correct thoughts about their nature. The most substantiated theory of the origin of coral reefs was put forward by Charles Darwin. In many respects adhere to it and today .

The formation of atolls does not always fit into the scheme put forward by Darwin. Some of them originate on the tops of underwater volcanoes or on sea shallows. This is evidenced, for example, by the results of drilling of the coastal reef at Pago Pago in Samoa, where the bedrock (not corals) is already at a depth of 35 m from the surface.

The English scientist J. Murray made significant additions to Darwin's theory. He proved that a solid coral reef is bound to transform into a ring reef, and here's why. Corals in the middle part of the reef do not have enough food, they gradually die and collapse, because carbon dioxide accumulates here - a product of the respiration of polyps, which dissolves limestone, and the reef grows only from the outside. This creates a lagoon in the center of the reef.

V. N. Kosmynin, who studied in detail the geomorphology of coral reefs Seychelles, found on them a number of successive stages of formation of the relief of the outer slope. At the first stages, spurs are bands of dense interweaving of branched corals stretched from top to bottom along the slope. Such corals are characterized by rapid growth, and for a long time they keep up with relatively short term form a so-called coral bush on the reefrock. Under the influence of waves, the delicate terminal branches of the colonies break off, and their bases, meanwhile, are cemented by calcareous algae and encrusting corals.

On this, as it were, compressed and therefore more dense vertical strip of coral limestone, branched corals grow again, as if on a burrow, and the formation of a spur passes into the second stage.

The emergence of channels, i.e., notches between the spurs, is partly due to erosion under the influence of water flowing from the reef, which, when the wave retreats, rushes precisely here, since it does not encounter obstacles in the form of coral thickets. However main reason The emergence of channels is still in the growth of corals on spurs. At the last stage, the width of the spurs along the front reaches 3-5 m, and sometimes more, and they begin to close with their sides, and then the channels between them turn into vertical or inclined tunnels.

From what has been said, it is obvious that the reef grows towards the sea due to the formation of spurs and their subsequent confluence. Of course, their erosional destruction is not excluded, but it, apparently, takes place only during very strong storms.

On the above-mentioned reef on the island of Hainan, the system of spurs and channels was in the third, most developed stage.

The ridge crowning the outer slope of the reef rises somewhat above the zero depth level, behind it a more or less flat calcareous platform, or rifflet, stretches towards the coast.

Directly behind the ridge on the rifflet there is almost always a depression with a depth of 50 cm to 1-2 m and a width of several meters. It runs in a winding channel parallel to the outer edge of the reef. As already mentioned above, the crest of the reef is the place of the most active growth of corals, and the so-called algae wall also develops on it due to calcareous algae.

The formation of a rising swell by calcareous red algae precisely at the seaward edge of the rifflet and on the ridge is explained ecological features these plant organisms. They tolerate overheating and drying out much more easily than stony corals. The conditions for periodic exposure and splashing by waves for calcareous crimson, apparently, should be considered optimal: on the one hand, intensive water exchange contributes to the production of calcium carbonate, and on the other hand, when the wave recedes, the plants receive maximum sunlight (V. Kosmynin).

These hermatypic organisms raise the ridge above the level of the reef platform. At a distance of several meters from the edge of the outer slope, there was usually a second, less pronounced ridge. It is obvious that the edge of the reef used to pass along this line, but as a result of the development of the current generation of the spur system, it ended up in the immediate rear.

Since both ridges are located on a horizontal plane, they must be considered in the rifflet structure, however, the genesis different parts the reef platform itself is not the same. If its seaward part arises as a result of the active growth of corals and algae, then the areas lying closer to the coast owe their origin to the accumulation and partial cementation of detrital material, which is formed mainly on the outer slope and ridge and transported from there by waves.

So, on a reef, two main parts should be distinguished - the outer, bioconstructional, created as a result of the vital activity of hermatypic organisms, and the inner - accumulative, formed by the accumulation of material that comes from its outer part. B. V. Preobrazhensky notes (1979) that the first is inhabited mainly by producers, i.e. producers organic matter, while the other serves as the main place for the settlement of consumers - consumers of ready-made organic substances.

The accumulative part of the rifflet, in turn, consists of three belts or zones. The uppermost of them, closely adjacent to the shore, lies near the boundary of the upper standing of the water at high (tropical) tides. It is represented by ancient limestone and covered with a layer of the purest coral sand. This is the beach area. Directly adjacent to it from the sea side is a strip of rifflet, covered with large and small coral fragments that are not interconnected. The fact is that this high-lying part of the reef platform dries up daily for a long time and within its limits the calcareous algae cementing the fragments can no longer exist. There are no live corals here either. Between this dead zone of the ripplet and the ridge, there is a more or less wide living zone, on which individual massive corals take root, and a special fauna of lagoon corals develops in pools and pools on the silted bottom. There are both solitary mushroom corals and many finely branched bushy corals. Dying off, they are cemented and also enter the structure of the platform, but the latter is still primarily formed from debris that falls here from the rifrock.

Thus, the lagoonal reef, which is so different from the surf one, is genetically closely related to it and arises from the interior of the latter.

Having studied a large number of coral reefs, we came to the conclusion that all the diversity of their geomorphological types can be reduced to a combination in different proportions of the main elements that make up a characteristic surf fringing reef.

Depending on the strength of the impact of the waves and on the profile of the bottom, reefs of various types arise.

Coral islands are created by organisms (polyps) capable of secreting a calcareous substance. They live in colonies. New developing organisms remain in connection with dead ones and form a common trunk. For the life of corals, and, consequently, for the formation of the island, some favorable conditions are needed. It is necessary that the water temperature on average does not fall below 20 °. Therefore, polyps can develop only in warm tropical seas, and even then not everywhere. Where the coasts are washed by cold currents, they are not, as, for example, off the coast of Peru. In addition, most polyps need a solid bottom in order to take root, and are comparatively pure water; as a result, in those places where rivers flow into the sea, bringing with them turbidity, the reef is interrupted. Coral structures can be divided into two categories. The first category includes coral reefs that border an island or mainland - these are coastal and barrier reefs. The second category includes independent islands known as atolls. Atolls are more or less round or oval in shape, triangular or quadrangular in shape are less common. Coastal reef borders some island or coast of the mainland. This shaft barely rises above the water, but even then it is far from everywhere, and for the most part it is a shallow, since corals in general can only live under water. Living corals can exist at depths of up to 90 m, but at this depth they are quite rare, and for the most part they do not fall below 30-40 m. The ebb tide is their upper limit. But some polyps can also be exposed from under the water and subjected to a short time of insolation. A number of processes lead to the fact that the coral shoal rises. The sea runs ashore, tears away pieces of polypnyak, grinds them into sand and throws them aground, filling up voids; other organisms settle on the surface of the reef - mollusks, crustaceans, shells and skeletons of which, in turn, go to raise the reef. Besides, warm water dissolves limestone, wind and waves throw substances brought from the shore aground. As a result, the reef as a whole becomes denser and sometimes rises somewhat above the sea surface, separated from the shore by a narrow channel. The barrier reef is much further from the coast than the coastal reef. Between it and the coast there is a lagoon, in some places also filled with reefs and sediments. The biggest barrier reef stretches along the northeast coast of Australia for 2000 km. The width of the lagoon here is 40-50 km, sometimes it expands even up to 180 km; its depth in some places reaches 100 m, so that steamboats can enter the lagoon, although swimming is dangerous, since there are many coral shoals. The width of the reef itself is several tens of kilometers. If we look at a map of the Pacific Ocean, we can see how many barrier reefs are found there. All large islands and a lot of small ones are bordered by coral buildings.

Atolls represent the third group of coral structures. Actually, the entire ring of atolls is stranded, and the islands rise out of the water only in places. The atolls make a very strong impression. Darwin also says: “It is difficult to imagine, without seeing with your own eyes, the infinity of the ocean and the fury of the waves in sharp contrast to the low border of land and the expanse of light green water inside the lagoon.” If there is a significant break in the atoll ring, then ships can find a calm pier in its lagoon.

In cross section, the atoll is first a steep slope, then a flat shoal with islands rising on it, and, finally, a deepening of the lagoon. The sizes of the atolls are very different: from 2x1 km to 25x10 km and even 90x35 km. The emergence of atolls can be explained as follows: if there is a shoal in the sea, barely covered with water, then in the case of a solid bottom, corals can settle on it and form an atoll. The atoll gets an oval shape because corals settle mainly along the edges of the shoal, since the sea waves here, if it is not excessively strong, and sea currents bring food supplies without hindrance (Figure 5). A strand can arise both as a result of the rise of the sea bottom, and as a result of the formation of an underwater volcano, or as a result of the compaction of ash on a cone that barely rises above the surface. If initially corals settle evenly over the entire surface of the shoal, then soon the marginal corals will be in a more advantageous position: food is freely delivered to them, and they grow faster than the corals located in the middle. A lagoon is created in the middle, however, it is rather shallow, since the shoal is not deep under water. The thickness of such polyps is small and rarely reaches 10 m. Such formations are called coral reefs. It is more difficult to explain the origin of atolls among deep sea. Darwin, like many other scientists, noticed that coral islands often rise very steeply; their slope reaches 30°. At first it was believed that only coral islands had such steep slopes, but now we know that volcanic and sometimes continental islands are not inferior to them in this respect. Another fact that makes it difficult to explain the origin of the atolls is that dead polyps are sometimes found at depths of 100-200 m or more, and we know that corals cannot live at such depths.

All these difficulties were eliminated by Darwin's theory of reef formation, which linked all three types of coral formations together. He believed that every polypnyak begins its existence in the form of a coastal reef, then passes into a barrier reef, and then turns into an atoll, and that this transformation is due to the sinking of the sea bottom in a given area. Corals begin their construction around some island, most often of volcanic origin, and first form a coastal reef.

As the island slowly sinks, the lower parts of the polyp forest die, and new corals breed above them, which have time to build up the reef. At the same time, the distance between the outer edge of the reef and the bedrock increases, and a barrier reef is already formed. A small part of the island still remains, rising among the lagoon. Further subsidence then occurs and an atoll is formed; the island has already completely disappeared under water, and in its place is a lagoon.

Naturally, with such an atoll formation, its outer slopes are steep. Many scientists recognized this theory, which was elaborated especially in detail in 1885 by Dan, but then objections were also raised against it. Darwin's theory was opposed by the fact that often in the same group of islands we meet all the transitional stages of reefs. atolls (Figure 6).

mainland volcanic coral island

Figure 5. - Scheme of the formation of attol.

However, this objection, based on the existence of different forms of reefs in close proximity to each other, is easily eliminated if we assume that in this place there were uneven vertical movements of the seabed. Thanks to this, they could form nearby various forms polypnyakov. In favor of the Darwinian theory is also the fact that although sometimes they are found in the neighborhood different forms reefs, but much more often one form dominates over vast areas, as, for example, is observed in Oceania. The drilling of a polypiak on the island of Funafuti (in the Ellis Islands group) also confirmed the correctness of Darwin's views. The well passed 334 m in a continuous polypnyak.

Therefore, in this place there was a real subsidence of the bottom, since corals cannot live at such a depth.

Figure 6. - Caroline Islands.

According to the observations of Murray, Guppy and Agassiz, there is no need for an atoll to develop without fail from a coastal and barrier reef - it can also arise independently, moreover, not only in shallow water, but also in deep sea areas. If a volcanic eruption occurs at the bottom of the sea, then corals can create an atoll on the edge of an emerging underwater volcano, around its crater. Already Chamisso, during his travels in Oceania, pointed out that the formation of a lagoon is often due to the fact that the crater of a volcano serves as the bottom of the lagoon. Sometimes the underwater hill is still very deep, at a depth of several hundred meters. Corals cannot live at such a depth, but many other organisms can exist there: crustaceans, mollusks and algae that have a calcareous skeleton; the skeletons of these organisms increase the height of the underwater reef, so that corals can eventually settle on it (Murray's theory). As for the formation of the lagoon, Agassiz believed that the sea tides contribute to its deepening. The atoll does not represent a closed ring, but has breaks. A tidal current penetrates them, produces a strongly eroding effect and cleans the lagoon from sediments. Despite the objections and additions made, Darwin's theory was generally fully confirmed. latest research, and it can be considered the most correct explanation of the origin of the atolls.

This reef, in fact, is represented by only one constituent element, namely, the outer slope with a ridge on top. At this point, the coastal cliffs plunge steeply into the sea, and hermatypic corals develop on them. Fragments of these corals, which inevitably arise as a result of the action of the coast and during storms, due to the steepness of the rocks rising from the sea, do not accumulate at the top, but roll down the slope.

Their heaps are visible at a depth of about 20 m, where the flat bottom begins. Only in some areas behind the crest of the reef can one find small (no more than 3-5m wide) areas - the beginnings of the future riffle.

Unlike surf reef corals, lagoon species are able to remain dry for several hours at low tide. The excitement in the lagoon is weaker, and water does not fall on exposed corals in low water.

Sometimes it is completely separated from the ocean by a ring reef, and sometimes it is connected to it by a wide strait, sufficient for the passage of boats and even ships. There are a lot of fish here edible shellfish, crayfish, algae; found in some places sea ​​turtles and dugongs.

Lagoons and channels between reefs and land are often used as safe harbors, hydrodromes and bases for ships and submarines.

Corals also cause a lot of trouble: reefs are difficult to notice from afar, they appear suddenly in front of the ship; since the depth near them sharply hits, and the sailing directions and maps of coral areas become outdated very quickly. Therefore, many ships suffered accidents near reefs.

Interesting case happened to famous Captain J. Cook during his first world travel. On June 11, 1770, not far from the Great Barrier Reef, the frigate Endeavre suddenly ran into a coral reef. Only a day later, having completely unloaded the ship, it was possible to remove it from the reef and take it to the mouth of the river, where the Australian city of Cooktown now stands. During repairs, Cook discovered that the main hole in the ship's hull was almost completely plugged with a large coral fragment. This circumstance helped to save the ship.

The economic importance of all coral islands is small; their population is also small: before the Second World War, about 100 thousand people lived here. Copra is exported from here - the core of coconuts, trepangs; mother-of-pearl, mainly from pearl shells. Pearls are also mined here. On a small atoll off the west coast of Australia, in 1917, one of the most beautiful pearls in the world, the Star of the West, was found. It is the size of a sparrow's egg and is valued at £14,000.

Coral limestone is used in some places as a building material; when ground, it is used to polish wood and metal. In Ceylon, cement is produced from it. From madrepore corals, just like from red ones, everyday items, jewelry, vases, etc. are made. They are also used in Chinese medicine.

In addition to corals with a calcareous skeleton, there are also corals with horn. From gorgonine, the horny substance of black coral, in Indochina and Malaya, for example, they make room decorations, weapons, knife handles, beads, bracelets.

Small size, remoteness from the continents, endemicity and poverty of the biological diversity of flora and fauna create very big problems in cases of irrational use of natural resources, serious violations of the ecological balance and intense pollution environment. After all, the ecosystems of these islands were formed long time in conditions of limited connections with other islands and the mainland. Therefore, it is very difficult to restore disturbed ecosystems here. The nature of atolls is especially vulnerable, firstly, because of their very small size. Secondly, because of the instability of their ecosystems, the primitiveness of relations between organizations and the presence of ecological niches that allow organisms that are alien to island landscapes to invade. Thirdly, due to the limited fresh water resources on the atolls, which significantly limits the possibilities economic activity. Therefore, most of the atolls are inhabited with little or even no permanent population, but are used for seasonal work on coconut plantations.

Conclusion

Islands are small isolated areas of land. The area of ​​the islands is 9.9 million km 2, about 78% of this area is 28 large islands. Of these, the largest is Greenland.

Island groups are called archipelagos. They can be compact, such as Franz Josef Land, Svalbard, the Greater Sunda Islands, or elongated, such as Japanese, Philippine, Greater and Lesser Antilles. In Russian, such islands are called ridges (Kuril ridge). The archipelagos of small islands scattered in the Pacific Ocean are combined into three large groups - Melanesia, Micronesia and Polynesia.

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

• a) Mainland: platform, continental slope, orogenic, island arcs, coastal:
  • - skerries,
  • - fjords,
  • - braids and arrows,
  • - deltaic.
• b) Independent:
  • 1 Volcanic:
    • - fissure effusion,
    • - central outpouring,
    • - shield and conical,
• 2 coral:
  • - coastal reefs,
  • - barrier reefs
  • - atolls.

mainland islands genetically related to the continents, but these connections are of a different nature and this 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. Islands of the mainland slope are also parts of the continent, but their separation happened earlier. They are usually separated not by a gentle inflection of the mainland, but by a deep split. The straits between the island and the mainland are oceanic in nature. The flora and fauna of such islands is very different from the mainland. This group includes Madagascar and Greenland. Orogenic islands are a continuation of the mountain folds of the continents. island arcs- parts of transitional areas. Mainland offshore islands.

Independent islands have never been part of the continents and in most cases formed independently of them.

Volcanic islands- the main mass of volcanic islands is formed by eruptions of the central type. Naturally, these islands cannot be very large.

coral islands- coastal reefs, barrier reefs and lagoon islands. Coastal reefs begin directly at the coast. Barrier reefs are located at some distance from the land and are separated from it by a strip of water - a lagoon.

Atolls (lagoon islands) are located in the middle of the ocean. These are low islands in the form of an open ring or ellipse. Inside the atoll is a lagoon less than 100m deep. The island is composed of sandy or pebble-blocky material - products of coral destruction. The bottom of coral lagoons is flat, covered with coral sand or accumulations of calcareous algae residues.

No less interesting is another group of independent islands - coral islands. They are created by organisms (polyps) capable of secreting a calcareous substance. They live in colonies. New developing organisms remain in connection with dead ones and form a common trunk. For the life of corals, and therefore for the formation of the island, certain favorable conditions are needed. It is necessary that the water temperature on average does not fall below 20 °. Therefore, polyps can develop only in warm tropical seas, and even then not everywhere. Where the coasts are washed by cold currents, they are not, as, for example, off the coast of Peru. In addition, most polyps require a solid bottom to take root and comparatively clear water; as a result, in those places where rivers flow into the sea, bringing with them turbidity, the reef is interrupted.
Coral structures can be divided into two categories.
The first category includes coral reefs that border an island or mainland - these are coastal and barrier reefs. The second category includes independent islands, known as atolls. Atolls are more or less round or oval in shape, triangular or quadrangular in shape are less common.
Coastal reef borders some island or coast of the mainland. This shaft barely rises above the water, but even then it is far from everywhere, and for the most part it is a shallow, since corals in general can only live under water. Living corals can exist at depths of up to 90 m, but at this depth they are quite rare, and for the most part they do not fall below 30-40 m. The ebb tide is their upper limit. But some polyps can also be exposed from under the water and subjected to a short time of insolation.
A number of processes lead to the fact that the coral shoal rises. The sea runs ashore, tears away pieces of polypnyak, grinds them into sand and throws them aground, filling up voids; other organisms settle on the surface of the reef - mollusks, crustaceans, shells and skeletons of which, in turn, go to raise the reef. In addition, warm water dissolves limestone, wind and waves throw substances brought from the shore aground. As a result, the reef as a whole becomes denser and sometimes rises somewhat above the sea surface, separated from the shore by a narrow channel.
The barrier reef is much further from the coast than the coastal reef. Between it and the coast there is a lagoon, in some places also filled with reefs and sediments. The largest barrier reef stretches along the northeast coast of Australia for 2000 km. The width of the lagoon here is 40-50 km, sometimes it expands even up to 180 km; its depth in some places reaches 100 m, so that steamboats can enter the lagoon, although swimming is dangerous, since there are many coral shoals. The width of the reef itself is several tens of kilometers.
If we look at a map of the Pacific Ocean, we can see how many barrier reefs are found there. All large islands and a lot of small ones are bordered by coral buildings.
Atolls represent the third group of coral structures. Actually, the entire ring of atolls is stranded, and the islands rise out of the water only in places. The atolls make a very strong impression. Darwin also says: “It is difficult to imagine, without seeing with your own eyes, the infinity of the ocean and the fury of the waves in sharp contrast to the low border of the land and the expanse of light green water inside the lagoon.” If there is a significant break in the atoll ring, then ships can find a calm pier in its lagoon.
In cross section, the atoll is first a steep slope, then a flat shoal with islands rising on it, and, finally, a deepening of the lagoon. The sizes of the atolls are very different: from 2 X 1 km to 25 X 10 km and even 90 X 35 km.
The emergence of atolls can be explained as follows: if there is a shoal in the sea, barely covered with water, then in the case of a solid bottom, corals can settle on it and form an atoll. The atoll acquires an oval shape because corals settle mainly along the edges of the shallows, since here the excitement of the sea, if it is not excessively strong, and sea currents bring food supplies without hindrance. A strand can arise both as a result of the rise of the sea bottom, and as a result of the formation of an underwater volcano, or as a result of the compaction of ash on a cone that barely rises above the surface. If initially corals settle evenly over the entire surface of the shoal, then soon the marginal corals will be in a more advantageous position: food is freely delivered to them, and they grow faster than the corals located in the middle. A lagoon is created in the middle, however, it is rather shallow, since the shoal is not deep under water. The thickness of such polypnyak is small and rarely reaches 10 m.
Such formations are called coral reefs.
It is more difficult to explain the origin of atolls in the deep sea. Darwin, like many other scientists, noticed that coral islands often rise very steeply; their slope reaches 30°.
At first it was believed that only coral islands had such steep slopes, but now we know that volcanic and sometimes continental islands are not inferior to them in this respect.
Another fact that makes it difficult to explain the origin of atolls is that dead polyp forest is sometimes found at depths of 100-200 m or more, and we know that corals cannot live at such depths.
All these difficulties were eliminated by Darwin's theory of reef formation, which linked all three types of coral formations together. He believed that every polypnyak begins its existence in the form of a coastal reef, then passes into a barrier reef, and then turns into an atoll, and that this transformation is due to the sinking of the sea bottom in a given area.
Corals begin their construction around some island, most often of volcanic origin, and first form a coastal reef. As the island slowly sinks, the lower parts of the polyp forest die, and new corals breed above them, which have time to build up the reef. At the same time, the distance between the outer edge of the reef and the bedrock increases, and a barrier reef is already formed. A small part of the island still remains, rising among the lagoon. Further subsidence then occurs and an atoll is formed; the island has already completely disappeared under water, and in its place is a lagoon. Naturally, with such an atoll formation, its outer slopes are steep.
Many scientists recognized this theory, which was elaborated especially in detail in 1885 by Dan, but then objections were also raised against it. Darwin's theory was opposed by the fact that often in the same group of islands we meet all the transitional stages of reefs. atolls. However, this objection, based on the existence of various forms of reefs in close proximity to each other, is easily eliminated by assuming that uneven vertical movements of the seabed have occurred at a given location. Due to this, various forms of polypnyaks could form nearby.
The Darwinian theory is also supported by the fact that although different forms of reefs are sometimes found in the neighborhood, much more often one form dominates over vast expanses, as, for example, is observed in Oceania. The drilling of a polypiak on the island of Funafuti (in the Ellis Islands group) also confirmed the correctness of Darwin's views. The well passed 334 m in a continuous polypnyak. Therefore, in this place there was a real subsidence of the bottom, since corals cannot live at such a depth.
According to the observations of Murray, Guppy and Agassiz, there is no need for an atoll to develop without fail from a coastal and barrier reef - it can also arise independently, moreover, not only in shallow water, but also in deep sea areas. If a volcanic eruption occurs at the bottom of the sea, then corals can create an atoll on the edge of an emerging underwater volcano, around its crater.
Already Chamisso, during his travels in Oceania, pointed out that the formation of a lagoon is often due to the fact that the crater of a volcano serves as the bottom of the lagoon.
Sometimes the underwater hill is still very deep, at a depth of several hundred meters. Corals cannot live at such a depth, but many other organisms can exist there: crustaceans, mollusks and algae that have a calcareous skeleton; the skeletons of these organisms increase the height of the underwater reef, so that corals can eventually settle on it (Murray's theory). As for the formation of the lagoon, Agassiz believed that the sea tides contribute to its deepening. The atoll does not represent a closed ring, but has breaks. A tidal current penetrates them, produces a strongly eroding effect and cleans the lagoon from sediments.
Despite the objections and additions made, the Darwinian theory has generally been fully confirmed by the latest research, and can be considered the most correct explanation of the origin of the atolls.

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If you had a bad dream...

There are three types of islands: mainland, volcanic and coral. The formation of islands took place not only many thousands of years ago, but even now new island territories are emerging.

How were the mainland islands formed?

Continental islands were formed due to the movement of tectonic plates of the earth's crust. Once the islands were part of large continents. The vertical movements of tectonic plates, together with the rise in the level of the world ocean, formed faults in the continents. Nature mainland islands and the nature of the mainland closest to them are almost identical. Continental or continental islands are located within a single shelf, or are separated from the mainland by a deep fault. The continental islands include Greenland, new earth, Madagascar, British Isles, etc.

How are volcanic islands formed?

Volcanic activity is constantly taking place in the oceans. An erupting volcano emits a huge amount of lava, which, solidifying on contact with water and air, forms new volcanic islands. Such islands experience great water erosion and gradually go under water. Volcanic islands are often far removed from the mainland and form a unique ecological system. An example of volcanic islands is the chain of Hawaiian Islands.

How are coral islands formed?

Such islands can form only in equatorial and tropical latitudes. The shoals are inhabited by corals and polyps, which hook on to the seabed with their roots. Over time, the bottom of the coral hardens, forming a solid foundation for the island. Such a base begins to retain the sand that the ocean carries with its course. Coral reefs are formed, which are inhabited by the most outlandish animals of the ocean. An excellent example of such islands is the Great Barrier Reef off the coast of Australia.

Coral reefs are structures made of organogenic limestones formed by polyps and other reef-forming organisms and inhabited by numerous forms of corals, algae and other living organisms.
It should be especially noted that the dominant role in the construction of coral reefs is played by madrepore corals and coralline algae. In addition to these two main "builders" of reefs, other organisms also take part in its construction - sponges, mollusks, foraminifers, etc.

The whole variety of buildings created by corals and other reef-forming organisms can be divided into several main types. Distinguish coastal reefs located directly on the coast of islands or continents, barrier reefs, some distance away from the shore, and atolls- ring-shaped coral islands. Finally, there are also significant coral banks that occur in the shallows, and dense coral colonies in lagoons and bays.

Three types of reefs give rise to many transitional forms. A fringing reef near one part of the shore may further merge into a barrier reef, and their boundary is difficult to distinguish. Some islands could actually be considered atolls, if it were not for the presence of one or more dissected massifs of volcanic rocks in the center of the reef ring. Submerged (submerged) reefs serve as indicators of submerged platforms located near the water surface, which promote coral growth, but are more likely to be submerged atolls or islands.

The process of formation of all these coral structures interested geologists and zoologists for a very long time, the origin of ring-shaped islands - atolls seemed especially incomprehensible. Several theories have been proposed to explain the formation of these islands, many of them rather naive. So, until the middle of the 19th century, the opinion prevailed that the atolls were coral fouling of the craters of underwater volcanoes.

The first convincing theory of the origin of coral structures of various types was put forward by C. Darwin. In his book "The Structure and Distribution of Coral Reefs", published in 1842, Charles Darwin gave not only detailed description various coral structures, but also showed how one type of coral settlements passes into another as it develops. Darwin collected a huge amount of material concerning the vital activity of the organisms that form the coral reef, their relationship to the conditions external environment, intensity of growth and distribution in the oceans.
He received part of the information from the captains of ships that plied the tropical latitudes of the oceans and seas, and from scientists who studied corals.
He made the most valuable observations himself during his round-the-world trip on the Beagle. According to Darwin, the first stage in the formation of coral islands is fringing reef . Corals in this case use the shores of the islands as a support, or, as experts say, a substrate. If conditions are favorable for coral development and the island does not rise or fall, the reef remains a fringing reef.
In those cases when the bottom of the sea as a result of processes occurring in the earth's crust begins to rise and the island seems to emerge from the water, the fringing reef grows along its new coastline. Parts of the reef that are out of the water die, and on the sea side the reef grows and grows, but the overall picture does not change.

The situation is quite different when the seabed sinks and the island sinks into the water. Reef-building organisms require a lot of food and clean, oxygen-rich sea water for their development. Due to this, the growth of the reef, which previously bordered the island, always goes along its periphery, washed by the sea, and almost does not go in the inner part of the formed ring, where the water is less saturated with oxygen due to heating in the sun and less mixing with air by surf waves. This is how barrier reef . The longer this process lasts, the further the barrier recedes from the island. Finally, there comes a moment when the island finally sinks into the sea, and the barrier reef turns into atoll - a ring island with a lagoon enclosed inside it.
Coral buildings in various parts of the ocean make it possible to trace all the stages of the gradual transformation of the coastal reef into an atoll. The growth of the reef upwards is sharply limited by sea level, but no matter how quickly the seabed sinks, corals always have time to grow to the upper limit at which they can exist. Part of modern coral islands rises from the bottom of the sea to a considerable height.

Later, some researchers put forward other theories of the origin of coral islands. The fact is that Darwin's theory is based on one indispensable condition - the subsidence of the seabed. Opponents of this theory argued that the relationship between uplift and subsidence of the seabed is much more complicated than Charles Darwin believed.
In this regard, other theories of the formation of reefs and atolls were put forward, based on volcanic activity in the earth's crust or tied to cooling and warming in the Earth's climate. However, they all had weaknesses in their bases. Darwin knew how to prove the legitimacy of his theory: to organize the drilling of some atolls in order to obtain samples for studying the soil at depths of 200 meters or more. If at such a depth the interior of the atoll is composed of coral limestone, then his theory will receive irrefutable proof.
Darwin's dream came true only in the middle of the 20th century. In 1951, two very deep drilling on Eniwetok Atoll (Marshall Islands).
It turned out that coral limestone is underlain by a basalt layer only at a depth of 1266 and 1380 m. The calculations made showed that the reef that gave rise to this atoll originated 60 million years ago. It is quite obvious that the accumulation of such thick limestones could have occurred only as a result of a long subsidence of the ocean floor.

It is highly probable that the most common way atolls were formed was through the sinking of volcanic islands. Interesting discoveries on seabed a large number flat-topped seamounts (called guyots), similar to deeply submerged atolls. Shallow corals were recovered from at least one of them.
From a review of the fossil reef literature, it appears that reef formation occurred mainly during those geologic epochs when weak subsidence of the earth's crust (or slow sea level rise) predominated.
AT geological periods, characterized by the uplift of coral structures or their rapid sinking, the reefs almost did not develop.

AT recent times a lot of new data appeared concerning geomorphology, oceanography, paleontology and biology of modern reef-forming animals. All of them are used to refine the Darwinian theory.