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 in size and have a very limited distribution in the coastal zone. Coral formations are fringing reefs that stretch along the coast, or barrier reefs located offshore and separated from them by lagoons. Most of the reefs are underwater, and above sea level, only their tops protrude in the form of small islands with intricate outlines, for example, on the Great Barrier Reef off the east coast of Australia. Ocean atolls arise on the tops of large volcanic seamounts or during the long evolution of an annular fringing reef around volcanic islands that subsequently submerged below sea level and overlapped by coral limestones. As a result, circular low islands are formed, composed of coral sand - the 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 the Indian Ocean, the Albuquerque islands, Saint-Andres, Roncador - in the Atlantic Ocean (Caribbean Sea) and others. These islands are young formations that arose as a result of the superstructure of coral reefs in 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 settles or collapses, but the reefs remain in place, repeating its shape, continuing to grow. In the end, only the "plate" 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 on the island and deserves a place of interest.
This type of islands is a favorite vacation spot for people from all over the world, being the hallmark of the beautiful Pacific islands (Figure 4)
Coral reefs grow not only by turning hard corals into sand, which 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 limestone residues into an unbreakable rock surface.
Figure 4. - Coral Islands. Maldives.
In the tropics, heavy rains are frequent. Then the concentration of salt in the surface layers of seawater drops sharply, and many polyps die. Sometimes clouds of silt and sand float down, which, settling, bury animals under them. Dead coral colonies crumble and turn into coral sand.
Thus, coral formations result from endless processes of creation and destruction.
People have long been interested in how reefs arise, especially atolls found right in the open ocean.
The famous Russian navigator FF Bellingshausen expressed a number of correct thoughts about their nature. The most grounded theory of the origin of coral reefs was put forward by Charles Darwin. To a large extent, they adhere to it 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 the coastal reef at Pago Pago in Samoa, where the bedrock (not coral) is already 35 m below the surface.
The English scientist J. Murray made significant additions to Darwin's theory. He proved that a solid coral reef necessarily transforms into a ring reef, and for this reason. Corals in the middle part of the reef do not have enough food, they gradually perish and collapse, because carbon dioxide accumulates here - the product of the respiration of polyps, which dissolves the limestone, and the reef grows only from the outside. This is how the lagoon appears in the center of the reef.
VN Kosmynin, who studied in detail the geomorphology of the coral reefs of the Seychelles, found on them a number of successive stages in the formation of the relief of the outer slope. In the early stages, the spurs are bands of dense weaving of branched corals elongated from top to bottom along the slope. Such corals are characterized by rapid growth, and for a long time they manage to form the so-called coral bush on the reefrock in a relatively short time. Under the influence of waves, the delicate end branches of the colonies break off, and their bases, meanwhile, undergo cementation with calcareous algae and encrusting corals.
On this, as it were, compressed and therefore denser vertical strip of coral limestone, as in a generation, branching corals grow again - and the formation of a spur passes into the second stage.
The emergence of channels, i.e., grooves between the spurs, is partly explained by erosion under the influence of water flowing from the reef, which, when the wave retreats, rushes here, since it does not meet obstacles in the form of coral thickets. However, the main reason for the formation of channels is still the growth of corals on the spurs. At the last stage, the width of the spurs along the front reaches 3-5 m, and sometimes even more, and they begin to close with their lateral sides, and then the channels between them turn into vertical or inclined tunnels.
From the above, it is obvious that the reef grows towards the sea due to the formation of spurs and their subsequent merging. Of course, their erosional destruction is not excluded, but it, apparently, takes place only during very strong storms.
On the Hainan Island reef mentioned above, the spur and canal system was in its third and most advanced 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 from 50 cm to 1–2 m deep and several meters wide. It runs in a winding channel parallel to the outer edge of the reef. As mentioned above, the crest of the reef is the place of the most active growth of corals, on which the so-called algal bank develops due to calcareous algae.
The formation of an uplifting rampart by the calcareous red algae just at the seaward edge of the rifflet and on the ridge is explained by the ecological characteristics of these plant organisms. They tolerate overheating and drying out much more easily than madrepore corals. The conditions of periodic exposure and splashing by waves for calcareous purple leaves, 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 waves recede, the plants receive maximum sunlight (V. Kosmynin).
It is these hermatipic organisms that 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. Obviously, the edge of the reef passed along this line before, but due to 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 should be considered in the structure of a rifflet, however, the genesis of different parts of 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 clastic material, which is formed mainly on the outer slope and ridge and is transported from there by waves.
So, on the reef, two main parts should be distinguished - the external, bioconstructive, created as a result of the vital activity of hermatipic organisms, and the internal - accumulative, formed by the accumulation of material that comes from its external part. B.V. Preobrazhensky notes (1979) that the former is populated mainly by producers, i.e., producers of organic matter, while the other serves as the main settlement site for consumers - consumers of ready-made organic substances.
The accumulative part of the rifflet, in turn, consists of three belts, or zones. The highest of them, closely adjacent to the coast, lies near the border 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 rifflet strip covered with large and small coral fragments not connected with each other. 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 living corals here either. Between this dead zone of the rifflet and the ridge, there is a more or less wide living zone in which individual massive corals take root, and in puddles and baths on the silted bottom, a special fauna of lagoon corals develops. Both solitary mushroom corals and many finely branched bushy forms are found here. When they die off, they become cemented and also enter the structure of the platform, but the latter is still primarily formed from debris that falls here from the reefrock.
Thus, the lagoon reef, which is so different from the surf, 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 the entire diversity of their geomorphological types can be reduced to a combination in different ratios of the main elements that make up the characteristic surf bordering reef.
Depending on the strength of the impact of the waves and on the bottom profile, reefs of various types appear.
Coral islands are created by organisms (polyps) capable of secreting calcareous matter. They live in colonies. New developing organisms remain in connection with the dead ones and form a common trunk. For the life of corals, and, therefore, for the formation of the island, some favorable conditions are needed. It is necessary that the water temperature does not drop below 20 ° on average. Therefore, polyps can develop only in warm tropical seas, and even then not everywhere. Where the coasts are washed by cold currents, there are none, as, for example, off the coast of Peru. In addition, most polyps require a solid bottom to root and relatively clear water; consequently, 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 have a more or less rounded or oval shape, less often a triangular or quadrangular shape. A coastal reef flanks an island or mainland coast. This rampart barely rises above the water, but even then it is far from everywhere, and for the most part it is a sandbank, since corals in general can only live under water. Live corals can exist at a depth of up to 90 m, but at such a depth they are quite rare, and for the most part they do not fall below 30-40 m. The low tide limit is their upper limit. But some polyps can be exposed from under the water and be exposed to insolation for a short time. A number of processes lead to the fact that the coral bank is rising. The sea rushes to the shore, rejects pieces of polypny, crushes them into sand and throws them aground, filling the voids; other organisms settle on the surface of the reef - molluscs, crustaceans, shells and skeletons of which, in turn, go to the rise of 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 is compacted and sometimes rises somewhat above the sea surface, being separated from the coast by a narrow channel. The barrier reef is much farther from the coast than the coastal one. Between it and the coast there is a lagoon, in places filled with reefs and sediments. The largest barrier reef stretches along the northeastern coast of Australia for 2000 km. The width of the lagoon here is 40-50 km, sometimes it even expands up to 180 km; its depth in some places reaches 100 m, so that steamers 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 take a look at the 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 structures.
The third group of coral structures is represented by atolls. Actually, the entire ring of atolls is shallow, and the islands rise from the water only in places. The atolls are very impressive. Even Darwin 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 with a smooth surface of light green water inside the lagoon." If there is a significant breakthrough in the atoll's ring, then ships can find a calm dock in its lagoon.
In cross-section, the atoll is first a steep slope, then a flat aground with islands towering 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 formation of atolls can be explained as follows: if there is a shallow in the sea, barely covered with water, then in the case of a hard 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 is rough here, if it is not excessively strong, and sea currents bring food supplies without hindrance (Figure 5). Shallowness 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 the corals settle evenly over the entire shallow surface, then soon the edge corals will find themselves in a better position: they have an unhindered food supply and they grow faster than the corals in the middle. In the middle, a lagoon is created, although it is quite shallow, since the shallow is shallow under water. The thickness of such a polypian is small and rarely reaches 10 m. Such formations are called coral reefs. It is more difficult to explain the origin of the atolls in the deep sea. Darwin, like many other scientists, noticed that often coral islands rise very steeply; their slope reaches 30 °. At first, it was believed that only coral islands have such steep slopes, but now we know that volcanic, and sometimes even 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 polypies are sometimes found at depths of 100-200 m and more, and we know that corals cannot live at such a depth.
All of these difficulties were removed by Darwin's theory of reef formation, which linked all three types of coral formations together. He believed that any polypnyak begins its existence in the form of a coastal reef, then turns 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 buildings around an island, most often of volcanic origin, and first form a coastal reef.
As the island slowly sinks, the lower parts of the polypnyts die, and new corals multiply above them, which have time to build on 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. There is still a small part of the island, rising among the lagoon. Then further subsidence occurs, and an atoll is formed; the island has completely disappeared under water, and in its place is a lagoon.
Naturally, with such a formation of the atoll, its outer slopes are steep. Many scholars recognized this theory, especially elaborated in 1885 by Dan, but then objections were also expressed against it. Against Darwin's theory, they cited the fact that often in the same group of islands we meet all the transitional stages of reefs For example, in the Caroline Islands group there are coastal reefs, nearby barrier reefs and atolls, from the lagoons of which small islands still peep out, and, finally, typical atolls (Figure 6).
mainland volcanic coral island
Figure 5. - Diagram of attol formation.
However, this objection, based on the existence of different forms of reefs in close proximity to each other, is easily eliminated if it is assumed that irregular vertical movements of the seabed have occurred at this location. Due to this, various forms of polypies could form side by side. In favor of the Darwinian theory is the fact that although sometimes different forms of reefs are found in the neighborhood, but much more often one form dominates over vast areas, as, for example, is observed in Oceania. Drilling a polypyak on Funafuti Island (in the Ellis group of islands) also confirmed the correctness of Darwin's views. The well drilled 334 m in a solid polypune.
Consequently, in this place there was an actual sinking 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 the atoll to develop necessarily from a coastal and barrier reef - it can arise independently, moreover, not only in shallow waters, but also in the deep sea. If a volcanic eruption occurs at the bottom of the sea, then corals can create an atoll at the edge of the emerged underwater volcano, around its crater. Already Chamisso, during his travels across Oceania, pointed out that the formation of a lagoon is often due to the fact that the crater of the volcano serves as the bottom of the lagoon. Sometimes the seamount lies very deep, at a depth of several hundred meters. Corals cannot live at such a depth, but many other organisms can exist there: crustaceans, molluscs and algae with 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 sea tides contributed to its deepening. The atoll does not represent a closed ring, but has breaks. A tidal current penetrates them, produces a highly erosive effect and cleans the lagoon of sediments. Despite the objections and additions made, Darwin's theory, in general, has been fully confirmed by the 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 component, namely an external slope with a ridge at the top. At this point, the coastal cliffs steeply go into the sea, and hermatip corals develop on them. Debris of these corals, inevitably resulting from the action of the roll and during storms, due to the steepness of the rocks rising from the sea, do not accumulate at the top, but slide 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 reef crest can one find small (no more than 3-5m wide) areas - the beginnings of the future rifflet.
Unlike surf reef corals, lagoon species are able to remain dry for several hours at low tide. The waves in the lagoon are weaker, and water does not get into the low water on the exposed corals.
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 many fish, edible shellfish, crayfish, algae; sea \u200b\u200bturtles and dugongs are found in some places.
Lagoons and canals between reefs and land are often used as safe harbors, hydrodromes and bases for ships and submarines.
Corals also cause a lot of troubles: reefs from afar are difficult to notice, they appear in front of the ship suddenly; since the depth near them drops sharply, and the directions and maps of the coral regions become outdated very quickly. Therefore, many ships suffered accidents near the reefs.
An interesting incident occurred with the famous captain J. Cook during his first voyage around the world. On June 11, 1770, not far from the Great Barrier Reef, the frigate Endeavor suddenly ran into a coral reef. Only a day later, having completely unloaded the ship, was it 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 sealed with a large piece of coral. This circumstance helped to save the ship.
The economic value 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 western coast of Australia in 1917, one of the most beautiful pearls in the world, the Star of the West, was found. It is about the size of a passerine egg and is priced at £ 14,000.
Coral limestone is used here and there as a building material; when ground, it serves for polishing wood and metal. In Ceylon, cement is produced from it. From madrepore corals, just as from red ones, they make everyday objects, jewelry, vases, etc. They are also used in Chinese medicine.
In addition to corals with a calcareous skeleton, there are corals with horns. From gorgonin, the horny substance of black coral, in Indochina and Malaya, for example, they make decorations for rooms, weapons, knife handles, beads, bracelets.
Small size, remoteness from continents, endemicity and poverty of 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 environmental pollution. After all, the ecosystems of these islands have been forming for a long time in conditions of limited connections with other islands and the mainland. Therefore, it is very difficult to restore damaged ecosystems here. The nature of the atolls is especially vulnerable, firstly, because of their very small size. Secondly, because of the instability of their ecosystems, the primitiveness of connections between organizations, and the presence of ecological niches that allow organisms alien to island landscapes to penetrate. Third, due to the limited resources of fresh water on the atolls, which significantly limits the possibilities of economic activity. Therefore, most of the atolls are inhabited with little or no permanent population, and are used for seasonal work on coconut plantations.
Conclusion
Islands are small isolated areas of land. The area of \u200b\u200bthe islands is 9.9 million km 2, about 78% of this area is 28 large islands. The largest of these is Greenland.
The island groups are called archipelagos... They can be compactsuch as Franz Josef Land, Svalbard, 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.
All islands can be grouped by origin as follows:
- and) Mainland: platform, continental slope, orogenic, island arcs, coastal:
- - skerries,
- - fjord,
- - braids and arrows,
- - delta.
- b) Independent:
- 1 Volcanic:
- - fissure effusion,
- - central outpouring,
- - panel board and conical,
- 1 Volcanic:
- 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 islandslie on a continental shelf and geologically represent an extension of the continent. Mainland slope islands are also parts of the continent, but their separation happened earlier. They are usually separated not by a gentle bend of the mainland, but by a deep split. The straits between the island and the mainland are oceanic. The flora and fauna of such islands is very different from the mainland. This group includes Madagascar and Greenland. Orogenic Islands represent the continuation of the mountain folds of the continents. Island arcs- parts of the transition areas. Mainland coastal islands.
Independent islands have never been parts 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 start directly off 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 there is a lagoon less than 100m deep. The island is composed of sandy or pebble-block material - products of coral destruction. The bottom of the coral lagoons is flat, covered with coral sand or accumulations of calcareous algae.
Coral islands
Coral island - an island that arose as a result of the vital activity of reef-building organisms in the oceans and seas of the tropical belt. A coral island in the form of a solid or broken ring is called an atoll.
Notes
- Ignatiev G. M. Tropical Islands of the Pacific Ocean. Moscow, publishing house "Mysl", 1978, 270 p.
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- Coral snakes
- Coral atoll
See what "Coral Islands" is in other dictionaries:
CORAL ISLANDS - the islands that have arisen as a result of the vital activity of reef-building organisms in the oceans and seas of the tropical belt ... Big Encyclopedic Dictionary
coral islands - Islands that have arisen as a result of the vital activity of reef-building organisms in the oceans and seas of the tropical belt. * * * CORAL ISLANDS CORAL ISLANDS, islands that have arisen as a result of the vital activity of reef-building organisms in the oceans and ... ... encyclopedic Dictionary
Coral islands - islands formed on the surface of coral structures (See Coral structures), as a result of wave activity and surf from the products of mechanical destruction of coral limestones and a colony of living corals ...
CORAL ISLANDS - Islands arising from the life of reef-building organisms in the oceans and seas of the tropics. belts ... Natural science. encyclopedic Dictionary
Coral structures - coral reefs, geological formations formed as a result of the life of colonial coral polyps (mainly madrepore corals (see madrepore corals)) and accompanying organisms capable of extracting ... Great Soviet Encyclopedia
islands - land areas, surrounded on all sides by the waters of oceans, seas, lakes, rivers. They differ from the continents in their relatively small size. There are single islands and their groups (archipelagos). Islands in the oceans and seas are subdivided into continental ... ... encyclopedic Dictionary
coral reefs - structures made of organogenic limestone, located near sea level or at shallow depths in the coastal zone of tropical seas or in shallow warm seas. They represent massive deposits of calcite (limestone), ... ... Geographical encyclopedia
CORAL POLYPS - (Anthozoa), class mor. cnidarians. Colonial, rarely solitary polyps; jellyfish do not form. Many have a calcareous or horny skeleton. Dept. individuals are usually cylindrical. forms, with their base grow together with the colony or (single, capable of slowly ... Biological encyclopedic dictionary
Friendship Islands
Tonga islands - Coordinates: 20 ° 35′16 ″ S sh. 174 ° 48'37 ″ W d. / 20.587778 ° S sh. 174.810278 ° W etc ... Wikipedia
Books
- Underwater kingdom. Red Sea, Maldives, Malaysia, Caribbean Sea, Angelo Moggetta, Andrea Ferrari, Antonella Ferrari. Bizarre reefs and coral platforms, high cliffs falling right into the abyss of turquoise water, picturesque straits where life unknown to us is in full swing - so you want to dive into this underwater ... Buy for 2300 rubles
- Underwater kingdom Red Sea Maldives, Mogetta A., Ferrari A. Bizarre reefs and coral platforms, high cliffs falling right into the abyss of turquoise water, picturesque straits - you really want to dive into this underwater paradise, admire its unearthly ...
On the coasts of tropical seas, an active role in the formation of sea coasts may belong to some marine organisms, and primarily to various reef builders - six- and eight-rayed corals, accompanying calcareous algae (Litotamnyon, Halimeda), various hydroids and bryozoans. These organisms are able to absorb lime from sea water and build their skeletons from it, from which during
the withering away of corals and algae, their destruction by waves and surf and
the subsequent cementation of the destruction products forms a massive rock - coral, or reef, limestone.
Accumulative forms built from reef limestone are called coral reefs. There are several types of coral structures: fringing, or coastal, barrier, ring and intra-lagoon reefs.
Fringing reefs are underwater coral-limestone terraces adjacent directly to the shore. Their outer zone is covered with living coral colonies. The surface of the reef - the so-called reef-flat - with distance from the outer zone is increasingly covered by a sediment cover of coral gravel and sand. At the coast, it is bordered by a snow-white sand and gravel beach.
On tectonically stable shores, the thickness of the coral fringing reef usually does not exceed 50 m. This is due to the habitat of reef-forming corals. Reef-building coral polyps live in symbiosis with the unicellular green alga Zooxantella, which lives in the polyp cavity and needs good light for photosynthesis. This most important ecological condition is no longer satisfied at depths of more than 50 m. Barrier reefs are coral-limestone ridges or
barriers located more or less significant distance from the coast. The thickness of the barrier reef is usually many times that of the fringing reefs. From the above-mentioned ecological features of the habitat of reef-forming corals, it follows that a large thickness of the reef limestone that composes the barrier reef can be achieved only under the condition of tectonic subsidence of the reef base. This is exactly how Charles Darwin, one of the first founders of the theory of the formation and development of coral reefs, explained this fact. So the barrier reefs
arise as a result of the sinking of the coastal reef, provided that its outer edge is constantly growing in height. The largest structure of this kind in the world is the Great Barrier Reef, which stretches along the northeastern edge of Australia for more than 2000 km. If a barrier reef forms around a small sinking island, then as the base sinks and the outer edge continues to build up, it transforms into a ring-shaped reef, or atoll.
The water area located inside the atoll or fenced off from the open sea by a barrier reef is called a coral lagoon. The lagoon is inhabited by special types of reef-forming corals, which, in the course of their life, create another kind of reef structures - intra-lagoon reefs. In most cases, they look like columns or giant pedestals, scattered randomly within the lagoon and are usually called pinnacles (from the English. - spire, peaked turret). Pinnacles merged with each other form larger formations in area -
coral jars patches. Sometimes intra-lagoon reefs are formed on the crests of submarine ridges built by tidal currents.
Coral islands are scattered in abundance both in the open ocean and in the coastal zones of tropical seas. It is generally believed that coral islands are built by corals, that they are former coral reefs. However, this is not the case. Although in the oceans there are sometimes islands - raised coral reefs (Nauru Island in the Pacific Ocean, Tromelin Island in the Indian Ocean, etc.), such formations are rare. Ordinary coral islands, including islands located on atolls, are typical island bars, built using the activity of sea waves from coral deposits - sand, gravel, pebbles, sometimes heaps of blocks of reef limestone. The bar formation scheme, which was discussed above, is generally applicable to the explanation of their formation.
