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Message on the topic of coral islands. Jellyfish, corals, polyps. See what “Coral reefs and islands” are in other dictionaries

In the morphology of many coasts tropical zone The limestone structures of reef-forming corals play an important role. Such coasts, due to their originality, can be distinguished into a special type called coral coasts. In addition, coral structures form many small low islands scattered over vast areas in the oceans and seas of the tropical zone. It is convenient to consider them in the same section, since in their genesis they essentially represent coastal forms of island hills that have sunk below ocean level.

Reef-building corals are colonial organisms that form a calcareous skeleton. This skeleton, remaining after the death of individuals, makes up the mass of the reef. Corals are divided into six- and eight-rayed. Reefs are formed predominantly by six-rayed corals, with eight-rayed corals playing a subordinate role.

Coral structures represent a unique living environment in which many other organisms, both leading an attached lifestyle and freely moving, find very favorable conditions in terms of shelter and abundance of food. These include numerous mollusks with thick-walled shells, sea urchins, crustaceans, bryozoans, calcareous sponges, calcareous algae, variegated fish, etc. As can be seen from the above list, many of these organisms also deposit lime and, therefore, can contribute to the growth of the reef. A particularly large role from plant organisms played by multicellular algae belonging to the red (purple) algae. The cells of these algae secrete, as it were, a cover of calcite and magnesite, which, however, does not destroy the flexibility and mobility of the algae branches, which allows them to withstand strong water movements in the surf zone without breaking. Typically these carbonate crusts are red or pink in color. Being less fastidious than corals, lithothamniaceae grow well in places where corals can no longer develop. They develop luxuriantly in the upper part of the reef in the tidal zone, where they can remain alive during low tide, wetted only by the spray of the surf. Lithothamniums feel best in shallow, clear waters, at depths not exceeding 10 m. In reef lagoons, a multicellular, lime-secreting green alga, halimeda, is also often found, forming calcareous, sieve-like perforated branches. Halimedas reproduce and grow so quickly that they literally entwine the bases of coral colonies with their branches.

Reef-forming corals are found only in sea waters whose temperature never drops below 20° (optimally 25-30°). Such temperature conditions They also determine the horizontal distribution of corals, limiting it only to the seas of the tropical zone. At the same time, coral structures are stronger and further from the equator along the eastern shores of continents, washed by warm currents, and are almost absent on the western shores, along which cold currents pass. In the Northern Hemisphere, the border of coral reefs passes through Bermuda (about 30° N), the northern part of the Red Sea (26-27° N) and the Hawaiian Islands (20° N). In the Southern Hemisphere, this border passes through Houtman (28 0 30 "S) off the western coast of Australia, Lord Howe Island at 31 ° 30" S. w. between Australia and New Zealand.

Most reef corals can live at depths not exceeding 40 m, and only a few were found at depths of 60-70 m. Coral polyps cannot withstand being out of water for any length of time, so the living parts of the reef start only from the water level at low tide. Further, corals love clean and transparent water, although animated by strong movement. Where the water becomes turbid, for example in front of the mouths of rivers carrying a lot of suspended sediment, the coral structures are usually interrupted. Corals also need light, since polyps are in symbiosis with one of the types of unicellular algae , which requires light. Water strongly enlivened by wave movement and currents contributes to an abundant supply of corals with food in the form of plankton and oxygen and therefore favors the enhanced growth of colonies. It is generally believed that a coral colony requires hard, rocky ground to initially establish itself. This is true where the waves and surf are strong enough to destroy the coral structure at an early stage of its development. But in the calm waters of the Red Sea lagoons you can often find small colonies of corals, the substrate for which is sand, so that they can be easily moved by hand. According to the observations of O.K. Leontyev, there are colonial corals that settle not only on sandy and silty soils, but sometimes even on plant substrates. Thus, off the southern coast of Cuba there are forms of corals that settle on the stems of seaweed (thalassia).

A coral reef is composed of colonies of many types of corals, having the shape of more or less compact masses, spherical or cake-shaped , or strongly branching like a bush , intertwining and merging with each other. Between the colonies and their branches there are often cavities and channels of varying sizes, filled with water and providing shelter and protection from enemy attacks for many other animal organisms. Thus, the reef has a generally loose structure.

As already mentioned, corals can only build their structures up to sea level when the water is low. Having reached the latter, the reef can only grow laterally, while its middle parts, to which access to fresh water and food is difficult, begin to die and collapse. Due to the growth laterally, individual stems of coral structures often acquire a mushroom-shaped shape, starting at the bottom with a relatively narrow trunk and expanding laterally in the upper part. Such coral formations are found, for example, off the coast of Brazil. The latter sometimes merge with each other in their upper parts into large reef massifs , resting below, on seabed, only on separate columns, between which cavernous underwater corridors stretch.

A reef rising from the bottom of the sea near the shore or at some distance from it has an inner side facing the land and an outer side towards open sea. The surf on the outer side of the reef is especially strong, and here the reef is subject to severe destruction. Its dead parts are broken off by the surf and, in the form of limestone rubble and sand, are thrown onto the surface of the reef during a splash; they fill the voids and depressions with a loose mass, which, however, is quickly cemented by the waters penetrating into it. The reef, which originally had a cavernous and spongy structure, is thus transformed into dense and compact calcareous rock.

Simultaneously with this process, the reef also rises and rises above sea level. The detritus material thrown by the surf onto the surface of the reef gradually forms a shaft rising above the water on its outer side. This shaft often serves as a source of coral sand, from which the wind forms dunes behind the shaft, which, however, are quickly cemented and fixed. On the surface thus formed, which in some places is not covered by water even during high tides, a soil cover can later form and vegetation can develop due to the introduction of seeds and fruits (coconut palm, etc.) by sea water.

Based on their location in plan, there are three main types of coastal coral structures, determined by the depth of the coastal part and the steepness of the underwater slope: 1) fringing reefs; 2) barrier reefs and 3) cortical reefs.

fringing reef is formed if the underwater slope is steep and the depths at which corals can develop are located only near the shore. The reef then, as it were, builds up the coastal slope, adjoining the main shore closely and forming a coastline with its outer edge. Two types of fringing reef can be distinguished: 1) reefs with their outer edge facing the open sea and not protected from its influence by any other barrier, and 2) reefs protected by a barrier reef. Not exposed to strong surf, such a fringing reef does not have a ridge formed by calcareous algae, although its outer edge is often almost vertical. The surface of the protected fringing reef is similar to the reef plateau of the atolls described below, has an uneven surface, and is often occupied by a series of small lakes or puddles at low waters. In some cases, the fringing reef is not quite adjacent to the main shore, but is separated from it by a very shallow (0.3-1.5 m) channel with a bottom covered with sand or gravel; This is the so-called “boat channel”. Often this phenomenon is associated with an abundance of sediment near the coast, creating unfavorable conditions for coral growth. Channels of this origin are developed, for example, on the eastern shore of the Red Sea north of Jeddah and on the northwestern coast of Madagascar.

barrier reef represents a shaft rising from the bottom of the sea and running parallel to the shore, separated from it by a channel or lagoon of greater or lesser width. The Great Barrier Reef, which stretches for almost 2000 km along the east coast of Australia, has an average lagoon width of 30-50 km. In some places it narrows to 7 km, in others it expands to 100 and even 180 km. The depth of the reef channel usually does not exceed 50 m. Australian barrier reef does not stretch continuously: it is composed of a huge number of individual reef massifs, located in a chain on the border with the open ocean, and, in addition, a mass of individual small reefs are scattered among the lagoon. Some of these structures represent underwater rocks, the surface of which is located at depths of 10-15 m. Such underwater rocks, which make navigation very difficult for ships in the areas where they develop, represent the early stages of reef development. Characteristic is the almost complete absence of participation of calcareous algae in their formation. As the corals continue to grow, the reef rocks reach the water level at low tide, which stops their further growth in height due to the vital activity of the corals. The surface of the building is now in different conditions than its slopes.

The strong movement of water in the surf zone causes fragmentation of the calcareous rock of the reef, the fragments are ground and crushed. turning into coral sand thrown by the surf onto the surface of the reef.

Miniature sand islands or coral shoals of several tens or hundreds of square meters are scattered in large numbers in the lagoon of Australia's Great Barrier Reef. Sometimes these lagoon reefs look like atolls in miniature, having a ring-shaped shape with a lagoon inside in the form of a small lake only a few decimeters or meters deep. Such lagoonal reefs are quite common in the epicontinental seas of Indonesia, the South China Sea, and the Antilles region. in places in the Red Sea, along the northwestern coast of Madagascar, in the reef channel of the Queensland coast. Typically these atoll-like reefs are higher on the windward side.

Apparently, those forms that are known in the literature as cortical reefs should also be included in the same category of shallow-water, lagoonal forms.

Despite the intensive destruction of the dead parts of the reef on the outer side by the action of the surf, the reef grows especially vigorously on this side. This is explained by the fact that the strong movement of water brings food to the polyps in the form of plankton. Due to this, the outer edge of the reef is usually very steep, often even overhanging, and on barrier and fringing reefs it often drops to quite significant depths.

With changing depth conditions along the same coastline, different types of reefs can transition into one another. Thus, the barrier reef fringing New Caledonia is under. 21° S w. adjoins directly to the western coast of the island and stretches for 100 km in the form of a fringing reef. In the same way, the Great Barrier Reef, which frames the islands of Viti Levu and Vanua Levu (Fiji) in the north, is directly adjacent to both islands for the rest of its length. Sometimes a fringing reef separates a bay from the sea, appearing in the form of a barrier reef, as is observed, for example, in one of the bays of the island. Oau (in the Hawaiian Islands). In the Red Sea, fringing, barrier and cortical reefs are developed nearby. The latter dominate the Gulf of Suez; along the coast of the Hijaz there is a barrier reef, separated from the coast by a channel 70 m deep, and on the opposite African coast the reef has the character of a fringing one.

The bedrock coast, bordered by reefs of one form or another, can be either low (for example, the Florida Peninsula) or high (the east coast of Australia, New Caledonia and the shores of many other Pacific islands).

In a number of cases, it is observed that the coastal slope of the coast, accompanied by coral structures, is also covered with the latter, located, however, already at a considerable height above sea level. These will be ancient, already dead coral reefs, formed at a different sea level and have since experienced uplift. Sometimes these raised coral reefs form, as it were, several floors or terraces, lying one above the other, corresponding to several fairly long-term stable positions of the coastline. Raised coral reefs are known almost exclusively on the shores of islands: on the Greater and Lesser Antilles (Cuba, Jamaica, Barbados, Leeward), on the southern coast of Java, etc. Karst phenomena are often observed in these coral limestones: small rivers, reaching the coastal strip of raised coral reefs, disappear from the surface and reach the sea via underground routes. The outer edge of the ancient reefs in some places seems to be raised like a swell and fences off dry depressions that drain through caves and channels in the limestone. These depressions apparently represent nothing more than reef channels and lagoons that dried up during uplift.

On the other hand, many reefs, with their morphological features, often clearly indicate that the bottom of the sea on which the corals originally settled has taken place and may still be continuing. It was already noted above that not only the depths immediately in front of the outer edge of the barrier reef, but often also the depths of the channel separating the reef from the bedrock shore, significantly exceed the depths at which corals can develop. It is quite obvious that this could only have happened as a result of the slow subsidence of the seabed, with the lower parts of the reef dying off and turning into simply coral limestone, while the upper part of the reef, consisting of still living colonies, was constantly building up the reef to sea level as it sank. In such cases, subsidence is evidenced by the significant width of the channel separating the reef from the shore, as well as the very young nature of the bedrock coast, often almost devoid of alluvial formations and rich in ingression bays, etc. (coasts of Australia, New Caledonia).

Atolls. The types of coral structures considered so far only complicated the structure of the coasts of continents and islands composed of rocks of inorganic origin. But in the so-called atolls, coral reefs already play a completely independent role. An atoll marks the presence of a shallow underwater rise, most often an underwater volcanic cone, plunging steeply at its edges to considerable depths. Coral structures are planted on this hill, which alone protrude from the water in the form of a ring of low coral islands or in the form of a continuous ring-shaped shaft enclosing the internal water space - the lagoon. This reef ring is actually called an atoll.

The sizes and shapes of atolls are varied: their diameter varies from 2-3 to several tens of kilometers. Suvodiva Atoll in the Maldives group reaches 217 km in circumference with a diameter of up to 71 km and consists of 102 coral islets. The Marshall Islands group has atolls reaching over 100 km in diameter. The shape of the atoll in plan is sometimes more or less round or oval, sometimes triangular, quadrangular, or irregularly lobed or angular. In the smallest atolls, the lagoon is sometimes absent, replaced by a dry saucer-shaped depression. If the lagoon is developed, then its depth is always insignificant - no more than 70-80 m, and in small atolls - several meters. The bottom of the lagoon is usually quite flat, slightly concave, smooth, usually composed of coral sand, and closer to the middle - fine calcareous silt. In cases where the reef ring is broken in several places by channels connecting the lagoon with the open sea, the depth of the lagoon always exceeds the depth of these channels. Thus, if the above-water "part of the atoll is not continuous, then the underwater part usually represents a continuous reef shaft, outlining the edge of the underwater volcanic rise of the seabed. If oceanic waters can freely penetrate into the lagoon through wide channels, then corals can develop in the lagoon, forming rising here and there reef islands. The individual islands that form the ring of the atoll are themselves often like an atoll in miniature, having their own lagoon in the middle, or they represent an incomplete ring, opening towards the main lagoon with a wide channel. Such atolls of the second order are called an atoll mi.

A clearly defined zonal structure is observed in the structure and topography of the atoll ring. The following zonal elements can be distinguished:

1. An outer (oceanic) slope, falling steeply to significant depths of several hundred meters. The angle of incidence often exceeds 45°, and in the upper part of the slope, where especially intensive reproduction of corals occurs, it often even forms a canopy.

2. A calcareous-algal ridge that forms the outer edge of the reef surface and is clearly expressed only on its windward side. This ridge represents the highest part of the reef and is composed mainly of calcareous algae deposits . The crest is absent on the atolls of the seas of Indonesia and the Red Sea, where the surf is never as powerful as on the atolls open ocean. The strength of the surf is such that it makes the reef completely inaccessible from the windward side, except in those places where it is broken through by channels. The surf destroys coral structures and makes their development almost impossible to a depth of 2-3 m. But algae grows luxuriantly in such conditions and, as already indicated, can remain alive above sea level, wetted only by splashes. On Bikini Atoll in the Marshall Islands, a red or pink algal ridge resembles a cuesta with its asymmetrical profile and rises 0.6-1.0 m above the reef plateau located behind it.

On the outer, windward side of the ridge they are sometimes observed; equidistant furrows and elevations separating them, absent on the leeward side of the ridge facing the lagoon. In addition to the Marshall Islands, similar grooves are observed on Funafuti Atoll in the Ellis Islands group, on Ra-roia in the Tuamotu group, and on Onotoa in the Gilbert Islands.

3. The reef plateau lies behind the algal ridge; it is usually several hundred meters wide, has an uneven surface, and is formed mainly by dead coral reefs and reef detritus that has undergone cementation and encrusted with calcareous algae. Sometimes the plateau is partially occupied by living corals, which, near the algal ridge, find favorable living conditions due to the abundant supply of water from the ocean. These coral colonies often take the form of microatolls - ring-shaped structures in which living polyps are located at the edges, and these edges rise slightly above the central part. The height of microatolls is usually only a few decimeters, and the diameter varies from a few decimeters to several meters. On the reef plateau there are islands composed of coral sand.

4. The inner slope of the reef ring is much flatter than the outer slope. Its upper part is formed by sand blown from the islands. On the slope there are colonies of living corals with flat top surfaces that almost reach the surface of the water.

5. Lagoon. The bottom of the lagoon is sometimes flat and covered with calcareous sand of unknown thickness. But steep tubers of living corals often protrude from the sandy bottom. In the Enewetak Lagoon (Marshall Islands), Emery counted about 2,300 such colonies.

Atolls are extremely widespread throughout the tropical Pacific and Indian oceans. In the Indian Ocean, between 70 and 100° east. e. atolls are the dominant form of islands. These include the groups of the Laccadive and Maldives islands, the Chagos Islands, etc. Pacific Ocean The atolls include all the islands of Tuamotu, Tokelau, Phoenix, the Central Polynesian Sporades, Ellis, Gilbert, Marshall, Hawaiian Islands and many others. As you can see, atoll islands are grouped into archipelagos that occupy vast areas. Outside the open ocean, true atolls are rare. One can note their presence in the seas of Indonesia, and several typical atolls are known in the Red Sea.

Real atolls rise only a few meters above sea level, and some do not even protrude to the surface at all, representing underwater banks. But along with this, there are also atolls that have undoubtedly experienced some uplift since their formation. They rise significantly greater height higher than sea level than normal reefs, their lagoon often has the appearance of a dry depression with underground drainage through cracks in the reef limestone. Examples of raised atolls include numerous small islands of the Central Polynesian Sporades, Christmas Island in the Indian Ocean (height 364 m), some of the Loyalty Islands, Eua Island (329 m) in the Tonga archipelago, etc. As already mentioned, on the slopes of such raised atolls reef limestones are located in several tiers, forming a series of terraces. Raised atolls are of particular interest for understanding their genesis, since erosion of reef limestones occasionally exposes their base, which usually consists of volcanic rocks. Such formations mark a series of transitions to high volcanic islands with modern fringing or barrier reefs. Such transitional formations are the so-called almost-atolls. An example of an almost-atoll is the Truk Atoll in the Caroline Islands group, in the lagoon of which, reaching 63 km in diameter, a number of islands of volcanic origin rise, one of which reaches a height of 530 m. The Admiralty Islands are known for their near-atoll, Ermit, with its four volcanic islands.

Until relatively recently, the question of the origin of atolls was solved by researchers in different ways. In the first half of the last century, the companion of the Russian navigator O. E. Kotzebue I. F. Eschscholz expressed the view that the formation of atolls occurs on mountain peaks rising from the ocean floor. This idea of I. F. Eshsholz was further developed in the views of a number of naturalists of the 19th century, who believed that the coral reefs of the atolls, with their ring-shaped shape, repeated the same shape of the edge of the volcano crater on which they were founded. This opinion, however, did not fit with the fact that the size of many atolls is much larger than the diameter of the craters of known volcanoes on the globe.

A coherent theory of the formation of atolls was given in 1842 by Charles Darwin after his observations of the structure and life of coral reefs during trip around the world on the Beagle. According to Darwin, the foundation of each atoll should be an island, most often representing the top of a volcano rising from the bottom of the sea. The corals that settled along the outskirts of this island initially formed a fringing reef, closely adjacent to the shore. With the subsequent slow sinking of the sea floor, the original island gradually sank and decreased in height and diameter. As the reef sank, the corals continually built it up to sea level, but the reef grew laterally only on the outer side, facing the open sea. In this regard, a channel should have formed between the reef ring and the island, which had decreased in size. The reef thus turned into a barrier. Later, the island could completely disappear under water, and only the reef, constantly built on top by living corals, remained in the form of a ring in place of the sunken island.

Darwin's theory of the origin of atolls, developed and supplemented later by D. Deng, was accepted unconditionally by everyone for some time and was recognized as universal. It was confirmed by the fact that dead reefs were observed beneath the living ones, going to depths at which corals can no longer live, as well as the presence of the almost-atolls described above. However, a number of new facts have caused criticism of this theory and cast doubt on its universal application.

An important event in the second half of the last century was the round-the-world oceanographic expedition of the research ship Challenger (1868-1872). A participant in this expedition, D. Murray, proposed a new theory of the origin of coral reefs and atolls. According to this theory, ring reefs form on underwater volcanic hills, confined to areas where the seabed rises. When the top of this hill reaches such a depth that shallow-marine bottom organisms can already settle on it, their calcareous skeletons begin to accumulate here, to which are also abundantly added the remains of dying planktonic organisms - shells of foraminifera, pteropods, etc. Thanks to these deposits, the surface is volcanic the elevation increases even more. Calcareous sediments usually do not reach deeper places on the seabed, being completely dissolved by sea water as they descend. When in this way the underwater elevation approaches sea level so much that reef-forming corals can settle on it, the latter begin to build their buildings throughout the entire shallow area. However, on the outer edges of the reef thus formed, the corals are in more favorable conditions, receiving constantly fresh water and abundant food, so the reef grows faster here. In the inner parts of the shallows, the corals soon begin to die, and the limestone of the dead reefs is gradually dissolved by sea water. Thus, a lagoon is formed here. On very small atolls, as already mentioned, there may not be a lagoon, since the breaking waves fill it with the thrown-in products of the destruction of the outer parts of the reef. The larger the size of the atolls, the more the accumulative activity of the surf lags behind the dissolving and eroding action of sea water, and the larger and deeper the lagoon becomes.

As can be seen from the above, Murray’s theory, in contrast to Darwin’s theory, assumes a relatively very small thickness of coral reef limestone, not exceeding several tens of meters. As we will see below, drilling undertaken on some atolls in some cases did not confirm this assumption. There are other weaknesses in Murray's theory. For example, the possibility of the formation of a lagoon by the dissolving action of sea water raises doubts.

Finally, it should be stated glacio-eustatic theory reefs, proposed by R. A. Daly. The latter’s views are based on the fact that a large number of lagoons have a depth of about 60 m. The level of the bottoms of these lagoons is considered by the named author as an abrasion surface formed during the Quaternary glaciation, when the level of the World Ocean was eustatically lowered due to the withdrawal of a significant volume of water from it, migrated to land in the form of thick ice layers of continental and mountain glaciers. A decrease in ocean water temperature during the glaciation maximum in the marginal parts of the modern coral distribution area more distant from the equator should have caused their death here , and they were kept alive only in a few “shelters” (refugia) near the equator. During the post-glacial period of climate warming and melting of glaciers, the sea level eustatically rose, corals spread from these refuges and occupied with their buildings the abrasive surfaces of the time the level decreased. Daly points out the vastness of atolls and barrier reefs that reach large sizes in the equatorial regions than in the marginal parts of their range, and explains this by the greater antiquity of the former. He also admits that movements could also play a role earth's crust, but does not attach much importance to them.

As can be seen from the above, the solution to the question of which of the proposed theories best answers known facts, requires, first of all, to determine the thickness of coral limestones lying on a volcanic basement or on a layer of shallow-marine sedimentary formations. Thus, according to Daly's theory, the thickness of the coral formation cannot be greater than the layer of water that returned to the ocean as a result of the melting of glaciers. On the other hand, the theory of subsidence (Darwin, Dana) requires a large thickness of coral limestone. This issue could only be resolved by drilling on the atolls. The first such drilling was carried out in 1896-1899. on the Funafuti Atoll in the group of Ellis Islands, rising from depths of 5000 m. With the then low drilling technology, it was possible to drill only to a depth of a little over 300 m, although the foundation of the coral formation had not yet been reached. The well revealed coral rocks interbedded with sandstones, fragments of protozoan skeletons, shells of bivalves and gastropods. The presence of coral rocks at depths exceeding the capacity of coral growth indicates the subsidence of the reef structure as it grows (, p. 18). This spoke in favor of Darwin's theory.

According to Vivald's research in the Indian Ocean, the decrease in surface water temperature in the area of the Laccadive and Maldives islands reached 8-9° during the glaciation period, i.e., the water temperature was below 18° - the limit of the possible existence of kooalls.

At a later time, drilling on reefs and atolls was carried out in many areas, and the results in most cases speak in favor of the Darwinian theory of subsidence. Thus, drilling at Kito Daito Cima, south of Japan, showed the distribution of recent Plio-Pleistocene coral limestones to a depth of 103 m, i.e., to a depth somewhat greater than required by Daly’s eustatic theory. This drilling, carried out to a depth of 432 m, did not reach the basement of the reef limestones. Two drilling rigs installed in the lagoon of the Great Barrier Reef of Queensland showed the distribution of recent reef limestones to depths of 123 and 145 m. On the island. Maratua, northeast of Borneo (Kalimantan), there were corals to the very end of the well - to a depth of 429 m, on Oagu in the Hawaiian Islands group - to 319 m, on Bikini Atoll in the Marshall Islands, the deepest of four wells penetrated 777 m without reaching the base of the coral formation. Magnetic surveys at Bikini showed a presumably volcanic base of the atoll at depths of 1250 to 3950 m. All these facts indicate a significant amount of subsidence of the sea floor. It is impossible, however, not to mention the Bermuda Islands, the structure of which shows agreement with Daly’s glacio-eustatic theory. It turned out that the Bermuda reefs rested on a platform at a depth of 75 m, and seismic surveys in 1952 showed the presence of a leveling surface at this level under the entire archipelago.

Coral reefs and islands.

In their education main role solid polyps of coral polyps (see) and the products of their destruction play. Although coral polyps are common in the seas of all zones and are found at all kinds of depths, from the lower limit of low tides to the enormous ocean depths, however, their mass development is limited to relatively narrow horizontal and vertical limits. This applies especially to those K. polyps that form colonies, equipped with a dense calcareous skeleton, which develop in huge masses and islands. These animals find conditions favorable for their development in relatively shallow layers: from the low tide line to 20-30 fathoms; below this depth, living K. polyps, which take part in the construction of K. reefs, are found only as an exception (to a depth of about 90 m ); In general, below 20-30 fathoms we find only dead masses of K. polypnyaks. The most abundant growth of corals is limited to even tighter limits - from the low tide line to 10-15 fathoms. In the horizontal direction, the distribution area of reef-building corals is limited to a narrow strip on both sides of the equator; Only near the Bermuda Islands are there significant coral formations at 32° N. w. Within the specified zone of K., reefs and islands are not widespread; studies by the American zoologist Dan showed that coral reefs and islands are found only where the sea water temperature does not fall below 20 ° C (however, there is a known case of reef corals being found at several lowest temperature, about 18° C). Therefore, we do not find significant K. formations off the western coasts of America, Africa and Australia; due to the existence of cold currents here, the line connecting the points where the temperature does not fall below 20° C (“20° isocrime”) approaches the equator here and only at the west. On the coast of America there are poorly developed coral reefs between California and Guayaquil. Meanwhile, the eastern coasts of all these continents are surrounded by numerous and extensive K. buildings.

Fig. 1. General form coastal and barrier reef.

Corals of the Great Barrier Reef, Australia

The most developed K. buildings are in Great Ocean where they are found in all typical forms (coastal reefs, barrier reefs and K. islands - see below). The central and southern parts are dominated by atolls (Lowland, Alice, Gilbert, Marshall, and Caroline islands); coastal reefs fringe the Elizabeth Islands, the Mariners, Friendship, New Hebrides, Solomon, Sandwich, Mariana and some islands China Sea; in the Australian seas there are barrier reefs and partly atolls (the most important are the reefs off the eastern coast of Australia, off the western coast of New Caledonia and the reefs of the Fiji islands). Of the East Asian islands, coral formations (especially coastal reefs) are found in the Philippine Islands, Borneo, Java, Celebes, Timor, etc. Indian Ocean South coast Asia, in general, is poor in coral formations; Significant coastal reefs border individual points in the southwest. and southeast the coast of Ceylon; in the islands of the Maldives, Lakedives and Chagos there are extensive K. formations in the form of atolls; in the western part of the Indian Ocean, the islands are fringed mainly by coastal reefs (Seychelles, Mauritius, partly Bourbon); part of the coast of Madagascar is fringed by coastal reefs, the Comoros coastline is lined with barrier reefs, and the eastern coast of Africa has extensive coastal reefs. K. reefs abound in the Red Sea, where a little interrupted coastal reef stretches along the African coast from Suez to Bab el-Mandeb; in addition, there are formations similar to barrier reefs, and according to Walter, atolls. K. reefs are also common in the Persian Gulf. IN Atlantic Ocean significant K. buildings are located in the east. the coast of America, here significant reefs are found off the coast of Brazil, along the coasts of Yucatan and Florida, Cuba, Jamaica, Haiti, in the Bahamas and Bermuda Islands; There are coastal and barrier reefs here, and in the Bermuda Islands there are atolls.

A section of the Great Barrier Reef as seen from space. The Great Barrier Reef is not a single formation; it consists of thousands of interconnected segments, the most massive and oldest of which are located at its northern tip

Fig. 2. General view of the atoll.

Barrier Reef Islands.

The main role in the formation of K. structures is played by polypnyas of a number of forms from the group of 6-rayed or multi-tentacled polyps (Hexactinia s. Polyactinia), especially the families Astraeidae (Astraea, Meandrina, Diploria, Astrangia, Cladocora, etc.), Madreporidae (Madrepora, etc. ), Poritidae (Pontes, Goniopora, Montipora, etc.), partly Oculinidae (Orbicella, Stylaster, Poecillopora, etc.) and most of the representatives of Fungidae (Fungia, etc.). In addition, some 8-rayed polyps with a calcareous skeleton (for example, Heliopora, Tubipora), as well as horny polyps of gorgonids, take part in the formation of K. islands and reefs. In addition to the coral polyps themselves, representatives of one group of hydromedusae, distinguished by calcareous deposits - Hydrocorallinae (Millepora, etc.) are also important in the formation of reefs and islands. Finally, a significant part of the mass of reefs and islands consists of masses of calcareous algae, nullipores and partly coralline. Finally, the composition of coral structures includes shells of mollusks, calcareous skeletons of bryozoans (Bryozoa), shells of rhizopoda (Rhizopoda) and radiolarians (Radiolaria), and other solid parts of animals; these foreign elements can sometimes form a very significant part of the mass of coral structures. The composition of reefs and islands in different seas shows significant differences; Thus, in the Red Sea, the polypnyas Porites, Madrepora and Stylophora predominate and make up the bulk, in the reefs of the island of Mauritius - Porites and Montipora, in Ceylon - Madrepora and Poecilopora, in Singapore - Madrepora, on the Sandwich Islands - Poecillopora, in the west. the coast of America - Porites and Poecillopora, off Florida - Porites, Madrepora and Meandrina, etc.

Porite coral

For the most part, the base of a reef or island is formed by solid rocks - underwater mountains or the coastlines of continents and islands. Loose soil, especially silt, is unfavorable for the development of corals. However, the latest research by Sluiter off the coast of Java has shown that coral reefs can also arise on the bottom covered with silt, if there are shells, stones or pieces of pumice on its surface, to which young corals can attach. As the latter grow and the weight of the colony of polyps sitting on a piece of pumice, etc. increases, its base is pressed deeper and deeper into the silt, while on the upper parts of the polyp forest the coral polyps continue to successfully multiply and grow upward. Reaching denser soil with its base, the young reef receives a dense foundation, relying on which it can successfully grow further. Some polyps, according to other studies, can grow successfully on gravel soil if it is held together by algae (such as Psammocora, Montipora, Lophoseris off the eastern coast of Africa). Most coral polyps find the most favorable conditions in the upper layers, where there is strong water movement, and only a few, more fragile forms seek protection from the surf. At the same time, most of them strive for light (represents positive heliotropism - see). Therefore, polypnyaks continuously grow upward, while the parts lying below die off. Thus, living colonies of polyps form, so to speak, a living crust on the dead mass of the reef, containing various cavities and voids. Thick masses of coral structures are compacted due to the fact that the empty spaces between individual polypnyaks and their branches are gradually filled with coral fragments and other calcareous deposits. The strong surf to which the polypnyaks are exposed breaks off significant masses of them and the movement of water wears the fragments into smaller material. The process of destruction and change of the reef under the mechanical action of waves is greatly facilitated by various marine animals drilling into coral structures; such are boring sponges, some mollusks (for example, Lithodomus) and partly crustaceans. Some fish that feed on corals gnaw branches and, crushing them, give rise to the formation of thin calcareous silt, which also cements the fragments of polypnyaks. Some role in the formation of this thin silt is also played by sea cucumbers, which are found in abundance on the K. reefs, from where hundreds of centners of some species are annually taken to China under the name sea cucumber. The growth of K. polypnyaks occurs at different speeds. Branched tree-like forms grow fastest; so in one case, on the remains of a broken ship at 64 years old, Madrepora grew up to 1 6 ft. in height; Madrepora alcicornis in Haiti formed branches 7-12 cm long at 3 months; Usually, branched polyp forests lengthen by a smaller amount per year. The growth of massive polypnyaks, such as Astraea, Meandrina, etc., occurs much more slowly; Thus, there is a known case when Meandrina grew 6 inches at the age of 12, but usually the polyp grows thicker by a small part of an inch per year. K. polyps can live only below the low tide line, and for the most part, even a short stay out of water entails the death of animals (only some forms, like Porites, Goniastraea, Coeloria, Tubipora, can remain alive out of water for whole hours). The polyps themselves can, therefore, build their buildings only to the lower low tide line, and any elevation of reefs and islands above this level can only be determined by the action of other factors. Pieces of polypnyaks, broken off by the surf, are thrown out by the sea onto the surface of the reefs and, gradually piling up, give rise to the above-water parts of K. buildings. And here the gaps are filled with smaller fragments, sand and other dense remains of animals, and the individual pieces are finally cemented, merging into solid rock, thanks to the release of lime from the solution in water. Another reason that can cause a strong increase in the altitude of buildings above the sea is a negative fluctuation in sea level, due to which the altitude of buildings can rise to 80 or more m above sea level. seas. The dissolution of part of the dead polypnyaks in water containing carbon dioxide occurs as at eq. sea, and on the surface of the surface parts of K. buildings. The accumulation of K. sand on the surface of K. islands can reach such a size that real dunes are formed, which, under the influence of prevailing winds, gradually move inland, filling up plantations and farms; this was the case, for example, at Paget Parish in the Bermuda Islands, where the movement of the “sand glacier,” as they called the moving dune that covered the farms, was stopped only by planting trees. The surface of islands and reefs, being covered with a layer of humus, provides soil on which very luxurious tropical vegetation often develops. Coastal structures are found in a wide variety of forms, which can be reduced to three main types: 1) coastal reefs, 2) barrier reefs, and 3) individual coastal islands and shoals. Coastal reefs are formed in cases where K. buildings are directly adjacent to the shores of islands or continents and border them, interrupted in those places where streams and rivers flow in (since polyps for the most part cannot live in muddy and especially desalinated water) or where their development is hampered by the quality or structure of the bottom ( e.g. steep cliff). Coastal reefs can either remain under water or, for the above reasons, become above water. Sluiter's research on the formation of K. reefs off the coast of Krakatoa Island, after the famous eruption of this volcano, proved that reefs can arise at some distance from the coast and gradually grow towards it. A study of the bottom surrounding the coastal reef shows that it gradually decreases towards the open sea. Barrier reefs (also underwater or surface) stretch along the shores of an island or mainland, remaining separated from them by a relatively shallow channel of varying widths (10-15 and up to 50 nautical miles). The depth of the channel can be very different, but is always relatively small. Sometimes its bottom dries up at low tide, but usually its depth is several fathoms and can even reach 40-50 fathoms. Meanwhile, outside the reef, the depths are relatively great and can reach several hundred fathoms, and the outer edge of the reef falls very steeply into the depths. In some places the barrier reefs are interrupted. Sometimes they surround the island on all sides. In some cases, barrier reefs reach enormous sizes; so at the east. coast of Australia from Cape Kar Sunday (24 about 40" S) to the southern coast of New Guinea stretches the "Great Australian Reef" about km long, separated from the coast by a channel 25-160 km wide; its main passage with a lighthouse lies under 11°35" S. w. (Raines Inlet), the depth of the channel is 10-60 fathoms, and in places outside the reef it is more than 300 fathoms. Very various shapes represent K. islands (and individual shoals); the predominant forms are round, oblong, ring-shaped (“atolls”) and semi-lunar. Atolls have the most characteristic appearance; it is a ring-shaped strip of land, usually no more than 100-200 m wide, surrounding a central basin ("lagoon"), which is usually connected to the surrounding sea by several passages lying on the side opposite to that from which the prevailing winds blow. Rarely (eg Whitsunday Island) atolls form a continuous continuous ring. The sizes of the lagoons are very different and their diameter can reach 75 km. or more (and a diameter of 30-45 km is not uncommon). The depth of the lagoon is generally insignificant, usually several fathoms, but can reach up to 50 fathoms; meanwhile, on the outer side of the atoll we find, as with barrier reefs, for the most part very significant depths. The bottom of the lagoon is covered (like the barrier reef channel) with sand and calcareous silt and contains relatively few living corals, the advantage of more delicate forms. Sometimes there may be small islands in the lagoon. The height of the atolls above sea level is mostly insignificant, no more than 3-4 m; Sometimes surf waves crash through the atoll into the lagoon. The windward side of the atoll is generally higher. Relatively rarely, the Caucasian islands reach a significant height above sea level (which is explained by negative fluctuations in sea level: the resulting reefs move out of the sea). So at Vanikoro, according to Darwin, the wall of the K. reef reaches 100 m in height, according to Dana in Metia, in the Low Islands, rocks from K. limestone are 80 m high. Sometimes underwater atolls are also found, such as, for example, a large reef in the Chagos Islands, lying at a depth of 5-10 fathoms. below sea level. Other forms of islands and shoals are also very common, and sometimes also reach significant sizes; Thus, the reef lying to the west of the two main islands of the Fiji group represents a surface of about 3000 square meters. English miles; The coastline of the Saya de Malha bank, NE of Madagascar, extends from 60°20"E to 62°10" (GMT) and from 8°18"S to 11°30", and then to South lies the Nazarethbank, about 400 km long. Seas overflowing with coastal reefs generally pose significant dangers to navigation, especially since coastal islands and reefs often rise steeply from considerable depths and there is nothing to indicate the proximity of the reefs, except for breakers in case of waves. On the other hand, barrier reefs in some cases allow ships to safely pass along the coast while the open sea is in severe weather. Fencing the shores with reefs prevents the eroding effect of waves on the shores. In addition, thanks to reefs, in some cases, erosion products brought from land are deposited off the coast and cause a significant increase in land mass; Thus, Tahiti is surrounded by a strip of land with a width of 0.5 to 3 English. miles, which occurred this way and was covered with rich vegetation.

mushroom coral

Black corals

Along with the process of formation of the K. islands (for example, near Florida), we encounter phenomena of their destruction in other places (for example, on the Bermuda Islands); in these cases, the formation of caves (sometimes stalactites and stalagmites), arches, etc. is observed; at the same time, on the surface of the island there is a special red soil, in which they see a residue from the erosion and dissolution of the lime of the reef. The peculiar structure of K. reefs and islands, their importance and enormous distribution have long aroused interest in these formations, especially in atolls; to explain the shape of the latter, some resorted (with Steffens, in) to the hypothesis that atolls are crowned with underwater craters; others believed that K. polyps, due to a special instinct, erect their buildings in the shape of a ring in order to enjoy protection from the surf. The theory of coral formations, given by Darwin, explained the mysterious fact of the existence of coral structures at great depths, where corals that build reefs cannot live, explained the reason for the significant thickness of coral deposits (which, by the way, was confirmed by the latest drilling experiments in coral. reefs), as well as the shape of K. buildings and the connection between them. Despite a number of recent objections, Darwin's theory remains dominant. Darwin's theory is the name. the theory of immersion (Senkungstheorie), the essence of which is as follows. If coastal structures arise near the shore of an island or mainland, where the water level remains more or less constant (the bottom does not sink), then, as they grow, they should give rise to a coastal reef. If the bottom goes down, the reef will continue to grow upward and should take on the character of a barrier reef, separated from the land by a channel. This will be facilitated by the fact that K. polyps will be found Better conditions for life on the outer side of the reef, which will therefore grow stronger. If, finally, with further subsidence, the island, surrounded by a ring-shaped reef, completely disappears under the surface of the sea, an atoll (underwater or above-water, depending on the speed of the dive) will remain in its place. This explanation of the origin of K. buildings and the connections between them explains many of their features and is based on a number of diverse facts. However, extensive K. formations, in the form of barrier reefs, are also observed in places where, on the contrary, a rise in the bottom obviously occurs, and atolls are also observed in such areas. In general, it must be admitted that various forms of K. buildings can occur in another way, in addition to any lowering of the bottom, for example, on underwater banks and mountains, and the shape of islands (including atolls) is sometimes determined by the direction sea currents or the fact that the corals of a given reef grow more successfully at its edges than in the middle, the middle ones die off and are subject to the destructive action of currents and water containing carbon dioxide, which leads to the formation of a lagoon. Be that as it may, the latest objections to Darwin's theory represent rather additions and amendments to it than a new explanation that could completely replace that given by Darwin. Extensive K. formations existed in previous geological periods, and in many sediments we find clear traces of reefs. In the most ancient periods of Cyprus, reefs occupied a relatively large area. Paleozoic reef corals are found in Scandinavia and Russia well beyond 60° N. w. and some genera even on Spitsbergen, Novaya Zemlya and the Barents Islands; Lithost r otion was found during the Nares expedition to the north of 81° N. w. In the Silurian and Devonian periods, corals abounded in the seas over a wide range. Canada and Scandinavia. In later geological periods, we see that the K. reefs are retreating more and more towards the equator, which was, in all likelihood, due to a decrease in sea temperature at high latitudes. During the Triassic period, reefs were abundant in central and southern Europe; in the Jurassic period, the vast K. Sea occupied a significant part of western and central Europe, and traces of reefs remained in England, France, summaries of the most important data in Keller, “Leben des Meeres” (unfinished edition), Marschell in Bram’s “Thierleben” (Bd. X ; new edition, ends in Russian), as well as in Kingsley, “The Riverside Zoology” (vol. I); Heilprin, "The Distribution of Animals" (1887) and Nicholson's article in Encyclopedia Britannica.

I will never forget my holiday in Egypt! He was amazing! Most of all I remember the sea; I have never seen such a sea anywhere else! In Egypt, it is simply teeming with colorful, eye-catching inhabitants. I consider corals to be one of the most beautiful inhabitants of the Red Sea.

Corals: animals or plants

When I returned from Egypt and showed my friends the photographs, for some reason almost everyone was worried about this question. So, corals are living microorganisms that live in colonies.

By the way, we found this out not so long ago. It was only in 1982 in France that they proved that corals are not plants. They are based on invertebrate polyps. These organisms originated back in the days when mammoths lived on Earth. They have a single cavity - the intestine, which is responsible for digesting food.

The size of polyps is not always small. More often they range from a millimeter to a couple of centimeters, but sometimes there are also huge ones up to half a meter.

Formation of coral reefs

Polyps have a very delicate body. To protect yourself from predatory fish, they have to build a protective cell of limestone. This cell is called a calyx. Polyps mainly lead a colonial lifestyle. They glue their cups together, thereby forming coral reefs of fantastic beauty.

Do you know how corals reproduce? In fact, they do this in several ways:

Sexual method. When corals live together, the male is with the female. As a result, small larvae are formed that swim in the sea. This is not observed in all subtypes of polyps.
Budding. The appearance of a baby followed by its separation from the parental polyp. In this case, a shoot is formed at the base of the coral, which over time detaches and takes root as an independent individual on the bottom.
Division. This method of reproduction is characteristic of some solitary soft individuals.

Amazing, right? The sexual reproduction of corals is actually a very beautiful sight.

This usually occurs under the cover of darkness at the end of spring and coincides with the full moon. Many tourists come to see it.

Coral reefs and islands

In their formation, the main role is played by solid polypnyaks of coral polyps (see) and the products of their destruction. Although coral polyps are common in the seas of all zones and are found at all kinds of depths, from the lower limit of low tides to the enormous ocean depths, however, their mass development is limited to relatively narrow horizontal and vertical limits. This applies especially to those of the K. polyps that form colonies equipped with a dense calcareous skeleton, which, developing in huge masses, lead to the formation of thick calcareous deposits - K. reefs and islands. These animals find conditions favorable for their development in relatively shallow layers: from the low tide line to 20-30 fathoms; below this depth, living K. polyps, which take part in the construction of K. reefs, are found only as an exception (to a depth of about 90 m ); In general, below 20-30 fathoms we find only dead masses of K. polypnyaks. The most abundant growth of corals is limited to even tighter limits - from the low tide line to 10-15 fathoms. In the horizontal direction, the distribution area of reef-building corals is limited to a narrow strip on both sides of the equator; Only near the Bermuda Islands are there significant coral formations at 32° N. w. Within the specified zone of K., reefs and islands are not widespread; Research by the American zoologist Dan showed that coral reefs and islands are found only where the sea water temperature does not fall below 20° C (however, there is a known case of reef corals being found at a slightly lower temperature, about 18° C). Therefore, we do not find significant K. formations off the western coasts of America, Africa and Australia; due to the existence of cold currents here, the line connecting the points where the temperature does not fall below 20° C (“20° isocrime”) approaches the equator here and only at the west. On the coast of America there are poorly developed coral reefs between California and Guayaquil. Meanwhile, the eastern coasts of all these continents are surrounded by numerous and extensive K. buildings.

Fig. 1. General view of the coastal and barrier reef.

The most developed K. buildings are in Great Ocean where they are found in all typical forms (coastal reefs, barrier reefs and K. islands - see below). The central and southern parts are dominated by atolls (Lowland, Alice, Gilbert, Marshall, and Caroline islands); coastal reefs fringe the Elizabeth Island, the Navigators Island, the Friendship Island, the New Hebrides, the Solomon Islands, the Sandwich Islands, the Mariana Islands and some islands in the China Sea; in the Australian seas there are barrier reefs and partly atolls (the most important are the reefs off the eastern coast of Australia, off the western coast of New Caledonia and the reefs of the Fiji islands). Of the East Asian islands, coral formations (especially coastal reefs) are found in the Philippine Islands, Borneo, Java, Celebes, Timor, etc. Indian Ocean the southern coast of Asia is generally poor in coral formations; Significant coastal reefs border individual points in the southwest. and southeast the coast of Ceylon; in the islands of the Maldives, Lakedives and Chagos there are extensive K. formations in the form of atolls; in the western part of the Indian Ocean, the islands are fringed mainly by coastal reefs (Seychelles, Mauritius, partly Bourbon); part of the coast of Madagascar is fringed by coastal reefs, the Comoros coastline is lined with barrier reefs, and the eastern coast of Africa has extensive coastal reefs. K. reefs abound in the Red Sea, where a little interrupted coastal reef stretches along the African coast from Suez to Bab el-Mandeb; in addition, there are formations similar to barrier reefs, and according to Walter, atolls. K. reefs are also common in the Persian Gulf. IN Atlantic Ocean significant K. buildings are located in the east. the coast of America, here significant reefs are found off the coast of Brazil, along the coasts of Yucatan and Florida, Cuba, Jamaica, Haiti, in the Bahamas and Bermuda Islands; There are coastal and barrier reefs here, and in the Bermuda Islands there are atolls.

For the most part, the base of a reef or island is formed by solid rocks - underwater mountains or the coastlines of continents and islands. Loose soil, especially silt, is unfavorable for the development of corals. However, the latest research by Sluiter off the coast of Java has shown that coral reefs can also arise on the bottom covered with silt, if on its surface there are shells, stones or pieces of pumice to which young corals can attach. As the latter grow and the weight of the colony of polyps sitting on a piece of pumice, etc. increases, its base is pressed deeper and deeper into the silt, while on the upper parts of the polyp forest the coral polyps continue to successfully multiply and grow upward. Reaching denser soil with its base, the young reef receives a dense foundation, relying on which it can successfully grow further. Some polyps, according to other studies, can grow successfully on gravel soil if it is held together by algae (such as Psammocora, Montipora, Lophoseris off the eastern coast of Africa). Most coral polyps find the most favorable conditions in the upper layers, where there is strong water movement, and only a few, more fragile forms seek protection from the surf. At the same time, most of them strive for light (represents positive heliotropism - see). Therefore, polypnyaks continuously grow upward, while the parts lying below die off. Thus, living colonies of polyps form, so to speak, a living crust on the dead mass of the reef, containing various cavities and voids. Thick masses of coral structures are compacted due to the fact that the empty spaces between individual polypnyaks and their branches are gradually filled with coral fragments and other calcareous deposits. The strong surf to which the polypnyaks are exposed breaks off significant masses of them and the movement of water wears the fragments into smaller material. The process of destruction and change of the reef under the mechanical action of waves is greatly facilitated by various marine animals drilling into coral structures; such are boring sponges, some mollusks (eg Lithodomus) and partly crustaceans. Some fish that feed on corals gnaw branches and, crushing them, give rise to the formation of thin calcareous silt, which also cements the fragments of polypnyaks. Some role in the formation of this thin silt is also played by sea cucumbers, which are found in abundance on the K. reefs, from where hundreds of centners of some species are annually taken to China under the name sea cucumber. The growth of K. polypnyaks occurs at different speeds. Branched tree-like forms grow fastest; so in one case, on the remains of a broken ship at 64 years old, Madrepora grew up to 16 feet high; Madrepora alcicornis in Haiti formed branches 7-12 cm long at 3 months; Usually, branched polyp forests lengthen by a smaller amount per year. The growth of massive polypnyaks, such as Astraea, Meandrina, etc., occurs much more slowly; Thus, there is a known case when Meandrina grew 6 inches at the age of 12, but usually the polyp grows thicker by a small part of an inch per year. K. polyps can live only below the low tide line, and for the most part, even a short stay out of water entails the death of animals (only some forms, like Porites, Goniastraea, Coeloria, Tubipora, can remain alive out of water for whole hours). The polyps themselves can, therefore, build their buildings only to the lower low tide line, and any elevation of reefs and islands above this level can only be determined by the action of other factors. Pieces of polypnyaks, broken off by the surf, are thrown out by the sea onto the surface of the reefs and, gradually piling up, give rise to the above-water parts of K. buildings. And here the gaps are filled with smaller fragments, sand and other dense remains of animals, and the individual pieces are finally cemented, merging into solid rock, thanks to the release of lime from the solution in water. Another reason that can cause a strong increase in the altitude of buildings above the sea is a negative fluctuation in sea level, due to which the altitude of buildings can rise to 80 or more m above sea level. seas. The dissolution of part of the dead polypnyaks in water containing carbon dioxide occurs as at eq. sea, and on the surface of the surface parts of K. buildings. The accumulation of K. sand on the surface of K. islands can reach such a size that real dunes are formed, which, under the influence of prevailing winds, gradually move inland, filling up plantations and farms; this was the case, for example, at Paget Parish in the Bermuda Islands, where the movement of the “sand glacier,” as they called the moving dune that covered the farms, was stopped only by planting trees. The surface of islands and reefs, being covered with a layer of humus, provides soil on which often very luxurious tropical vegetation . Coastal structures are found in a wide variety of forms, which can be reduced to three main types: 1) coastal reefs, 2) barrier reefs, and 3) individual coastal islands and shoals. Coastal reefs are formed in cases where coastal structures are directly adjacent to the shores of islands or continents and border them, interrupted in those places where streams and rivers flow in (since polyps for the most part cannot live in muddy and especially desalinated water). or where their development is hampered by the quality or structure of the bottom (for example, a steep cliff). Coastal reefs can either remain under water or, for the above reasons, become above water. Sluiter's research on the formation of K. reefs off the coast of Krakatoa Island, after the famous eruption of this volcano, proved that reefs can arise at some distance from the coast and gradually grow towards it. A study of the bottom surrounding the coastal reef shows that it gradually decreases towards the open sea. Barrier reefs (also underwater or surface) stretch along the coast of an island or mainland, remaining separated from them by a relatively shallow channel of varying widths (10-15 and up to 50 nautical miles). The depth of the channel can be very different, but is always relatively small. Sometimes its bottom dries up at low tide, but usually its depth is several fathoms and can even reach 40-50 fathoms. Meanwhile, outside the reef, the depths are relatively great and can reach several hundred fathoms, and the outer edge of the reef falls very steeply into the depths. In some places the barrier reefs are interrupted. Sometimes they surround the island on all sides. In some cases, barrier reefs reach enormous sizes; so at the east. coast of Australia from Cape Kar Sunday (24 about 40 "S) to the southern coast of New Guinea stretches the "Great Australian Reef" about 1725 km long, separated from the coast by a channel 25-160 km wide; its main passage with a lighthouse lies at 11°35" south. w. (Raines Inlet), the depth of the channel is 10-60 fathoms, and in places outside the reef it is more than 300 fathoms. The islands (and individual shoals) represent a very diverse form; the predominant forms are round, oblong, ring-shaped (“atolls”) and semi-lunar. Atolls have the most characteristic appearance; it is a ring-shaped strip of land, usually no more than 100-200 m wide, surrounding a central basin ("lagoon"), which is usually connected to the surrounding sea by several passages lying on the side opposite to that from which the prevailing winds blow. Rarely (eg Whitsunday Island) atolls form a continuous continuous ring. The sizes of the lagoons are very different and their diameter can reach 75 km. or more (and a diameter of 30-45 km is not uncommon). The depth of the lagoon is generally insignificant, usually several fathoms, but can reach up to 50 fathoms; meanwhile, on the outer side of the atoll we find, as with barrier reefs, for the most part very significant depths. The bottom of the lagoon is covered (like the barrier reef channel) with sand and calcareous silt and contains relatively few living corals, the advantage of more delicate forms. Sometimes there may be small islands in the lagoon. The height of the atolls above sea level is mostly insignificant, no more than 3-4 m; Sometimes surf waves crash through the atoll into the lagoon. The windward side of the atoll is generally higher. Relatively rarely, the Caucasian islands reach a significant height above sea level (which is explained by negative fluctuations in sea level: the resulting reefs move out of the sea). So at Vanikoro, according to Darwin, the wall of the K. reef reaches 100 m in height, according to Dana in Metia, in the Low Islands, rocks from K. limestone are 80 m high. Sometimes underwater atolls are also found, such as, for example, a large reef in the Chagos Islands, lying at a depth of 5-10 fathoms. below sea level. Other forms of islands and shoals are also very common, and sometimes also reach significant sizes; Thus, the reef lying to the west of the two main islands of the Fiji group represents a surface of about 3000 square meters. English miles; The coastline of the Saya de Malha bank, NE of Madagascar, extends from 60°20"E to 62°10" (GMT) and from 8°18"S to 11°30", and then to South lies the Nazarethbank, about 400 km long. Seas overflowing with coastal reefs generally pose significant dangers to navigation, especially since coastal islands and reefs often rise steeply from considerable depths and there is nothing to indicate the proximity of the reefs, except for breakers in case of waves. On the other hand, barrier reefs in some cases allow ships to safely pass along the coast while the open sea is in severe weather. Fencing the shores with reefs prevents the eroding effect of waves on the shores. In addition, thanks to reefs, in some cases, erosion products brought from land are deposited off the coast and cause a significant increase in land mass; Thus, Tahiti is surrounded by a strip of land with a width of 0.5 to 3 English. miles, which occurred this way and was covered with rich vegetation.

Along with the process of formation of the K. islands (for example, near Florida), we encounter phenomena of their destruction in other places (for example, on the Bermuda Islands); in these cases, the formation of caves (sometimes stalactites and stalagmites), arches, etc. is observed; at the same time, on the surface of the island there is a special red soil, in which they see a residue from the erosion and dissolution of the lime of the reef. The peculiar structure of K. reefs and islands, their importance and enormous distribution have long aroused interest in these formations, especially in atolls; to explain the shape of the latter, some resorted (with Steffens, in 1822) to the hypothesis that atolls are crowned with underwater craters; others believed that K. polyps, due to a special instinct, erect their buildings in the shape of a ring in order to enjoy protection from the surf. The theory of coral formations, given by Darwin, explained the mysterious fact of the existence of coral structures at great depths, where corals that build reefs cannot live, explained the reason for the significant thickness of coral deposits (which, by the way, was confirmed by the latest drilling experiments in coral. reefs), as well as the shape of K. buildings and the connection between them. Despite a number of recent objections, Darwin's theory remains dominant. Darwin's theory is the name. the theory of immersion (Senkungstheorie), the essence of which is as follows. If coastal structures arise near the shore of an island or mainland, where the water level remains more or less constant (the bottom does not sink), then, as they grow, they should give rise to a coastal reef. If the bottom goes down, the reef will continue to grow upward and should take on the character of a barrier reef, separated from the land by a channel. This will also be facilitated by the fact that K. polyps will find better living conditions on the outer side of the reef, which will therefore grow stronger. If, finally, with further subsidence, the island, surrounded by a ring-shaped reef, completely disappears under the surface of the sea, an atoll (underwater or above-water, depending on the speed of the dive) will remain in its place. This explanation of the origin of K. buildings and the connections between them explains many of their features and is based on a number of diverse facts. However, extensive K. formations, in the form of barrier reefs, are also observed in places where, on the contrary, a rise in the bottom obviously occurs, and atolls are also observed in such areas. In general, it must be admitted that various forms of K. buildings can occur in another way, in addition to any lowering of the bottom, for example, on underwater banks and mountains, and the shape of islands (including atolls) is sometimes determined by the direction of sea currents or by those that the corals of a given reef grow more successfully at its edges than in the middle, the middle ones die off and are subject to the destructive action of currents and water containing carbon dioxide, which leads to the formation of a lagoon. Be that as it may, the latest objections to Darwin's theory represent rather additions and amendments to it than a new explanation that could completely replace that given by Darwin. Extensive K. formations existed in previous geological periods, and in many sediments we find clear traces of reefs. In the most ancient periods of Cyprus, reefs occupied a relatively large area. Paleozoic reef corals are found in Scandinavia and Russia well beyond 60° N. w. and some genera even on Spitsbergen, Novaya Zemlya and the Barents Islands; Lithostrotion was found during the expedition of Nares to the north of 81° N. w. In the Silurian and Devonian periods, corals abounded in the seas over a wide range. Canada and Scandinavia. In later geological periods, we see that the K. reefs are retreating more and more towards the equator, which was, in all likelihood, due to a decrease in sea temperature at high latitudes. During the Triassic period, reefs were abundant in central and southern Europe; In the Jurassic period, the vast K. Sea occupied a significant part of western and central Europe, and traces of reefs remained in England, France, Germany and Switzerland. IN Cretaceous period there were already few reefs here, but they abounded in southern Europe. In the Eocene, they abounded in southern Europe, but were also found in England; in the Miocene, they were found only in southern and central Europe, and in the Pliocene, reefs are no longer found in the present extent of Europe.

Literature. The most important works about coral reefs and islands: Darwin, “On the Structure and Distribution of Coral Reefs” (1st ed., 1842); Dana, "Corals and Corals Islands" (1872); Semper, "Die Palau-Inseln" (Lpc., 1873); Semper, "Die natürlichen Existenzbedingungen der Thiere" (Lpc., 1880); Rein, "Die Bermudasinseln und ihre Korallenriffe" (Berl., 1881); Guppy, "Salomon-Islands" (Lond., 1887); Langenbeck, "Die Theorien über die Entstehung der Koralleninseln" (Lpc., 1890); Böttger, "Geschichtliche Darstellung unserer Kenntnisse und Meinungen von der Korallenbauten" ("Zeitschrift für Naturwissenschaften" vol. LXIII); Murray and Irvine, "Coral Reefs and other Carbonate of Lime Formations in Modern Seas" (Nature, XLII; other papers in the same journal); Sluiter, "Einiges über die Entstehung d. Korallenriffe in d. Java See" ("Biol. Centralblatt", Bd. IX); Kent, "The Great Barrier Reef of Australia" (1893); a number of works on corals in the “Challenger” reports, etc. Good summaries of the most important data in Keller, “Leben des Meeres” (unfinished edition), Marschell in Bram’s “Thierleben” (Bd. X; new edition, ending in Russian), and also in Kingsley, "The Riverside Zoology" (Vol. I); Heilprin, "The Distribution of Animals" (1887) and Nicholson's article in the Encyclopedia Britannica.

N. Knipovich.

encyclopedic Dictionary F. Brockhaus and I.A. Efron. - S.-Pb.: Brockhaus-Efron. 1890-1907 .

See what “Coral reefs and islands” are in other dictionaries:

CORAL REEFS, underwater or partially above-water calcareous structures formed mainly by the skeletons of colonial coral polyps (see CORAL POLYPS) in shallow areas of tropical seas. Within the ecosystem (see... ... encyclopedic Dictionary

Structures made of organic limestones located near sea level or at shallow depths in the coastal zone tropical seas or in shallow water warm seas. They are massive deposits of calcite (limestone),... ... Geographical encyclopedia

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, foraminifera, etc.

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

Three types of reefs generate many transitional forms. The fringing reef near one part of the coast can then turn into a barrier reef, and their border is difficult to distinguish. Some islands could actually be considered atolls, if not for the presence of one or more dissected volcanic rock masses in the center of the reef ring. Submerged (submerged) reefs are indicators of underwater platforms located near the water surface that support coral growth, but are more likely to be submerged atolls or islands.

The process of formation of all these coral structures has been of interest to geologists and zoologists for a very long time; the origin of the ring-shaped islands - atolls - seemed especially incomprehensible. Several theories have been proposed to explain the formation of these islands, many of them quite naive. Thus, until the middle of the 19th century, the prevailing opinion was that atolls are 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 Charles Darwin. In his book “The Structure and Distribution of Coral Reefs,” published in 1842, Charles Darwin not only gave detailed description various coral structures, but also showed how one type of coral settlements transitions into another as they develop. Darwin collected a wealth of material concerning the life activity of organisms that form a coral reef, their relationship to environmental conditions, the intensity of growth and distribution in the World Ocean.
He received some of the information from captains of ships that sailed the tropical latitudes of the oceans and seas, and from scientists who studied corals.
He made the most valuable observations himself during his trip around the world on the Beagle. According to Darwin, the first stage in the formation of coral islands is fringing reef . In this case, corals 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 experience uplift or subsidence, the reef remains a fringing shore reef.
In those cases when the seabed, 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. Sections of the reef that are out of water die, and on the sea side the reef grows and grows, but the overall picture does not change.

The situation is completely different in cases where the seabed drops and the island is submerged. Reef-forming organisms require a lot of food and clean seawater rich in oxygen for their development. Due to this, the growth of the reef that previously bordered the island always occurs along its periphery, washed by the sea, and almost does not occur 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 it arises barrier reef . The longer this process lasts, the further the barrier retreats from the island. Finally, the moment comes 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 structures in various parts of the ocean make it possible to trace all stages of the gradual transformation of a coastal reef into an atoll. The upward growth of the reef is sharply limited by sea level, but no matter how quickly the seabed sinks, the corals always have time to grow to the upper limit at which they can exist. Some of the modern coral islands rise from the seabed to a considerable height.

Later, some researchers put forward other theories about 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 the uplift and subsidence of the seabed is much more complex 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 validity of his theory: organize 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 bowels of the atoll are composed of coral limestone, then his theory will receive irrefutable evidence.
Darwin's dream came true only in the middle of the 20th century. In 1951, two very deep drillings were carried out on Enewetak Atoll (Marshall Islands).
It turned out that the coral limestone is underlain by a basalt layer only at a depth of 1266 and 1380 m. 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 only have occurred as a result of prolonged subsidence of the ocean floor.

It is very likely that the most common method of forming atolls was the submergence of volcanic islands. Interesting discoveries on the seabed large quantity flat-topped seamounts (called guyots) that look like deeply submerged atolls. Shallow-water corals were recovered from at least one of them.
From a review of the literature on fossil reefs, it appears that the formation of reefs occurred mainly during geological epochs when slight subsidence of the earth's crust (or slow rise in sea level) prevailed.
During geological periods characterized by the uplift of coral structures or their rapid submergence, reefs hardly developed.

Recently, a lot of new data has emerged concerning the geomorphology, oceanography, paleontology and biology of modern reef-forming animals. All of them are used to clarify Darwin's theory.

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