How are coral islands formed? How are corals formed and where do they live? There are large islands in the ocean, the builders of which are small creatures whose size does not exceed the size of a pin. How coral islands are formed mini project

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,

• atolls - ring-shaped coral 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 last century, the prevailing assumption 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 the greatest naturalist of the last century, Charles Darwin. In his book The Structure and Distribution of Coral Reefs, published in 1842, Darwin not only gave detailed description various coral structures, but also showed how one type of coral settlements transitions into another as they develop. (The coastal reef (1) gradually turns into a barrier reef (2), and then into an atoll (3))

Darwin collected a wealth of material concerning the life activity of the organisms that form a coral reef and their relationship to environmental conditions. external environment, intensity of growth and distribution in the World Ocean. He received some of the information from captains of ships sailing in tropical seas and from scientists studying corals. He himself made the most valuable observations during trip around the world on his ship Beagle.

According to Darwin, the first stage in the formation of coral islands is the fringing coastal 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 the development of corals, and the island does not experience uplift or subsidence, the reef remains fringed coastal 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 raised above sea level die, and on the sea side the reef grows and increases in size. But the overall picture does not change.
• The situation is completely different in cases where the seabed drops and the island is submerged. As mentioned above, reef-forming organisms require a lot of food and clean water for their development. sea ​​water rich in oxygen. Thanks to this, the growth of the reef always occurs along its periphery, washed by the sea. As a result, between the growing outer edge of the coastal reef and the sinking island, a space flooded with water soon forms, in which corals grow less intensively. This is how it arises barrier reef. The longer this process lasts, the further the barrier retreats from the island.
• Finally, a moment may come when the island finally sinks into the sea, and the barrier reef turns into atoll- a ring island with a lagoon enclosed inside it.

Later, other theories of the formation of coral islands were put forward, but they have not received recognition in our time.

Externally, all coral islands are very similar to each other. When approaching such an island, rows of coconut palms, a white strip of coastal beach and breakers on the edge of the reef become visible from a distance.

Coral islands usually rise very slightly above sea level, their vegetation is quite monotonous: in addition to coconut palms, tall pandanus bushes grow here. The leaves of this plant are wide and long, equipped with numerous very sharp serrated spines along the edges. On the bushes hang fruits similar in color, size and shape to pineapple. Closer to the shore you can see tall, tough grass and fleshy prickly pears, similar to some types of cacti. All this vegetation is satisfied with poor soil and can get by with a minimum amount of fresh water, which falls during rare rains.

The beach is sharply demarcated from a strip of green plants; it consists almost exclusively of coral sand - the skeletons of madrepore corals ground by waves, but there are also shells of marine protozoan foraminifera and fragments of mollusk shells. (The photo shows a photograph of sand under a microscope with 250x magnification.)

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 a detailed description of various coral structures, but also showed how one type of coral settlements changes 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 weak spots 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 seabed a large number of flat-topped seamounts (called guyots), similar to deeply submerged atolls. Shallow-water corals were recovered from at least one of them.
From a review of the literature on fossil reefs, it follows that the formation of reefs occurred mainly during those geological epochs when weak subsidence predominated earth's crust(or slow sea level rise).
IN geological periods, characterized by the uplifting of coral structures or their rapid subsidence, the reefs hardly developed.

IN Lately Much new data has emerged regarding geomorphology, oceanography, paleontology and the biology of modern reef-forming animals. All of them are used to clarify Darwin's theory.

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, thus forming a fantastic beauty Coral reefs.

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.

Biogenic islands are found only in the tropical and equatorial latitudinal zones of the ocean with warm waters. Based on 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 stretching along the coast, or barrier reefs located at a distance from the coast and separated from them by lagoons. Most of the reefs are underwater, and only their tops protrude above the ocean level in the form of small islands of intricate shapes, for example, on the Great Barrier Reef off the eastern coast of Australia. Atolls in the ocean arise on the tops of large underwater volcanic mountains or during the long-term evolution of a ring fringing reef around volcanic islands, which subsequently sank below ocean level and were covered by a layer of coral limestone. As a result, circular low islands are formed, composed of coral sand - a product of the destruction of reefs, which surround the inner shallow lagoon, for example, the Caroline, Marshall, Gilbert, Line, Tuamotu islands - in Pacific Ocean, Malvinas and Chagos Islands - in Indian Ocean, Albuquerque Islands, Saint-Andrés, Roncador - in Atlantic Ocean(Caribbean Sea) and others. These islands are young formations resulting from the superstructure of coral reefs during the Holocene.

From the name it is clear that such names were given to islands that “grew” from coral reefs. It looks like this. First, an active underwater volcano, having completed its last eruption, rises above the surface of the water and fades away. It is surrounded on all sides by coral reefs, whose roots reach ocean floor. Over time, the volcano settles or collapses, but the reefs remain in place, repeating its contours and continuing to grow. In the end, only a “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 a nice place island, which is deservedly its attraction.

This type of island is favorite place holidays for people from all over the world, being business card beautiful Pacific islands (Figure 4)

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

Figure 4. - Coral islands. Maldives.

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

Thus, coral formations arise from endless processes of creation and destruction.

People have long been interested in how reefs form, especially atolls found right in the open ocean.

The famous Russian navigator F. F. Bellingshausen expressed a number of correct thoughts about their nature. The most substantiated theory of the origin of coral reefs was put forward by Charles Darwin. In many respects they still 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 on the Samoan Islands, where bedrock (not corals) is already at a depth of 35 m from the surface.

The English scientist J. Murray made significant additions to Darwin's theory. He proved that a continuous coral reef will necessarily transform into a ring one, and for this reason. The corals in the middle part of the reef do not have enough food, they gradually die and are destroyed, because carbon dioxide accumulates here - a product of the respiration of polyps, which dissolves the limestone, and the reef grows only from the outside. This is how a lagoon appears in the center of the reef.

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

On this seemingly compressed and therefore denser vertical strip of coral limestone, as if on a pock, branched corals grow again - and the formation of the spur moves into the second stage.

The appearance of channels, i.e., recesses between the spurs, is partly explained by erosion under the influence of water flowing from the reef, which, when the wave recedes, rushes here, since it does not encounter obstacles in the form of coral thickets. However, the main reason for the appearance 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 more, and they begin to close with their 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 fusion. Of course, their erosional destruction is not excluded, but this, apparently, occurs only during very strong storms.

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

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

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

The formation of a rising ridge by calcareous red algae precisely at the seaward edge of the rifflet and on the ridge is explained by environmental features 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 scarlet plants should apparently 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).

These hermatypic organisms raise the ridge above the level of the reef platform. At a distance of several meters from the edge of the outer slope there was usually a second, less pronounced ridge. It is obvious that previously the edge of the reef passed along this line, 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 rifflet structure, but the genesis different parts the reef platform itself is not the same. If its seaward part arises as a result of the active growth of corals and algae, then the areas lying closer to the shore owe their origin to the accumulation and partial cementation of clastic material, formed mainly on the outer slope and ridge and transported from there by waves.

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

The accumulative part of the rifflet, in turn, consists of three belts, or zones. The highest of them, closely adjacent to the shore, lies near the boundary of the upper water level at high (tropical) tides. It is represented by ancient limestone and covered with a layer of pure coral sand. This is the beach area. Directly adjacent to it from the sea is a strip of riffle, covered with large and small coral fragments that are not interconnected. The fact is that this high-lying part of the reef platform dries out every day for a long time and within its boundaries the calcareous algae that cement the fragments can no longer exist. There are no living corals here either. Between this dead zone rifflet and ridge extends a more or less wide living zone, on which individual massive corals take root, and a special fauna of lagoonal corals develops in puddles and baths on the silted bottom. There are both solitary mushroom corals and many finely branched bushy forms. When they die, they become cemented and also become part of the structure of the platform, but the latter is still primarily formed from fragments that fall here from the reefrock.

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

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

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

Coral islands are created by organisms (polyps) capable of secreting a calcareous substance. They live in colonies. New developing organisms remain in connection with dead ones and form a common trunk. For the life of corals, and, consequently, for the formation of an island, certain favorable conditions are needed. It is necessary that the water temperature does not drop below 20° on average. Therefore, polyps can only develop 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 take root and relatively clean water to survive; As a result, in those places where rivers flow into the sea, bringing with them turbidity, the reef is interrupted. Coral structures can be divided into two categories. The first category includes coral reefs fringing an island or continent - these are coastal and barrier reefs. The second category includes independent islands, known as atolls. Atolls have a more or less round or oval shape; less commonly, a triangular or quadrangular shape is found. A coastal reef fringes an island or the shore of a mainland. This shaft barely rises above the water, but even then it is not everywhere, and for the most part it is a shallow, since corals in general can only live under water. Living corals can exist at depths of up to 90 m, but at such depths they are quite rare, and for the most part they do not fall below 30-40 m. The low tide boundary 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 rise of the coral shelf. The sea runs onto the shore, tears off pieces of polypnyak, crushes them into sand and throws them aground, filling up the voids; other organisms settle on the surface of the reef - mollusks, crustaceans, shells and skeletons of which, in turn, go to increase the reef. In addition, warm water dissolves limestone, and wind and waves throw substances brought from the shore aground. As a result, the reef as a whole becomes denser and sometimes rises somewhat above the sea surface, separated from the shore by a narrow channel. barrier reef much further from the shore than the coastal one. Between it and the shore there is a lagoon, in some places also filled with reefs and sediment. The largest barrier reef stretches along the north-eastern coast of Australia for 2000 km. The width of the lagoon here is 40-50 km, sometimes it expands even to 180 km; Its depth in some places reaches 100 m, so steamboats can enter the lagoon, although swimming is dangerous, as there are many coral shoals. The width of the reef itself is several tens of kilometers. If we look at a map of the Pacific Ocean, we will see how a large number of barrier reefs are found there. All large islands and a mass 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 a shoal, and the islands rise out of the water only in places. The atolls make a very strong impression. Darwin also says: “It is difficult to imagine, without seeing with your own eyes, the infinity of the ocean and the fury of the waves in sharp contrast to the low border of land and the smooth surface of light green water inside the lagoon.” If there is a significant break in the atoll's ring, then ships can find a calm haven in its lagoon.

In cross-section, the atoll first appears as a steep slope, then as a flat shoal with islands rising on it, and finally as a deepening of the lagoon. The sizes of the atolls vary greatly: from 2x1 km to 25x10 km and even 90x35 km. The emergence of atolls can be explained as follows: if there is a shoal in the sea, barely covered with water, then if the bottom is hard, corals can settle on it and form an atoll. The atoll gets its oval shape because corals settle mainly along the edges of the shallows, since the sea waves come here, if they are not excessively strong, and sea ​​currents bring food supplies freely (Figure 5). A strand can arise either as a result of the rising of the seabed, or 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 surface of the shallows, then soon the marginal corals will find themselves in a more advantageous position: food is easily delivered to them, and they grow faster than the corals located in the middle. A lagoon is created in the middle, although it is quite shallow, since the shoal is not deep under water. The thickness of such a polypnyak is small and rarely reaches 10 m. Such formations are called coral reefs. It is more difficult to explain the origin of atolls among deep sea. Darwin, like many other scientists, noticed that coral islands often rise very steeply; their slope reaches 30°. At first it was believed that only coral islands had such steep slopes, but now we know that volcanic and sometimes continental islands are not inferior to them in this regard. Another fact that makes it difficult to explain the origin of atolls is that dead polypnyak is sometimes found at depths of 100-200 m or greater, and we know that corals cannot live at such depths.

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

As the island slowly sinks, the lower parts of the polypnyak die, and new corals multiply above them, which manage to build on the reef. At the same time, the distance between the outer edge of the reef and the bedrock shore increases, and a barrier reef is formed. There is still nothing left of the island most of, rising among the lagoon. Then further subsidence occurs and an atoll is formed; The island has already completely disappeared under water, and in its place there is a lagoon.

Naturally, with such an atoll formation, its outer slopes are steep. Many scientists recognized this theory, especially developed in detail in 1885 by Deng, but then objections were raised against it. Darwin's theory was argued against by the fact that often in the same group of islands we encounter all the transitional stages of reefs. Thus, in the group of the Caroline Islands there are coastal reefs, nearby barrier and atolls, from the lagoons of which small islands still peek out, and, finally, typical atolls (Figure 6).

mainland volcanic coral island

Figure 5. - Scheme of attol formation.

However, this objection, based on the existence of various forms of reefs in close proximity to each other, is easily eliminated if it is assumed that uneven vertical movements of the seabed have occurred in a given place. Thanks to this, they could form nearby various shapes polypnyaks. The Darwinian theory is also supported by the fact that although they sometimes meet in the neighborhood different shapes reefs, but much more often over vast areas one form dominates, as is, for example, observed in Oceania. Drilling a polyppyak on the island of Funafuti (in the Ellis Islands group) also confirmed the correctness of Darwin's views. The well penetrated 334 m in a continuous polypnyak.

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

Figure 6. - Caroline Islands.

According to the observations of Murray, Guppy and Agassiz, there is no need for an atoll to develop necessarily from a coastal and barrier reef - it can arise independently, moreover, not only in shallow water, but also in the deep sea. If a volcanic eruption occurs at the bottom of the sea, then corals can create an atoll on the edge of the emerging underwater volcano, around its crater. Already Chamisso, during his trip to Oceania, pointed out that the formation of a lagoon is often due to the fact that the crater of a volcano serves as the bottom of the lagoon. Sometimes the underwater hill 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, mollusks and algae that have a calcareous skeleton; the skeletons of these organisms increase the height of the underwater reef, so that corals can eventually settle on it (Murray's theory). As for the formation of the lagoon, Agassiz believed that sea tides contributed to its deepening. The atoll does not represent a closed ring, but has breaks. The tidal current penetrates them, produces a strong eroding effect and clears the lagoon of sediment. Despite the objections and additions made, Darwin’s theory has generally been fully confirmed by the latest research, and can be considered the most correct explanation of the origin of atolls.

This reef, in fact, is represented by only one component element, namely outer slope with a ridge on top. In this place, the coastal cliffs go steeply into the sea, and hermatypic corals develop on them. The fragments of these corals, which inevitably arise as a result of the action of the run-up and during storms, due to the steepness of the cliffs rising from the sea, do not accumulate at the top, but roll down the slope.

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

Unlike surf reef corals, lagoon species are able to remain dry for several hours during low tide. The waves in the lagoon are weaker, and in low water Water does not reach 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 a lot of fish here edible shellfish, crayfish, algae; found in some places sea ​​turtles and dugongs.

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

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

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

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

Coral limestone is used here and there as construction material; when ground, it is used to polish wood and metal. In Ceylon, cement is produced from it. Madrepore corals, like red ones, are used to make everyday items, jewelry, vases, etc. They are also used in Chinese medicine.

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

Small size, remoteness from continents, endemicity and poverty biological diversity flora and fauna create very big problems in cases of irrational use of natural resources, serious disturbances of ecological balance and intense pollution environment. After all, the ecosystems of these islands were formed 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 atolls is especially vulnerable, firstly, due to their very small size. Secondly, due to 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 invade. Thirdly, due to the limited fresh water resources on the atolls, which significantly limits the possibilities economic activity. Therefore, most of the atolls are sparsely populated or even have no permanent population, but are used for seasonal work on coconut plantations.

Conclusion

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

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

By origin, all islands can be grouped as follows:

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

Mainland Islands are genetically connected with the continents, but these connections are of a different nature and this affects the nature and age of the islands, their flora and fauna.

Platform Islands lie on a continental shelf and geologically represent a continuation of the mainland. Islands of the continental slope are also parts of the continent, but their division occurred earlier. They are usually separated not by a gentle bend of the continent, but by a deep split. The straits between the island and the mainland are of an oceanic nature. The flora and fauna of such islands is very different from the mainland. This group includes Madagascar and Greenland. Orogenic islands represent a continuation of the mountain folds of the continents. Island arcs- parts of transition areas. Mainland offshore islands.

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

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

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

Atolls (lagoon islands) are located in the ocean. These are low islands in the shape 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 coral lagoons is flat, covered with coral sand or accumulations of calcareous algae remains.

There are three types of islands: continental, volcanic and coral. The formation of islands occurred not only many thousands of years ago, but also now new island territories are emerging.

How were mainland islands formed?

Continental islands were formed due to the movement of tectonic plates in the earth's crust. The islands were once part of large continents. Vertical movements of tectonic plates, together with rising sea levels, formed faults in the continents. The nature of the mainland islands and the nature of the continent closest to them are almost identical. Mainland or continental islands are located within a single shelf, or are separated from the mainland by a deep fault. Continental islands include Greenland, New land, Madagascar, British Isles, etc.

How are volcanic islands formed?

There is constant volcanic activity in the oceans. An erupting volcano releases a huge amount of lava, which solidifies upon contact with water and air, forming new volcanic islands. Such islands experience great water erosion and gradually go under water. Volcanic islands are often significantly removed from the continents and form a unique ecological system. An example of volcanic islands is the Hawaiian Island chain.

How are coral islands formed?

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