Pacific Ocean coral reefs. Coral reefs - amazing photos

Coral reefs- calcareous organogenic geological structures formed by colonial coral polyps and some types of algae that can extract lime from sea water.

Coral reefs are fantastically beautiful and the most "densely populated" ecosystems of the oceans. The biomass of benthic invertebrates and fish in them is estimated at hundreds of grams per square meter sea ​​bottom. The total number of species on coral reefs can reach one million or even more.

The reef ecosystem owes such a vibrant life to unicellular algae (symbiont algae) living in corals, whose photosynthetic activity does not stop throughout the year.

The first corals on Earth, tabulates, appeared in the Ordovician period of the Paleozoic era, about 450 million years ago. Together with stromatoporid sponges, they formed the basis of reef structures.

Throughout the history of their existence, corals have repeatedly experienced periods of decline and mass death - the climate has changed, the level of the World Ocean has repeatedly risen and fallen. But neither the changing climate, nor the lowering or rising of the ocean level led to the complete and final death of reef-forming corals - they adapted to new conditions. A balanced reef ecosystem has great restoration potential.

In 1997-1998, there was an abnormal increase in temperature surface water tropical zone of the World Ocean, which led to the mass death of corals in the vast waters of the Indian and Pacific Oceans.

Then, in the coastal regions of Bahrain, the Maldives, Sri Lanka, Singapore and near Tanzania, up to 95% of all corals in shallow reef zones died. In other coastal areas tropical zone 20 to 70% of corals suffered the same fate. In the early 2000s, the tragedy repeated itself twice, especially affecting the central and southern parts of the Pacific Ocean, and by the middle of the 2000s, according to scientists, a little less than half of the previously existing reefs remained on Earth.

But not in all reef areas the situation is so catastrophic. For example, in the Maldives, the coral cover has completely recovered due to the rapid growth of acroporids (acropor corals), whose growth reaches 20-25 cm per year.

A different picture is observed in the region of Bahrain and Sri Lanka, where the reefs were under very strong anthropogenic pressure.

Thus, not sharp climatic fluctuations, but high level anthropogenic pressure leads to fatal consequences for this ecosystem.

What are coral reefs?

Coral reefs are massive deposits of calcite (limestone) formed from the skeletal remains of some colonial marine organisms - plants and animals - among which madrepore corals and coralline algae stand out. In addition to these two dominant groups of reef-forming organisms, the proportion of other animal and plant species, such as mollusks, sponges, foraminifera, and some green algae, is also significant in the composition of reefs.

There are three main types of interconnected coral reefs: fringing, barrier and atolls.

Fringing, or coastal, reefs

Located along a recently drained or stable coast. Such a reef is a terrace-like platform at sea level or somewhat below it, extending from the shore and often ending in an elevated edge, strongly indented by channels, from which the reef slope drops rather steeply into the water, and then abruptly breaks off to great depths. Corals grow fastest on the outer slope of the reef and very slowly on the shallow platform.

barrier reefs

They often surround individual islands, but are located at some distance from the coast, separated from it by a strait or a calm, medium-deep lagoon. The largest and most famous is the Great Barrier Reef - a complex system of reefs with a length of more than 1600 km, located along the east coast of Australia.

atolls

Ring-shaped reefs surrounding lagoons, inside which there are no land areas. They are ring-shaped with an inner shallow lagoon; annular with one or more ring breaks through which tidal waters enter the lagoon; in the form of a half ring, a quarter ring or annular, flooded during high tides; or in the form of giant rings, consisting of individual small atoll-like reefs, fringing huge lagoons.

coral varieties

In general, hard corals that form a reef can be divided into branched brittle corals (madrepore corals) and massive, rocky corals (brain and mendrin corals). Branching corals tend to be found on shallow and level bottoms. They come in blue, lavender, purple, red, pink, light green and yellow. Sometimes the tops have a contrasting color, for example, green branches with purple tops.

Brain corals can reach over 4 meters in diameter. They live at greater depths than the branched ones. The surface of brain corals is covered with sinuous cracks. The color is dominated Brown color, sometimes combined with green. Dense porites form a kind of bowl, the base of which consists of dead corals, and living corals are located at the edges. The edges grow, increasing the diameter of the bowl, which can reach 8 m. Living porous colonies are painted in a pale purple color, the tentacles of the polyps are greenish-gray.

At the bottom of the bays, individual mushroom corals are sometimes found. Their lower flat part fits snugly to the bottom, and the upper one consists of vertical plates converging in the center of the circle. Mushroom coral, unlike branching and massive hard corals, which are colonies, is an independent living organism. Only one polyp lives in each such coral, the tentacles of which reach a length of 7.5 cm. Mushroom corals are painted in greenish and brownish colors. The coloration is retained even when the polyp retracts the tentacles.

Coral reefs are calcareous organic geological structures. They are mainly made up of corals, which are formed with the help of small marine animals. An individual coral, also called a polyp, is cylindrical in shape with an exoskeleton. The exoskeletons give each polyp a hard, rock-like outer body. Corals produce calcium carbonate from their bodies. As the corals remain immobile, the individual polyps cluster together. They form colonies that allow the release of calcium carbonate and the formation of new reefs.

Corals depend on algae and in turn, algae take refuge in coral reefs. Living corals and algae form closest to the surface of the water on top of old, dead corals. They release limestone during their life cycle, which helps develop reefs. Because corals need algae to survive, they are more common in calm, shallow, clear waters where there is enough sunlight.

Coral reefs form in waters dominated by warm ocean currents, which largely limits their distribution to no more than 30°N. sh. and yu. sh. develops rapidly along the reefs, making them one of the most diverse in the world. In general, coral reefs attract almost a quarter of the species of living organisms.

Types of coral reefs

Fringing reef off the coast of Eilat (Israel)

Some coral reefs take thousands of years to form. During this time, they can develop into several different forms depending on the location and surrounding geological features. There are 4 main types of coral reefs:

• Fringing (coastal) reefs composed of platform-like coral rocks. They are usually connected to or close to the coast, separated by a semi-enclosed lagoon with deeper water.
• barrier reefs extend along the island or continental shallows, in the warm waters of the Pacific, Indian and Atlantic oceans, at a distance of several to tens of kilometers from the coast. Barrier reefs are hundreds of meters wide. The distance between the reef and the shore is filled with a lagoon. The largest barrier reef on Earth is about 2000 km long, located off the eastern coast of Australia. Barrier reefs also occasionally extend above the surface of the water.
• atolls are ring reefs that completely cover the lagoon. The lagoons within the atolls are more brackish than the surrounding sea waters and often attract fewer species than the surrounding coral reef.
• Intralagoonal reefs(patch reefs) form on shallow areas of the seafloor separated by deeper water from adjacent fringing and barrier reefs.

Functions of coral reefs

Coral reefs have several different functions. They help prevent sediment washout and shoreline damage. Coral reefs act as a physical barrier that helps create a healthier, more protected habitat near the coast. They also capture carbon dioxide, which helps create conditions for the sea. Coral reefs also have economic benefits for nearby communities. They are collected for the production of medicines and jewelry. Exotic species of fish and marine plants are caught for keeping in aquariums. Also, the magnificent underwater life of coral reefs attracts tourists from all over the world.

Environmental threats to coral reefs

Many coral reefs have experienced a phenomenon known as bleaching, in which corals turn white and die after the algae disappears. The bleached coral weakens and eventually dies, resulting in the death of the entire reef. The exact cause of the bleaching remains unclear, although scientists suggest that it may be directly related to changes in sea temperature. Global climate events such as El Niño and climate change have increased the temperature of the world's oceans. After the events of El Niño in 1998, approximately 30% of coral reefs were permanently lost by the end of 2000.

Sedimentation also poses a threat to coral reefs around the world. Although they only form in clear waters, soil erosion due to mining, agriculture/forestry causes rivers to carry sediment to the ocean. Natural vegetation, such as that growing along waterways, acts as a barrier to rainfall. due to the construction and development of settlements increases the amount of precipitation in the sea.

Pesticides also enter the ocean through runoff from agricultural land, which increases the amount of nitrogen in the sea, causing corals to get sick and die. Careless management practices such as overfishing and uncontrolled coral mining also disrupt fragile marine ecosystems.

Conservation and restoration of coral reefs

One of the proposals to save coral reefs is to take care of them like a garden. The introduction of plants to remove sediment and overgrowth of algae can help temporarily keep coral reef ecosystems in balance. Reducing pesticide runoff from fields can also help reduce nitrogen levels in the sea. Reducing carbon dioxide emissions from human activities can improve the overall health of coral reefs.

Corals are not just calcareous structures created by polyps. This is one of the most incredible life forms on earth. Our top 10 includes the most beautiful coral reefs in the world.

Why are corals so attractive? Firstly, with its amazing natural beauty, variety of shapes, the number of colors and shades, which reaches 400. Secondly, they do not appear anywhere: corals need pure water and warm climate. And, perhaps, the main thing is fish and other fauna of the most incredible shapes and colors, which, by their presence, creates an integral picture of indescribable beauty.

10. Barrier in the Red Sea (Egypt)

The coral reefs of the Red Sea may not be the most beautiful in the world, but they have very rare and interesting shades of some colors, such as yellow, red and pink. Corals located close to the coast attract snorkelers and divers from many countries, especially post-Soviet ones. Frankly, it was the latter fact that caused the inclusion in our top 10.

9. Aldabra Atoll in the Indian Ocean (Seychelles) It occupies 200 km² and is notable for the fact that it has survived to this day almost in its original state. All thanks to the fact that, once a pirate island, long time was well hidden and was an inaccessible place. Now the beauty of this amazing coral reef is under the protection of the local government and UNESCO. The atoll is also notable for its huge population of turtles, reaching 150,000 individuals.

8. Roatan Island in the Caribbean (Honduras)

Here is the Mesoamerican Barrier Reef, one of the largest in the world. Diving began to develop here relatively recently, some 10 years ago, and man has not yet managed to exert his detrimental effect on this beautiful underwater corner of the planet. Therefore, if you want to admire the fish surrounded by pristine beauty, hurry up!

7. Palancar Reef in the Caribbean (Mexico)

Stretching for 5 km, the reef is notable for its structure: there are many underwater caves, branches and crevices where you can see quite rare black corals, and at the same time get acquainted with a variety of marine life forms. Here you can meet giant barracuda, moray eels, spotted eagle rays, spiny lobsters, crabs, lobsters, as well as many colorful tropical fish.

6. Tubbataha in the Sulu Sea (Philippines)

This is a whole national park, a World Heritage Site UNESCO. He is about 15 million years old, so he takes pride of place among the most beautiful coral reefs in the world, and the most ancient. On a relatively small area (7,030 hectares) there are 75% of the corals existing on the planet (almost 400 species), and about 40% of the studied reef fish (more than 500 varieties). About 1000 species of marine animals live in the local waters, including dolphins, sharks and whales. The fact that the schedule of local tourist boats for diving is scheduled for a year in advance suggests that Tubbataha rightly got into our TOP-10.

5. Raja Ampat in the Indian Ocean (Indonesia)

According to some estimates, it is here that you can find the largest variety of marine life. No wonder this place is called World Richest Reefs(eng. the richest reefs in the world). There are over 1200 species of reef fish (25 of which are endemic). The diversity of corals is so great that it is 10 times greater than all the existing species in the Caribbean combined! On top of that, at least a hundred sunken ships and aircraft have been recorded in the vicinity. Isn't this every diver's dream?

4. Coral reefs of the Andaman Sea (India)

At one time, they delighted the famous Jacques-Yves Cousteau, who filmed a documentary here. It was here that 111 varieties of corals were discovered, previously unknown to science. The most beautiful site is located on the territory of the National Marine Park named after Mahatma Gandhi. Here you can admire stingrays, dolphins, turtles, as well as schools of colorful fish swimming calmly around enthusiastic divers.

3. Apo in the South China Sea (Philippines)

It occupies 34 km², is home to several different ecosystems at once. The title of one of the most beautiful coral reefs on the planet Apo owes much to the water with its unique transparency. On clear and calm days, visibility can exceed 50 meters. And there is something to see here: several varieties of sharks and rays, huge tunas, lobsters, gorgonians, as well as many sea snakes that make you shudder at every accidental touch, especially at night.

2. Belize Barrier Reef in the Atlantic Ocean (Belize)

It ranks second in the ranking of the best coral reefs in the world. Its length is 280 km, and on such a vast territory there is an unimaginable variety of living organisms. And the most interesting thing is that, according to scientists, the reef has been studied by only 10-15%. Therefore, among the 140 thousand divers who annually visit this place, there are many who want to become pioneers.

The highlight of this beautiful place known as the "Great Blue Hole". It is a blue-black circle with a diameter of 300 meters, which seems to be bottomless. Almost the only inhabitants of the cave (which is actually a “hole”) are sharks, they can be seen even from the surface. The sight is very impressive!

1. Great Barrier Reef in the Coral Sea (Australia)

The largest coral reef in the world is also considered the most beautiful. Its area is almost 345,000 km², this miracle of nature can be seen even from space. As a colony of coral polyps, this aquatic oasis is considered the largest ecosystem on the planet. There are over 400 varieties of corals and more than one and a half thousand species of inhabitants, including whales and sharks, octopuses and squids, parrot fish and butterfly fish, rays, crustaceans, turtles ... The Great Barrier Reef is rightly considered a valuable world heritage site.

They are called the oases of the ocean, and there is little in this world that can compare with them in beauty. They are the longed-for dream of every diving enthusiast, and they are also home to millions of fish ... This article will focus on one of the most amazing creations of nature - reefs.

Meaning and definition of the term

Reefs - what is it? From the Dutch language the word "reef" is translated as "rib". The term was originally used by geographers and oceanographers to refer to narrow, rocky terrain that was dangerous to navigation. Today, reefs are mainly called coral-algae formations on the ocean floor. They can be of different configurations and sizes, have a different location feature. But not only corals can serve as “building materials” for reefs.

reef types

Depending on what nature has created reefs from, they are divided into several types.

So, for example, a rocky reef is formed as a result of the destruction of rocky shores or the bottom. This natural phenomenon can be observed off the coast of Canada. In some areas of the waters of Scotland there are reefs created by tube worms. Still sometimes the "builders" of this miracle of nature are oysters and bryozoans. Sometimes you can find a barrier reef made of sea grass. Sometimes sponges serve as the basis for its creation. Such a reef, respectively, is called sponge, and if cyanobacteria “work hard”, then the formation is called stromatolite. And, finally, reefs are called artificial, which are the result of human labor.

But all of the above phenomena in nature are extremely rare. The vast majority of the planet's reefs are a product of the vital activity of corals.

Coral reefs - what is it?

Underwater or partially surface limestone formations, the material for which were colonies of polyps and certain types algae are called

Polyps belong to the class of marine invertebrates. They live in shallow waters in the warm waters of the tropics. When a colony dies, many skeletons remain. And the descendants of the dead settle on top, who eventually replenish the “bunch” with their remains. And so on, ad infinitum. It turns out that the coral reef is nothing more than a concentrated mass, consisting of a huge number of living and dead polyps.

But not every coral is suitable as a building material for a reef, but only one that is able to absorb calcium carbonate from ocean water. It is calcium that is responsible for the formation of the skeleton. And the latter, in turn, then becomes the basis for the coral formation.

Scientists say that the polyp in the body is responsible for the ability to absorb calcium. If such an element is absent, then the coral will not be able to participate in the creation of the reef.

Corals can have a variety of bizarre shapes, as well as the configurations they form. As for size, real giants grow over millions of years. And sometimes such intricate "laces" create reefs at the bottom of the sea, which is hard to even imagine. You only need to see it with your own eyes.

Where are coral reefs found?

Photos of them can be seen in our article. One of the features of coral reefs is their thermophilicity. They are fragile structures, cold water with high or low salinity, as well as a lack of sunlight contribute to their rapid destruction. Therefore, they choose warm and comfortable seas as the main place for their habitat, where almost 45% of the world's "collection" is collected. 18% of reefs "live" in pacific ocean, 17% - in the Indian, 14% - in the Atlantic, and 6% - in the Red Sea.

But there are exceptions. reef colonies that live in cold water. They are scattered all over the world and lurk at great depths (about a kilometer). This is another one of them distinguishing feature, because reefs that are not prone to winter swimming usually choose shallow water for life. The total area of ​​​​the reserves of these amazing marine objects of the planet is approximately 27 million square kilometers.

The population of coral castles

Coral reefs are called oases of sea deserts for a reason, but because of the richest stocks of fish that live in "beautiful castles". They are attracted here, of course, not by the beauty of nature, but tasty food in the form of polyps. Thus, the house also serves as a dinner for the fish.

Hundreds live in each of the reefs. They are usually small and very bright. Red, poisonous yellow, green, purple, black... The constant flickering of these "lanterns" around the body of the reef creates an indescribable picture.

The scale is also impressive. Of the 20,000 bony fish on the planet, about a third live in coral reefs. In addition to fish, a huge number of worms, mollusks, sponges, crustaceans, and algae live here.

Types of coral reefs

Depending on the location, several main varieties of coral reefs are distinguished:

• Coastal (or fringing) reefs encircle the islands and are located in shallow water. They are a narrow terrace that begins on the shore and ends at some distance, already in the water.
• Barrier reefs are much more remote from the coastline and are separated from it by a deep depression.
• completely hidden underwater. Not a single part of it protrudes above the sea surface. Such configurations are also called atolls.

The largest barrier reef in the world

The most famous and largest barrier reef on the planet is considered to be Australian. It is located in the northeastern part of the water area surrounding this mainland, and stretches along Queensland, occupying about 435 square kilometers. Such an area could accommodate the ships of all the states of the world, and there would still be room for a small country.

This giant conglomeration of colorful corals began to form 18 million years ago and over such a long period of time has grown to an unprecedented size that impresses even experienced divers. consists of many large and small, unique in shape islets. You can even see it from space!

Coral Treasures of the Red Sea

Not every traveler can afford to go to Australia to admire the reefs. But there is a wonderful alternative - Egypt. The untold riches of the Red Sea are impressive and therefore attract millions of tourists every year.

Coral reefs in Egypt are distinguished by their diversity and magical pink and blue colors. In addition, they are a magnet for marine life. Diving to the bottom, you can see not only beautiful coral castles, but also a huge number of rare fish. Dolphins frolic in shallow water, and the shore is literally dotted with resting turtles. They, too, like tourists, arrived here, responding to the "coral call".

In this article, we looked at reefs of different sizes and shapes, what reefs are and what their charm is. Described all their varieties. You can also admire the reefs, the photos of which we provide, in this article. Once again, we remind you that coral varieties are the most common. Therefore, when talking about reefs, in 90% of cases, it is corals that are meant. You can see from the photos that this is a truly breathtaking sight. Today, coral reefs are considered almost one of the wonders of the world.

The content of the article

CORAL REEFS, organogenic limestone structures located near sea level or at shallow depths in coastal zone tropical seas or in shallow warm seas. They are massive deposits of calcite (limestone) formed from the skeletal remains of some colonial marine organisms - plants and animals - among which madrepore corals and coralline algae stand out. In addition to these two dominant groups of reef-forming organisms, the proportion of other animal and plant species, such as mollusks, sponges, foraminifera, and some green algae, is also significant in the composition of reefs.

The main reef builders develop best at depths of no more than 50 m, in clear water normal salinity with a temperature not lower than 20 ° C, abundantly saturated with dissolved gases and the smallest organisms (plankton) that serve as food for them. Minor growth of reef corals may occur up to penetration depth sun rays- up to a maximum of 185 m, but practically deeper than 120 m, limestones are almost not formed. The rather wide distribution of reefs within tropical and subtropical shallow waters is interrupted where cold currents pass (for example, along the west coast South America) or the turbidity of the water prevents their growth.

Structure.

The modern coral reef can be considered as a well-organized biogeocenosis. It is unlikely that corals would be able to build reefs if there were not extremely favorable conditions, because corals are very fragile and have a large surface area, which is affected by waves that tend to break and crush them. However, coralline algae, which form a calcareous crust, grow abundantly on the surface of corals and in the spaces between them, especially where the wave action is strongest and the water is most saturated with dissolved gases. They form a continuous, smooth, extremely dense and durable coating, excellent for cementing corals.

Best of all, corals and algae grow along the periphery of the reef and on its outer slopes. On the reef platform, a small water column, warming up, becomes depleted in gases; in addition, silt accumulates there. At low tide, the reef surface is exposed, which is deadly for corals. Some types of corals and algae (especially green algae Halimeda) under such conditions produce less lime. In addition, on the reef platform, waves raise silty sandy sediments from the bottom, consisting of finely crushed limestones and organic remains. Abrupt shifts temperatures lead to the dissolution and precipitation of calcium carbonate, which fills and compacts the interior of the reef. The accumulation of debris and coral sand often continues above sea level, forming coral islands. A significant part of the coral sand is sometimes made up of foraminifera shells. In the relatively shallow, calm waters of the lagoons, the growth of some coral and algae species also lead to lime accumulation. At the bottom of individual lagoons, a large number of destroyed algae skeletons accumulate Halimeda. In other lagoons, fantastically beautiful underwater forests of branching "coral trees" and columns called coral heads grow. However, usually lagoons are not filled with clastic material, since, in all likelihood, finer material is carried out to the open sea by tidal currents.

Types.

There are three main types of interconnected coral reefs: fringing, barrier and atolls.

Fringing, or coastal, reefs

usually located along a recently exposed (drained) or stable coast. Such a reef is a terrace-like platform at sea level or somewhat below it, extending from the shore and often ending in an elevated edge, strongly indented by channels, from which the reef slope drops rather steeply into the water, and then abruptly breaks off to great depths. Corals grow fastest on the outer slope of the reef and very slowly on the shallow platform.

barrier reefs

like borders. They often surround individual islands, but are located at some distance from the coast, separated from it by a strait or a calm, medium-deep lagoon. The largest and best known is the Great Barrier Reef, a complex reef system stretching over 1,600 km along the east coast of Australia.

atolls

- These are usually ring-shaped reefs surrounding lagoons, inside of which there are no land areas. They are ring-shaped with an inner shallow lagoon; annular with one or more ring breaks through which tidal waters enter the lagoon; in the form of a half ring, a quarter ring or annular, flooded during high tides; or in the form of giant rings, consisting of individual small atoll-like reefs, fringing huge lagoons.

transitional forms.

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

Origin of coral reefs.

A coral reef can form in any tropical or subtropical water where the depth, temperature, and salinity allow reef-building organisms to thrive. Thus, the genesis of fringing reefs in shallow waters surrounding islands and continents is fairly obvious. However, the specific forms of atolls and barrier reefs have given rise to endless discussions and reasoning about their origin. There are many hypotheses, but none of them is recognized as completely satisfactory in explaining the conflicting geological facts.

Charles Darwin was the first to note that if a barrier reef did not surround a high island, it would be a real atoll. His observations on several coral reefs, and especially on the Cocos (Keeling) Islands (southwest of Sumatra), suggested that fringing reefs, barrier reefs, and atolls may represent different stages of the same process. He argued that if the entire coral structure was sinking at a speed not exceeding the growth rate of corals, then a barrier reef would form from the fringing reef, and if further subsidence continued until the island inside it was under water, then from the barrier reef reef would form an atoll. The atolls are characterized by a calmer water regime, and the former barrier reef in such conditions receives less dissolved gases and less abundant food for corals in the form of plankton, hiding from the ocean waves behind it. This shortage will result in slower growth of reef-forming corals on the inside of the atoll, and thus a lagoon is formed. To explain the huge number of atolls, Darwin assumed that large swaths of the seafloor had subsided in the recent past. When this theory, later called the immersion theory, was first published, it was widely accepted and was seen as a triumph of deduction.

However, this theory of Darwin was not long accepted as true, and a series of alternative hypotheses was put forward to replace it. K. Semper and H. Guppy believed that reefs grow on rising grounds. Other researchers (including J. Reine, J. Murray, W. Wharton, and F. Wood-Jones) thought that atolls were formed by coral growth on submerged banks, an idea revived this century in J. Hoffmeister's antecedent platform theory. and G. Ladd, who, after a long and careful study of the raised reefs of the Fiji Islands, prove that modern reefs grow on pre-existing underwater platforms. The origin of banks, or platforms (small underwater plateaus), was explained in its own way in each theory. Semper, Murray, and Wharton believed that lagoons formed as a result of the dissolution of limestone in marine environments or within flat limestone islands. S. McNeil revived this idea and put forward a number of evidence in its favor, obtained as a result of the study of uplifted coral islands. Older versions of these theories, other than those proposed by Hoffmeister, Ladd, and McNeil, developed until the concept of changing sea levels was widely accepted.

In 1910, R. Daly suggested that the effect of sea level rise, which occurred as a result of the melting of the Pleistocene ice sheets, is similar to the result of sinking earth's crust. Based on this assumption, he developed the theory of glacial control, which dominated all subsequent discourse on the genesis of atolls. He assumed that during low sea levels, the leveling of the islands occurred everywhere, but when the sea level rose again, these flat banks became the foundations of the atolls. In a series of articles culminating in a book The problem of coral reefs (The Coral Reef Problem, 1928), W. Davis combined the theories of Darwin's sinking and Daly's glacial control and put forward a more plausible theory, confirmed by many geological facts.

From the very beginning of research, in order to find out what is under the coral reefs, the idea arose to drill the entire thickness of the coral structure. E. Belcher was the first who tried to implement it in 1840 on the Khao Atoll on the Tuamotu Islands. At a depth of 14 m, which the primitive drill managed to pass, he found nothing but corals. In 1896-1898, a group funded by the Royal Scientific Society and the government of New South Wales (Australia) made a series of determined attempts to drill a well to the foundation of Funafuti atoll (Tuvalu). In this way, it was possible to reach a depth of 340 m in a homogeneous layer of coral limestones. This borehole, penetrating far below the maximum depths of possible growth of reef-building organisms, was considered by some researchers as sufficient proof of the subsidence theory. Other scientists believed that since the drilling was carried out on the periphery of the reef, the hole passed only a layer of detrital material that crumbled and accumulated on the outer slope of the island. They refused to consider the borehole as definitive evidence of subsidence. But later it was confirmed by Japanese scientists who drilled a 432 m deep well on the uplifted atoll of Kito-Daito-Shima in the eastern part of the Ryukyu Islands. This well also did not reach the underlying bedrock of the atoll.

Seismic data obtained during testing atomic bomb on Bikini Atoll in 1946 also indicate that limestone extends to great depths. In 1947, several wells were drilled in Bikini, one of which, at a depth of 779 m, penetrated exclusively limestone and reached Early Miocene deposits, which are approx. 25 million years. A study of the reflection of seismic waves, carried out by the expedition of the second research vessel Challenger in 1951, showed that the limestones on the Funafuti and Nukufetau atolls (Tuvalu Islands) have a thickness of approx. 600 m and possibly lie on volcanic rocks. And finally, in 1954, two boreholes on Eniwetok Atoll (Marshall Islands) penetrated Eocene limestones (about 50 million years old), which at depths of 1266 and 1405 m are in contact with bedrock basalts - rocks of volcanic origin. Judging by the thickness of limestone deposits, the formation of the Funafuti and Nukufetau atolls is explained by the gradual lowering of the coral structure below sea level (or significant fluctuations in sea level). Basalts under the Eniwetok Atoll testify to the volcanic genesis of its base. Thus, it is highly likely that the most common way atolls formed was through the subsidence of volcanic islands.

Interesting discoveries on the seabed of a large number of flat-topped seamounts (called guyots), similar to deeply submerged atolls. Shallow corals were recovered from at least one of them.

Finally, it appears from a review of the fossil reef literature that reef formation occurred mainly during those geologic epochs when weak crustal subsidence (or slow sea level rise) predominated. During geological periods characterized by the uplift of coral structures or their rapid subsidence, reefs hardly developed.