What does atoll mean. The meaning of the word atoll. North Male Atoll

Atoll

Atoll

coral building in the form of a solid or broken ring, bordering the water basin - lagoon. The atoll ring is formed by separate islands. Some of them have their own lagoon or represent an incomplete ring that opens towards the main lagoon with a wide channel. They are found only in the tropical zone. The atolls can vary in size, are usually small, but some are up to 50 km across. The underwater base of the atoll is usually the heights of the oceanic bed. volcanic origin. The genesis of the atoll, according to the hypothesis of Charles Darwin, is associated with the slow sinking of a tropical island surrounded by a ring of barrier coral reef, which is gradually built up by buildings of colonial corals. According to another concept, the formation of atolls occurs due to a rise in the level of the World Ocean as a result of the melting of ice sheets, and not the sinking of the bottom, as evidenced, in his opinion, by the same depths from which coral structures begin in different atolls. Atolls are very demanding on the complex natural conditions: the temperature of air and water, the constancy of salinity and dynamics of ocean waters, their purity, etc. They are found mainly in the waters of the Pacific and Indian oceans.

Plan and vertical section of the atoll (along the line AB)

Geography. Modern illustrated encyclopedia. - M.: Rosman. Under the editorship of prof. A. P. Gorkina. 2006 .

Atoll

ring-shaped coral reef surrounding the water area. Coral reefs are common in tropical and subtropical latitudes, and most widely in the Pacific and Indian Oceans. The word "atoll" comes from the name used by the peoples of Oceania for the reefs that form a chain small islands, towering 4–6 m above the water surface. Sometimes reefs are flooded, and between their individual parts there are deep straits. The lake-like waters within the atolls are called lagoons. Usually their depth is 18–30 m, but sometimes it reaches 90 m.
Structure. The main material from which coral reefs are built is limestone, formed as a result of the accumulation of skeletons of marine animals and plants containing calcium carbonate, mainly corals and coralline algae. Islets scattered along reefs usually consist of limestone fragments and crumbs - products of wave activity. Often, small dunes are formed from this material under the influence of wind. The outer coast of most of the atoll islands is a rather narrow strip of fringing reefs, often with an even border of massive pink calcareous algae and steep, narrow beaches composed of limestone fragments. Behind the coastal ridge of the beach, coarse-grained material is replaced by crushed stone, which is further replaced by gravel as it is crushed. In the center of the islet there is often a depression filled with saline soil stained with humus. The interior of the island is usually composed of coarse coral sand or fine gravel, while the central parts of the beach are composed of fine calcareous sand. If the atoll is completely raised above sea level, then the limestones composing it usually erode and form a rough surface - “macatea”. The groundwater level is very close there, and small wells are often dug near the center of the islands. If the island is large or there is a lot of rainfall, then the groundwater is almost fresh, but if it is small or located in an area with little rainfall, then the groundwater is brackish or saline.
Flora. The vegetation cover is the most remarkable characteristic of the surface of the atolls. It is sparse and most often consists of several species of widespread herbaceous plants and tree species that live in the coastal strip. In the distribution of vegetation, a certain zonality is observed in the direction from the outer edge of the atoll to the inner one. She may be in in general terms described for the entire Indo-Pacific region, where atolls are especially numerous, and their flora is of the same type. The outer zone is represented by shrubs, among which scaevola shrubs predominate ( Scaevola frutescens). This is followed by forests, mainly from halophytes such as silver messerschmidia ( Messerschmidia argenta) and pandanus roofing ( Pandanus tectorius). Even farther from the coast, there are fewer halophytes, and cordia appear ( Cordia), pisonia large ( Pisonia grandis), ochrosia ( Ochrosia) and other types. In the center, in more mesophytic conditions, the primary vegetation probably consisted of trees such as splendid coetard ( Guetta speciosa), hibiscus lime-shaped ( Hibiscus tiliacens), ficuses, pisonias, pandanuses, Asian barringtonia ( Barringtonia asiatica), terminals ( Terminalia), etc. On most of the islands, these forests were subsequently replaced by plantations of coconut palms and breadfruit. The inner zone of the beaches is characterized by a narrow strip of trees such as hernandia, calophyllum, pandanus, thespesia, cordia. On sandbars between the islets are usually found suriana maritime ( Suriana maritima) and scaevola, and on rock outcrops - sour pemphis ( Pemphis acidula). In places, in the inner parts of the islets, waterlogged depressions are found, and in some of them they are confined to hard rocks. In such conditions, some tree species typical of mangrove swamps are found. Other islets are swampy and treeless, dominated by grasses and sedges. On inhabited islands, soil is taken out of these depressions and filled with humus, and then used to grow taro, sugar cane and other plants. Due to the salinity of the soil, few (except those listed above) cultivated plants can grow on low coral islets. Taro, breadfruit and coconuts are the main food of the population, in addition, coconuts are a source of drinking water.
Other natural resources. The vast majority of the atolls are inhabited, and there, crop production depends on the potential fertility of nutrient-poor saline soils. The main crop is coconuts. Along with the above plants, the basis of the diet of the population is fish and shellfish, which abound in lagoons, reefs and adjacent open water areas. Other trade items are guano, pearls, shells for making buttons and dried holothurians.
The most fertile are the large atolls that rise above sea level, especially if they are confined to very humid areas. They feature much more lush vegetation, richer flora, and a much more diverse array of crops. Apparently, all this is associated with lower soil salinity. On the uplifted atolls, there are often large deposits of phosphorites (probably formed from ancient deposits of guano). Such atolls include the islands of Makatea, Nauru, Ocean, Angaur. Guano deposits on many atoll islands were formed as a result of the life of sea birds - gannets, frigatebirds, terns and tropical birds that breed here. In addition to the listed birds and land crabs, including the palm thief, the terrestrial fauna is poor: rats dominate, and in the western part Pacific Ocean fruit bats (fruit-eating bats) are numerous. The waters abound with turtles, spiny lobsters, giant bivalves, and many kinds of fish. The rich fauna of the reefs includes many species of invertebrates.
famous atolls. The largest groups of atolls in the Indian Ocean are the Maldives and Laccadives; in the Pacific Ocean - the Caroline, Marshall, Gilbert Islands in Micronesia, Tuamotu and others in Polynesia; a large number of atolls scattered throughout Melanesia. IN Atlantic Ocean there are several atolls in the Lesser Antilles group, in addition, the Bahamas include a number of atolls that are not typical. The largest atoll in terms of area (with a relatively small lagoon in the center) is Christmas Island in the equatorial Pacific Ocean. The largest lagoon is apparently located on Kwajalein Atoll (Marshall Islands), although lagoons of almost the same size are found on some other atolls. see also Coral reefs .
LITERATURE
Ravikovich A.I. Modern and fossil reefs. M., 1954
Geography of the atolls of the Southwest Pacific. M., 1973

Encyclopedia Around the World. 2008 .

See what "atoll" is in other dictionaries:

Modern Encyclopedia

- (Atoll) lagoon reef, a special kind of island. A ring-shaped strip of land surrounding an inland shallow lake or lagoon. Usually A. rises above sea level by several meters. The depth in the lagoon is small; 60 150 m (30 80 sea fathoms), while ... ... Marine Dictionary

Coral island, reef Dictionary of Russian synonyms. atoll n., number of synonyms: 9 bikinis (5) ... Synonym dictionary

atoll- a, m. atoll m., eng. Coral island. Atolian oh, oh. Sometimes the tentacles of an octopus were pushed into the hole, which, apparently, methodically ransacked the atoll jar. A. Kim Fish Simplicitas. // NM 1997 4 102. Lex. Dal: atol; Toll 1863: atoll; SAN 1891 ... Historical Dictionary of Gallicisms of the Russian Language

Atoll- (from the Malay adol closed), a coral building with a shallow lagoon in the middle, connected to the ocean through one or more straits. The base of an atoll is usually the summit. underwater volcano. Distributed in tropical latitudes ... ... Illustrated Encyclopedic Dictionary

A coral structure, in the form of a solid or broken ring, surrounding a shallow lagoon. The base of an atoll is usually the summit of an underwater volcano. Widespread in the tropical latitudes of the Pacific and Indian Oceans ... Big Encyclopedic Dictionary

ATOLL, a ring-shaped CORAL REEF surrounding a shallow LAGOON. The reef appears initially as a frame for an island slowly sinking into the water, usually of volcanic origin. As the island sinks, the coral continues to grow... Scientific and technical encyclopedic dictionary

There are a huge number of islands in the world, some of them appeared tectonically, that is, due to the movements of the earth's crust. Others broke away from the mainland. Finally, there are those for the origin of which living organisms are responsible. The article will tell you what atolls are, how they appeared, what are their features.

Etymology

The noun "atoll" comes from the Maldivian word "atolhu" (atolhu), which, in turn, comes from the root atollon, which in the language of the natives means "closed, closed." Already this information is enough to suggest that the word "atoll" means something closed.

Meaning

Referring to any dictionary, you can find out the meaning of the term. What is an atoll? This is an island created by corals, in the form of a ring (often closed, although an interrupted version is acceptable), located in the lagoon. It should also be noted that this is a certain elevation in the ocean, on which, thanks to coral organisms, a kind of land arose. Since such an elevation is located on ocean floor most often on the crater of a dormant volcano, its corresponding shape is cone-shaped.

Considering what an atoll is, we note that it belongs to biogenic islands, that is, created by living organisms.

Structure

Of particular interest is the structure of such an island. It has 3 parts:

1. The outer slope of the reef. Most of the time, he's pretty cool. It is here that two opposite processes take place - the formation of coral material and its destruction under the influence of waves.
2. Platform. It is a platform, the width of which can reach several hundred meters and even kilometers.
3. Lagoon (water area inside the atoll). As a rule, it has a shallow depth and a flat bottom.

The average height of the atoll, as a rule, is 3-4 meters, the sizes can be very different. So, Kwajalein - the largest of the atolls - exceeds 2300 km, more than 90% of which falls on the lagoon, which stretches for 300 km.

In the next video you can see amazing beauty atoll and learn some interesting facts about it.

Briefly about education

Considering what an atoll is, it is important to pay attention Special attention how the island was formed. Most often, the process looked like this: around the crater extinct volcano a coral reef gradually appeared, forming a ring. With a slight decrease in the water level, such an “island” appeared on the surface. And so the atoll was born. There are a lot of similar formations in the Pacific and Indian oceans, in tropical and subtropical latitudes.

The reverse process is also possible - immersion of the atoll in water. In this case, the so-called bank, or submerged atoll, is formed.

The main material from which such unusual islands, is limestone accumulated from the skeletons of dead plants and animals, primarily corals. The surface of such islands, as a rule, is adorned with a rather poor vegetation cover, which includes several species of trees and grasses located in zones. The most common are shrub scaevolla, roofing pandanus, ochrosia and others. Closer to the center, away from the coast, you can find ficuses, hibiscus, as well as artificially planted breadfruit trees and coconut palms.

Other atolls are swampy, and other plants, except for sedge, cannot survive on them.

So, considering the meaning of the word "atoll", it should be noted that this is an oceanic island created from limestone - the remains of dead living organisms, primarily corals, which has a lagoon and is covered with some types of vegetation. Because of its picturesque and unusual similar islands very attractive to tourists.

Let's continue our acquaintance with the atolls. What this word means, we have already considered, now it is necessary to distinguish the term from a group of similar ones, first of all, it is a volcanic island and a coral reef.

So, an island of volcanic origin is a piece of land located on the crater of an extinct volcano; it has nothing to do with the life of corals and other organisms. A coral reef is a deposit of dead corals located underwater on a volcanic crater.

If the water level drops and sediments are partially on the surface, an atoll is obtained.

The sequence can be represented as follows:

1. Corals settled on the crater of an extinct volcano, gradually a kind of underwater organic backbone formed from their remains. This is a coral reef.
2. When it was partially above the water column, an atoll turned out.

Therefore, these terms are not synonymous, although they are close in meaning. We can say that the atoll is a certain type of coral reef, having a round shape with a lagoon located in the central part, on which several tiny islands can be located.

In the photo you can see that the ocean waters surrounding the atolls have a striking turquoise color. It is connected with the fact that the inner part of the island is gradually dying and decomposing.

We looked at what an atoll is, how it differs from similar terms, and what it consists of.

In our minds, picturesque atolls with their turquoise lagoons and colorful reefs are closely associated with sea ​​adventures and rich beach holiday.


For many, these strange formations are the embodiment of a dream of living on paradise island, overgrown with tall palm trees and tropical flowers. But what are atolls? geographic point vision? Where are they located and how do they appear on the surface of the oceans?

What does the word "atoll" mean?

Term "atoll" is of Maldivian origin. In the language of the local tribes, dhivehi, the word atholhu means the administrative unit constituting the Maldives archipelago. IN English language the concept first hit in 1625 thanks to Charles Darwin, who was closely involved in the study of the structure of atolls and their distribution around the world.

Initially, the term was synonymous with a lagoon island, and later came to mean "a ring-shaped reef ribbon containing a lagoon."

What is an atoll?

Today, an atoll is understood as a coral island in the form of a solid or broken ring surrounding a small body of water (lagoon). Most often, such islands are located either as an underwater mountain, which was partially eroded and submerged under water. The lagoon in this case is a volcanic crater, and the land is its rim, rising above the water.


The height of the atoll, as a rule, does not exceed 3-4 meters above sea level. If the island ring is solid, then the water in the lagoon is less salty than in the ocean. If the ring is broken, then straits appear between individual sections of the island, thanks to which salt water enters the lagoon. ocean water.

What is an atoll made of?

Structurally, the atoll includes three separate parts - the outer slopes of the island, the island itself and the lagoon. The rim of the volcano is crowned with a superstructure formed by coral polyps.

In the course of their life, these invertebrate creatures form large colonies, which, as they develop and increase, create coral reefs. The rate of rifting in the atolls is sufficient to make up for the height of the islands, which gradually decreases with erosion and subsidence. mountain top.

How are atolls formed?

According to the theory of Charles Darwin, volcanoes and mountains once existed on the site of the atolls, which, as a result of eruptive and other geological processes, sank under water.


In the case of volcanoes, subsidence could cause eruptions that caused the top of the summit to collapse under its own weight and form a caldera. Over time, the volcanic edges overgrown coral reefs, forming an annular belt.

If the volcano does not completely sink under water, then a so-called nuclear atoll is formed, in the lagoon of which an island of volcanic origin remains. Due to the decrease in sea level, some ring-shaped islands rise above the water and form raised atolls, which are an ordinary island (without a lagoon) with high marginal ramparts around the perimeter. This is a rather rare occurrence, but similar atoll formations can be found in the Tonga archipelago and in southern Micronesia.

Where do atolls form?

Reef-building corals can only exist in warm waters oceans and seas, so atolls are formed exclusively in tropical and subtropical latitudes. The northernmost in the world is Kure Atoll, located in the northwestern part of Hawaiian archipelago. South of all are the atoll reefs of Elizabeth and Middleton, extending into the waters coral sea.

Most of the ring-shaped islands are concentrated in the Pacific and Indian Oceans - in the Maldives, the Marshall and Caroline Islands, the Seychelles, in the Chagos Archipelago and east of Australia. In the Atlantic there are several small atoll groups located east of Nicaragua.


Kwajalein Atoll is one of the largest in the world. Marshall Islands, consisting of 92 rings. His total area- more than 2300 km², of which over 90% are lagoons.

Charles Darwin's theory of the formation of atolls

famous biologist C. Darwin in 1842 formulated immersion theory. It explains how atolls are formed, namely, the uplift and subsidence of the crust. Earth in the oceans. According to him, atolls are formed in 3 stages:

1. fringing reef(the volcano fades, the bottom settles);
2. barrier reef(continuation of subsidence);
3. atoll.

Also, Darwin believed that under all lagoons lies stone foundation. Subsequently, his theory was confirmed.

Fresh water, flora and fauna of the atolls

In fact, streams, rivers and similar sources fresh water there isn't. Where does it come from on the coral islands drinking water ? Drinkable water to the atolls bring the rains. What about flora? Seeds of plants with a high level of survival, fall into a mixture of old coral and sand. Mainly coconuts. Over time, shrubs and palm trees begin to sprout. If speak about fauna, then its presence is not typical for atolls. But, there are a large number of different insects and many kinds fish.

The famous bathing suit "bikini" named thanks to the sensational, at one time, atomic testing on an atoll called Bikini. Countries like Tuvalu And Kiribati, are located entirely on the atolls. And, here, the atoll, on which the human foot has never set foot - Caroline. It is part of the state Kiribati. But, soon, it will be flooded due to the constant rise water level in the global ocean.

The most famous atoll in the world is Lighthouse having annular shape. At its center is the famous Big lagoon blue hole . And the largest atoll on Earth is Kiritimati. The second largest atoll is Aldabra where they found their home more150 thousand giant turtles. The largest raised atoll is considered Niue where the country with the same name is located.

atolls — coral islands in the form of continuous or discontinuous rings surrounding the lagoon. In essence, an atoll is an elevation at the bottom of the ocean, which, as it were, is crowned with a coral superstructure. She, in turn, forms a reef with a group of small islands "motu", which are separated from each other by straits. The straits connect the inner lagoon to the surrounding ocean. If there are no such straits, then the land forms a continuous ring. In this case, the water in the lagoon is often less salty than the ocean. Elevation at the bottom of the ocean usually has a cone shape, formed on the site of an extinct volcano.

Major atolls of the Maldives

North Male Atoll

The city of Male is the capital of the Maldives, located on the main atoll - North Male. It is 69 km long and 39 km wide. This atoll includes 50 enough major islands and a few smaller ones. For tourists atoll North Male attractive for its white sandy beaches, turquoise lagoons and cozy atmosphere.

South Male Atoll

This atoll consists of three dozen islands, and only three of them are inhabited. 10 islands are uninhabited and 17 are hotels. South Male It is 36 km long and 19 km wide. The resorts are usually reached by speed boats, and between the nearby islands, movement is carried out using motor boats.

Baa Atoll

Baa Atoll has 10 inhabited and 41 desert island. The atoll is 42 km long and 32 km wide. Basic concept local resort- Modern comfort combined with Robinson Crusoe lifestyle.

Ari and Rasdhoo atolls

Ari Atoll is 33 km long and 96 km wide (from north to south). The atoll is located to the east of Male and includes the large Ari and the small Rasdhoo atolls. Ari has approximately 70 islands. 18 of them are inhabited by people, and 26 are purely tourist area. These are fairly new resorts, the construction of which took place in the 90s. Now the influx of tourists to Ari has reached its peak and resort area is very popular. locals they make sails, collect and process corals, hunt sharks and turtles, from which a special oil is extracted to lubricate local boats.

Dhaalu Atoll

The main island of Dhaalu Atoll is Velavaru. It takes about 40 minutes to reach it by seaplane. The locals used to call it "turtle island". Here reigns lush tropical vegetation, tall palm trees, sandy beaches and an attractive picturesque lagoon. All this attracts turtles to the island (“velaa” - translated from local language dvivehi - "turtle") lovers of wildlife.

Laviani Atoll

This atoll is located far north of North Male at a distance of about 120 km from the latter. The Laviani Atoll consists of 63 islands, most of which are concentrated around barrier reef over 30 km. This is one of the best places for diving throughout the Maldives archipelago. Here incredible amount beautiful corals and colorful tropical fish. IN clear waters Laviani Atoll is home to some of the most beautiful creatures in the ocean, including glassfish, moray eels or large nurse sharks.

Addu Atoll

Addu Atoll is located 480 km from the capital Maldives, the cities of Male and international airport Hulule. This is the southernmost of the elements of the Maldives archipelago, located south of the equator. The island has a well-developed network of communications and roads. Most big Island in the atoll - Gan, very different from the rest. It is 5 km long and 3 km wide. The island is covered with banana plantations and dense flowering vegetation. Gan is connected by dams to four neighboring islands, only slightly inferior to it in size. You can ride a bike along them, and the entire route will take only 20 km. The most beautiful lake here from October to March it receives a lot of wintering birds, and there are also places on the islands where you can swim.