What is an atoll? Structure and stages of education

Every man has heard the word "atoll" once in his life. It came to us from the Maldivian language. Do you want to get to know this concept more closely and find out what an atoll is? Then let's start with a brief description.

Describe the concept of

Atoll is called a coral island, which is similar to a full or broken ring. Inside it is a lagoon, that is, a smaller pond that is separated from the sea or ocean waters by a narrow strip of shore. A more precise description of what an atoll is is an elevation rising from the ocean floor on which a coral superstructure was formed. In shallow water, corals form reefs and small groups of islands, between which there are straits. At their expense, lagoons communicate with the ocean. But if the atoll is formed as a closed ring, then the waters in the lagoon have a lower salinity than in the surrounding ocean. They are usually formed on extinct volcanoes. The meaning of the word "atoll" can be found in any dictionary.

The structure and stages of atoll formation

Charles Darwin tried to explain the stages of atoll formation. Later, his assumptions were confirmed by numerous observations of modern scientists.

At the first stage, the volcano begins to act on the ocean floor. Volcanic islands rise somewhat above the surface of the ocean. Slopes gradually become overgrown with coral reefs, and the volcano slowly sinks. Before the colony of polyps reaches the surface, it takes a long time. Most of today's atolls are formed after the glacial period. In order for such formation to take the form of a closed ring, the volcano's immersion and the growth of corals should go at approximately the same speed, otherwise the ring will be torn.

However, the immersion of volcanoes in the ocean may not occur, in this case, inside the lagoon there is a volcanic island. Such an education is called somewhat aggressively - a nuclear atoll. The islands formed by the coral colonies can be many.

Each atoll has 3 components:

• External reef slope;
• Dense reef platform;
• An internal pond, that is, a lagoon.

The average height rarely exceeds 4 meters above the ocean level, but the area of such formations may be large enough. So, the largest atoll on the planet is Kwajalein, which is included in the Marshall Islands archipelago. Its area is more than 2300 km ². But 92% of this area falls on the lagoon. And on the island land remains about 15 km ².

Building material for reefs

Have you already understood what an atoll is? And what does the building material from which reefs are built look like? Coral polyps belong to the class of invertebrate benthic organisms. In the reef formation, those species of polyps that have a calcareous skeleton participate. Most often, the reefs consist of madreporous corals and several species of algae that are capable of releasing lime from the surrounding water. The place of formation of coral reefs is the shallow waters of the tropical seas. Most of them are located in the Indian and Pacific oceans.

Where does fresh water come from? How does vegetation appear?

Knowing what an atoll is, many wonder why coral islands take fresh water and vegetation. There are practically no rivers, streams and other sources of drinking fresh water on atolls. Fresh water comes here only in the form of rains.

Waves, like giant millstones, grind some of the hardened corals and put a layer of sand on the surface of the atolls. In this mixture seeds of various unpretentious plants get. Coconuts brought by the ocean most often sprout. Gradually, calcareous cliffs are covered with palms and shrubs. Usually there are no animals on atolls, but there is a huge variety of insects. And in the surrounding waters, many species of fish are found.