Every person has heard the word "atoll" at least once in their life. It came to us from the Maldivian language. Want to get to know this concept better and find out what an atoll is? Then let's start with a brief description.
Describing the concept
An atoll is a coral island that looks like a complete or broken ring. Inside it is a lagoon, that is, a shallower body of water, which is separated from sea or ocean waters by a narrow strip of coast. A more accurate description of what an atoll is is a rise rising from the ocean floor on which a coral superstructure has formed. In shallow water, corals form reefs and groups of islands, between which there are straits. At their expense, the lagoons communicate with the ocean. But if the atoll is formed as a closed ring, then the waters in the lagoon have less salinity than in the surrounding ocean. They usually form on extinct volcanoes. The meaning of the word "atoll" can be found in any dictionary.
Structure and stages of formation of atolls
Ch. Darwin tried to explain the first stages of the formation of atolls. Subsequently, his assumptions were confirmed by numerous observations of modern scientists.
At the first stage, a volcano begins to act on the ocean floor. Volcanic islands rise slightly above the surface of the ocean. The slopes are gradually overgrown with coral reefs, and the volcano itself is slowly sinking. Before a colony of polyps reaches the surface, a long period of time passes. Most of today's atolls were formed after the Ice Age. For such a formation to take the form of a closed ring, the sinking of the volcano and the growth of corals must proceed at approximately the same speed, otherwise the ring will be broken.
However, volcanoes may not sink into the ocean, in which case a volcanic island remains inside the lagoon. This formation is called somewhat aggressively - a nuclear atoll. There can be many islands formed by coral colonies.
Each atoll has 3 components:
- outer reef slope;
- dense reef platform;
- an inland body of water, that is, a lagoon.
The average height rarely exceeds 4 meters above sea level, but the area of such formations can be quite large. So, the largest atoll on the planet is Kwajalein, which is included in the Marshall Islands archipelago. Its area is more than 2300 km². But 92% of this area falls on the lagoon. And about 15 km² remains on the island land.
Reef building material
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. Those types of polyps are involved in reef formation,which have a calcareous skeleton. Most often, reefs consist of stony corals and several types of algae that are able to release lime from the surrounding water. The place of formation of coral reefs is the shallow waters of 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 are surprised at where fresh water and vegetation come from on coral islands. There are practically no rivers, streams and other sources of drinking fresh water on the atolls. Fresh water comes here only in the form of rain.
Waves, like giant millstones, grind some of the hardened corals and apply a layer of sand to the surface of the atolls. Seeds of various unpretentious plants get into this mixture. Most often, coconuts brought by the ocean sprout. Gradually limestone rocks are covered with palm trees and shrubs. Usually there are no animals on the atolls, but there is a huge variety of insects. And there are many types of fish in the surrounding waters.
