The word "atoll" at least once in a life has been heard by everyone. It came to us from the Maldivian language. Want to get to know this concept and find out what an atoll is? Then let's start with a brief description.
We describe the concept
An atoll is a coral island that looks like a full or torn ring. Inside it is a lagoon, that is, a shallow body of water that is separated from sea or ocean waters by a narrow strip of shore. A more accurate description of what an atoll is is an elevation rising from the bottom of the ocean on which a coral superstructure has formed. In shallow water, corals form reefs and groups of islands, between which straits exist. 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. Usually they form on extinct volcanoes. The meaning of the word "atoll" can be found in any dictionary.
The structure and stages of the formation of atolls
For the first time C. Darwin tried to explain the stages of formation of the 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 the colony of polyps reaches the surface, a long period of time passes. Most of today's atolls formed after the ice age. In order for such a formation to take the form of a closed ring, the dive of the volcano and the growth of corals should go at about the same speed, otherwise the ring will be broken.
However, immersion of volcanoes in the ocean may not occur, in this case a volcanic island remains inside the lagoon. Such a formation is called a somewhat aggressive - nuclear atoll. There can be many islands formed by coral colonies.
Each atoll has 3 components:
- external reef slope;
- dense reef platform;
- inland pond, i.e. 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 is in the lagoon. And on island land remains about 15 km².
Reef Building Material
Do you already understand what an atoll is? And what does the building material from which reefs look like look like? Coral polyps belong to the class of invertebrate bottom organisms. Those types of polyps that have a calcareous skeleton are involved in reef formation. Most often, reefs consist of madrepore corals and several types of algae, which are capable of releasing 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 Ocean.
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 the coral islands. In fact, rivers, streams and other sources of drinking fresh water on atolls do not exist. Fresh water gets here only in the form of rains.
Waves, like giant millstones, grind part of the hardened corals and apply a layer of sand to the surface of the atolls. The seeds of various unpretentious plants fall into this mixture. Most often, coconuts brought by the ocean sprout. Gradually, calcareous 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 in the surrounding waters there are many species of fish.