Oceans and seas are the property of mankind, since not only most of all known (and unknown) species of living creatures live in them. In addition, only in the gloomy depths of sea waters can one sometimes see such pictures, the beauty of which sometimes can simply stun even the most indifferent person. Look at the coral reef, and you will see that nature is many times greater than the creation of any talented artist.
What it is?
Coral reefs are called colonies of coral, which sometimes form really gigantic formations, similar in size to the rocks.
Note that the real corals that can form reefs are Scleractinia, belonging to the class Anthozoa, type Cnidaria. Single individuals form giant colonies of polyps, and calcareous colonies of older individuals provide support for the development and growth of young animals. Contrary to popular belief, polyps are found at all depths, and not just in shallow water. So, the most beautiful black coral lives at such a depth that no sunbeam penetrates.
But a real coral reef can be formed only by those species that live in the shallow waters of tropical seas.
What reefs exist?
There are three main varieties of them: bordering, barrier and atolls. As you might guess, the fringing species is found in shallow water near the coast. The most impressive formations are barrier reefs that look like a breakwater. They are located along the coast of continents or large islands. As a rule, they are very important. Firstly, millions of species of living creatures take refuge there, and secondly, these formations play an important role in shaping the region’s climate, preventing ocean currents.
The largest and most famous is the Great Barrier Reef, which stretches over 2000 km, forming the eastern edge of the Australian mainland. Other not so significant and large "relatives" are located along the coast of the Bahamas, as well as in the western Atlantic.
Atolls are small ring-shaped islands. Their coast is protected by coral reefs that form a natural barrier that prevents strong tides and ocean currents from washing away the fertile layer from the land surface. Where do reefs come from? What is the mechanism of their formation?
The emergence of coral reefs
Since most polyps need a relatively shallow environment, it is ideal for them to have a small and flat base, preferably located near the coast. However, many scientists believe that the conditions under which the formation of a colony of polyps is possible is much more diverse.
Thus, many atolls, by all indications, should have arisen on the tops of old volcanoes, but far from everywhere traces of really high lava formations were found that could completely confirm this theory. The famous scientist Charles Darwin, traveling on the no less famous Beagle, was engaged not only in the formation of an evolutionary view of the development of mankind. Along the way, he managed to make many discoveries, one of which was an explanation of how the world of coral reefs arose.
The “Reef” Theory of C. Darwin
Suppose that the volcano that arose in antiquity gradually increased due to lava, which fell into the external environment as a result of numerous eruptions. As soon as about 20 meters remain to the surface of the ocean, optimal conditions will arise for the coral to populate the top of the seamount. They begin to quickly build up the colony, gradually completely modifying the primary relief that arose after the eruptions.
When the young coral reef reaches a critical mass, the volcano, the upper part of which by that time had already practically collapsed, begins to gradually plunge back into the ocean. As the dive begins, corals begin to grow more intensively, and therefore the reef begins to become even more massive, remaining at approximately the same level relative to the surface of the water.
Dynamic formation theory
Near the reef, sand begins to accumulate, most of which are the skeletons of the corals themselves, ground by erosion and some species of sea creatures. There are more and more shallows; over time, the reef begins to protrude above the surface of the ocean, gradually forming an atoll. The dynamic model suggests that the elevation of the colony of polyps above the surface of the water is due to the constant change in the level of the World Ocean.
Many geologists and geographers of that time immediately became interested in this theory. If it is true, then every large coral reef had to carry at least some remnants of a volcanic core.
Is the volcanic theory of reef origin true?
To test this, in 1904, trial drilling was organized on the island of Funafuti in the Pacific Ocean. Alas, the technologies that existed at that time made it possible to reach a depth of only 352 meters, after which the work was stopped, and scientists could not get to the proposed core.
In 1952, Americans began drilling on the Marshall Islands for the same purpose. At a depth of about 1.5 kilometers, scientists found a layer of volcanic basalt. It has been proven that a coral reef formed over 60 million years ago when a colony of polyps settled on top of an extinct volcano. Darwin was right again.
How reefs changed during periods of decline in sea level
It is known that the amplitude of ocean oscillations in different periods reached one hundred meters. The current level stabilized only six thousand years ago. Scientists believe that 15 thousand years ago, the ocean level was at least 100-150 meters below modern. Thus, all the coral reefs that formed at that time are now 200-250 meters below the modern edge. After this mark, the formation of colonies of polyps becomes impossible.
In addition, the former coral reefs (photo is in the article), which were formed in even more ancient periods, are often found on present land. They were formed at a time when the ocean level was as high as possible, and there were no ice caps at the earth's poles. Note that between the ice ages, polyps did not actually form any more significant colonies, since the water level changed too quickly.
Egypt is especially indicative in this regard. Coral reefs in the Red Sea are sometimes found at great depths, which several million years ago were the bottom of ordinary shallow seas.
The main components of a coral reef
To understand how a colony of polyps is built specifically, consider the coast of Jamaica as an example. In any photo of the classic atoll, a sand spit steeply rising from the depths is first visible. Dark stripes parallel to the atoll are traces of the destruction of corals that occurred during different geological periods due to fluctuations in ocean level.
Sailors identify this area by the breakers: even at night, the sound of the surf, which is heard long before the coast, warns of reefs. After the protected zone, a plateau begins, on which corals open at low tide. Oddly enough, but in the water area of the lagoon, the depth increases sharply, colonies of polyps in this area are not so developed, at low tide they continue to remain under water. The area near the coast, which constantly opens during low tides, is called littoral. There are few corals there.
The largest and most branched corals grow on the outer edges, which look into the open ocean. The highest concentration of marine inhabitants is observed in the littoral region. By the way, who can you meet visiting a coral reef? The underwater world of Egypt and other popular tourist countries is so rich that your eyes will run wide! Yes, you cannot refuse these places in the richness of fauna.
