prefomnalna.tk: David Alexander - Hardcover / Geology / Earth Sciences: Books
However, Bermuda is termed a pseudo-atoll because its general form, while resembling that of an atoll, has a very different mode of formation. In most cases, the land area of an atoll is very small in comparison to the total area. Atoll islands are low lying, with their elevations less than 5 meters 9. The remains of an ancient atoll as a hill in a limestone area is called a reef knoll. The largest atoll in terms of island numbers is Huvadhu Atoll in the south of the Maldives with islands.
Accepted as basically correct, his explanation involved considering that several tropical island types—from high volcanic island , through barrier reef island, to atoll—represented a sequence of gradual subsidence of what started as an oceanic volcano.
Reef Madness: Charles Darwin, Alexander Agassiz, and the Meaning of Coral
He reasoned that a fringing coral reef surrounding a volcanic island in the tropical sea will grow upward as the island subsides sinks , becoming an "almost atoll", or barrier reef island, as typified by an island such as Aitutaki in the Cook Islands, Bora Bora and others in the Society Islands. The fringing reef becomes a barrier reef for the reason that the outer part of the reef maintains itself near sea level through biotic growth, while the inner part of the reef falls behind, becoming a lagoon because conditions are less favorable for the coral and calcareous algae responsible for most reef growth.
In time, subsidence carries the old volcano below the ocean surface and the barrier reef remains. At this point, the island has become an atoll. Atolls are the product of the growth of tropical marine organisms, and so these islands are only found in warm tropical waters.
Volcanic islands located beyond the warm water temperature requirements of hermatypic reef-building organisms become seamounts as they subside and are eroded away at the surface. An island that is located where the ocean water temperatures are just sufficiently warm for upward reef growth to keep pace with the rate of subsidence is said to be at the Darwin Point. Islands in colder, more polar regions evolve toward seamounts or guyots ; warmer, more equatorial islands evolve toward atolls, for example Kure Atoll.
Darwin's theory starts with a volcanic island which becomes extinct. As the island and ocean floor subside, coral growth builds a fringing reef , often including a shallow lagoon between the land and the main reef. As the subsidence continues the fringing reef becomes a larger barrier reef farther from the shore with a bigger and deeper lagoon inside. Ultimately the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon.
Discovery of the great depth of the volcanic remnant beneath many atolls such as at Midway Atoll favors the Darwin explanation, although there can be little doubt that fluctuating sea level has had considerable influence on atolls and other reefs. Coral atolls are also an important place where dolomitization of calcite occurs. At certain depths water is undersaturated in calcium carbonate but saturated in dolomite.
Convection created by tides and sea currents enhance this change. Hydrothermal currents created by volcanoes under the atoll may also play an important role. In , and , the Royal Society of London carried out drilling on Funafuti atoll in Tuvalu for the purpose of investigating the formation of coral reefs to determine whether traces of shallow water organisms could be found at depth in the coral of Pacific atolls.
This investigation followed the work on the structure and distribution of coral reefs conducted by Charles Darwin in the Pacific. Professor Edgeworth David led the expedition in On January 6, , U. President George W. From Wikipedia, the free encyclopedia. Ring-shaped coral reef, generally formed over a subsiding oceanic volcano, with a central lagoon and perhaps islands around the rim.
And now, David is providing on his blog a series of stand-alone excerpts from the book for your enjoyment. The first one is here. Personally, I think you should buy the book to support David and his excellent science journalism, but while you are waiting for it to arrive in the mail or be downloaded to your kindle, you can whet your appetite.
Rocky and Bullwinkle? Soupy Sales? Anyone out there remember? If anyone else has used it, they may have changed it ….
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How big are they? Mr Darwin must have had high testosterone to have such a redcnieg hairline at 31! Notify me of follow-up comments by email.
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Related Reef Madness: Charles Darwin, Alexander Agassiz, and the Meaning of Coral
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