在2014年6月21日的托福阅读考试中有这样一道题：珊瑚礁。针对这道托福考题，出国留学网(m.liuxue86.com)小编为大家搜集整理了珊瑚礁的背景知识，这样有助于考生在面对这类题目时方便作答。
托福阅读真题再现：
版本一：讲三种海洋reef的形成。这篇阅读是关于coral reef的 有三种类型atoll、fringe、和barrier reef。文章重点讨论atoll 因为它在非常深的水底生长 并且会包围一片水域成为lagoon 文章主要讨论了atoll形成的原因，解释是达尔文提出的一个理论：A其实是由F形成的。，后来虽然被科学家怀疑，但不久后的发现又证实了达尔文是对的。
版本二：
珊瑚礁(重点讲atoll(环礁)这个种类)
第一段说珊瑚礁有三种，atoll，barrier reef(堡礁) 和 fringing reef(岸礁)。atoll主要在印度洋附近，里面有大大的lagoon。barrier reef和fringing reef就哪里都有了。
第二段说根据达尔文的猜想，atoll说长在volcanic island上的fringing reef由于海水沉积放慢，所以慢慢形成barrier reef，最后形成了fringing reef。也解释了lagoon怎么产生的。虽然这可以解释三种coral的进化关系，但是并不能解释所有barrier reef和fringing reef的形成。
第三段说很多科学家质疑达尔文的猜想，但是一个科学家钻了3000miles的limestone然后发现了一个在volcanic island上面的atoll layer，证实了darwin的想法是对的。
第四段又重申这不能解释所有barrier reef 和 fringing reef的形成。
最后一段说在很多沉积很快的ocean里是不会积成atoll的。
解析：
解析：托福阅读考查对于学科类说明文字的理解。有时理论随着时间变迁会发生变化，或是与之相对的新理论产生。在解答文章时，可利用但并不能过于依赖自己已有的学科知识，避免出现过度推断，或是答非所问的现象，一定要注意尊重原文。
相关背景：
Atoll
Formation
See also: Formation of coral reefs
In 1842, Darwin explained the creation of coral atolls in the southern Pacific Ocean based upon observations made during a five-year voyage aboard the HMS Beagle from 1831 to 1836. 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. He reasoned that a fringing coral reef surrounding a volcanic island in the tropical sea will grow upwards 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 towards seamounts or guyots; warmer, more equatorial islands evolve towards atolls, for example Kure Atoll.
Reginald Aldworth Daly offered a somewhat different explanation for atoll formation: islands worn away by erosion, by ocean waves and streams, during the last glacial stand of the sea of some 900 feet (270 m) below present sea level developed as coral islands (atolls), or barrier reefs on a platform surrounding a volcanic island not completely worn away, as sea level gradually rose from melting of the glaciers. 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.
Coral Reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in
groups. The polyps belong to a group of animals known as Cnidaria, which also includes sea anemones and jellyfish. Unlike sea anemones, coral polyps secrete hard carbonate exoskeletons which support and protect their bodies. Reefs grow best in warm, shallow, clear, sunny and agitated waters.
Often called "rainforests of the sea", coral reefs form some of the most diverse ecosystems on Earth. They occupy less than 0.1% of the world's ocean surface, about half the area of France, yet they provide a home for 25% of all marine species, including fish, mollusks, worms, crustaceans, echinoderms, sponges, tunicates and other cnidarians. Paradoxically, coral reefs flourish even though they are surrounded by ocean waters that provide few nutrients. They are most commonly found at shallow depths in tropical waters, but deep water and cold water corals also exist on smaller scales in other areas.
Lagoons
Lagoons are shallow, often elongated bodies of water separated from a larger body of water by a shallow or exposed shoal, coral reef, or similar feature. Some authorities (such as Nybakken) include fresh water bodies in the definition of "lagoon", while others explicitly restrict "lagoon" to bodies of water with some degree of salinity. The distinction between "lagoon" and "estuary" also varies between authorities. Richard A. Davis Jr. restricts "lagoon" to bodies of water with little or no fresh water inflow, and little or no tidal flow, and calls any bay that receives a regular flow of fresh water an "estuary". Davis does state that the terms "lagoon" and "estuary" are "often loosely applied, even in scientific literature." Kusky characterizes lagoons as normally being elongated parallel to the coast, while estuaries are usually drowned river valleys, elongated perpendicular to the coast. When used within the context of a distinctive portion of coral reef ecosystems, the term "lagoon" is synonymous with the term "back reef" or "backreef", which is more commonly used by coral reef scientists to refer to the same area. Coastal lagoons are classified as inland bodies of water.
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