The Origin of Coral Reefs
1 Coral reefs are natural structures formed from deposits of the calcium carbonate secretions of coral, a marine animal that lives in colonies. In general, coral reefs are grouped into one of three categories, atolls, barrier reefs, and fringing reefs. Atolls are usually easily distinguished because they are modified horseshoe-shaped reefs that rise out of very deep water far from land and enclose a lagoon (a body of certain water surrounded by a coral reef). With few exceptions, atolls are found only in the Indo-Pacific area. Barrier reefs and fringing reefs, on the other hand, tend to grade into each other and are not readily separable. Some scientists would prefer to group them into a single category. Both types occur adjacent to a landmass, with a barrier reef being separated from the landmass by a greater distance and deeper water channel than the fringing reef. Fringing reefs and barrier reefs are common throughout the coral reef zones in all oceans.
Different types of reefs and reefs in different oceans may have diverse origins and histories. The greatest interest in the origin of reefs has centered on atolls. For many years, humans speculated as to how such reefs could develop in such deep water, miles from the nearest emergent land. This interest was heightened when it was discovered that reef corals cold not live deeper than 50-70 meters. This led to the development of several theories concerning the origin of atolls. Only one need be discussed here——the the theory proposed that atolls grow on the shores of newly formed volcanic islands that have pushed to the surface from deep water. These islands often begin to subside, and if the subsidence is not too fast, reef growth will keep up with the subsidence. The reef growth will then form a barrier reef and, ultimately, an atoll as the island disappears beneath the sea. When the island has disappeared, corals continue to grow on the outside and keep the reef at the surface. On the inside, where the island used to be, quiet water conditions and high sedimentation prevail. These conditions prevent continued vigorous coral growth, hence, a lagoon develops. This theory links all three reef types into evolutionary sequence, but is not an explanation for all fringing and barrier reef types.
Since the current surface features of atolls give no evidence of a volcanic base, in the years after the development of Darwin’s theory other explanations were offered, and the whole concept of the origin of atolls became embroiled in the controversy over the origin of coral reefs. If Darwin’s theory was correct, it must be assumed that drilling down through the current atoll reefs would yield layer after layer of reef limestone until, finally, volcanic rock would be encountered. The ability to drill to the base of atoll reefs and resolved the problem had to wait until the mid-twentieth century in 1953. Ladd and other geologists reported borings at Eriwetok atoll in the Marshall Islands that penetrated 1,283meters of reef limestone and then hit volcanic rock. ■This was the evidence that Darwin’s theory was substantially correct. ■The correctness of this theory has been strengthened by the discovery of flat-topped mountains or guyots that, at present, have their tops many hundreds or thousands of meters below the ocean surface, but have on their surface the remains of shallow water corals. ■Evidently, these mountains sank too fast for reef growth to keep above the ocean surface. ■
Although the subsidence theory links all three reef types in a successional sequence, not all barrier reefs and fringing reefs can be explained by this mechanism. Indeed, the reasons barrier and fringing reef types occur around continental margins and high non-volcanic islands are simply that these areas offer suitable environmental conditions for the growth of reefs and a suitable substrate (surface) on which to begin growth. The extensive reefs around the Indonesian Islands, the Philippines, New Guinea, Fiji and most of the Caribbean Islands are there because a suitable substrate in shallow water existed on which they could initiate growth. In none of these areas are large land areas subsiding, not will these reefs ultimately become atolls.
1. According to paragraph 1, all of the following are true of atolls EXCEPT
A. Most atolls occur in the Indo-Pacific area.
B. Atolls occur only in very deep waters.
C. Atolls are more common than other types of reefs.
D. Atolls occur far from a landmass.
2. According to paragraph 1, fringing reefs and barrier reefs can be identified by
A. their similarities to atolls
B. their organization into groups of separable parts
C. their distance from the nearest landmass
D. whether or not they enclose a lagoon
3. The phrase “was heightened” in the passage is closest in meaning to
4. The word “prevail” in the passage is closet in meaning to
5. Which of the following can be inferred from paragraph 2 as the reason that the origin of atolls has generated more interest than has the origin of other types of reefs?
A. Most reefs have a predictable shape, but atolls do not.
B. Atolls appear to form more rapidly than other kinds of reefs.
C. The animals that form coral reefs cannot live in water below 70 meters, but atolls only form in very deep water.
D. Atolls have more diverse origins and histories than other types of reefs.
6. According to paragraph 2, Darwin’s subsidence theory provided an answer to each of the following questions EXCEPT
A. How does a lagoon develop?
B. Why do volcanic islands subside?
C. How does a fringing reef become a barrier reef?
D. What happens to coral growth when a volcanic island subsides slowly?
7. What can be inferred from paragraph 2 about the development of atolls according to Darwin’s theory?
A. All atolls begin as fringing reefs.
B. An atoll will not develop if an island subsides too slowly.
C. Most atolls will eventually become barrier reefs.
D. The formation of a lagoon accelerates the overall development of an atoll.
8. The word “substantially” in the passage is closet in meaning to
9. Which of the following is mentioned in paragraph 3 as the reason many scientists doubted Darwin’s theory of how atolls form?
A. After Darwin’s five-year voyage, different kinds of atolls than those he observed were discovered.
B. Scientists discovered guyots, or flat-topped mountains, below the ocean’s surface.
C. Surface features of atolls do not show any signs of a volcanic base.
D. Drilling through layers of atoll reef did not produce the results Darwin predicted.
10. What can be inferred from paragraph 2 and 3 about the volcanic rock found by the geologists in the Marshall Islands?
A. It extends from the surface to 1,283 meters deep.
B. It was the first guyot ever discovered by researchers.
C. It formed after the limestone layers of the Eriwetok atoll.
D. It was once an island existing above the ocean surface.
11. The word “initiate” in the passage is closet in meaning to
12. In paragraph 4, why does the author discuss the reefs around the Indonesian Islands, the Philippines, New Guinea, Fiji, and the Caribbean Islands?
A. To argue that these islands have suitable environmental conditions for subsidence
B. To support the claim that the subsidence theory cannot explain the formation of all barrier and fringing reefs.
C. To indicate how different barrier and fringing reefs could be from atolls
D. To identify reefs that grow around non-volcanic islands
13. Look at the four squares [■] that indicate where the following sentence could be added to the passage.
Had subsidence occurred more slowly, the reefs would have eventually become atolls.
Where would the sentence best fit? Click on a square [■] to add the sentence to the passage.
14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
A. Because of its many marine animal colonies, the Indo-Pacific area contains the world’s largest number of atolls, barrier reefs, and fringing reefs.
B. In the years after Darwin’s voyage, researchers discovered that Darwin was incorrect in believing that all fringing reefs and barrier reefs will eventually become atolls.
C. The discovery of guyots suggested to researchers that atolls would never form in non-volcanic islands where fringing and barrier reefs are common.
D. Darwin argued that atolls are formed when volcanic islands surrounded by fringing reefs begin to slowly sink, forming first a barrier reef and finally an atoll.
E. Darwin’s theory was confirmed by the discovery of volcanic rock underneath the limestone layers of an atoll in the Marshall Islands.
F. The fringing reefs and barrier reefs that grow in the waters of continental margins and around non-volcanic islands will never become atolls.
Q1C Q2C Q3D Q4A Q5C Q6B Q7A Q8B Q9C Q10D
Q11A Q12B Q13D Q14 BDE