托福閱讀真題Official 47 Passage 3（一）

托福閱讀真題Official 47 Passage 3（一）

Coral Reefs

An important environment that is more or less totally restricted to the intertropical zone is the coral reef.Coral reefs are found where the ocean water temperature is not less than 21°C,where there is a firm substratum,and where the seawater is not rendered too dark by excessive amounts of river-borne sediment.They will not grow in very deep water,so a platform within 30 to 40 meters of the surface is a necessary prerequisite for their development.Their physical structure is dominated by the skeletons of corals,which are carnivorous animals living off zooplankton.However,in addition to corals there are enormous quantities of algae,some calcareous,which help to build the reefs.The size of reefs is variable.Some atolls are very large—Kwajalein in the Marshall Islands of the South Pacific is 120 kilometers long and as much as 24 kilometers across—but most are very much smaller,and rise only a few meters above the water.The 2,000 kilometer complex of reefs known as the Great Barrier Reef,which forms a gigantic natural breakwater off the northeast coast of Australia,is by far the greatest coral structure on Earth.

Coral reefs have fascinated scientists for almost 200 years,and some of the most pertinent observations of them were made in the 1830s by Charles Darwin on the voyage of the Beagle.He recognized that there were three major kinds:fringing reefs,barrier reefs,and atolls;and he saw that they were related to each other in a logical and gradational sequence.A fringing reef is one that lies close to the shore of some continent or island.Its surface forms an uneven and rather rough platform around the coast,about the level of low water,and its outer edge slopes downwards into the sea.Between the fringing reef and the land there is sometimes a small channel or lagoon.When the lagoon is wide and deep and the reef lies at some distance from the shore and rises from deep water it is called a barrier reef.An atoll is a reef in the form of a ring or horseshoe with a lagoon in the center.

Darwin’s theory was that the succession from one coral reef type to another could be achieved by the upward growth of coral from a sinking platform,and that there would be a progression from a fringing reef,through the barrier reef stage until,with the disappearance through subsidence(sinking)of the central island,only a reef-enclosed lagoon or atoll would survive.A long time after Darwin put forward this theory,some deep boreholes were drilled in the Pacific atolls in the 1950s.The drill holes passed through more than a thousand meters of coral before reaching the rock substratum of the ocean floor,and indicated that the coral had been growing upward for tens of millions of years as Earth’s crust subsided at a rate of between 15 and 51 meters per million years.Darwin’s theory was therefore proved basically correct.There are some submarine islands called guyots and seamounts,in which subsidence associated with sea-floor spreading has been too speedy for coral growth to keep up.

Like mangrove swamps,coral reefs are extremely important habitats.Their diversity of coral genera is greatest in the warm waters of the Indian Ocean and the western Pacific.Indeed,they have been called the marine version of the tropical rain forest,rivaling their terrestrial counterparts in both richness of species and biological productivity.They also have significance because they provide coastal protection,opportunities for recreation,and are potential sources of substances like medicinal drugs.At present they are coming under a variety of threats,of which two of the most important are dredging and the effects of increased siltation brought about by accelerated erosion from neighboring land areas.

Question 1 of 14

According to paragraph 1,all of the following are needed for the growth of coral reefs EXCEPT

A.a solid base to grow on

B.exposure to light

C.the presence of river-borne sediment

D.ocean temperatures of 21oC or higher

正確答案：C

題目詳解

題型分類：否定事實信息題

原文定位：第二句Coral reefs are found where the ocean water temperature is not less than 21°C,where there is a firm substratum,and where the seawater is not rendered too dark by excessive amounts of river-borne sediment。

選項分析：

C選項出自第二句where the seawater is not rendered too dark by excessive amounts of river-borne sediment過多的沉積物會遮擋光，不利于珊瑚礁生長，與原文矛盾。

A選項出自第二句where there is a firm substratum。

B選項出自第二句where the seawater is not rendered too dark說明不能太暗。

D選項出自第二句Coral reefs are found where the ocean water temperature is not less than 21°C。說明溫度不能低于21℃。

Question 2 of 14

The word“prerequisite”in the passage is closest in meaning to

A.precaution

B.preparation

C.requirement

D.achievement

正確答案：C

題目詳解

題型分類：詞匯題

選項分析：

原文說它們不會生長在非常深的水中，所以在離地表30至40米范圍內(nèi)的平臺是它們發(fā)展的prerequisite，推測珊瑚礁需要在離地表30至40米范圍內(nèi)生長，推測prerequisite意思是requirement，因此選C。

A選項，預(yù)防措施。

B選項，準(zhǔn)備。

D選項，成就。

帶入原文均不符合語境。

>> 雅思 托福 免費測試、量身規(guī)劃、讓英語學(xué)習(xí)不再困難<<