托福阅读真题Official 47 Passage 3（一）

托福阅读真题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

正确答案：C

题目详解

题型分类：否定事实信息题

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。

选项分析：

C选项出自第二句where the seawater is not rendered too dark by excessive amounts of river-borne sediment过多的沉积物会遮挡光，不利于珊瑚礁生长，与原文矛盾。

A选项出自第二句where there is a firm substratum。

B选项出自第二句where the seawater is not rendered too dark说明不能太暗。

D选项出自第二句Coral reefs are found where the ocean water temperature is not less than 21°C。说明温度不能低于21℃。

Question 2 of 14

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

A.precaution

B.preparation

C.requirement

D.achievement

正确答案：C

题目详解

题型分类：词汇题

选项分析：

原文说它们不会生长在非常深的水中，所以在离地表30至40米范围内的平台是它们发展的prerequisite，推测珊瑚礁需要在离地表30至40米范围内生长，推测prerequisite意思是requirement，因此选C。

A选项，预防措施。

B选项，准备。

D选项，成就。

带入原文均不符合语境。