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  3. New Tools to Research Earthquake On December 16th, 1811, the residents of New Madrid, Missouri, were startled out of their b

New Tools to Research Earthquake On December 16th, 1811, the residents of New Madrid, Missouri, were startled out of their b

admin2013-07-08  71
问题                      New Tools to Research Earthquake
    On December 16th, 1811, the residents of New Madrid, Missouri, were startled out of their beds by a huge earthquake, which was quickly followed by a second. Those who survived the catastrophe reported that cracks opened in the earth’s surface, that the ground rolled visibly in waves and that large areas of land sank downwards. People in places as far away as Boston declared that they heard church bells ringing at the time the earthquake happened.
    That huge earthquakes occur in parts of North America outside their traditional habitat on the Pacific margin is well known from accounts like those from New Madrid. Such records, however, have been kept only since Europeans arrived, so it has been hard to work out how active the faults that cause them are. Now it is a little easier. A study carried out by Samuel Panno of the Illinois State Geological Survey and his colleagues has revealed that nearby caves store the dates of past earthquakes in stone.
    Dr. Panno and his team found their recording angel in the form of stalagmites, the conical projections that grow on the floors of limestone caves. They made their discovery when using a radioactive dating technique to check the ages of small stalagmites occupying a number of caves in the Midwest. Many of these stalagmites had come into existence at about the same moment, and that moment coincided with the New Madrid earthquake.
    This makes sense. Stalagmites form when water trickles through crevices in a cave’s ceiling and drips to the floor. Each drop carries with it a quantity of dissolved calcium carbonate that it has picked up while flowing through the rock above. When a drop lands, some of this mineral is deposited at the landing site, where it accumulates, forming a stalagmite. A paper to be presented at a meeting of the Geological Society of America in Houston on October 5th by Dr. Panno’s colleague Keith Hackley suggests that when large earthquakes shake the ground, new cracks in cave ceilings open. The result is the formation of a new generation of stalagmites.
    Like trees, stalagmites are often composed of concentric layers that represent annual growth periods. Counting the layers is one way of assessing how old a stalagmite is. But radioactive dating provides a second, and sometimes more accurate, assessment.
    In this case the geologists drilled into the stalagmites and estimated their age from the way that uranium decays into an isotope of thorium. Many, they found, dated back to 1811, while others began life in 1917, the date of another nearby earthquake.
    Subsequent investigation has confirmed a further seven big earthquakes previously suspected to have happened over the course of the past 18, 000 years. An average interval between quakes of 2,500 years is a hopeful sign for New Madrid’s immediate future. But if the technique can be tried out in other places it might reveal areas now thought safe, precisely because there has not been a recent earthquake, that are actually under threat.
Which of the following is true according to para. 5?
选项 A、Stalagmites are often composed of concentric tree layers that represent annual growth periods.
B、Counting the layers is the only way of assessing how old a stalagmite is up to now.
C、Radioactive dating is a more accurate assessment concerning the age of stalagmite.
D、With radioactive dating, age of stalagmite comes from the relation between uranium and thorium.
答案D
解析 事实细节题。根据题干提示定位至第五段。本段第一句提到“像树木一样，石笋是由很多每年长一层的同中心的层构成”，但这并不等于石笋是由树木的轮构成，所以[A]不正确；由“放射性年代测定技术是第二种”，并非唯一的方法，[B]不正确；原文提到“放射性年代测定技术有时甚至是更准确的评估方法”，[C]模糊了“有时”这一概念，说法绝对，故排除；[D]与原文“通过测量石笋所包含的铀和钍同位素之间的关系估算石笋的年龄”相符，是正确答案。
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