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　　The Earth comprises three principal layers: the dense, iron-rich core, the mantle made of silicate (硅酸盐) that are semi-molten at depth, and the thin, solid-surface crust There are two kinds of crust, a lower and denser oceanic crust and an upper，lighter continental crust found over only about 40 percent of the Earth’s surface. The rocks of the crust are of very different ages. Some continental rocks are over 3,000 million years old, while those of the ocean floor are less than 200 million years old. The crusts and the top, solid part of the mantle, totaling about 70 to 100 kilometers in thickness, at present appear to consist of about 15 rigid plates, 7 of which are very large. These plates move over the semi-molten lower mantleto produce all of the major topographical(地形学的)features of the Earth. Active zones where intense deformation occur are confined to the narrow, interconnecting boundaries of contact of the plates.

　　There are three main types of zones of contact: spreading contacts where plates move apart, converging contacts where plates move towards each other, and transform contacts where plates slide past each other. Newoceanic crust is formed along one or more margins of each plate by material issuing from deeper layers of the Earth’ s crust, for example, by volcanic eruptions (爆发) of lava (火山熔岩) at mid-ocean ridges. If at such a spreading contact the two plates support continents，a rift(裂缝) is formed that will gradually widen and become flooded by the tea. The Atlantic Ocean formed like this as the American and Afro-European plates moved in opposite directions. When two plates carrying continents collide, the continental blocks,too light to be drawn down, continue to float and therefore buckle (起褶皱) to form a mountain chain along the length of the margin of the plates.

　　练习题：

　　Choose correct answers to the question:

　　1.The Earth’s crust______.

　　A.can be classified into two types

　　B.is formed along the margins of the plates

　　C.consists of semi-molten rocks

　　D.is about 70 to 100 kilometers thick

　　2.The 15 plates of the Earth are formed from ___.

　　A.the oceanic crusts and continental crusts

　　B.the crusts and the mantle

　　C.the crusts and the top and solid part of the mantle

　　D.the continental crusts and the solid part of the mantle

　　3.Seriously-deformed zones appear _______

　　A.whenever the crusts move over mantle

　　B.when the plates move towards each other

　　C.in the narrow boundaries where two plates meet

　　D.to be the major topographical feature of the Earth

　　4.According to the second paragraph, the formation of the Atlantic Ocean is the example of_______.

　　A.spreading contacts

　　B.the influence of volcanic eruptions

　　C.converging contacts

　　D.transform contacts

　　5.This passage is probably_______.

　　A.a newspaper advertisement

　　B.a chapter of a novel

　　C.an excerpt from a textbook

　　D.a scientific report of new findings

　　1.[A] 事实细节题。选项A与文章第1段第2句所述一致，为正确答案。此类题可采用排除法。根据第2段第2句的内容可以知道选项B所指的只是地壳中的一种.而不是所有地壳。选項C与文章第1句的内容不符。选項D的主语应该是地壳和上层的地幔，不仅仅是地壳。

　　2.[C] 推理判断题。此题可定位到第1段倒数第3句。题干中的15 plates是原文该句中的宾语，且该句的谓语中心词是consist of(由......组成)，题干的谓语是are formed from，这意思一致，但题目用的是被动语态。表明答案可从原文该句的主语得到。主语中的top和solid并列修饰part of the mantle。选项C为该句的同义替换，故正确。

　　3.[C] 亊实细节题。选项C是对第1段最后一句的同义改写。文章中没有提到选项A中the crusts move over mantle 这种现象;选项B提到的现象在第2段第1句才讨论到，与题干无关;文中也没有提到严重变形的地区是地球的主要地形特征，因此选项D不正确。

　　4.[A] 亊实细节题。第2段主要分成两部分：第一部分谈板块活动的三种形式，第二部分分别举例说明其中在说明spreading contacts时，引用了大西洋的例子："The Atlantic Ocean formed like this..."，所以答案为A。

　　5. [C] 主旨大意题。由本文的语言风格和描述内容可以判定这篇文章既不是商业性的，也不是文学性的同时文中叙述的内容具有科普性质，不可能是太前沿性的，因此A、B、D三项都不正确，选项C为此题答案。