The Earth comprises three principal layers: the dense, iron-rich core, the mantle made of silicate (硅酸鹽) 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(地形學的)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 (爆發(fā)) of lava (火山熔巖) at mid-ocean ridges. If at such a spreading contact the two plates support continents，a rift(裂縫) 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 (起褶皺) to form a mountain chain along the length of the margin of the plates.

　　練習題：

　　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] 事實細節(jié)題。選項A與文章第1段第2句所述一致，為正確答案。此類題可采用排除法。根據(jù)第2段第2句的內(nèi)容可以知道選項B所指的只是地殼中的一種.而不是所有地殼。選項C與文章第1句的內(nèi)容不符。選項D的主語應該是地殼和上層的地幔，不僅僅是地殼。

　　2.[C] 推理判斷題。此題可定位到第1段倒數(shù)第3句。題干中的15 plates是原文該句中的賓語，且該句的謂語中心詞是consist of(由......組成)，題干的謂語是are formed from，這意思一致，但題目用的是被動語態(tài)。表明答案可從原文該句的主語得到。主語中的top和solid并列修飾part of the mantle。選項C為該句的同義替換，故正確。

　　3.[C] 亊實細節(jié)題。選項C是對第1段最后一句的同義改寫。文章中沒有提到選項A中the crusts move over mantle 這種現(xiàn)象;選項B提到的現(xiàn)象在第2段第1句才討論到，與題干無關;文中也沒有提到嚴重變形的地區(qū)是地球的主要地形特征，因此選項D不正確。

　　4.[A] 亊實細節(jié)題。第2段主要分成兩部分：第一部分談板塊活動的三種形式，第二部分分別舉例說明其中在說明spreading contacts時，引用了大西洋的例子："The Atlantic Ocean formed like this..."，所以答案為A。

　　5. [C] 主旨大意題。由本文的語言風格和描述內(nèi)容可以判定這篇文章既不是商業(yè)性的，也不是文學性的同時文中敘述的內(nèi)容具有科普性質(zhì)，不可能是太前沿性的，因此A、B、D三項都不正確，選項C為此題答案。

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