专八阅读练习之Exploration on the Origin of Continents
<p> <span word="The">The</span> <span word="origin">origin</span> <span word="of">of</span> <span word="continental">continental</span> <span word="nuclei">nuclei</span> <span word="has">has</span> <span word="long">long</span> <span word="been">been</span> <span word="a">a</span> <span word="puzzle">puzzle</span>. <span word="Theories">Theories</span> <span word="advanced">advanced</span> <span word="so">so</span> <span word="far">far</span> <span word="have">have</span> <span word="generally">generally</span> <span word="failed">failed</span> <span word="to">to</span> <span word="explain">explain</span> <span word="the">the</span> <span word="first">first</span> <span word="step">step</span> <span word="in">in</span> <span word="continent">continent</span> <span word="growth">growth</span>， <span word="or">or</span> <span word="have">have</span> <span word="been">been</span> <span word="subject">subject</span> <span word="to">to</span> <span word="serious">serious</span> <span word="objections">objections</span>. <span word="It">It</span> <span word="is">is</span> <span word="the">the</span> <span word="purpose">purpose</span> <span word="of">of</span> <span word="this">this</span> <span word="article">article</span> <span word="to">to</span> <span word="examine">examine</span> <span word="the">the</span> <span word="possible">possible</span> <span word="role">role</span> <span word="of">of</span> <span word="the">the</span> <span word="impact">impact</span> <span word="of">of</span> <span word="large">large</span> <span word="meteorites">meteorites</span> <span word="or">or</span> <span word="asteroids">asteroids</span> <span word="in">in</span> <span word="the">the</span> <span word="production">production</span> <span word="of">of</span> <span word="continental">continental</span> <span word="nuclei">nuclei</span>. <span word="Unfortunately">Unfortunately</span>， <span word="the">the</span> <span word="geological">geological</span> <span word="evolution">evolution</span> <span word="of">of</span> <span word="the">the</span> <span word="Earths">Earths</span> <span word="surface">surface</span> <span word="has">has</span> <span word="had">had</span> <span word="an">an</span> <span word="obliterating">obliterating</span> <span word="effect">effect</span> <span word="on">on</span> <span word="the">the</span> <span word="original">original</span> <span word="composition">composition</span> <span word="and">and</span> <span word="structure">structure</span> <span word="of">of</span> <span word="the">the</span> <span word="continents">continents</span> <span word="to">to</span> <span word="such">such</span> <span word="an">an</span> <span word="extent">extent</span> <span word="that">that</span> <span word="further">further</span> <span word="terrestrial">terrestrial</span> <span word="investigations">investigations</span> <span word="have">have</span> <span word="small">small</span> <span word="chance">chance</span> <span word="of">of</span> <span word="arriving">arriving</span> <span word="at">at</span> <span word="an">an</span> <span word="unambiguous">unambiguous</span> <span word="answer">answer</span> <span word="to">to</span> <span word="the">the</span> <span word="question">question</span> <span word="of">of</span> <span word="continental">continental</span> <span word="origin">origin</span>. <span word="Paradoxically">Paradoxically</span>， <span word="clues">clues</span> <span word="to">to</span> <span word="the">the</span> <span word="origin">origin</span> <span word="and">and</span> <span word="early">early</span> <span word="history">history</span> <span word="of">of</span> <span word="the">the</span> <span word="surface">surface</span> <span word="features">features</span> <span word="of">of</span> <span word="the">the</span> <span word="Earth">Earth</span> <span word="may">may</span> <span word="be">be</span> <span word="found">found</span> <span word="on">on</span> <span word="the">the</span> <span word="Moon">Moon</span> <span word="and">and</span> <span word="planets">planets</span>， <span word="rather">rather</span> <span word="than">than</span> <span word="on">on</span> <span word="the">the</span> <span word="Earth">Earth</span>， <span word="because">because</span> <span word="some">some</span> <span word="of">of</span> <span word="these">these</span> <span word="bodies">bodies</span> <span word="appear">appear</span> <span word="to">to</span> <span word="have">have</span> <span word="had">had</span> <span word="a">a</span> <span word="much">much</span> <span word="less">less</span> <span word="active">active</span> <span word="geological">geological</span> <span word="history">history</span>. <span word="As">As</span> <span word="a">a</span> <span word="result">result</span>， <span word="relatively">relatively</span> <span word="primitive">primitive</span> <span word="surface">surface</span> <span word="features">features</span> <span word="are">are</span> <span word="preserved">preserved</span> <span word="for">for</span> <span word="study">study</span> <span word="and">and</span> <span word="analysis">analysis</span>. <span word="In">In</span> <span word="the">the</span> <span word="case">case</span> <span word="of">of</span> <span word="both">both</span> <span word="the">the</span> <span word="Moon">Moon</span> <span word="and">and</span> <span word="Mars">Mars</span>， <span word="it">it</span> <span word="is">is</span> <span word="generally">generally</span> <span word="concluded">concluded</span> <span word="from">from</span> <span word="the">the</span> <span word="appearance">appearance</span> <span word="of">of</span> <span word="their">their</span> <span word="heavily">heavily</span> <span word="cratered">cratered</span> <span word="surfaces">surfaces</span> <span word="that">that</span> <span word="they">they</span> <span word="have">have</span> <span word="been">been</span> <span word="subjected">subjected</span> <span word="to">to</span> <span word="bombardment">bombardment</span> <span word="by">by</span> <span word="large">large</span> <span word="meteoroids">meteoroids</span> <span word="during">during</span> <span word="their">their</span> <span word="geological">geological</span> <span word="history">history</span>. <span word="Likewise">Likewise</span>， <span word="it">it</span> <span word="would">would</span> <span word="appear">appear</span> <span word="a">a</span> <span word="reasonable">reasonable</span> <span word="hypothesis">hypothesis</span> <span word="that">that</span> <span word="the">the</span> <span word="Earth">Earth</span> <span word="has">has</span> <span word="also">also</span> <span word="been">been</span> <span word="subjected">subjected</span> <span word="to">to</span> <span word="meteoroid">meteoroid</span> <span word="bombardment">bombardment</span> <span word="in">in</span> <span word="the">the</span> <span word="past">past</span>， <span word="and">and</span> <span word="that">that</span> <span word="very">very</span> <span word="large">large</span> <span word="bodies">bodies</span> <span word="struck">struck</span> <span word="the">the</span> <span word="Earth">Earth</span> <span word="early">early</span> <span word="in">in</span> <span word="its">its</span> <span word="geological">geological</span> <span word="history">history</span>.</p><p> <span word="The">The</span> <span word="large">large</span> <span word="crater">crater</span> <span word="on">on</span> <span word="the">the</span> <span word="Moon">Moon</span> <span word="listed">listed</span> <span word="by">by</span> <span word="Baldwin">Baldwin</span> <span word="has">has</span> <span word="a">a</span> <span word="diameter">diameter</span> <span word="of">of</span> 285 <span word="km">km</span>. <span word="However">However</span>， <span word="if">if</span> <span word="we">we</span> <span word="accept">accept</span> <span word="the">the</span> <span word="hypotheses">hypotheses</span> <span word="of">of</span> <span word="formation">formation</span> <span word="of">of</span> <span word="some">some</span> <span word="of">of</span> <span word="the">the</span> <span word="mare">mare</span> <span word="basins">basins</span> <span word="by">by</span> <span word="impact">impact</span>， <span word="the">the</span> <span word="maximum">maximum</span> <span word="lunar">lunar</span> <span word="impact">impact</span> <span word="crater">crater</span> <span word="diameter">diameter</span> <span word="is">is</span> <span word="probably">probably</span> <span word="as">as</span> <span word="large">large</span> <span word="as">as</span> 650<span word="km">km</span>. <span word="Based">Based</span> <span word="on">on</span> <span word="a">a</span> <span word="lunar">lunar</span> <span word="analogy">analogy</span>， <span word="one">one</span> <span word="might">might</span> <span word="expect">expect</span> <span word="several">several</span> <span word="impact">impact</span> <span word="craters">craters</span> <span word="of">of</span> <span word="at">at</span> <span word="least">least</span> 500<span word="km">km</span> <span word="diameter">diameter</span> <span word="to">to</span> <span word="have">have</span> <span word="been">been</span> <span word="formed">formed</span> <span word="on">on</span> <span word="Earth">Earth</span>. <span word="By">By</span> <span word="applying">applying</span> <span word="Baldwins">Baldwins</span> <span word="equation">equation</span>， <span word="the">the</span> <span word="depth">depth</span> <span word="of">of</span> <span word="such">such</span> <span word="a">a</span> <span word="crater">crater</span> <span word="should">should</span> <span word="be">be</span> <span word="about">about</span> 20<span word="km">km</span>. <span word="Baldwin">Baldwin</span> <span word="admits">admits</span> <span word="that">that</span> <span word="his">his</span> <span word="equation">equation</span> <span word="gives">gives</span> <span word="excessive">excessive</span> <span word="depths">depths</span> <span word="for">for</span> <span word="large">large</span> <span word="craters">craters</span> <span word="so">so</span> <span word="that">that</span> <span word="the">the</span> <span word="actual">actual</span> <span word="depth">depth</span> <span word="should">should</span> <span word="be">be</span> <span word="somewhat">somewhat</span> <span word="smaller">smaller</span>. <span word="Based">Based</span> <span word="on">on</span> <span word="the">the</span> <span word="measured">measured</span> <span word="depth">depth</span> <span word="of">of</span> <span word="smaller">smaller</span> <span word="lunar">lunar</span> <span word="crater">crater</span>. <span word="Baldwins">Baldwins</span> <span word="equation">equation</span> <span word="gives">gives</span> <span word="the">the</span> <span word="depth">depth</span> <span word="of">of</span> <span word="the">the</span> <span word="zone">zone</span> <span word="of">of</span> <span word="brecciation">brecciation</span> <span word="for">for</span> <span word="such">such</span> <span word="a">a</span> <span word="crater">crater</span> <span word="as">as</span> <span word="about">about</span> 75<span word="km">km</span>. <span word="The">The</span> <span word="plasticity">plasticity</span> <span word="of">of</span> <span word="the">the</span> <span word="Earths">Earths</span> <span word="mantle">mantle</span> <span word="at">at</span> <span word="the">the</span> <span word="depth">depth</span> <span word="makes">makes</span> <span word="it">it</span> <span word="impossible">impossible</span> <span word="to">to</span> <span word="speak">speak</span> <span word="of">of</span> <span word="bracciation">bracciation</span> <span word="in">in</span> <span word="the">the</span> <span word="usual">usual</span> <span word="sense">sense</span>. <span word="However">However</span>， <span word="local">local</span> <span word="stresses">stresses</span> <span word="may">may</span> <span word="be">be</span> <span word="temporarily">temporarily</span> <span word="sustained">sustained</span> <span word="at">at</span> <span word="that">that</span> <span word="depth">depth</span>， <span word="as">as</span> <span word="shown">shown</span> <span word="by">by</span> <span word="the">the</span> <span word="existence">existence</span> <span word="of">of</span> <span word="deep">deep</span>-<span word="focus">focus</span> <span word="earthquakes">earthquakes</span>. <span word="Thus">Thus</span>， <span word="short">short</span>-<span word="term">term</span> <span word="effects">effects</span> <span word="might">might</span> <span word="be">be</span> <span word="expected">expected</span> <span word="to">to</span> <span word="a">a</span> <span word="depth">depth</span> <span word="of">of</span> <span word="more">more</span> <span word="than">than</span> 50<span word="km">km</span> <span word="in">in</span> <span word="the">the</span> <span word="mantle">mantle</span>.</p>
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