石油地质研究生专业英语学习第1课地质学基础.docx

石油地质研究生专业英语学习第1课地质学基础.docx

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石油地质研究生专业英语学习第1课地质学基础

SectionA

EARTHINSPACEANDTIME

TheEarlySolarSystem

Inrecentdecades,scientistshavebeenabletoconstructanever-clearerpictureoftheoriginsofthesolarsystemand,beforethat,oftheuniverseitself.Mostastronomersnowacceptsomesortof"BigBang"astheoriginoftoday'suniverse.Atthattime,enormousquantitiesofmatterwerecreatedandflungviolentlyapartacrossanever-largervolumeofspace.ThetimeoftheBigBangcanbeestimatedinseveralways.Perhapsthemostdirectistheback-calculationoftheuniverse'sexpansiontoitsapparentbeginning.Othermethodsdependonastrophysicalmodelsofcreationoftheelementsortherateofevolutionofdifferenttypesofstars.Mostageestimatesoverlapintherangeof15to20billionyears,althoughafewresearcherssuggestanagecloserto10billionyears.

StarsformedfromthedebrisoftheBigBang,aslocallyhighconcentrationsofmasswerecollectedtogetherbygravity,andsomebecamelargeanddenseenoughthatenergy-releasingatomicreactionsweresetoffdeepwithinthem.Starsarenotpermanentobjects.Theyareconstantlylosingenergyandmassastheyburntheirnuclearfuel.Themassofmaterialthatinitiallyformedthestardetermineshowrapidlythestarburns;somestarsburnedoutbillionsofyearsago,whileothersareprobablyformingnowfromtheoriginalmatteroftheuniversemixedwiththedebrisofolderstars.

Oursunanditssystemofcirclingplanets,includingtheearth,arebelievedtohaveformedfromarotatingcloudofgasanddust.Mostofthemassofthecloudcoalescedtoformthesun,whichbecameastarandbeganto"shine,"orreleaselightenergy,whenitsinteriorbecamesodenseandhotfromthecrushingeffectsofitsowngravitythatnuclearreactionsweretriggeredinsideit.Meanwhile,dustcondensedfromthegasesremainingintheflattenedclouddiskrotatingaroundtheyoungsun.Thedustclumpedintoplanets,theformationofwhichwasessentiallycompleteover41/2billionyearsago.

ThePlanets

Thecompositionsoftheplanetsformeddependedlargelyonhowneartheyweretothehotsun.Theplanetsformednearesttothesuncontainedmainlymetallicironandafewmineralswithveryhighmeltingtemperatures,withlittlewaterorgas.Somewhatfartherout,wheretemperatureswerelower,thedevelopingplanetsincorporatedmuchlargeramountsoflower-temperatureminerals,includingsomethatcontainwaterlockedwithintheircrystalstructures.（Thisdevelopmentlatermadeitpossiblefortheearthtohaveliquidwateratitssurface.）Stillfartherfromthesun,temperaturesweresolowthatnearlyallofthematerialsintheoriginalgascloudcondensed—evenmaterialslikemethaneandammonia,whicharegasesatnormalearthsurfacetemperaturesandpressures.

Theresultwasaseriesofplanetswithavarietyofcompositions,mostquitedifferentfromthatofearth.Thisisconfirmedbyobservationsandmeasurementsoftheplanets.Forexample,theplanetarydensitieslistedintable1.1areconsistentwithahighermetalandrockcontentinthefourplanetsclosesttothesunandamuchlargerproportionoficeandgasintheplanetsfartherfromthesun.Thesedifferencesshouldbekeptinmindwhenitisproposedthatotherplanetscouldbeminedforneededminerals.Boththebasicchemistryoftheseotherbodiesandthekindsofore-formingorotherresource-formingprocessesthatmightoccuronthemwoulddifferconsiderablyfromthoseonearth,andmaynotleadtoproductswewouldfinduseful.（Thisisleavingasideanyquestionsoftheeconomicsortechnicalpracticabilityofsuchminingactivities!

）Inaddition,ourprincipalcurrentenergysourcesrequiredlivingorganismstoform,andsofar,nosuchlife-formshavebeenfoundonotherplanetsormoons.Venus—closetoEarthinspace,similarinsizeanddensity—showsmarkeddifferences:

Itsdense,cloudyatmosphereisthickwithcarbondioxide,producingplanetarysurfacetemperatureshotenoughtomeltleadthroughrunawaygreenhouse-effectheating.Marswouldlikewisebeinhospitable,aswillbeseenlaterinthechapter.

Earth,ThenandNow

Theearthhaschangedcontinuouslysinceitsformation,undergoingsomeparticularlyprofoundchangesinitsearlyhistory.Theearlyearthwasverydifferentfromwhatitistoday,lackingthemodernoceansandatmosphereandhavingamuchdifferentsurfacefromitspresentone,probablymorecloselyresemblingthebarren,crateredsurfaceofthemoon.Theplanetwasheatedbytheimpactofthecollidingdustparticlesandmeteoritesastheycametogethertoformtheearth,andbytheenergyreleasefromdecayofthesmallamountsofseveralnaturallyradioactiveelementsthattheearthcontains.

Theseheatsourcescombinedtoraisetheearth'sinternaltemperatureenoughthatpartsofit,perhapseventuallymostofit,melted,althoughitwasprobablynevermoltenallatonce.Densematerials,likemetalliciron,wouldhavetendedtosinktowardthemiddleoftheearth.Ascoolingprogressed,lighter,low-densitymineralscrystallizedandfloatedouttowardthesurface.Theeventualresultwasanearthdifferentiatedintoseveralmajorcompositionalzones:

thecentralcore;thesurroundingmantle;andathincrustatthesurface.Theprocesswascompletebefore4billionyearsago.

Althoughonlythecrustandafewbitsofuppermostmantlethatarecarriedupintothecrustbyvolcanicactivitycanbesampledandanalyzeddirectly,weneverthelesshaveagooddealofinformationonthecompositionoftheearth'sinterior.First,scientistscanestimatefromanalysesofstarsthestartingcompositionofthecloudfromwhichthesolarsystemformed.Geologistscanalsoinferaspectsoftheearth'sbulkcompositionfromanalysesofcertainmeteoritesbelievedtohaveformedatthesametimeas,andunderconditionssimilartotheearth.Geophysicaldatademonstratethattheearth'sinterioriszonedandalsoprovideinformationonthedensitiesofthedifferentlayerswithintheearth,whichfurtherlimits,theirpossiblecompositions.Theseandotherkindsofdataindicatethattheearth'scoreismadeupmostlyofiron,withsomenickelandafewminorelements,andthatthemantleconsistsmainlyofiron,magnesium,silicon,andoxygencombinedinvaryingproportionsinseveraldifferentminerals.Theearth'scrustismuchmorevariedincompositionandverydifferentchemicallyfromtheaveragecompositionoftheearth.Asisevidentfromthattable,manyofthemetalswehavecometoprizeasresourcesarerelativelyuncommonelementsinthecrust.

Theheatingandsubsequentdifferentiationoftheearlyearthledtoanotherimportantresult:

formationoftheatmosphereandoceans.Manymineralsthathadcontainedwaterorgasesintheircrystalsreleasedthemduringtheheatingandmelting,andastheearth'ssurfacecooled,thewatercouldcondensetoformtheoceans.Withoutthisabundantsurfacewater,whichinthesolarsystemisuniquetoearth,mostlifeasweknowitcouldnotexist.Theoceansfilledbasins,whilethecontinents,buoyantbecauseoftheirlower-densityrocksandminerals,stoodabovetheseasurface.Atfirst,thecontinentswerebarrenoflife.

Theearth'searlyatmospherewasquitedifferentfromthemodernone,asidefromtheeffectsofmodempollution.Thefirstatmospherehadlittleornofreeoxygeninit.Itprobablyconsisteddominantlyofnitrogenandcarbondioxide（whichisthegasmostcommonlyreleasedfromvolcanoes,asidefromwater）withminoramountsofsuchgasesasmethane,ammonia,andvarioussulfurgases.Humanscouldnothavesurvivedinthisearlyatmosphere.Oxygen-breathinglifeofanykindcouldnotexistbeforethesingle-celledblue-greenalgaeappearedinlargenumberstomodifytheatmosphere.Theirremainsarefoundinrocksasmuchasseveralbillionyearsold.Theymanufacturefoodbyphotosynthesis,usingsunlightforenergy,consumingcarbondioxide,andreleasingoxygenasaby-product.Intime,enoughoxygenaccumulatedthattheatmospherecouldsupportoxygen-breathingorganisms.

LifeonEarth

Therockrecordshowswhendifferentplantandanimalgroupsappeared.Somearerepresentedschematicallyinfigure1.4.Theearliestcreaturesleftveryfewremainsbecausetheyhadnohardskeletons,teeth,shells,orotherhardpartsthatcouldbepreservedinrocks.Thefirstmulticelledoxygen-breathingcreaturesprobablydevelopedabout1billionyearsago,afteroxygenintheatmospherewaswellestablished.Byabout600millionyearsago,marineanimalswithshellshadbecomewidespread.

Thedevelopmentoforganismswithhardparts—shells,bones,teeth,andsoon—greatlyincreasedthenumberofpreservedanimalremainsintherockrecord;consequently,biologicaldevelopmentssincethattimearefarbetterunderstood.Drylandwasstillbarrenoflargeplantsoranimalshalfabillionyearsago.Inrocksabout500millionyearsoldisthefirstevidenceofanimalswithbackbones—thefish—andsoonthereafter,earlylandplantsdeveloped,before400millionyearsago.Insectsappearedapproximately300millionyearsago.Later,reptilesandamphibiansmovedontothecontinents.Thedinosaursappearedabout200millionyearsagoandthefirstmammalsatnearlythesametime.Warm-bloodedanimalstooktotheairwiththedevelopmentofbirdsabout150millionyearsago,andby100millionyearsago,bothbirdsandmammalswerewellestablished.

Suchinformationhascurrentapplications.Certainenergysourceshavebeenformedfromplantoranimalremains（seechapter13）.Knowingthetimesatwhichparticulargroupsoforganismsappearedandflourishedishelpfulinassessingtheprobableamountsoftheseenergysourcesavailableandinconcentratingthesearchforthesef