A Short History of Nearly Everything-第45章

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just　the　rightspeed　and　cool　down　with　the　necessary　swiftness　to　bee　a　diamond。　it　was　such　a　pipethat　made　johannesburg　the　most　productive　diamond　mining　city　in　the　world；　but　there　maybe　others　even　bigger　that　we　don’t　know　about。　geologists　know　that　somewhere　in　thevicinity　of　northeastern　indiana　there　is　evidence　of　a　pipe　or　group　of　pipes　that　may　be　trulycolossal。　diamonds　up　to　twenty　carats　or　more　have　been　found　at　scattered　sites　throughoutthe　region。　but　no　one　has　ever　found　the　source。　as　john　mcphee　notes；　it　may　be　buriedunder　glacially　deposited　soil；　like　the　manson　crater　in　iowa；　or　under　the　great　lakes。

so　how　much　do　we　know　about　what’s　inside　the　earth？　very　little。　scientists　aregenerally　agreed　that　the　world　beneath　us　is　posed　of　four　layers—rocky　outer　crust；　amantle　of　hot；　viscous　rock；　a　liquid　outer　core；　and　a　solid　inner　core。

1we　know　that　thesurface　is　dominated　by　silicates；　which　are　relatively　light　and　not　heavy　enough　to　accountfor　the　planet’s　overall　density。　therefore　there　must　be　heavier　stuff　inside。　we　know　that　togenerate　our　magnetic　field　somewhere　in　the　interior　there　must　be　a　concentrated　belt　ofmetallic　elements　in　a　liquid　state。　that　much　is　universally　agreed　upon。　almost　everythingbeyond　that—how　the　layers　interact；　what　causes　them　to　behave　in　the　way　they　do；　whatthey　will　do　at　any　time　in　the　future—is　a　matter　of　at　least　some　uncertainty；　and　generallyquite　a　lot　of　uncertainty。

even　the　one　part　of　it　we　can　see；　the　crust；　is　a　matter　of　some　fairly　strident　debate。

nearly　all　geology　texts　tell　you　that　continental　crust　is　three　to　six　miles　thick　under　theoceans；　about　twenty…five　miles　thick　under　the　continents；　and　forty　to　sixty　miles　thickunder　big　mountain　chains；　but　there　are　many　puzzling　variabilities　within　thesegeneralizations。　the　crust　beneath　the　sierra　nevada　mountains；　for　instance；　is　only　aboutnineteen　to　twenty…five　miles　thick；　and　no　one　knows　why。　by　all　the　laws　of　geophysics　thesierra　nevadas　should　be　sinking；　as　if　into　quicksand。　（some　people　think　they　may　be。）1for　those　who　crave　a　more　detailed　picture　of　the　earths　interior；　here　are　the　dimensions　of　the　variouslayers；　using　average　figures：　from　0　to　40　km　（25　mi）　is　the　crust。　from　40　to　400　km　（25　to　250　mi）　is　theupper　mantle。　from　400　to　650　km　（250　to　400　mi）　is　a　transition　zone　between　the　upper　and　lower　mantle。

from　650　to　2；700　km　（400　to　1；700　mi）　is　the　lower　mantle。　from　2；700　to　2；890　km　（1；700　to　1；900　mi）　is　the〃d〃　layer。　from　2；890　to　5；150　km　（1；900　to　3；200　mi）　is　the　outer　core；　and　from　5；150　to　6；378　km　（3；200　to3；967　mi）　is　the　inner　core。

how　and　when　the　earth　got　its　crust　are　questions　that　divide　geologists　into　two　broadcamps—those　who　think　it　happened　abruptly　early　in　the　earth’s　history　and　those　who　thinkit　happened　gradually　and　rather　later。　strength　of　feeling　runs　deep　on　such　matters。　richardarmstrong　of　yale　proposed　an　early…burst　theory　in　the　1960s；　then　spent　the　rest　of　hiscareer　fighting　those　who　did　not　agree　with　him。　he　died　of　cancer　in　1991；　but　shortlybefore　his　death　he　“lashed　out　at　his　critics　in　a　polemic　in　an　australian　earth　science　journalthat　charged　them　with　perpetuating　myths；”　according　to　a　report　inearth　magazine　in　1998。

“he　died　a　bitter　man；”　reported　a　colleague。

the　crust　and　part　of　the　outer　mantle　together　are　called　the　lithosphere　（from　the　greeklithos；　meaning　“stone”）；　which　in　turn　floats　on　top　of　a　layer　of　softer　rock　called　theasthenosphere　（from　greek　words　meaning　“without　strength”）；　but　such　terms　are　neverentirely　satisfactory。　to　say　that　the　lithosphere　floats　on　top　of　the　asthenosphere　suggests　adegree　of　easy　buoyancy　that　isn’t　quite　right。　similarly　it　is　misleading　to　think　of　the　rocksas　flowing　in　anything　like　the　way　we　think　of　materials　flowing　on　the　surface。　the　rocksare　viscous；　but　only　in　the　same　way　that　glass　is。　it　may　not　look　it；　but　all　the　glass　on　earthis　flowing　downward　under　the　relentless　drag　of　gravity。　remove　a　pane　of　really　old　glassfrom　the　window　of　a　european　cathedral　and　it　will　be　noticeably　thicker　at　the　bottom　thanat　the　top。　that　is　the　sort　of　“flow”　we　are　talking　about。　the　hour　hand　on　a　clock　movesabout　ten　thousand　times　faster　than　the　“flowing”　rocks　of　the　mantle。

the　movements　occur　not　just　laterally　as　the　earth’s　plates　move　across　the　surface；　but　upand　down　as　well；　as　rocks　rise　and　fall　under　the　churning　process　known　as　convection。

convection　as　a　process　was　first　deduced　by　the　eccentric　count　von　rumford　at　the　end　ofthe　eighteenth　century。　sixty　years　later　an　english　vicar　named　osmond　fisher　prescientlysuggested　that　the　earth’s　interior　might　well　be　fluid　enough　for　the　contents　to　move　about；but　that　idea　took　a　very　long　time　to　gain　support。

in　about　1970；　when　geophysicists　realized　just　how　much　turmoil　was　going　on　downthere；　it　came　as　a　considerable　shock。　as　shawna　vogel　put　it　in　the　book　naked　earth：　thenew　geophysics：　“it　was　as　if　scientists　had　spent　decades　figuring　out　the　layers　of　theearth’s　atmosphere—troposphere；　stratosphere；　and　so　forth—and　then　had　suddenly　foundout　about　wind。”

how　deep　the　convection　process　goes　has　been　a　matter　of　controversy　ever　since。　somesay　it　begins　four　hundred　miles　down；　others　two　thousand　miles　below　us。　the　problem；　asdonald　trefil　has　observed；　is　that　“there　are　two　sets　of　data；　from　two　different　disciplines；that　cannot　be　reconciled。”　geochemists　say　that　certain　elements　on　earth’s　surface　cannothave　e　from　the　upper　mantle；　but　must　have　e　from　deeper　within　the　earth。

therefore　the　materials　in　the　upper　and　lower　mantle　must　at　least　occasionally　mix。

seismologists　insist　that　there　is　no　evidence　to　support　such　a　thesis。

so　all　that　can　be　said　is　that　at　some　slightly　indeterminate　point　as　we　head　toward　thecenter　of　earth　we　leave　the　asthenosphere　and　plunge　into　pure　mantle。　considering　that　itaccounts　for　82　percent　of　the　earth’s　volume　and　65　percent　of　its　mass；　the　mantle　doesn’tattract　a　great　deal　of　attention；　largely　because　the　things　that　interest　earth　scientists　andgeneral　readers　alike　happen　either　deeper　down　（as　with　magnetism）　or　nearer　the　surface　（aswith　earthquakes）。　we　know　that　to　a　depth　of　about　a　hundred　miles　the　mantle　consistspredominantly　of　a　type　of　rock　known　as　peridotite；　but　what　fills　the　space　beyond　isuncertain。　according　to　a　nature　report；　it　seems　not　to　be　peridotite。　more　than　this　we　donot　know。

beneath　the　mantle　are　the　two　cores—a　solid　inner　core　and　a　liquid　outer　one。　needless　tosay；　our　understanding　of　the　nature　of　these　cores　is　indirect；　but　scientists　can　make　somereasonable　assumptions。　they　know　that　the　pressures　at　the　center　of　the　earth　aresufficiently　high—something　over　three　million　times　those　found　at　the　surface—to　turn　anyrock　there　solid。　they　also　know　from　earth’s　history　（among　other　clues）　that　the　inner　coreis　very　good　at　retaining　its　heat。　although　it　is　little　more　than　a　guess；　it　is　thought　that　inover　four　billion　years　the　temperature　at　the　core　has　fallen　by　no　more　than　200°f。　no　oneknows　exactly　how　hot　the　earth’s　core　is；　but　estimates　range　from　something　over　7；000°fto　13；000°f—about　as　hot　as　the　surface　of　the　sun。

the　outer　core　is　in　many　ways　even　less　well　understood；　though　everyone　is　in　agreementthat　it　is　fluid　and　that　it　is　the　seat　of　magnetism。　the　theory　was　put　forward　by　e。　c。

bullard　of　cambridge　university　in　1949　that　this　fluid　part　of　the　earth’s　core　revolves　in　away　that　makes　it；　in　effect；　an　electrical　motor；　creating　the　earth’s　magnetic　field。　theassumption　is　that　the　convecting　fluids　in　the　earth　act　somehow　like　the　currents　in　wires。

exactly　what　happens　isn’t　known；　but　it　is　felt　pretty　certain　that　it　is　connected　with　the　corespinning　and　with　its　being　liquid。　bodies　that　don’t　have　a　liquid　core—the　moon　and　mars；for　instance—don’t　have　magnetism。

we　know　that　earth’s　magnetic　field　changes　in　power　from　time　to　time：　during　the　age　ofthe　dinosaurs；　it　was　up　to　three　times　as　strong　as　now。　we　also　know　that　it　reverses　itselfevery　500；000　years　or　so　on　average；　though　that　average　hides　a　huge　degree　ofunpredictability。　the　last　reversal　was　about　750；000　years　ago。　sometimes　it　stays　put　formillions　of　years—37　million　years　appears　to　be　the　longest　stretch—and　at　other　times　it　hasreversed　after　as　little　as　20；000　years。　altogether　in　the　last　100　million　years　it　has　reverseditself　about　two　hundred　times；　and　we　don’t　have　any　real　idea　why。　it　has　been　called　“thegreatest　unanswered　question　in　the　geological　sciences。”

we　may　be　going　through　a　reversal　now。　the　earth’s　magnetic　field　has　diminished　byperhaps　as　much　as　6　percent　in　the　last　century　alone。　any　diminution　in　magnetism　is　likelyto　be　bad　news；　because　magnetism；　apart　from　holding　notes　to　refrigerators　and　keeping　ourpasses　pointing　the　right　way；　plays　a　vital　role　in　keeping　us　alive。　space　is　full　ofdangerous　cosmic　rays　that　in　the　absence　of　magnetic　protection　would　tear　through　ourbodies；　leaving　much　of　our　dn