EXPANDI NG Ghiselin, Michael T. The Ti,iumph of the Darwinian Meth od. Berkeley: Universiry of California Press,1969. Glen, \7illiam, ed. TheMass-extinction Debates:How ScienceVorhs in a Cisis. Stanford: Stanford University Press,

1994. Gould, StephenJay. Time'sAnow Time's Cycle:Myth and Metaphor in the Discoaery of Geological Time. Cambridge: Harvard University Press, 1987. \umbers, Ronald. The Creationists:The Euolution of Scientif c Creationism. NewYork: AlfredA. Knopf, 1992. Lossi, Paolo. The Dark AbyssofTime: The History of the Earth and the History of Nations from Hooke to Vico. Chicago: Universiryof Chicago Press,1984. Rudwick, Martin J.S. TheMeaning of Fossils:Episodesin the History of Pakeontology.2d ed. Chicago: Universiry of Chicago Press,1976. Scenes fom Deep Time: Early Pictorial Representationsof the Prehistoic World. Chicago: University of Chicago Press,1992. R.use.Michael , The Darwinian Reuolution: ScienceRed in Tboth and Ckw. Chicago: Universiry of Chicago Press,

1979. Seeaho Age of the Earth; Climates, Pleis:oceneand Recent; Climate Change; Deluge; .cology; Foucault's Order of Things; Geog:aphy and Natural Theology; Geography; Geological Periodization; Geological Time; Geolory; Humboldtian Science;MassExtincjons and the Impact-Volcanic Controversy; Paleontology;Plutonists, Neptunists, Vul;anists; Stratigraphy Expandingl Earth Theorics Family of twentieth-century theories basedon :he prernisethat the Earth has expandedover rime from a smaller globe, perhaps aslittle as i5 percent of the Earth's present diameter. First developedin detail during the 1920s and 1930s,expansiontheoriesenjoyed a revival in :he I 950s and 1960sand continue to sustain a modest but persistent researchprogram. -\lthough sometimesmotivated by other concerns, the chiefhistorical significance ofexpansion theories has been their potential to accommodate the evidence for continental displacement and provide at least a partial account of the underlying mechanism. \Tilliam Lowthian Green (d. 1890)

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probably deservescredit as the first serious proponent of Earth expansion, introducing the idea initia.lly 1n 1857.I.O. Yarkovski advocated expansion in 1899 as a consequence of the transformation of weightlessether into matter. Severalmore theoristsproposed some form ofexpansion during the first quarter of the twentieth century, but their work went largely unnoticed. Credit for the first sustained focus on Earth expansiont potential as a mechanisrn for explaining the movement of the continents relative to each other goesto Bernhard Lindemann (b. 1871-?) in 1927 and Otto C. Hilgenberg in 1933, rwo German scienrists working independently in Gtittingen and Berlin, respectively.Unlike most of their contemporaries,both Lindemann and Hilgenberg were completely persuaded by the empirical argumentsfor continental displacement compiled by Alfred Wegener in Die Entstehungder Kontinente und Ozeane Qhe origin ofcontinents and oceans,l9l5). But like most of their contemporaries,they found 'Wegenert account of the possible mechanisms for moving the continents completely implausible . Instead of requiring the continents to plough through the ocean floor, as 'Wegener did, Lindemann and Hilgenberg suggestedthat the continents were driven apart from each other as the ocean floors expanded to accommodate a growing Earth. Hilgenberg deservesspecialmention for three reasons.He was the first to attempt ro fit the continents rogether in a continuous sialicshell, using the continental shelfoudines on a modern dimension globe astemplatesfor papier-m6.chdcontinents that he then transferred to a globe 60 percent the sizeofhis reference globe. Moreover, by conceiving of expansion on such a scalefrom the early Mesozoic, Hilgenberg was also the first to suggesta rapid expansionmodel. And finally, his model of expansionwasradial: the continents should be conceived asconic wedgesanchored in the Earth's core, being driven apart from eachother in all directions by expanding oceanic crust (Hilgenberg, Vom wachsenden Erdball, pp. 2-9,22-29). In all three respecrs Hilgenberg anticipated rapid expansionmodels developedin the 1960sand 1970s. Hilgenberg's notion of a contiguous globe-encircling continental crust also provides an elegant explanation of the Eartht dual hypsometric curve. In this view, as our planet cooled and stabilized, the lighter sialic material rose to the surface and was evenly distributed acrossthe Earth's face. Subse-

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quently, as this continental shell was rifted apart in various placesby expansionaryforces within the Earth, a second layer of crustal material was createdto fill the expanding rifts, but at a much lower elevation than the original continental shell. This two-stage process of crustal development would correspond naturally to the dual mean elevationsof the continental plateausand the ocean floors (see Carey, The Expanding Earth, p.3l). In addition. the sialic shell has been cited, at leastby the advocatesofrapid expansion, to explain the otherwise statisticallyodd fact that all the continents could accreteon one side of the globe to form Pangaea, 'W'egener's super continent. If the Earth was once completely coveredby a sialic shell, then Pangaeadid not accreteat all. The emergence of the Pacific at the beginning of the Mesozoic "created" the other side ofthe globe, and Pangaeawas merely the intact residue of the original sialic shell. The work of the early expansionists,including Hilgenberg's, failed to generate any real interest, and was completely forgotten by the time a secondwave of expansiontheorists reinvented the tradition, and subsequently rediscoveredHilgenberg and his predecessors. This second wave emerged during the late 1950s,matured in the 1960sand 1970s,and hasbeenfighdng an uphill banle againstskeptics in the dominant plate-tectonics research tradition ever since. There are actually three distinct subspecies of modern expansion theories, distinguishable both by the rate ofexpansion and by what they are designed to explain. Slow expansion,the first to emerge,was developed byLazlo Egyed (b. l9l4) ac Eciwrts Universiry in Budapest.In 1956 Egyed beganpublishing a series of papers arguing that paleoclimatic evidencesuggestsgradually diminishing cyclesof marine transgression on continental areasduring Phanerozoictimethat is, a net decreasein sealevel. But Eryed also calculated that the volume of seawater had increasedduring this period by atleast 4 percent. To explain this apparent paradox, he suggestedthat the Earths radius has been undergoing gradual expansionat a constant rate of less than a millimeter a year, which translatesinto lessthan a 3 percent increase in the Earth'sdiameter over the past two hundred million years.He alsoarguedthat expansion would explain the fossil evidence for a seculardecreasein the Earth'srotational speed. Devonian and Carboniferous corals exhibit alternating sequencesof minute ridges and

furrows that are assumedto constitute daii: growth lines (as they do in modern corals This record suggeststhat 350 million yea;' ago the terrestrial year was 400 days lons gradually diminishing to 365 days as th= Earth completed progressivelyfewer revolutions during its annual orbit around the Sur Egyed's arguments for slow expansio:were subsequentlyadvocated by the Englisi-geologistArthurHolmes (1890-1965) in th: final (1965) edition ofhis influential textboo,: Principlesof PhysicalGeologybp.702-70At that time a varianr of slow expansionr. ialso defended by the English geophysicist Kc'b-. Creer, who was influenced not only Egyedt views, but also by the speculation . i physicistsPaul Dirac (1902-1984\and Ror ert Dicke (b. 1916) concerningthe possib:-iry of gradual diminution in g, the value c: the universal gravitational constant. Simil:' speculation motivated Canadian geologisr Tirzo \Wilson (b. 1908) to entertain slow er-pansion briefly during the early 1960s. Creer was also influenced by goodnerof-fit arguments propounded by fast expr-sion advocateS.'WarrenCarey (b. 1911) (*below). In the early 1960s Carey met Crer: while on a lecture tour in England. Obser. ing Creert efforts to reconstruct Pangaeac: a modern dimensions globe, Carey said: "O: you ll never fit the continents tog€ther on : globe ofthe presentsize;youll have to go :: a smaller globe." Careyk remark convince: Creer to try what Hilgenberg had done: :: reconstrucr a sialic shell by fitting the coninents on a smaller globe. But Creer never aisumed that expansion could be used r: explain post-Pangaeacontinental displactment. Unlike Hilgenberg (and Carey), ri'L: thought the rate ofexpansion has been expcnentially increasingover time, so that its mo.-: dramatic geological effects weren't evider-until the breakup of Pangaea,Creer took rh= opposite view, assuming that most of the erpansion took place during the Archeozoi; Creer held that the Earth had achievedabou: 94 percent of its presentdiameter by the eari. Paleozoic, and 96 or 97 percent by the enc ofthe Paleozoic.\Vith only 3 or 4 percenr o: the expansionoccurring during the Mesozoi: and Cenozoic, expansion could be no mor; than a background phenomenon to the di-+ persal ofPangaea. Fast expansion is basedon the idea tha: the Earth was almost entirely covered bv ; sialic shell as recently astwo hundred millior years ago, and that this shell, broken onlt along one hemisphereby a narrow Eo-Pacifi;

EXPANDI NG ocean,constituted Pangaeabefore its decomposition. In this view, the Earth's radius is currently increasing at a rate of 2 to 4 centimetersa yeat but really significant expansion didnt begin until the Jurassicdismantling of Pangaea,when the Earth's diameter was still not much more than 60 percent of its Present ;ize.Exponentiallyincreasingexpansionwas tirtually imperceptible throughout the Precambrian, and still very gradual for most of with the first true oceaniccrust rhe Paleozoic, emergingin an Eo-Pacificrift during the early Permian, 260 to 280 million yearsago. Although Hilgenberg anticipated this riew, the earliest of the second wave of fast expansiontheoristswere the Australian geologst S.\Tarren Carey and the American oceanographerBruce Heezen (1924-1977). Carey .arne to this idea after convening a continenral drift symposium at his home institution, the University ofThsmania, in 1956- He was then by the inability of the conferees 'rruck himself included) to achievea really good fit oF the continents to improve on \(egener's sketchyoutline of Pangaea.All more precise reconstructionson a globe of modern dimensionsleft ineliminable triangular goresofvarying sizesbetweenneighboringcontinental th. most dramatic of which was the -"rser, putative Tethys Ocean seParatingAustralia, lndia, and Antarctica from the Asian cratonic block-6,000 kilometers acrossat the broad end ofthe triangle. (Seethe shaded areasin

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245

anomalies that would not exist but for the misguided attempt to fit the continental "skin' of a smaller Earth on a spheretoo large to accommodateit. An appropriatemetaphor would be the prospect of trying to fit the intact peel ofan orange over a grapefruit. fu information about the constitutionof oceanic crust accumulated in the 1960s and 1970s,the gore argument has beenelaborated further, especially by Hugh Owen, a stratigrapher and cartographer at the British Museum. Owen has pointed out ("Constant Dimensions," pp. 180-182) that there is no sea-floor material old enough to occupy the spacewhere the Triassicgoressupposedlyexist. This lacuna might be explained if there were any evidence of ancient subduction trenchesalong the margins of thesegoresthat might have consumed the missing crust or, alternatively, if there were evidence of compression at those margins, in which casethe missing crust might have been pushed up againstthe adjacent continents. But Owen arguesthat neither of these is the case,that the putative gore seParadngArgentina from Angola, for example (seefigure 1), was passive-margined all the way back to the Tiiasslc. Carey alsoconcluded that expansionwas dictated by geological evidence that the Pacific perimeter had been growing rather than shrinking since the Jurassic.On a constantdimensionsEarth the Mesozoicand Cenozoic growth of the Atlantic can be accommodated only if there is a compensatingdiminution of the Pacific.Carey presentedtheseideasfor the first time in the 1958 published revision of his presentationto the 1956 syrnposium"The Figure 1 Standard reconstruction of Pangaea about 180 million years ago,assumingan Earth of moderndirnensions Oceaniccrustrequiredto be Presentin tlte reconstruc' tion, but ofuhich no eaidenceexists(Paleozoic Arctic and 7lthys oceans, and the *iangular gaPs beweenAfica and South America, Africa, and Antarctica), is shaded. Frorn Ouen, 1981,P 184, courtesl of The Natural HistoryMuseum,London

,r-ffi#'*BtLt'

[:] ?r'u^"t-"

246 Figure2 Expansionist of Pangaea reconstruction about 180 million Tears ago,asuming an Earth uith a diameter80 Percent ofmodern ua/ue Shaded oceaniccrustPresentln no longer reconstruction exists,but could hauebeen theoreticalfu subductedduring the interueningperiod at the tenchesinsertedoffthe an contin eilt aI : helf Pangae (ofuhich there is still euidencein thePresent distribution of subduction trenches)From Owen, of 1981,P 185,courtesy The Natural History Museum,London

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liri,l Leg€nd.....: iI- t - l0O0 rn ,Ft6rb md l Y l m d s n c *r t l h e Mrjor @r taut L " - 'z o n 6 -

TectonicApproach to Continental Drift" (pp' 311-319). Sincethat time he haslectured and written extensivelyon the subject, and argu\r)Tegener of ably deservescredit as the Alfred tradition. this Carey'sversion of fast radial exPansion is just one ofseveralthat havebeendeveloped in the secondwave of expansiontheories.It is not really even a pure version of radial expansion,for Carey is willing to countenance rotation ofcontinental blocks (although not subducrion;seeTheExpandingEarth,PP'5+ 79,390). A purer versionof the theory,closer to Hilgenberg's initial concePtion of radial expansion, has been developed by Klaus Vogel.a reriredconcreteengineerand ama'Werdau, teur geologist living in the town of in southeasternGermany. Like Hilgenberg, but initially unawareof his predecessortwork, Vogel began by experimenting with the task of reconstructing Pangaeaon small-earth the globesin the early1970s.To demonsrrate nature of radial expansion,Vogel came up with the idea of inserting his mesozoicglobe reconstructioninsidea transParentmodern dimensionsglobe.Unlike Carey'smodel, the relativepositions that Vogel'sTiiassiccontinents bear to one another are basicallythe sameasthe relativepositions of the continents today (with the exception, of course, of the distancebetweenthem). Bruce Heezen came to embrace fast Earth expansionas a result of his familiariry with the newiy emerging data about the existenceof a worldwide belt of oceanicridges. 'Working at the Lamont GeologicalObserva-

rl:;;l l:;:;r;l

httu JumsK @nrc (run rr end sffidlt@ ,n rhc pac.rK

ffi:g#;ml*" tory of Columbia Universiry Heezendiscc eredin 1953 that a deeprift valleyruns do'" the center of the mid-Atlantic ridge, and :.. recognizedsubsequentlythat other oce. rift valleys,and th. floor ridgesalso possess most seismicact.iviryon the oceanfloor a:' pearsto be restrictedto the immediatev.icr'.ity of these rifts. As data from ocean flo-: explorationaccumulatedin the late 1950s also becameapparenrthat the entire oce. floor is relativelyyoung, with no core samp-: predating the Jurassic,and that the age oceanic crust increaseswith distance fro oceanicridge lines. Basedon this informatic Heezenreasonedthat the oceanicridges .:' zones of crustal creation, with new crL:emerging from the rift valleys and thspreadingout acrossthe ocean floors. On t:- ' assumptionthat all the oceanbasinsmLlst:: growlng to accommodatethis new materiHeezen argued verbally for fast expansion 1958 and in print by 1960. Although Heezenintroduceda novel s. of reasonsfor accepting expansion,his a:' count was neveraselaborateasCarey's.Mor.' over, he gaveup the idea in the late 196[, having becomeconvinced that Harry Hes, 1960 model of sea-floorspreadingwas bascally correct, whereby crustal growth at tl, oceanicridge lines might be offsetby crustdestructionin subductiontrenches,primarr' around the Pacific margin. Nonetheles. Heezen'stemporary advocacyof expansic: was historically significant because,in rLNorth American geosclencecommunlty, i': had a higher professionalprofile than Care

EXPANDI NG Heezen'ssympathy for the view gavefast expansionenoughvisibiliry to encouragescientists to take note of Carey'smore detailed account as well. Between Heezent prestige and Carey'senergyfor proselytization,fast expansion became, for a few years in the early 1960s, a serious (although never popular) candidatefor explaining the mounting evidencefor continental disp.lacement. Unlike Heezen, Carey has never acceptedthe subduction hlpothesis, and consequentlyrejectsall plate tectonic accountsof continental disolacement.Subduction is indeed incompaiible with radial expansion models of the sort advocated by Carey, Hilgenberg,andVogel. If continental blocks sit atop wedges of material being simultaneouslydriven apart from eachother by newly emergent oceanic crust, then they can never meet again once oceanic rifts have separated them. If, on the other hand, subduction has occurred,then continencalblocks may have repeatedlymet, separated,and met again throughout the Phanerozoic,as subduction trenchesgraduallyclosedancient oceansand brought the bordering continents into collision to createnew suture zonesofcomoressionalorogenesis. The Himalayanmountain rangesare often cited as a recent example of this phenomenon, the result of the Indian subcontinentkcolliding with Asia during the Paleocene.The Urals and the Appalachians areregardedasPaleozoicinstancesofthe same phenomenon. The Appalachians, for example, arewidely assumedto be the product of a Devonian collision between North America, Europe, and Africa, after a subduction-induced closureofthe IapetusOcean, an Ordovicianproto-Atlantic. Fastexpansionistslike Carey are not especially impressedwith compressionalaccounts o-f orogeny, becausethey can offer alternativeaccountsof Paleozoicorogenic "sutures" as predominantly extensionairather than compressionalphenomena. In Careyt view, the Appalachians and the Urals are the product of diapiric uplift, accompaniedby graviry-driven lateral flow of extruded material ("T ecton ic Ap pro ach, " pp. j22- 338;

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by Indias Cenozoic rotation into its presenr position, around a hinge located ar rhe apex of the Arabian Sea("Tectonic Approach," pp. 'Wegener 264-271) . Nfred alsoassumedthat India was attached to Asia since the Tiiassic. He labeledthe conjoining continental crust the Lemurian zusammenzog(Lemurian compression). Ancient faunaldistriburionoatrernsmav providea greaterchallenge.J. iuro \filson pointed out in 1966 that the suture-zoneaccount of the formation of the Appalachians would also serve to explain the otherwise anomalous distribution of shallow-water Ordovicianmarine fossils.Distinctive soeciesof trilobites from rhat period have been Found to predominateon opposite sidesof the Atlantic. But those found primarily on the North American side ("Pacificprovince" trilobires) are also found in Norway, Scotland, and the northern half of Ireland, while 'Atlantic province" trilobites, found mainly in Europe and Africa, are also found in southern Newfoundland, Nova Scotia,coastalNew England, and southern Florida. lffilson suggestedin "Did the Atlantic Close and rhen Reopen?"that the distribution could easilybe explained if residual bits of proto-North America and proto-Africa/Eurasia, with their previouslyembeddedOrdovician fossils,became attachedto opposite shoresduring the putative Devonian collision, and were chen left behind when the continents broke aoart againduring the Arlanticrifting of Pangaea. Carey and other expansionistshave, however,exhibitedconsiderableingenuity in addressingsuch anomalies.In the caseof the Ordovician trilobite distributions,Carey has argued that the pattern can be explained in terms of separation along the a-xisof the Appalachian and Caledonian orogeny rather than acrossit-north/south faunal distribution rather than an east/wesrone, orovided that we recognizethat rhis mounrain belt underwent a massivesinistral shearduring the Devonian. Scandinavianand Scottish trilobites would then match trilobites found in Pennsylvaniabecausethe regionswould have been adjacentduring the relevantperiod in the Ordovician, before Scandinavia,Britain, Theoriesof the Earth, pp. 205-250). In the caseoflndia, Carey expiainsthe and Ireland were shifted 3,000 kilometersto unusually massiveuplift and clearevidenceof the north along this Devonian megashear compressionby suggesting thar,while ancient (Theoriesofthe Earth and [Jniuerse,pp. 180India was never separatedfrom the rest of l 84) . Asia, the subcontinentwas spreadout clockIn some casespaleontologyand paleowise in the direction ofArabia andAfrica, and geology actually work to the advantageof connected to Asia by a triangular block of expansionists. Many specialists of the Himacontinentalmaterialthat becamecomoressed layan region, for example, are convinced that

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there never was a wide seoaration berween lndia and the rest of Asia (SeeGansser). Nonetheless,evidenceof subductioncontinues to remain a potential sourceof embarrassment for fast-expansion theories, and certainly undercurs prospects for reconciliation berweenthe expansionand the plate tectonics researchprograms. To take just one illustration, consider rhe magnetic anomaly lineations produced in alternating parallel bands on eachside ofactive oceanicridge lines.This pattern is now widely understoodto be the effectofperiodic polarity reversalsin the geomagnetic field, imprinted on new oceaniccrust as it emerged from an oceanic rift valley, cooled past the Curie point, and gradually flo*ed away on eachside of the ridge line. But whiie the pattern is completelysymmetric acrossthe midAtlantic ridge, the corresponding Pacific pattern is severelytruncated in the eastern Pacific, where polariry reversalsextend back only as far as the Eocene. In the western Pacific, oceanic crust datesback to the Jurassic. Plate tectonicsaccommodaresthis asymmetry by assumingthat the missingportions of the eastern South Pacific have been subducted down the Peru-Chile trench, and those in the eastern North Pacific have been subducted and overrun by the western lareral motion of North America. Carey suggests insread that the Pacific simply grew asymmetrically. Advocates of plate tectonics, of course, are likely to dismiss such an explanarion asan ad hoc deviceto circumvent uncooperative facts. Primarily to deai with the evidence for subduction, and to securean even berter fir ofthe piecescomposingthe Pangaeansuperconrinent, some recent expansiontheorists, Iike Hugh Owen, haveadvocateda third class of theories, which we might call moderateexpansion. Instead of assuming that Pangaea constituted an aimost completely unbroken sialic shell covering a spherelessthan rwothirds of the Eartht present diameter, Owen arguesthat the earlyJurassicEarth must have beenat 80 percentof its presentdiameter.He speculatesthat the sialic shell started to break up much earlier, ar the beginning of the Paleozoic.OFcoursear that rare.exoansioncan explain only a portion oF rhe Phanerozoic continental displacement for which we now have evidence. But advocatesof moderate expansiondo not deny the occurrenceofsubduction, which they take to explain rhe balanceof the displacement.Owent only quarrel with plate tectonicsis the assumption that the

rate ofsubduction in the oceanictrenchesis equal to the rate ofcrustal creation at the oceanic ridges. Like Carey, he regardsthis assumption as an unexamined dogma of plate tectonics,and one that is not supportedby the evidence.(The nature ofthat evidenceis beyond the scopeofthis essay,but it is cataloged in detail in Carey, The Expanding Earth lpp 39-79), and summarized more informally in his Theoriesofthe Earth [pp. 150-189]). Owen arrived at the conclusion that the Earth wasroughly 80 percentofits presentsize. rather than the 60 percenrfigure offast expansionists,as a consequenceof meticulous map reconstructions employing traditional cartographic methods rather than commonly used computer programs.Most computer-generated cartography fails to reproject continents after they have been rotated or shifted with respecr to the projection pole usedin the original map construcdon. Without reprojection, a rotated or shifted continent has been distorted from its original shape,reducing the accuracyofthe fic. !7ith appropriate reprojections incorporated, Owen concluded that the goresof missing Tiiassic crust would be completelr' eliminated on an 80 percent globe (Atltu, pp 3-5, 8-1 1, I 5). (Figure 2 acrually depicts Owen's model of Pangaea,with a large Paleozoic Pacific. Hilgenberg, Carey, and Vogel would have at most a narrow Eo-Pacific lune separating Pangaeatwestern and easternextremities.) No expansiontheoriesenjoywidespread support in the 1990s.Although empiricalarguments againstmoderate and fast expansion were offered in the 1960s and 1970s, they are of debatablemerit (seeCarey, TheExpanding Earth, pp. 16-22; Theoriesofthe Earth, pp. 190-201). The realreasonsfor current indifference to this researchtradition seem to be more rheoreticalin narure. Ken Creer, for example, gaveup pursuing the idea of expansionwhen he realized how much distortion had to be accommodated in the continents if they were to be molded onro a globe of smallerdimensions. Unlike Hilgenberg, who worked with the malleablemedium of papier mAchd,Creer was forced to contend with the problem when he usedfiberglassto constructhis continents.In order to bend theselessflexiblepuzzlepieces to a smallerglobe,he simply took up the distortion in the center ofhis condnents. Recognizing later that this was an arbitrarv assumptionon his part, and that the distortion mighr have as easilyaffectedcontinents ar their edges,he gave up the project as a

EXPANDING

hopelessenterprise. (For the same reason, Owen refusesto speculateabout the precise fit of the continentsduring the Paleozoicera. But he chinlcsthat, while this distortion makes proiecruntenrhe expansionreconstruction able on a 60 percentglobe, distortion is still minimal at 80 percent.) The absenceof a physical mechanism to explain why expansionshould occur is a probiem more often cited by detractors. Indeed, expansion advocatesconcede that they can offer nothing more than speculation on this issue.Some of the speculationhasbeen pretry implausible. Hilgenberg, like Yarkovski,relied on the idea of transforming masslessether into matter, so that the Earth would have increased in mass as well as vohme (Vom wachsendznErdball, pp.29-35). The most widely shared explanation entertained today seemsto be the hypothesisthat the Earths core consists of disassociatedand densely packed subatomic particles that are undergoing a pha sech an gea nd as s um ingat om ic structure at the core/mantle boundary. In atomic form, these particles occupy more volume, thus exerting expansionarypressure on the Earth as a whole. This idea was proposedfirst by Lindemann, and by South African astronomerJ.K.E. Halm (1866-1944) in 1935 in "An Astronomical Aspect of the Evolution of the Earth," writing apparently in ignorance of both Lindemann's and Hilgenbergt work. But hereagain,thereis no developedphysical theory to explain why the core should be so constituted, and in standard views it is not. Ultimately, the problem with expansion for most Earth scientistsmay be Ockham's ruori they are convinced that, in the lateral convection belt models associatedwith plate tectonics,they alreadyhavea reasonablyplausible explanationof continental displacement. tWhy then should they rely on two distinct mechanisms,when one may suffice,and especially when the other seemsso bizarre? There is, at the level oftheory at least, a lack of eleganceand simpliciry in moderateexpansion in particular.Not only doesit require two apparently independent enginesof continental displacement, but it also lacks the internal aestheticuniry ofpure radial expansion. Owen cannot appeal to his theory, for example, to explain why all the continents should happen to accreteinto a singlesupercontinent ar the end of the Paleozoic. But such aesthedcjudgments are a matter of degree.Plate tectonics cannot explain why all the continentsshould congregateon

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one side of the globe either. On the other end of the spectrum, while Careyt model of fast expansionwould, ifcorrect, requireonly one mechanism for continental displacement, even his theory is more complicated, and arguably lesselegant, than the purely radial expansion proposedby Vogel. In the final analysis,regardlessof the aesthetic appeal of simplicity, expansion's prospects for supplanting plate tectonics, or even sharing the stagewith it, wiil be determined by additional empirical tests.It may be that the Earth is a lesstidy place than either expansion or plate tectonics purists would have us believe; it may be more like Owent somewhat messierplanet. Despite the current level ofunderexposure from which expansion theoriessuffet the jury is still out on the question of their empirical validiry.The fortunes of either moderate or fast exoansion could improveduring the coming deiades,although moderateexpansionlooks more promising at Present' Richard Nunan Bibliography Carey, S. 'Warren. The Expanding Earth. Developmentsin Geotectonics10. Amsterdam: Elsevier,1976. 'A Tectonic Approach to Continental Drift." ln Continental Drifi: A Symposium,edited by S.'W.Carey, pp. 177-355. Hobart: Universiryof Thsmania,1958. Theoriesofthe Earth and the Uniuerse. Stanford: Stanford Universiry Pr es s , 19 8 8 . Creer, Kenneth M. "Arr Expanding Earth?" Nature 205 (1965): pp. 539-544. Egyed, Lazlo. "The Change in the Eartht Dimensions Determined from PaleogeographicalData." Geofisica Pura e Applicata33 (1956): pp.4248. Gansser,Augusto. "Factsand Theorieson the Himalayas." EclogaeGeologicae Heluetiae 84 (1991):pp.33-59. Halm, J.K.E. 'An AstronomicalAspect of the Evolution of the E arth." Astronomical Societyof SouthAfica 4

(1935):pp.r-28. Heezen,Bruce C. "The Rift in the Ocean Floor." ScientificAmerican 203 (October 1960):pp. 98-l10. Hilgenberg, Otto C. Vom wachsenden Erdball. Berlin: Giessmann and Bar t s c h, 1 9 3 3 . Holmes, Arthur. Principlesof Physical Gnhgt. NewYork: Ronald Press,1965.

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Lindemann, B. Kettengebirge,hontinentale Zerspahung und Erdexpansion. J ena Fischer,1927. Owen, H.G. Atlas of Continental Displacement, 200 Million Yearsto the Present, Cambridge:Cambridge Universiry Press,1983. "Constant Dimensronsor an Expanding Earth?" In The Euoluing Earth, edited by L.R.M. Cocks, pp. 179-192. London: British Museum and Cambridge Universiry Press,1981. Vogel, Klaus. "The Expansionofthe Earth-An A.lternative Model to the Plate Tectonics Theory." In Critical Aspecx of the Plate TectonicsTheory Volume II (Ahernatiue Theories), edited by S.S.Augustithis et al., pp. 19-34. Athens: TheophrastusPublications, 1990. 'W'ilson, Tuzo. "Did the Atlantic Close J. and then Reopen?"Nature2ll (1966):

pp.676-68r. Seeaho Cartography; Continental Drifr and PlateTectonics;Earth, Sizeof; Oceanography, Physical;Paleomagnetism;Paleontology;SeaLevel; Shifting Crust Theory Exploration, A6!e of Critical period in the dwelopment of European consciousness. The study of the Earth was greatly affected by interest in navigation and cartography, the concept of the terraqueous globe was first discussed,effbctsof climate on human beings was investigared,and theories of ethnographl/were proposed. The Age of Exploration (fifteenth through sevenreenthcenturies)was a critical period in the developmentof Europeanconsciousness. Although the Chinesehad probably sailedfarther and many fishing peoples had been traversingthe Atlantic Ocean for centuries,the achievementsofVasco da Gama (ca. 1469-1524) and ChristopherColumbus (1451-1506), aswell as thosewho followed, fundamentally changed the way Europeans understood che Earth and their relationship to it. These early explorers,equippedwith a Christian and imperial belief in the righreousnessoftheir causeand the superioriryoftheir understanding,challengedthe authority of the ancients,especiallyClaudius Ptolemy (fl. secondcentury c.B.). Columbus and those who came after demonsrratedto Eurooeans the exisrence of a continentcomoierelvunknown to the ancients(rhough well known to its inhabitants).More imporrant for Earth

In thesixteenth centurl,m/e andglobemahers snugledto keepabrea*ofexplorationJohannes (1477-1547) produced Schtjner a numberofmaps andglobr that re;flexed hischangingideas.Source: A E. Nordenshiald, Facsimile Atlasto theEarly Historyof Cartography with Reproductions of the MostImportantMapsPrintedin theXV andXVI 1889.Tianslatedfom the CenruriesStocbholm, Swedish originalbyJohanAdolf Ekeldfand Clements R Markham,witha newintoductionbyJ.B,Post. Neu Yorh:Douer,1973,p 83. Vitb permission of DouerPublications studies,theseexplorersdisproveda number of ancient and medievaltheoriesof the Earth most particularly by demonstratingthat the globe has a much larger proportion of drr land than had hirherto been suspected,tha: it is possibleto sail rhrough the equatoriaregionswithout burning up, and that peopi. can and did live south of that eouacorialregion in the landsknown as the antipodes The "Voyagesof Discovery" were nc: principaily concernedwith investigatingth. Earth, so it.is not surprisingthat rhe geoloe:cal information they coliectedwas minim. The prime motivating factor for thesevo' ' ageswas amassinggreatwealth, both for t--.. individualand for the counrry sponsori:.. the enterprises.At firsr, the destination ". the Far East-Cathay and the SpiceIslan., The Portuguesewere most successful.' r e a c h i n gt h e s ea r e a s s, e t t i n g u p i m p o r c : trading depotsin Goa (India), Malacca(\ I . laysia),and the Moluccas (SpiceIslanc, The Spanish,having reachedthe Amer.,by mistake,soon modified their mission. although they continued to seek gold . -