Today, Antarctica seems to be part of asingle tectonic plate. But there is clearpalaeomagnetic evidence that, in thepast, there has been a large relative rotationbetween different parts of the continent1. Acentral issue has been how much, if any, ofthis motion has occurred during the past 80million years or so. Cande et al.2 (page 145 ofthis issue) now present data that reveal anextinct plate-tectonic boundary between thetwo parts of Antarctica, East and West, there-by enabling them to estimate that motion.East Antarctica is the larger and has been astable part of a nearly rigid plate for hun-dreds of millions of years. West Antarctica,by contrast, is probably an amalgamation ofmany smaller pieces that were assembled inthe past few hundred million years. The sig-nificance of Cande and colleagues studygoes well beyond Antarctica, however,because motion in Antarctica is a centralelement in reconstructing plate-tectonicmotions around the globe.

To understand the evolution of the platemargins surrounding the Pacific Ocean, onewould like to know the displacement historyof the plates of the Pacific basin relative tothe surrounding continental plates. Duringseafloor spreading the polarity of the geo-magnetic field, which reverses on averageevery half million years or so, is locked intonewly created and cooling crust, thus pre-serving a record of the history of relativemotion of the sea floor flanking a mid-oceanridge. Processes at converging plate bound-aries, however, leave no useful record for esti-mating the relative plate motions that haveoccurred across them. To estimate themotion across such boundaries, a so-calledplate-motion circuit through a series ofmid-ocean ridges is needed.

The present circum-Pacific margin hasonly one very small segment of mid-oceanridge, in the Gulf of California along thecoast of Mexico, and it has existed for only afew million years. So, to estimate the motionbetween Pacific-basin plates and the sur-rounding continental plates for any timeexcept the past few million years, a plate-motion circuit must be constructed throughmid-ocean ridges3.

Figure 1 illustrates a set of such plate-motion circuits. It shows the central role ofthe Antarctic plate, which is nearly sur-rounded by mid-ocean ridges. To estimatethe motion of the Pacific plate relative to theNorth American plate, for example, the rele-vant circuit is the Pacific plate to the Antarc-tic plate (via the PacificAntarctic Rise) tothe African plate (via the Southwest Indian

Ridge) to the North American plate (via theMid-Atlantic Ridge), with the relativemotion between each of these plate pairsbeing summed. Similarly, the circuit mustpass through Antarctica to estimate themotion between any Pacific-basin plate andany of the surrounding continental plates North America, South America, Eurasia orAustralia. These plate-motion circuits can beused, at most, for the past 80 million years,the time during which the PacificAntarcticRise has existed.

These same plate-motion circuits are themain yardsticks that one has for estimatingthe relative motion between the hotspots inthe Pacific Ocean and those in the Atlanticand Indian oceans. Hotspots are long-livedsources of volcanism, such as that which isnow active at Kilauea volcano in the Hawai-ian islands. Hotspots are thought to be thesurface manifestations of plumes rising fromdeep in the mantle. As a plate moves relativeto a plume beneath it, a trail of extinct volca-noes, such as the HawaiianEmperor islandand seamount chain, is created.

The original plate reconstructions for thesouth Pacific, Antarctica, Australia and NewZealand, published in 1975, implied thatabout 500 km of motion was requiredbetween East and West Antarctica sinceabout 80 million years ago4. Estimates of thepast motions of Pacific-basin plates relative

to the circum-Pacific plates showed thatthese reconstructions are extremely sensitiveto assumed motion between East and WestAntarctica5.

Furthermore, two approaches for esti-mating the northward motion of the Pacificplate relative to Earths spin axis have pro-duced estimates that disagree; motionbetween East and West Antarctica mightpartly explain this disagreement6. In the firstapproach, the northward motion is estimat-ed directly from Pacific plate palaeomag-netic data, which record the ancient time-averaged direction of Earths magnetic field.The time-averaged inclination, which is theangle that the magnetic field makes with thehorizontal, varies in a predictable waywith palaeolatitude, from which northwardmotion can be inferred. In the secondapproach, the amount of northward motionof the Pacific plate is inferred from palaeo-magnetic data reconstructed from otherplates through the global circuit throughAntarctica, assuming that Antarctica is a sin-gle plate6.

There also has long been a large misfitbetween observed and hypothetical Pacifichotspots tracks predicted in a referenceframe fixed to Atlantic and Indian Oceanhotspots. One explanation7 for the misfit issubstantial motion between Pacific and non-Pacific hotspots. A second explanation8,9 isthat the hotspots in the Pacific are fixed rela-tive to non-Pacific hotspots, and that motionbetween East Antarctica and West Antarcticacauses the misfit.

Since 1975, new data and interpreta-tions have reduced the size of the plate-reconstruction misfits that required motion

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NATURE | VOL 404 | 9 MARCH 2000 | www.nature.com 139

Plate tectonics

The Antarctic connectionRichard G. Gordon

Figure 1 Global plate-motion circuits for much of the past 80 million years. The double-headedarrows point to pairs of plates separated by a mid-ocean ridge from which the relative plate motion ofthe plate pair can be determined. For example, the motion of the Kula plate relative to the NorthAmerican plate is determined by the plate-motion circuit Kula (Ku) to Pacific (Pa) to West Antarctic(WAn) to East Antarctic (EAn) to Africa (Af) to North America (NA). Au, Australia; Eu, Eurasia; Fa,Farallon; Gr, Greenland; In, India; SA, South America. (Redrawn from refs 6, 12.)

NA

SA

Gr

Eu

In

AuWAn EAn

Fa

Ku

Pa

Af

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between East and West Antarctica. Someworkers have gone so far as to assert thatthere has been no such motion during thepast 80 million years. Much of the ambiguityhas been due to a lack of essential geophysicaldata from high-latitude regions, which aredifficult to survey because of the presence ofsea ice.

Cande et al.2 have produced such datafrom marine geophysical surveys north ofthe Ross Sea and in the South Tasman Sea(see Figs 1 and 2 of the paper on pages 145and 146). These data allow them to directlyestimate motion between East and WestAntarctica. They have discovered an extinctspreading centre north of the western RossSea embayment, which is flanked by a set oflinear magnetic anomalies that formedbetween 43 and 26 million years ago. Theseanomalies indicate a spreading rate of some12 mm per year, about the same rate as theslowest rates of seafloor spreading observedtoday.

Cande et al. combine these and other datato show that there was about 180 km of sepa-ration in the western Ross Sea embayment,and hence 180 km of motion between Eastand West Antarctica, between 43 and 26 mil-lion years ago. This work not only places firmbounds on how much motion could haveoccurred between East and West Antarctica;it also for the first time permits rigor-ous estimates of this motion to be incorpo-rated into the global plate-motion circuitand into estimates of the motion of Pacifichotspots relative to non-Pacific hotspots.

The results bear directly on the questionof the origin of the elbow in the Hawai-ianEmperor chain in the north Pacific(Fig 2). Ages along the chain, which recordthe motion of the Pacific plate relative to theHawaiian hotspot, increase monotonicallyto the northwest from zero (that is, activevolcanism) at its southeast end (for exampleKilauea on the big island of Hawaii) to about80 million years near where it intersects theAleutian trench. The chain consists mainlyof two nearly straight segments, the oldernorth-northwest-striking Emperor segmentwith ages from about 80 to 43 million years,and the younger west-northwest-striking

Hawaiian segment with ages less than 43 mil-lion years. These two straight segments meetat an angle of about 120 in an elbow, whichis widely, but not universally, interpreted asrecording a 60 change at 43 million yearsago in the direction of Pacific plate motionrelative to the Hawaiian and other Pacificplumes.

Reconstructions10 of the Pacific plate rel-ative to Atlantic and Indian Ocean hotspotsthat use Cande and colleagues estimatesindicate no sharp change in motion of thePacific plate relative to the Atlantic and Indi-an Ocean hotspots at about 43 million yearsago. If so, it implies that there was a sharp andsudden change in the motion of Pacifichotspots relative to those in other oceanbasins, which is difficult to understand inview of what is known about convection inEarths mantle11.

In any event, the new work of Cande et al.constitutes great progress. It will have apivotal influence on the future course ofresearch on global tectonics. nRichard G. Gordon is at the Department of Geologyand Geophysics, University of California, Berkeley,California 94720-4767, USA, on sabbatical leavefrom the Department of Geology and Geophysics,Rice University.e-mail: rgg@rice.edu

1. Luyendyk, B. et al. Tectonics 15, 122141 (1996).

2. Cande, S. C., Stock, J. M., Mller, D. & Ishihara, T. Nature 404,

145150 (2000).

3. Atwater, T. & Molnar, P. in Proceedings of Conference on Tectonic

Problems of San Andreas Fault System (eds Kovach, R. L. & Nur,

A.) 136148 (Stanford Univ. Publ. Geol. Sci., 1973).

4. Molnar, P., Atwater, T., Mammerickx, J. & Smith, S. M. Geophys.

J. R. Astron. Soc. 40, 383420 (1975).

5. Jurdy, D. M. J. Geophys. Res. 84, 67966802 (1979).

6. Acton, G. D. & Gordon, R. G. Science 263, 12461254 (1994).

7. Molnar, P. & Stock, J. Nature 327, 587591 (1987).

8. Morgan, W. J. in Oceanic Lithosphere (The Sea, Vol. 7) (ed.

Emiliani, C.) 443487 (Wiley, New York, 1981).

9. Duncan, R. A. Tectonophysics 74, 2942 (1981).

10.Raymond, C. A., Stock, J. M. & Cande, S. C. in History and

Dynamics of Global Plate Motions (eds Richards, M., Gordon,

R. G. & van der Hilst, R. D.) (Am. Geophys. Un., Washington

DC, in the press).

11.Bercovici, D., Ricard, Y. & Richards, M. in History and

Dynamics of Global Plate Motions (eds Richards, M., Gordon,

R. G. & van der Hilst, R. D.) (Am. Geophys. Un., Washington

DC, in the press).

12.Engebretson, D. C., Cox, A. & Gordon, R. G. Geol. Soc. Am.

Spec. Pap. 206 (1985).

13.http://www.ngdc.noaa.gov/mgg/announcements/

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Daedalus

Expiry date guaranteedLast week Daedalus invented the short-lifepet. By repeatedly dividing a newlyfertilized ovum in vitro, he obtained largenumbers of single-cell ova, each of whichhad already undergone many celldivisions. Every time a cell divides, each ofits chromosomes loses a telomere unitfrom its termination; when they have allgone, it can divide no longer, and isdoomed. So Daedaluss telomere-depletedova have a limited life ahead of them.Brought to term in a surrogate mother,they are sure to die young making themperfect pets.

This process has further implications.Do even human identical twins, who haveundergone one more cell division than therest of us, have a slightly reduced lifespan?Probably not. Few of us die from telomereexhaustion; indeed, even Daedalus finds ithard to guess how it would show itself.With luck it will be a mercifully swiftextinction, as one whole class of body cells in the gut, say, or the liver suddenlyceases to replicate. If so, Daedalus seesapplications in humane animalhusbandry. At present, sadly, all farmedanimals have to be deliberately killed. Anyanimal that dies a natural death hasprobably succumbed to some pathogen,and its meat is unsafe to eat. But an animalthat dies of telomere exhaustion would beentirely safe. With proper timing, it couldbe arranged to drop dead just after havingreached its ideal size and condition.

Even better, repeated ovum divisioncreates unlimited numbers of suchanimals, all identical. Implanted intosurrogate mothers, they would all have thesame predictable lifespan. DREADCOagronomists are now studying the logisticsof a humane agriculture in which identicalova are implanted at regular intervals intoa production line of surrogate mothers.The animals are brought to term andallowed to grow up normally. On thepredicted day of their death, they aretransported to the slaughterhouse bymodern just in time delivery methods.They walk onto the conveyor belt in thecorrect order, and each drops dead just atit reaches the butchery section. Noviolence is necessary. Animal-rightsenthusiasts will cheer, consumers willwelcome the reliable consistency of theproducts, and accountants will revel in theutter predictability of it all. David Jones

The Further Inventions of Daedalus (OxfordUniversity Press), 148 past Daedalus columnsexpanded and illustrated, is now on sale.Special Nature offer: m.curtis@nature.com

Figure 2 Topography of the north Pacific sea floor, showing the elbow where the Hawaiian island andseamount chain meets the Emperor seamount chain. (Map reproduced from ref. 13, courtesy ofWalter H. F. Smith.)

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