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ia before shooting up the west coast of the united states to alaska。
occasionally its higher peaks poked above the water as an island or archipelago鈥攖he azoresand canaries in the atlantic； hawaii in the pacific； for instance鈥攂ut mostly it was buriedunder thousands of fathoms of salty sea； unknown and unsuspected。 when all its brancheswere added together； the network extended to 46；600 miles。
a very little of this had been known for some time。 people laying ocean…floor cables in thenineteenth century had realized that there was some kind of mountainous intrusion in the mid…atlantic from the way the cables ran； but the continuous nature and overall scale of the chainwas a stunning surprise。 moreover； it contained physical anomalies that couldn鈥檛 be explained。
down the middle of the mid…atlantic ridge was a canyon鈥攁 rift鈥攗p to a dozen miles widefor its entire 12；000…mile length。 this seemed to suggest that the earth was splitting apart atthe seams； like a nut bursting out of its shell。 it was an absurd and unnerving notion； but theevidence couldn鈥檛 be denied。
then in 1960 core samples showed that the ocean floor was quite young at the mid…atlanticridge but grew progressively older as you moved away from it to the east or west。 harry hessconsidered the matter and realized that this could mean only one thing： new ocean crust wasbeing formed on either side of the central rift； then being pushed away from it as new crustcame along behind。 the atlantic floor was effectively two large conveyor belts； one carryingcrust toward north america； the other carrying crust toward europe。 the process becameknown as seafloor spreading。
when the crust reached the end of its journey at the boundary with continents； it plungedback into the earth in a process known as subduction。 that explained where all the sedimentwent。 it was being returned to the bowels of the earth。 it also explained why ocean floorseverywhere were so paratively youthful。 none had ever been found to be older than about175 million years； which was a puzzle because continental rocks were often billions of yearsold。 now hess could see why。 ocean rocks lasted only as long as it took them to travel toshore。 it was a beautiful theory that explained a great deal。 hess elaborated his ideas in animportant paper； which was almost universally ignored。 sometimes the world just isn鈥檛 readyfor a good idea。
meanwhile； two researchers； working independently； were making some startling findingsby drawing on a curious fact of earth history that had been discovered several decades earlier。
in 1906； a french physicist named bernard brunhes had found that the planet鈥檚 magnetic fieldreverses itself from time to time； and that the record of these reversals is permanently fixed incertain rocks at the time of their birth。 specifically； tiny grains of iron ore within the rockspoint to wherever the magnetic poles happen to be at the time of their formation； then staypointing in that direction as the rocks cool and harden。 in effect they 鈥渞emember鈥潯here themagnetic poles were at the time of their creation。 for years this was little more than acuriosity； but in the 1950s patrick blackett of the university of london and s。 k。 runcorn ofthe university of newcastle studied the ancient magnetic patterns frozen in british rocks andwere startled； to say the very least； to find them indicating that at some time in the distant pastbritain had spun on its axis and traveled some distance to the north； as if it had somehowe loose from its moorings。 moreover； they also discovered that if you placed a map ofeurope鈥檚 magnetic patterns alongside an american one from the same period； they fit togetheras neatly as two halves of a torn letter。 it was uncanny。
their findings were ignored too。
it finally fell to two men from cambridge university； a geophysicist named drummondmatthews and a graduate student of his named fred vine； to draw all the strands together。 in1963； using magnetic studies of the atlantic ocean floor； they demonstrated conclusively thatthe seafloors were spreading in precisely the manner hess had suggested and that thecontinents were in motion too。 an unlucky canadian geologist named lawrence morley cameup with the same conclusion at the same time； but couldn鈥檛 find anyone to publish his paper。
in what has bee a famous snub； the editor of the journal of geophysical research toldhim： 鈥渟uch speculations make interesting talk at cocktail parties； but it is not the sort of thingthat ought to be published under serious scientific aegis。鈥潯ne geologist later described it as鈥減robably the most significant paper in the earth sciences ever to be denied publication。鈥�
at all events； mobile crust was an idea whose time had finally e。 a symposium ofmany of the most important figures in the field was convened in london under the auspices ofthe royal society in 1964； and suddenly； it seemed； everyone was a convert。 the earth； themeeting agreed； was a mosaic of interconnected segments whose various stately jostlingsaccounted for much of the planet鈥檚 surface behavior。
the name 鈥渃ontinental drift鈥潯as fairly swiftly discarded when it was realized that thewhole crust was in motion and not just the continents； but it took a while to settle on a namefor the individual segments。 at first people called them 鈥渃rustal blocks鈥潯r sometimes 鈥減avingstones。鈥潯ot until late 1968； with the publication of an article by three americanseismologists in the journal of geophysical research ； did the segments receive the name bywhich they have since been known： plates。 the same article called the new science platetectonics。
old ideas die hard； and not everyone rushed to embrace the exciting new theory。 well intothe 1970s； one of the most popular and influential geological textbooks； the earth by thevenerable harold jeffreys； strenuously insisted that plate tectonics was a physicalimpossibility； just as it had in the first edition way back in 1924。 it was equally dismissive ofconvection and seafloor spreading。 and in basin and range； published in 1980； john mcpheenoted that even then one american geologist in eight still didn鈥檛 believe in plate tectonics。
today we know that earth鈥檚 surface is made up of eight to twelve big plates （depending onhow you define big） and twenty or so smaller ones； and they all move in different directionsand at different speeds。 some plates are large and paratively inactive； others small butenergetic。 they bear only an incidental relationship to the landmasses that sit upon them。 thenorth american plate； for instance； is much larger than the continent with which it isassociated。 it roughly traces the outline of the continent鈥檚 western coast （which is why thatarea is so seismically active； because of the bump and crush of the plate boundary）； butignores the eastern seaboard altogether and instead extends halfway across the atlantic to themid…ocean ridge。 iceland is split down the middle； which makes it tectonically half americanand half european。 new zealand； meanwhile； is part of the immense indian ocean plate eventhough it is nowhere near the indian ocean。 and so it goes for most plates。
the connections between modern landmasses and those of the past were found to beinfinitely more plex than anyone had imagined。 kazakhstan； it turns out； was onceattached to norway and new england。 one corner of staten island； but only a corner； iseuropean。 so is part of newfoundland。 pick up a pebble from a massachusetts beach； and itsnearest kin will now be in africa。 the scottish highlands and much of scandinavia aresubstantially american。 some of the shackleton range of antarctica； it is thought； may oncehave belonged to the appalachians of the eastern u。s。 rocks； in short； get around。
the constant turmoil keeps the plates from fusing into a single immobile plate。 assumingthings continue much as at present； the atlantic ocean will expand until eventually it is muchbigger than the pacific。 much of california will float off and bee a kind of madagascar ofthe pacific。 africa will push northward into europe； squeezing the mediterranean out ofexistence and thrusting up a chain of mountains of himalayan majesty running from paris tocalcutta。 australia will colonize the islands to its north and connect by some isthmianumbilicus to asia。 these are future outes； but not future events。 the events are happeningnow。 as we sit here； continents are adrift； like leaves on a pond。 thanks to global positioningsystems we can see that europe and north america are parting at about the speed a fingernailgrows鈥攔oughly two yards in a human lifetime。 if you were prepared to wait long enough；you could ride from los angeles all the way up to san francisco。 it is only the brevity oflifetimes that keeps us from appreciating the changes。 look at a globe and what you areseeing really is a snapshot of the continents as they have been for just one…tenth of 1 percentof the earth鈥檚 history。
earth is alone among the rocky planets in having tectonics； and why this should be is a bitof a mystery。 it is not simply a matter of size or density鈥攙enus is nearly a twin of earth inthese respects and yet has no tectonic activity。 it is thought鈥攖hough it is really nothing more