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s more like half an inch。） above all； he could offerno convincing explanation for how the landmasses moved about。 to believe in his theory youhad to accept that massive continents somehow pushed through solid crust； like a plowthrough soil； without leaving any furrow in their wake。 nothing then known could plausiblyexplain what motored these massive movements。
it was arthur holmes； the english geologist who did so much to determine the age of theearth； who suggested a possible way。 holmes was the first scientist to understand thatradioactive warming could produce convection currents within the earth。 in theory thesecould be powerful enough to slide continents around on the surface。 in his popular andinfluential textbook principles of physical geology ； first published in 1944； holmes laid outa continental drift theory that was in its fundamentals the theory that prevails today。 it wasstill a radical proposition for the time and widely criticized； particularly in the united states；where resistance to drift lasted longer than elsewhere。 one reviewer there fretted； without anyevident sense of irony； that holmes presented his arguments so clearly and pellingly thatstudents might actually e to believe them。
elsewhere； however； the new theory drew steady if cautious support。 in 1950； a vote at theannual meeting of the british association for the advancement of science showed that abouthalf of those present now embraced the idea of continental drift。 （hapgood soon after citedthis figure as proof of how tragically misled british geologists had bee。） curiously；holmes himself sometimes wavered in his conviction。 in 1953 he confessed： 鈥渋 have neversucceeded in freeing myself from a nagging prejudice against continental drift； in mygeological bones； so to speak； i feel the hypothesis is a fantastic one。鈥�
continental drift was not entirely without support in the united states。 reginald daly ofharvard spoke for it； but he； you may recall； was the man who suggested that the moon hadbeen formed by a cosmic impact； and his ideas tended to be considered interesting； evenworthy； but a touch too exuberant for serious consideration。 and so most american academicsstuck to the belief that the continents had occupied their present positions forever and thattheir surface features could be attributed to something other than lateral motions。
interestingly； oil pany geologists had known for years that if you wanted to find oil youhad to allow for precisely the sort of surface movements that were implied by plate tectonics。
but oil geologists didn鈥檛 write academic papers； they just found oil。
there was one other major problem with earth theories that no one had resolved； or evene close to resolving。 that was the question of where all the sediments went。 every yearearth鈥檚 rivers carried massive volumes of eroded material鈥�500 million tons of calcium； forinstance鈥攖o the seas。 if you multiplied the rate of deposition by the number of years it hadbeen going on； it produced a disturbing figure： there should be about twelve miles ofsediments on the ocean bottoms鈥攐r； put another way； the ocean bottoms should by now bewell above the ocean tops。 scientists dealt with this paradox in the handiest possible way。
they ignored it。 but eventually there came a point when they could ignore it no longer。
in the second world war； a princeton university mineralogist named harry hess was putin charge of an attack transport ship； the uss cape johnson。 aboard this vessel was a fancynew depth sounder called a fathometer； which was designed to facilitate inshore maneuvers during beach landings； but hess realized that it could equally well be used for scientificpurposes and never switched it off； even when far out at sea； even in the heat of battle。 whathe found was entirely unexpected。 if the ocean floors were ancient； as everyone assumed； theyshould be thickly blanketed with sediments； like the mud on the bottom of a river or lake。 buthess鈥檚 readings showed that the ocean floor offered anything but the gooey smoothness ofancient silts。 it was scored everywhere with canyons； trenches； and crevasses and dotted withvolcanic seamounts that he called guyots after an earlier princeton geologist named arnoldguyot。 all this was a puzzle； but hess had a war to take part in； and put such thoughts to theback of his mind。
after the war； hess returned to princeton and the preoccupations of teaching； but themysteries of the seafloor continued to occupy a space in his thoughts。 meanwhile； throughoutthe 1950s oceanographers were undertaking more and more sophisticated surveys of theocean floors。 in so doing； they found an even bigger surprise： the mightiest and mostextensive mountain range on earth was鈥攎ostly鈥攗nderwater。 it traced a continuous pathalong the world鈥檚 seabeds； rather like the stitching on a baseball。 if you began at iceland； youcould follow it down the center of the atlantic ocean； around the bottom of africa； and acrossthe indian and southern oceans； below australia； there it angled across the pacific as ifmaking for baja california 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。