lifetimes 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’s 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—venus is nearly a twin of earth inthese respects and yet has no tectonic activity。 it is thought—though it is really nothing morethan a thought—that tectonics is an important part of the planet’s organic well…being。 as thephysicist and writer james trefil has put it； “it would be hard to believe that the continuousmovement of tectonic plates has no effect on the development of life on earth。” he suggeststhat the challenges induced by tectonics—changes in climate； for instance—were animportant spur to the development of intelligence。 others believe the driftings of thecontinents may have produced at least some of the earth’s various extinction events。 innovember of 2002； tony dickson of cambridge university in england produced a report；published in the journal science； strongly suggesting that there may well be a relationshipbetween the history of rocks and the history of life。 what dickson established was that thechemical position of the world’s oceans has altered abruptly and vigorously throughoutthe past half billion years and that these changes often correlate with important events inbiological history—the huge outburst of tiny organisms that created the chalk cliffs ofengland’s south coast； the sudden fashion for shells among marine organisms during the cambrian period； and so on。 no one can say what causes the oceans’ chemistry to change sodramatically from time to time； but the opening and shutting of ocean ridges would be anobvious possible culprit。
at all events； plate tectonics not only explained the surface dynamics of the earth—how anancient hipparion got from france to florida； for example—but also many of its internalactions。 earthquakes； the formation of island chains； the carbon cycle； the locations ofmountains； the ing of ice ages； the origins of life itself—there was hardly a matter thatwasn’t directly influenced by this remarkable new theory。 geologists； as mcphee has noted；found themselves in the giddying position that “the whole earth suddenly made sense。”
but only up to a point。 the distribution of continents in former times is much less neatlyresolved than most people outside geophysics think。 although textbooks give confident…looking representations of ancient landmasses with names like laurasia； gondwana； rodinia；and pangaea； these are sometimes based on conclusions that don’t altogether hold up。 asgeorge gaylord simpson observes in fossils and the history of life； species of plants andanimals from the ancient world have a habit of appearing inconveniently where they shouldn’tand failing to be where they ought。
the outline of gondwana； a once…mighty continent connecting australia； africa；antarctica； and south america； was based in large part on the distribution of a genus ofancient tongue fern called glossopteris； which was found in all the right places。 however；much later glossopteris was also discovered in parts of the world that had no knownconnection to gondwana。 this troubling discrepancy was—and continues to be—mostlyignored。 similarly a triassic reptile called lystrosaurus has been found from antarctica allthe way to asia； supporting the idea of a former connection between those continents； but ithas never turned up in south america or australia； which are believed to have been part ofthe same continent at the same time。
there are also many surface features that tectonics can’t explain。 take denver。 it is； aseveryone knows； a mile high； but that rise is paratively recent。 when dinosaurs roamedthe earth； denver was part of an ocean bottom； many thousands of feet lower。 yet the rockson which denver sits are not fractured or deformed in the way they would be if denver hadbeen pushed up by colliding plates； and anyway denver was too far from the plate edges to besusceptible to their actions。 it would be as if you pushed against the edge of a rug hoping toraise a ruck at the opposite end。 mysteriously and over millions of years； it appears thatdenver has been rising； like baking bread。 so； too； has much of southern africa； a portion ofit a thousand miles across has risen nearly a mile in 100 million years without any knownassociated tectonic activity。 australia； meanwhile； has been tilting and sinking。 over the past100 million years as it has drifted north toward asia； its leading edge has sunk by some sixhundred feet。 it appears that indonesia is very slowly drowning； and dragging australia downwith it。 nothing in the theories of tectonics can explain any of this。
alfred wegener never lived to see his ideas vindicated。 on an expedition to greenland in1930； he set out alone； on his fiftieth birthday； to check out a supply drop。 he never returned。
he was found a few days later； frozen to death on the ice。 he was buried on the spot and liesthere yet； but about a yard closer to north america than on the day he died。
einstein also failed to live long enough to see that he had backed the wrong horse。 in fact；he died at princeton； new jersey； in 1955 before charles hapgood’s rubbishing of continentaldrift theories was even published。
the other principal player in the emergence of tectonics theory； harry hess； was also atprinceton at the time； and would spend the rest of his career there。 one of his students was abright young fellow named walter alvarez； who would eventually change the world ofscience in a quite different way。
as for geology itself； its cataclysms had only just begun； and it was young alvarez whohelped to start the process。
part iv dangerous planetthe history of any one part of theearth； like the life of a soldier； consistsof long periods of boredom andshort periods of terror。
…british geologist derek v。 ager
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13BANG！
@小‘说〃网
people knew for a long time that there was something odd about the earth beneathmanson； iowa。 in 1912； a man drilling a well for the town water supply reported bringing up alot of strangely deformed rock—“crystalline clast breccia with a melt matrix” and “overturnedejecta flap；” as it was later described in an official report。 the water was odd too。 it wasalmost as soft as rainwater。 naturally occurring soft water had never been found in iowabefore。
though manson’s strange rocks and silken waters were matters of curiosity； forty…oneyears would pass before a team from the university of iowa got around to making a trip to themunity； then as now a town of about two thousand people in the northwest part of thestate。 in 1953； after sinking a series of experimental bores； university geologists agreed thatthe site was indeed anomalous and attributed the deformed rocks to some ancient； unspecifiedvolcanic action。 this was in keeping with the wisdom of the day； but it was also about aswrong as a geological conclusion can get。
the trauma to manson’s geology had e not from within the earth； but from at least 100million miles beyond。 sometime in the very ancient past； when manson stood on the edge of ashallow sea； a rock about a mile and a half across； weighing ten billion tons and traveling atperhaps two hundred times the speed of sound ripped through the atmosphere and punchedinto the earth with a violence and suddenness that we can scarcely imagine。 where mansonnow stands became in an instant a hole three miles deep and more than twenty miles across。
the limestone that elsewhere gives iowa its hard mineralized water was obliterated andreplaced by the shocked basement rocks that so puzzled the water driller in 1912。
the manson impact was the biggest thing that has ever occurred on the mainland unitedstates。 of any type。 ever。 the crater it left behind was so colossal that if you stood on oneedge you would only just be able to see the other side on a good day。 it would make the grandcanyon look quaint and trifling。 unfortunately for lovers of spectacle； 2。5 million years ofpassing ice sheets filled the manson crater right to the top with rich glacial till； then graded itsmooth； so that today the landscape at manson； and for miles around； is as flat as a tabletop。
which is of course why no one has ever heard of the manson crater。
at the library in manson they are delighted to show you a collection of newspaper articlesand a box of core samples from a 1991–92 drilling program—indeed； they positively bustle toproduce them—but you have to ask to see them。 nothing permanent is on display； andnowhere in the town is there any historical marker。
to most people in manson the biggest thing ever to happen was a tornado that rolled upmain street in 1979； tearing apart the business district。 one of the advantages of all thatsurrounding flatness is that you can see danger from a long way off。 virtually the whole townturned out at one end of main street and watched for half an hour as the tornado came toward them； hoping it would veer off； then prudently scampered when it did not。 four of them； alas；didn’t move quite fast enough and were killed。